John Preskill: From the Early Universe to the Future of Quantum Computing

[Music] hi I'm Lawrence Krauss and welcome to the origins podcast John presco is the Richard P Feynman professor of physics at Caltech and also director of its Institute for Quantum information and matter where he directs a program in Quantum Computing and Quantum information and that's what he's perhaps most well known for now an incredibly exciting area which we spent a lot of time in this podcast talking about as I'll get to but John's background is actually in fundamental particle physics and cosmology which is what he was working on when I first got to know him we were together in in Boston when he was a graduate student at at Harvard and I was a graduate student at MIT and then we were together at Harvard for several years when he was ultimately an assistant professor at Harvard and uh I have to say we've written one paper together but beyond that I've learned more detailed physics from John then perhaps any other collaborator because what he does when he's teaching or working is produces the most amazing set of of of notes lecture notes and otherwise and I've actually found his his notes useful in my own teaching as well John actually has a graduate student did some work in cosmology having to do with things called magnetic monopoles that basically change the future of cosmology because his work on monopolis in the Euro Universe motivated in some sense Alan Guth to think about a problem that he eventually solved with his theory of inflation so inflation was partly motivated by resolving a profound problem in cosmology that that John had demonstrated in in his early work as a graduate student and uh and John's worked in a variety of areas of fundamental physics but eventually moved to the area of quantum information and Quantum Computing where he has helped establish and lead the one of the leading centers in that area and I wanted to talk to him about his experience as a scientist early on but ultimately to focus on Quantum Computing which is a subject you hear about a lot in them in the press and there's lots of hype and it's hard to separate the hype from reality and I thought it'd be best to go to the horse's mouth and John and I talk about the future of quantum computing its present State the challenges and the opportunities and it's a a it's a I believe a fascinating uh introduction into that field with a very clear thinker and speaker careful to to be accurate at all times it was a real joy to spend some time with John again uh during this episode I think you'll uh really enjoy it and I hope you can watch it uh ad free on our critical mass sub stack site by subscribing to critical mass and and those subscriptions uh those paid subscriptions support the origins project Foundation that runs that produces the origins podcast and also our public programming and travel adventures and other opportunities to try and connect science and culture and the most interesting ideas in the 21st century and bring them to the public you can of course watch it for free on on YouTube and or listen to it on any uh standard podcast site so however you watch it or listen to it I hope you'll really enjoyed this episode with John presco foreign thank you so much for uh for joining me it's been quite a while since we've been together but it's great to see you again it's a great pleasure and we go back we go back in fact you were you won't remember this but I will well I bet I do okay I the first very first I remember the very first time I met you I think I do too but we'll see if our stories aligned you were in your you were in the office you shared it with my friend Ian Affleck exactly and I would I've just been accepted to MIT and I came down and stayed with Ian and then visited Harvard where you and you were there and I remember I remember how intimidating you seemed at the time to me you were kidding me you know because you're working we were talking and you were just at your desk working I thought my God this guy can work through all this I really it's really I don't remember the part about working but I do remember being introduced by Ian and I I gathered that the two of you had a rapport being fellow Canadians fellow Canadians although he and Ian was uh he actually even grew up in the city I went to college in Ottawa and um and had it you know very my neither will get you your background neither my parents finished high school Ian's came from a different family he told me once I don't know if you ever knew this but he wrote a poem when he was in kindergarten or Raid one which said when I will grow up I want to be a doctor of philosophy and I thought wow I never even knew what that was so much later anyway I remember you then and of course you know when I was a student coming down up the river all the time and then we were in we were we we're in the Society of fellows for I think a year or two together before you um moved to before you became on the faculty at Harvard right you were 80 to 83 or something yeah one year I was in the society oh that's right you got you got promoted or seduced away from from The Good Life yes yeah yeah and and until you actually had to work for a living and uh and yeah well I remember uh you know Sydney Cole moment uh called to offer me this assistant professorship and I I said Do you happen to know what the salary is and he said no I have no idea but I'm sure it's absolutely pathetic compared to anything about the salary of a junior fellow and he was right he was right and you got to you had to teach on top but would you which the next thing I want to point out I remember is which you were you are a uh a fantastic teacher which one of the reasons I wanted to talk to you about the physics today I am your lecture now I I remember actually sitting in one while I was in the society sitting in a class that you taught on on a sort of advanced Quantum field there more it was a class on basically the work of a toast as far as I can remember um because it was sort of all of the theft sort of led a whole series of in the late 70s mid late 70s for uh new new ways of thinking about Quantum field Theory and and um of course like many people I've you I borrowed from your lecture notes when I was later on teaching it it's uh the fact that you crew season Immaculate lecture notes there's help me time time again including in fact in preparing for this this podcast so you've done the world and and your fellow physicists a great a great service but you know you know who learned the most from those lecture notes me yeah yeah well I think that's the that's the point of teaching after all one of the points is you finally when you have to write it down and you think when you have to try and explain it you realize how many things you don't understand and I think it was probably 1982 when I was probably 82. and I think I still draw on the knowledge and insights I got from that experience I worked very hard to you know uh digest and synthesize a lot of material and it works incredibly hard it was an amazing course and and when you think back maybe you still do this but if you think back you say wow it's amazing I worked that hard to I mean what it the energy of a young faculty member to work to learn things is is really impressive I don't maybe you've maintained that and you still think well it's hard to know for sure but I do have the impression I have a little less stamina now than I did 40 years ago you have a great well the subject one I want to talk to you about which we'll get to will be quantum computers and Quantum Computing and and you you have been teaching class in that for a long time and I've been looking at those lecture notes as they've been developing and no just superb so in any case you did you did move from the society fellows but before that I want to go back so I want to go back because this is an Origins podcast as you know um so you to your own Origins because I don't know a lot about it I knew you grew up in Highland Park in Illinois is that right that's that's right yeah and um I was born there I went to the public schools there all the way through high school what is it one of these good Suburban public high schools yeah that was a very good school system and I think I I got a pretty good education there yeah it seemed like it you and not only that you did pretty well though you were the valedictorian as my research seems to have indicated all right your staff uh did their homework yes my staff that's right um but I I am and yeah they're good big Suburban schools when I taught a case you know there was Shaker Shaker Heights High School which is another you know some of these good public Suburban public schools in the United States still still do produce a good education and um and you went from there to to Princeton but you already you went and majored in physics let's go back uh what got you interested in physics were your parents uh either your parents what do your parents do what did they do my parents were both trained as lawyers and they both had law degrees in my mom's case that was very unusual she was the only woman in her law school class wow in fact her law degree is from Western Reserve now Case Western Reserve where you were you know at one time on the faculty yeah she grew up in Cleveland and her father was an attorney and she was I I think she might have you know joined his practice except uh World War II intervened my dad uh was a bit of a prodigy he graduated from law school at age 20. wow he went to the University of Chicago he grew up in Chicago and um it was possible then to concurrently get a bachelor's degree and a law degree wow oh 1932 he's a 20 year old lawyer except he couldn't take the bar exam because he was a minor he had to wait until he was 21 for that under at least the rules that held in in those days um now he practiced law for for nearly 10 years but then when the war came uh he was 4f because of a medical condition so he couldn't serve in the military but he did work for the federal government and that's where he met my mom he went to Baltimore for what was then called the Federal Security Agency which is the Forerunner of what became health education and Welfare and uh anyway they their main legal uh agenda was to flesh out the Social Security program which was still kind of new at the time of which there were many you know legal issues and anyway they worked in that same law office and and that's where they met wow and then after the war they they came back to Chicago where my dad was from oh and then they moved to the suburbs Highland Park before my uh brothers and I were born and that's where I was raised presumably moved because of the education or the opportunities for a place to bring up kids okay wow wow your father's not still alive is he a father is not alive uh he was born in 1911 so it would be quite a Quite a feat if he were yeah we didn't live to be to live to be 90. well that's good my mom just died last year she was 1921 but she made it to 100 at least but uh no I was going to say I would have thanked him for um social security has been good to me lately um and uh uh so now so they were both well that they were both trained at you know in in college as lawyers uh you did they where did you get an interest in physics well I think it was the space program for my generation that was very transformational I became fascinated when I was in second grade with human space flight which was sort of the biggest uh you know news of the day I can remember uh hearing about Yuri Gagarin being the first human in orbit and then you know there was the Mercury program with uh Alan Shepard and Gus Grissom and John Glenn and so on each one of those flights was a huge National event oh yeah where you know in those days we we had uh three television networks and they'd all stopped regular programming to follow these space flights and uh so I became very curious about rockets and space and that Drew me to the public library where I started reading books about things like that so I think that was the uh the earliest uh thing that really got me started I actually when I was a fourth grader you know when I was 10 years old I read a book which turned out to be very influential and I didn't understand until many years later the origin of this book it was called the world of science and um it contained chapters on various scientific disciplines and the one that one of the ones which I found especially interesting was about theoretical physics and it doesn't mention the name of any scientist but it does mention various scientific developments and achievements and considering this book was actually published in 1958 it's a rather amazing thing that it contained a detailed and accurate discussion of the discovery of parity violation in the weak interactions wow physics is different in a mirror than it is in real life and I thought that was the most amazing thing it is but what I found out years later uh is that all of the scientists who were interviewed to provide the content of this book the author was um Jane Werner Watson and her her husband was a Caltech faculty member and all of the information came from Caltech faculty and the chapter on theoretical physics the main sources were Richard Feynman and Murray Gelman oh really wow wow that's kind of poetic isn't it yeah I think this got me excited about theoretical physics and then you know 20 years later I joined the Caltech faculty yeah Marie Billman and Richard Feynman uh became my colleagues wow that that must have been wow did you know did you know about that before you joined the Caltech faculty did you know about this it was only after well actually let me think about that um here's how I figured it out in this book on theoretical physics it starts out with a story which you may recognize about a boy pulling a little red wagon with a ball in the back and when he would pull the wagon forward the ball would roll to the back and when he'd stop pulling the ball would roll to the front and then as the story was told he went to his father and said why does that happen and his father said well that's called inertia but nobody knows why and then years later when Christopher Sykes did an interview with Feynman which became a TV show on BBC Horizon which was being broadcast in the U.S on Nova uh Feynman tells this exact same story and when I first heard that interview I thought Feynman stole the story from that golden book that I read when I was a little kid but that then that was what stirred me to look at the book after many years it was still on my shelf and there are front notes and very tiny print which say you know who was interviewed for each one of the chapters and not only that there is a photo of two unidentified physicists in front of a Blackboard making diagrams and that was fine wow I could now recognize years later but of course when I was 10 years old I had no idea who they were wow that that's that is an amazing story yeah I know the story obviously but uh that's fair well you now did you read so that book was instrumentally in grade four for me it was a book about Galileo in grade five or six that was really influential but it was did you I wrote a book I don't know I wonder if you ever read this one we are pretty much contemporary yeah we're with you're a part of there was a book for children by Isaac Asimov which was called breakthroughs in science yeah it had a chapter about Galileo and about Newton and about you know yeah all those people yeah and it was it was a fantastic Book for a kid my age you know a like fourth or fifth grade and yeah biographies of all these uh great scientists and the things they accomplished and that was also uh very fascinating to me at the time so did you read widely your parents did they encourage you to read did you was it reading about scientists I mean in addition to the the space program for me it was reading about scientists and then books by scientists like azimov George Gamel uh also a little bit and uh that really had a big influence and then Feynman later on but I'll tell you um I had another passion besides science and that was baseball oh yeah I think I read more about baseball than about anything else from about you know the age of of six till sure 14. and I was very interested in statistics um baseball is a very quantitative sport I collected baseball cards yeah and the way I did it was the way one used to do it in those days I you know I take a nickel down to the stationary store and buy a pack which had five cards and a piece of very stale bubble gum oh when you open the pack that you you know see one of your favorite players and uh I bought a lot of gum and uh me too collected a lot of cards which which I still have and on the back of these cards there were the you know career statistics of um well whoever the player was on the front and for some reason I memorized many of these sure and one thing that's interesting is back in the early 1960s when I was really into baseball people didn't really understand baseball very well you know there was kind of a revolution in the 1980s in uh baseball analysis where uh people had their amazing Insight that in order to win you have to score runs and stop the other things and so you need if you want to quantify the value of a player you should figure out how many runs they helped score or prevent it and uh the statistics are much more in line with an actual player value now after that Revolution than they were back in the 1960s that great book and then movie Moneyball I think it was uh uh about Moneyball is a really good book my my hero uh who actually uh was uh influence on Billy Bean though money Paul was is Bill James who was involved in in the 1980s and understanding uh you know what uh what's really important for scoring runs and preventing runs but you know it may have changed baseball but you know it's funny because I grew up in Canada but I baseball was the sport I also liked growing up and you know the thing I liked about baseball was one of the many things besides liking baseball was that it statistics were used more than any other area and as a kid it's a great way to teach kids statistics because learning mathematics often for many people even College even often they wonder why they're learning what they're learning and if they can see applications um then it makes it much more relevant and and and certainly you and to understand what a batting average of 300 is you have to know a little bit about statistics at least and um you know actually when I was chair of the physics department of case we we wanted to move um the the our physics class and electromagnetism down it was taught a little bit later after the mathematicians thought Vector calculus and we moved it to the same time and and the mathematicians were really concerned but the students has happened to me when I learned it found it much better because the real you really place you understand Vector calculus when you apply it in things like electromagnetism otherwise you'd sort of don't understand why you're doing it and so real world applications of math are a real good way to learn math and I think baseball therefore should be used more often in schools as an example well it was very inspiring for me when it came to understanding statistics and how to compute them myself you know which was very empowering incidentally there's a lot of interesting physics in baseball too but I was less attuned to that in those days my my late colleague at Yale Bob Adair wrote a book called the physics the book the physics of baseball yeah and he became because as you probably know the president of Yale became a commissioner of baseball uh Bart Giamatti the the actor's father um and who unfortunately died early but he appointed adara's official physi before that he was head of the national league and and he appointed uh uh Adair of physicist to the National League I think he had that official position whatever that was and they're like well qualified for that yeah yeah anyway so so um baseball science but you read more baseball than than you read science books by the way did you ever read the character of physical law that had a big impact on me feynman's book when you were younger not till much later no okay well in any case you went off to to um uh did your parents want you to do they want you to be a lawyer or did they not want you to be a lawyer because they were lawyers or they care they didn't well you know my dad uh left the law uh he decided the law was boring and he became a businessman he worked in marketing for a company called Allied radio for over 20 years and my mom uh did not um you know return to law after uh first child was born she was a very amazing woman though and she was very active in many ways she had natural leadership qualities and so she became the president of everything you know the PTA and the League of Women Voters and local philanthropic organization and and you know she was volunteering for everything um she has a very different personality than I do she or she did she was very outgoing and extroverted I'm quite introverted yeah you are it's funny interesting to me because you are introverted but you're a wonderful lecturer and teacher it's a nice combination you know it's really interesting well it seems to work for me yeah yeah no I think it I think because I suspect I don't know because I'm the opposite obviously in a ways I'm not an introvert my wife keeps reminding me but but uh maybe because you're introverted you put more energy into preparing things so that you're well prepared to speak about them well I I I have uh a constant fear of humiliation if I'm not prepared yeah yeah no and I I remember that I actually remember that once well anyway the answer to your question is no they didn't care whether I was a lawyer or anything else they just encouraged me to to do what I found interesting neither one had a scientific background at all or any special knowledge of science and did you know did you decide before we went to Princeton that you were going to study Physics when did you decide you want to study Physics as a matter of fact I became more interested in math than in physics when I was in high school okay I had learned about uh goodle's theorem you know and the idea that there are things that are true that can't be proven I thought this was the deepest Insight the human mind had ever achieved and that I was going to be a mathematician studying logic and set theory and so on and I had this view I'm not exactly sure what it was based on that if you were going to do math you had to go to Princeton maybe it was because Einstein had been there yeah yeah I assume that's the reason but uh yeah so I went to Princeton intending to be a math major and then I had to talk my way in as a freshman to a graduate course on set theory and logic and uh you know which I did well in but I started to realize when I was a freshman at Princeton I think I'd really known all along that I just am not cut out to be a mathematician I don't have the ability you know mathematicians and they have uh you know extraordinary uh powers of thought in their you know own Avenue of discourse which I really just could not match yeah and around that same time though I was getting more and more interested in physics I had some uh inspiring teachers oh it was then the first one was Val Fitch who you know won the Nobel Prize for discovering the CP violation and the neutral K on system he taught electricity and magnetism and he was great and then I took a full year course from John Wheeler wow that would be inspiring you know it's funny because at the time I thought he was ancient impossibly old how could anyone be that old he had worked with bells poor for goodness sake but in fact he was 61 then and you and I both know that's not really so old but at any rate uh he was um he was inspiring a very idiosyncratic yeah sure ways uh but uh he uh deepened my interest in physics so uh I didn't have any doubt at that point that I that I should study Physics that's it there's so there's boy there's so many interesting things about that I want to parse a little bit one was the mathematics thing it's interesting because I um I did a degree in math and a separate degree is in math and physics and the funny thing was that you're absolutely right I realized I wasn't a mathematician as you did I the difference for me was when I was doing physics I I could tell where I was gonna what was next what was gonna happen next where I was going in math I did very well in but I really never got a sense of where where what was on the horizon and I and I was just sort of doing it and that and and I and that was immediate for me to clear made it clear to me that I I wasn't really cut out to a mathematician the interesting thing for me was that there were people there were other people in in my in my math classes who were better mathematicians than me and they had to take a physics class as part of their coin and I thought it should be I just figured physics has applied math at that time and I thought it would be trivial for them but I was surprised at how many mathematicians had trouble in physics classes which is really a surprise to me it's still um yeah my experience is very similar and uh I I don't understand why the mathematicians don't uh you know beat the pants off the physicists when it comes to advancing physics but for some reason they don't yeah well you know and I think it's you know it you don't well anyway it's a demand for different levels of truth I guess or understanding um uh as as a number of people have talked about and um I was and John Wheeler I was influential me as in her graduate occurred to me I may have actually seen you if you did did undergraduates go to physics department picnics at Princeton yeah well I don't remember the picnics I do remember uh wheeler having us over to his house for tea yeah well that would be nice yeah I'm not sure I probably did go to the picnic if there was a picnic but I don't remember that I just remember going down at one point that's when I was trying to encourage him to come speak it I was a president of the Canadian undergraduate physics Association I got him to come up and speak and I went down to Princeton to try and convince him and it happened to be a physics department picnic and I remember I still have a picture that day and were you successful yeah yeah yeah yeah and he um and he came up and he you know when he was like just wonderful and Charming he brought I think he brought me the gravitation book or I or I owned it the big Bible and he and he wrote what is what it Still Remains the longest inscription I've ever had in any book it was about two pages because I don't think it's so difficult he was lovely anyway I I was probably too introverted to ask him to sign my copy of grip you see yeah I can tell you a lot of uh funny stories about wheeler maybe you'll appreciate this one um it's the first day of class we're we're sophomores John Wheeler is going to teach us classical mechanics the the book is Goldstein yeah yeah sure Annex which I dipped into uh before the first class and I thought oh this is going to be great we're going to learn about you know the principle of least action the oiler LaGrange equation I was excited and so wheeler comes in uh you know he's he's impeccably dressed and suit and tie and he had very uh he's very skillful with the chalk and he the first thing he did is he he made two dots on the board and labeled one of them a the other one b and he said all right there's an electron and it's going to go from a to B but how does it know what path to take there's no way to know so it takes all the past oh my God adds them all together with an e to the i s and that's how it knows where to go he was trying to explain to us that this principle of least possession uh really comes from quantum mechanics and it was the Insight of his uh most successful student Richard Feynman that we can think about quantum mechanics uh that way and how classical mechanics arises and to either it was very important that the students who were about to learn classical mechanics understand that it comes from something more fundamental well that's fascinating let's teach classical mechanics and in fact it was not appreciated by everybody in the class I thought it was wonderful but it didn't help you to do the problems yes write down the lagrangian for a bunch of masses and springs so well you know again this may be well for the physicists who are listening I'll mention for me that was another example of utility I I remember taking class out of Goldstein and well actually first one was a slightly lower level and Goldstein I've taught myself but but um and I just kept thinking well what's the point of all this you know the LaGrange and blah blah blah and it like many things was only when I got to quantum mechanics that I appreciated the utility of the class of the things I'd learned in classical mechanics because that's where you really use it hamiltonian lagrangian I really I mean they seem to abstract and useless or not useless but but pedantic to me as a as a when I was taking classical mechanics was only later so you know some people have I don't know if you've ever taught it I I think I don't know if Feynman ever tried to teach at Caltech I've known one one place where they try to teach classical mechanics or quantum mechanics before classical mechanics using feynman's book by the way it was an abject failure but but it would be an interesting thing to try um well I have taught classical mechanics and I enjoy teaching it probably because I don't mind being pedantic I'll tell you I had a role model in teaching classical mechanics because when I was at Princeton the next year after Wheeler's class I took a more advanced class In classical mechanics and it was one of the best lecture courses I ever attended and guess who the instructor was you're not going to be able to custom it was Alan Guth it was Alan Guth wow he was at that time an instructor at princetoni just finished his uh his PhD at MIT right yeah and yeah and he just come to Princeton and he worked very very hard on that class he told me years later which I can easily believe because he works very hard he's perfect and so I I stole a lot of the things that I do when teaching classical mechanics from Alan Guth this is now even more poetic God this whole your whole history is going to be poetry in so many ways this is great because I want to move on because you in my opinion so you have you owe Alan Goose that but I think Alan Guth owes you as we'll talk about um because um you you completed your your your undergraduate work and then you you went to Harvard as a graduate student which is where again where we first met and you achieved one of the mere impossible things which was to be a student of Steve weinberg's and actually get a BFD um very few um probably as we all know because you know if Steve was interested in something he did it and he generally could do it faster and than any graduate student and so it was hard to unless you found your own problem you know in some ways it was probably hard to to keep up I mean did you have a story about that too yeah good I want to hear it yeah I worked up my courage to go to Steve when I was a graduate student and you know it took me a while to build up to it and asked him if he could suggest a problem that was 1977. and he uh had just read a paper by two physicists named Roberto piche and Helen Quinn I think you know newspaper and uh he uh said to me you know it might be interesting to work out the phenomenology of this model that piche and Quinn have just constructed you know what experimentally detectical consequences and I thought oh well that sounds interesting so I did want any graduate student well not any but many graduate students would do I then obsessively read everything I could find about the phenomenology of the Higgs sector yeah and uh while I was doing so of course Steve was discovering what you know called the Axion he actually called it the higglet yeah so some weeks later he announced he was going to give a seminar and he had solved the problem he had suggested and discovered that there was a very interesting uh consequence of that model so that in a sense it could be ruled out but then it turned out you could the model could survive in a different form yeah but anyway uh you know at the time I felt a little resentful I thought boy Steve gave me this problem and um and then he did it himself of course he probably had no recollection he'd even mention it to me I just happened to walk into his office at the moment he had you know read the paper and was thinking about it and uh you know when it comes to working with Steve it's kind of like he said Steve would talk to me but he wanted to talk to me about what he was working on and get muted you know tell him things that would be useful for what he was like a vacuum cleaner yeah and that's how I first impressed him that I had studied you mentioned uh you mentioned it huffed yeah earlier he was one of my heroes and I had studied his work in much detail as a graduate student and Steve had not and Steve got interested in something that we call instantons which are an interesting Quantum phenomenon and article physics it was one of the things that was exciting in in the late 70s and I knew all about instantones and Steve didn't know anything about them so he we whenever we talk he would be pumping me for information about instant hunts you know it's it you know I was a I was a graduate student at MIT as you know at the time and I I was having a trouble finding us well being happy with the supervisor and at one point the great thing about Harvard MIT I did all my classes at Harvard I almost said no class at MIT because at the time Harvard had a much more in my opinion at the time a more powerful physics department at least in the areas I was interested in and um and so I took all my courses from Steve and um and I remembered a very similar thing I at the time I was very mathematical and I was working on the geometry of Gage called the geometry of gauge theories at Simon and and I was and that's what I was focusing on with my supervisor at the time at MIT and I learned all about something called fiber bundles and and Steve was getting very interested through instantones in fact and so I done well in this class and at some point I I brought up fiber bundles or something and it was amazing to me because again it was like a vacuum cleaner I remember Steve Weinberg phoning me at home at 11 o'clock at night to ask me a question and I as graduate student I felt like wow that I can't believe it it's so wonderful and and the interesting thing I wanted to work so I asked Steve if he would be my supervisor because I thought you know I wasn't really getting ahead at MIT and Steve gave me the greatest he was very honest he said I have a problem that I know I I would it'll be a good PhD problem it was have to do with what's called We Now call chiral Le Grand Jensen and and sort of understanding the phenomenology of these things and it would have been a great thesis he was fascinated by it he said I know won't interest you because it's it's not mathematical about it but if but I'll be willing to supervise you in that problem he said however I have an obligation to Harvard graduate students and if any Harvard graduate student asked me uh that I like asked me for a problem I'll give him I'll give them exactly the same problem and I'll say I never gave it to you so I sat it under those circumstances it was too much of a Gamble and I didn't work with Steve but at least he was honest about it and it was true in in retrospect it was a it would have been it was a very fruitful area obviously the uh uh and um as you pointed out Steve did not have many graduate students at Harvard at that time The Graduate students in particle theory flocked to Sydney Coleman and Howard George High powered because he was such a supportive mentor and Sydney because he knew everything and could answer off the cuff any question you could come up with but and I intended to work with Sydney actually but then I realized that the things that Steve was interested in uh resonated very well with what I was interested in and I don't know if you had this interest at that time in the late 70s but what I found particularly promising and fascinating in the late 70s was the connection between particle physics and cosmology which is exactly which he had pioneered it actually it's probably it was around that time that his book um first few minutes had just come out a little bit earlier and I remember reading it and then getting to sign it and yeah that was what got me interested in in cosmology too and and indeed I was going to say one of the ways to well I I alluded to earlier one of the ways to survive as a graduate student of Steve's was to have a problem that you that you know could build on that wasn't at the focus of his attention and and uh and I don't know if your thesis sorry go on I was going to say that book the book the uh the first three minutes was quite unusual because it was a book intended for a popular audience but it was it had a big influence on physicists yeah it really is physicists who came from a particle Theory background learned about cosmology and in particular the early Universe from that book and that uh launched some Scientific careers including Alan Guth I think he was very much influenced by that point yeah it is really it was really a powerful book and although he claimed it was for a written for a smart lawyer as you may know at the beginning of the book yeah I remember that well of course he must have had Louise in mind yeah yeah his wife yeah very smart lawyer yeah anyway you were asking about my thesis problem and that's also an interesting story because um it combined two different things I was interested in in the time of exciting developments in the late 70s and particle theory uh one was as you mentioned the fiber bundles the the ideas from topology which were becoming increasingly relevant to particle physics and the other the potential to learn about the very early universe and testing our ideas about particle physics by studying the the very early universe and in the case of the topological ideas I learned a lot about that from Sydney Coleman and from reading his papers and in the case of the connection between particle physics and cosmology I learned a lot about that by reading things that Steve had written but at that time at least there weren't many people who knew about both things and so I was able to arrive at a question that connected the two and in particular the possibility that what we call topological defects which are magnetic monopoles could be created in the early universe and I was interested in how many of those particles would be produced in the early universe and how many would still be left over today and I told Steve about this interest and he was very dismissive he really didn't seem to think it was a good question he didn't know anything at that time about these magnetic monopoles or about topological ideas in particle physics and he thought you know it seemed very speculative and not well founded you know he liked of course he liked the things that he understood yeah but you know he he liked arguments that were based on sort of the you know the most General Bedrock principles yeah and this involves speculations about Grand unified theories and uh he didn't really understand the topology so it was a little discouraging that my advisor was not enthusiastic about what I was doing but there were others who were including uh Howard Georgia and Sydney Coleman were supportive also Bert Halperin who has a background on particle physics but knew a lot about these topological things and also postdocs who were at Harvard at the time who were brilliant scientists especially Ed Whitton and Michael peskin so I had a community of people who were supportive but it wasn't really coming from Steve well that's probably lucky for you because if he had been he would have scooped you on it I think I mean he was a steamroller yeah and I have to say I dragged my feet because I you know I kept thinking I didn't understand it well enough so it took me quite a while well you know that go on here's something that seems amazing from the perspective of you know the young people who do theoretical physics today uh I had no papers after four years of graduate school I I wrote my of first paper the one about magnetic monopoles in the early Universe at the end of my fourth year submitted it to physical review letters and it was promptly rejected oh really as you know not being of sufficiently broad interest you know that story and uh is very discouraging because you know I was I thought I was going to finish the following year I'd be applying for postdocs in the fall I had no Publications that had at least appeared in journals so I remember uh Roberta my wife to cheer me up we went out and bought a colored television set which actually did cheer me up up until then we just had this little black and white carried around the apartment so baseball's much better in color oh absolutely and and then I I resubmitted the paper and it and it was accepted but you know I was applying for postdocs with just that just that one paper which at the time hadn't even been published and somehow I did okay yeah it's it it's it's it's it is interesting uh well it helps to have people especially good people who know you and and and and um you know they're famous yeah yeah I mean they're famous Stories the famous story about Iraq well I'll tell you that later but you probably know the story but um uh uh the that paper um which you which you did right I was gonna say I didn't know if that was your thesis per se but that it was not okay I bet it wasn't yeah I was gonna I didn't think it was but that paper that you wrote on monopoles which is the these these predictions of things that in some way in most of these in many theories of the universe must appear caused a problem in particle physics and I think that was probably I think it's fair to say the most single influential paper that convinced that that convinced people there was something that particle physics had something to say about the early universe and needed to be thought about and the early Universe need to be thought about seriously not just not just speculatively and and back of the envelope the paper you produced produced ultimately a problem which for many of us I Remember When Alan Guth came out with inflation yeah we'd I had never heard of the flatness problem or the or the Horizon problem which are part of the things which we now think of inflation as solving the really significant implication of inflation at least for us in the Boston Community as I remember was was that it would solve the problem that you'd presented as a generic problem in cosmology and I think and I think it has to be understood as as having been profoundly important in that regard so you know the context was interesting because until a few years before I wrote that paper it would have seemed ridiculous and we wouldn't have really known how to get started to talk about the first 10 to the minus 35 seconds after the Big Bang Yeah but what it made it one of the things that made it possible to do that was the discovery of asymptotic Freedom which came in 1973. until then you know when the universe was uh uh a hundredth of a second older you know the things were so hot yeah that physicists had no idea what was going on but then with asymptotic freedom we understood that you could go to much much higher temperatures and therefore earlier times and still have theories that were predictive and made sense and the other important context was the idea of grand unification sure that the uh interactions that we knew about could be descended from some more unified theory and that unification would have consequences the one that was uh got a lot of attention from experimentalists at first was that the proton would be unstable and you could look for the decay of the proton and people did um but another one of those predictions was the magnetic monopoles and so this was a situation where we wanted to explore physics at fantastically High energies and we couldn't do that with accelerators but we could use the early universe as the accelerator and look for the um you know the Vestige of that very early period and learn something about fundamental physics which then I should say by the way personally sponsored my prompted my own conversion sort of movement out of mathematical physics to my thesis was on the early universe and and ways to try and resolve problems that that having to do with the monopol problem but also a problem with entropy so yeah they these things really got they were in the air and and the thought that the hook the hubris the hood Spa that the particle physics Community had at that time having just produced a standard model and then Grant unification it looked like literally the the you know the the grand synthesis was in the air in the late 70s and early 80s and everyone expected when these machines were built they'd discover proton decay and and even and as you know uh around that time um Blas Cabrera discovered what I guess is the only monopole in our universe um because he never saw another and and and in principle I suppose inflation would suggest there might be one monopole in our universe so he might have been lucky but but uh yeah yeah in any case that that so that was I I think it's really important and and from there I mean your work uh you know having no we work we we've worked together too but I've been having been close at Harvard the work continued to be um many a number of ideas related to particle physics and cosmology Axion physics um and and uh and then related to black holes I I we we we've our we wrote a paper which as you know I'd gotten interested in in um in in a topic that was relevant to Quantum effects and black holes uh uh something that our friend and colleague Frank Belcher not well check and I worked on something called discreet hair black holes and and the end and um and there was an aspect of it that was nagging at me was based on something Sydney Coleman had said and typical of Sydney he sort of threw off a remark and and it required and I thought well this is really a subject that should be explored which is what which is you know you can't have a it turns out in a closed Universe you can't have it have an electric charge but in in these weird kind of models we'd made those same arguments wouldn't tell us you couldn't have these things we called discrete charges and so I talked to Sydney about it and I guess you and I talked to you I think it might I just looked at that paper and I we thank the Aspen so it may have been an aspen that we started to talk about but I figured who who would be the person to flesh out this problem if Sydney wouldn't you know be interested and um and the fascinating question was I had for me at the time what was fascinating was whether this could be a way that black holes could store information Quant mechanically to solve the this the information Paradox problem in black holes where the information Paradox problem which I've talked about a number of people but for listeners I'll just remind them that material objects fall no black hole and in principle once they've fallen in all information about what fell in is is gone except for the math the mass the total mass of the black hole in its charge and its Spin and um and then if the black hole evaporates by Hawking radiation into the thermal radiation all that information goes away and there's a property of quantum mechanics called unitarity that says that shouldn't happen and so maybe so it was it you know that's why black holes become so fascinating because there are they're an area where general relativity confronts quantum mechanics the two Forefront fields of fundamental fields of physics and um and so at the time I thought well this is an interesting potential area of Interest which is maybe whether you could get Quantum information from these these this Quantum here and we talked about it and I was kind of fascinated because you began to I I don't know if it had any influence to you but when I look at your work you began to think more about black hole information around that time so I don't know if that those discussions ever had any imp if that if or you'd been thinking about it beforehand or not I meant to ask you that I was starting to think about it uh and part of what sparked my interest or accentuated it was the paper you were you wrote with Frank will check um actually that that's a that's a nice paper we wrote together there are a lot of interesting things it's nice because of you yeah there were a lot of interesting things and and if uh one of the things that uh we discussed is related to an interest that uh you know I continue to pursue which doesn't have to do specifically with black holes but with uh objects called anyons yeah articles that'll obey uh unusual statistics and in particular not a billionaions which are the particularly exotic uh form of any hunts and uh some of the mathematics we did in that paper was related to anyons and I continued to pursue that we should mention that really in a way the hero of this whole discussion about black hole information with Stephen Hawking yeah until 1974 one could just say Okay information goes into a black hole and never comes out again and fine uh we could say it's it's locked inside the black hole behind the Event Horizon but when Hawking discovered that in fact because of quantum effects black holes evaporate and can eventually disappear uh this caused a very profound tension uh between the idea that information is not destroyed in quantum mechanics and on the other hand what comes out of a black hole doesn't seem to be related to what falls into it and you know we're still struggling with that I think there's been a lot of progress on that question um and it's a problem now which is you know nearly 50 years old yeah and still not solved in my opinion still not completely solved I I'm glad you agree there's been a lot of heat and a little bit of light in in the process but but I think do you think there's been a lot of light but it's nevertheless not completely solved yeah yeah and but it's interesting the reason it's a nice segue because I think it's important when I think of you may when I think of who might be these things might be useful for one of them is Young students and you hit one point when you talked about about uh what you did as a graduate student and it's and and I I it relate it it really relates to my feeling of being an early graduate student one of the problems of being a graduate student initially and I often try and encourage my students to get away from this is the first thing you want to do when you work on any problem is understand everything and it's a natural tendency when you're starting out saying before I bark this problem I have to read absolutely everything I have to understand absolutely everything and it takes a long time before you realize you actually just have to understand something and and and you can't and and you keep getting delayed by learning more and more and you really you really want to hit something concrete and I know you talked about that in your in in in in your you know early period with with with uh Weinberg and I think it is a it is a real it's probably the biggest change that a student needs to make from being an undergraduate to being a research graduate student is to realize is to is to begin to focus is to say I I can't learn everything it's I have to understand I have to actually do something no I know what it's like and it's very seductive and and but the other thing that that your history and now I want to move um to interestingly after an hour and 20 minutes to the topic I wanted to eventually get to but time flies huh when you're having fun I hope that I hope that you feel it that way um uh which is ultimately Quantum information Theory which I mean obviously become one of the world's leaders and you direct an Institute and at caltechn but I think it's another interesting thing is is I assume that your interest in information theory in general initiated with black holes and yeah and it's it's nice thing to know for students that things you learn um are often useful in ways you never expected them to be and and it's uh I off you know we we ran up we created a program when I was chair at case on physics entrepreneurship which the dean of business school said was an oxymoron but it isn't because because scientists have to learn that sometimes you saw the problem you think you're solving you you have a problem and you try and solve it but you're really trying to end up solving another problem and and the tools you've learned are are never wasted in that way and um sometimes you you uh you don't completely solve the problem but you partially saw that and that also is a valuable contribution and somebody can build on that and take it farther absolutely and maybe that's the thing maybe that's why mathematicians don't like to be physicists because physicist molten mind partially solving problems mathematicians I think probably have a harder time with partially solving something you know partial proof well you know if it's not a theorem then it then it doesn't count and yeah that's a very nice standard yeah exactly and a Higher One than we have um but around that time right around uh the time we were thinking about a little bit after we're all thinking about black hole information and and and um the world of quantum computing changed a world which of course had been created as you describe in in your in your beautiful many uh review articles and I guess I talked about it even in in my book about Feynman um well we'll talk about feynman's introduction in this field but for most of us for me and I suspect you I first learned about Quantum computation when I learned about this result by a guy named Peter Shore is that what I mean that's when the Peter Shore had show had a proof that these things called quantum computers could do something that at that time was protecting the world's and still is the world's banking system which was how to factor a large number into its product of primes which a classical computer can take longer than the age of the universe to do if the primes are if the number is big enough and that was a key that was the way to encrypt information and Peter Shore showed that a quantum computer could do that potentially in in a in a human lifetime or less and that was like a lightning bolt that reverberated throughout the community so for me that's where I first heard about quantum computers I don't know if it's where you did well it's not why I first heard about them but oops where I first began to uh to take the idea seriously and get deeply interested in it um there's several things Prime me to uh get so deeply interested one is what you mentioned I was interested in information because I wanted to understand how information can escape from a black hole and being the type of person who does these things I thought well I should learn everything about information and in particular about Quantum information which was not um a you know uh deeply developed subject at the time and had only a few practitioners but there were ideas like Quantum cryptography quantum teleportation that I learned about because I thought maybe that would help me to understand black holes better and then another thing was happening around that time as I'm sure you remember for my generation which is also your generation of particle physicists our great hope was to discover the physics beyond the standard model which had been established a little bit too early for you and me to contribute uh to erecting that uh core Theory beyond the standard model physics is where we would get our chance to understand new facts and principles about fundamental physics and where we thought that data was going to come from was something called the superconducting super collider which was already under construction in Texas and that project was canceled in 1993. for complicated reasons but at any rate it was canceled and many of us including me realize at that time that well our opportunity to have data about physics beyond the standard model has been pushed further into the future we anticipated as turned out to be the case that there would be a machine at CERN that would not quite as well as the SSC would have yeah uh explore some of that physics but in the meantime what we what were we supposed to do and so I was in a mood to learn about new things and maybe explore different possibilities I heard about Shore's algorithm in um in May of 1994 he had announced the discovery the previous month and I was immediately fascinated because the whole idea that the difference between a hard problem and an easy problem to solve between you know problems will never be able to solve in the age of the universe and problems we can solve uh very efficiently on future machines that that difference between hard and easy pivotably uh essentially depends on the fact that it's Quantum oral instead of a classical world what an amazing idea yeah I mean yeah I mean if I'm in I've read from you a quote for firemen really basically knocks that home but go on I'm sorry yeah so uh you know I was still thinking about black holes and it was kind of uh transitional time in our group because I had students working on all kinds of things you know uh black holes and particle physics and anyons and some of them got interested in Quantum Computing and and some of them did not but uh those who did really dove into the subject and you know when you're in the midst of of what turns out to be kind of career transition you're not so aware that it's happening because things seem more adiabatic at that time but it did turn out to be a major change in Direction in my my scientific uh Direction and The Shore's algorithm was was the key to spurring that well it was good for you I was happy to see that um and it was good for the world because you did it I I it is you know being able to change I I think it did affect the whole generation I I moved more towards astrophysics and for the same reason because it was clear to me that terrestrial we were I was trying to look for ways we could use the universe in other ways to try and constrain particle physics but but it was a similar motivation and we weren't going to get the results on the on the ground and and then I watched you from a distance because you already moved to Caltech and um begin to to take that and it's funny because I you know I guess I had an inner I we'd work together and we wrote that paper in 1990 and I saw I when I saw that field and saw you been doing I thought this is this is just what John is going to eat up this because it's exactly the type of thinking that I knew you carried out and I was therefore not surprised and I was really really happy that you that you were doing it it just seemed to me a field that was made for you and um and it has been and you've really helped lead that the developments in that field and that's what I want to spend the next you know four or six hours on no anyway but uh next little while talking about because Quantum Computing is a is a subject one people hear a lot about and there's always uh stuff in the new press about it because because it's one of these things that promises a lot it's not quite like Fusion which is always 25 years in the future but nevertheless it it there's it there's great promise but there are great challenges and so I want to talk about both and I want to spend a little time um going into what Quantum Computing is and where you think it's going if that's if that's okay with you now um You you the Feynman quote that you that you gave that you always say basically is some I think it's I was going to look it up I have it on my screen here somewhere but basically that that nature isn't classical damn it and if you want to simulate nature then you gotta gotta do it quantum mechanically more or less you may have the quote memorized in your mind because you've used it a lot do you have it well that was pretty close uh nature isn't classical damage so if you want to make a simile simulation of nature you better make it quantum mechanical and it's a wonderful problem because it doesn't look so easy that's right and by golly it's a wonderful problem because it doesn't look good by golly it's weird for him because he doesn't be really good I know it doesn't sound like him yeah it certainly does not like him well he was right about it not being easy yeah he was right about a lot of it and so and Feynman at first thought about this problem a long time ago and I you know we could talk about the history a little bit maybe well the the thing I've gotten out of uh you know I've obviously followed Quantum Computing and although I haven't written anything on it but um and I'd always thought of one aspect of quantum Computing and I guess I got to appreciate a second aspect by reading more of your review articles over the last few weeks um for me the power of quantum Computing was always the fact that um quantum mechanics because of the fact that Quantum objects are doing many things at the same time um can be doing many calculations at the same time whereas classical systems can do one and and we'll get into the the mechanics of how that happens but that's sort of my basic gut when people ask me a quantum mechanics I basically say uh you know a bit is one or zero but a A qubit can be in many different states at the same time and and therefore as it evolves it can be doing many different effectively calculations at the same time and so that so using quantum computers in a way to improve the calculational ability but the other aspect which I guess really I've come to appreciate from you is that Quantum systems themselves cannot really be ever practically um explored fully using uh uh classical computers as a matter of principle not just as a matter of of um of practicality and that there are therefore systems that you really need a quantum computer to understand if you understand the physics of those systems so if I have I encapsulated the two major briefly in my own way they have two major sort of strengths of quantum computers or maybe let me let me have you do it I didn't hear anything that I disagreed with okay but let elaborate because you'll do it much more beautifully well Let's uh dive in a little bit to what you said about uh you know Quantum system being many things at once yeah um well one way of saying why we think Quantum Computing is powerful is essentially the way you you put it a moment ago which is we don't know how to efficiently simulate what a quantum computer does with an ordinary classical computer and that's not for lack of trying because physicists and chemists have been trying for many decades to come up with better ways for computing how Quantum systems with many particles behave like a molecule that has many electrons we know the equations that describe that system we can write down down those equations with very high confidence but they're just too hard to solve and the root of that difficulty is that these Quantum systems particularly ones with many particles kind of speak a different language than the language that we know and understand or that are classical computers understand and in particular they have the capacity to become very highly entangled and what that word means is that they have very complex correlations among the particles which can't be easily captured in terms of classical data so if I have a system of just say a hundred qubits the quantum analog of bits which we call qubits in some highly entangled States if these qubits have been strongly interacting with one another for a while if I wanted to write down a complete description of that system in terms of ordinary bids it's completely infeasible to do so be it would require in fact more bits than the number of atoms in the visible universe so there's this kind of extravagant complexity in a many qubit or many particle Quantum system that we can only get a little glimpse of because while the quantum world has this great complexity our ability to interact with that Quantum world is quite limited what we can do is prepare a simple initial state of our qubits and we can measure them but if they're 100 qubits and we measure them all we get is a hundred bits of information which I could easily write down on a piece of paper so where's all this enormous us complexity well it has to do with how the system can evolve from some initial state to some final State this goes back to what I was saying about wheeler actually yeah earlier suppose um your looking at an electron and you see yesterday that it's at some point in space a and you'd like to predict uh where it's going to be today is it going to be at point B today and quantum mechanics modifies our notion of probability the best we can do is make some statement about the probability that the electron which was at a yesterday is at B today and how do we compute that well while we're not watching the electron we don't know what it was doing so it could have been anywhere so we have to consider all the possible paths that the electron takes from A to B and there are rules about how to assign numbers to all those paths which are called amplitudes and then we have to add together all those amplitudes two and then we Square it to find that probability but where these amplitudes are very different from probabilities is that they're not they can be negative they don't have to be zero or a positive number in fact they can even be complex numbers so while it doesn't make sense to say the probability it's going to rain today is minus 50 percent these amplitudes can be positive or negative they're different from probabilities and that means they can cancel out so when you add together a lot of amplitudes the positive ones can cancel against the negative ones and when that happens that means that the probability you're going to see the electron at B is very small but they can also add up to give a big number and that means the probability you're going to see the electron at B is actually fairly large and so what does that have to do with Quantum Computing well what we can do in a quantum computer is we can put in an initial state to our computation and then the quantum computer processes that state somehow and we're not watching it while that's happening but then at the end of the computation we measure all the qubits and we just get a bit string out which you know is not a very complex thing it's just a short list of bits but to determine the probability of the different possible bit strings we might see when we measure we have to consider all the possible paths the computation could have taken from the initial state to the final State and there are an enormous number of possible Pairs and we can't possibly add them all up with our classical computer uh there are just too many and when we do so it might be that the amplitudes give a large positive number and that's an outcome of the computation which is likely to occur but it might give almost zero and that's an outcome which is unlikely to occur and even though with a classical computer it's impossibly hard to add up all those amplitudes the quantum computer does it effortlessly just by following the rules of quantum mechanics which nature tells us or the fundamental arrivals for how a system of cupid should behave and so the art of quantum Computing and it's a big challenge is to figure out how to get those amplitudes to add up to a large number for the answers we want and to give something close to zero for the answers we don't want and we figured out how to do that in a few cases like the example you mentioned Shore's algorithm for factoring large integers and there are a lot of cases for which we really don't know how to do it and it might not even be possible um but uh Diamond's interest was in the applications of quantum Computing to understanding Quantum systems that chemists and physicists are interested in actually we used to talk about that in the uh in the 1980s when I was at Caltech I arrived in 1983 and he died in 1988 so yeah we overlapped for like four and a half years and we never talked about Quantum Computing per se but we did talk about computation he was very interested in Quantum chromodynamics yeah the theory of how nuclear particles okay that's another case where we know the equations we know with very high confidence what the correct equations are but if I want to describe say two protons colliding with one another at very high energy and predict what's going to come out we don't know how to solve those equations uh to predict that there are some things that we can compute but we can compute the outcome of a collision between two particles energy just like we can do with the Large Hadron Collider at CERN and the reason is that although we know the equations are just too darn hard to solve and that's because there are too many amplitudes to add up and so finally was very interested in doing that kind of computation and I think that was actually quite important for arousing in his interests in the concept of a quantum computer okay I oh let's see he he when did he write his uh I should remember this having written the book but but when when did he write the first about well the first time he talked about was the physics lots of physics down below or whatever that I love that piece that was that was 1959 yeah that was really early but he was thinking really early about at least about the physical life you know he was interested in computation his whole life yeah back at least to Los Alamos whereas the head of the computation group yeah that lecture in 1959 is quite uh remarkable yeah that he uh he speculates about computers and which information is stored in individual atoms which we can in fact do today at quantum computers but it wasn't until 1981 when he gave a talk at a conference at MIT May of 1981. which was transcribed and became a paper called simulating physics with computers yeah in which he uh discussed the idea that first of all Quantum Computing is um well that Quantum systems are very hard to simulate on ordinary computers well of course people who try to do that already knew why uh but then He suggests that as that a quote you mentioned indicates that if we want to do it we should have a Quantum system be our computer to in effect simulate another quantum system and that was the idea of a quantum computer and he deeply appreciated already in 1981 that a quantum computer would be capable of solving some problems that would be just too hard to solve with an ordinary computer including ones which are very interesting to chemists and materials scientists and so on I mean in a sense there's something called the strong Church Turing thesis which more which codifies that in some ways to basically say that that that that classical computers cannot do what what quantum computers can do when it comes to Quantum systems that's a well that's a very I think it's a that's a very interesting connection between computer science and physics and I really think the foundations of computer science are very much uh foundations of physics because Computing machine is undergoing some physical process the original Church Turing thesis which dates back to the 1930s was that a certain mathematical model what we've now called a turing machine could capture any computation which could be carried out uh in nature by any physical process the extended Church Turing thesis says something stronger it says that what can be computed efficiently with a turing machine and coincides exactly with the things that we can compute efficiently with any physically realizable device efficiently has to do with how many steps in the computation we need how that scales with the size of the input to the problem um and we think the extended Church Turing thesis as initially formulated is wrong wrong because it didn't take into account quantum physics so the modern version that has replaced it is the thesis that anything that can be solved efficiently in nature with any conceivable Computing machine can be simulated efficiently by a quantum computer and we can Define mathematically what we mean by quantum computer and we don't know for sure whether that's correct it's a statement about physics it's not just a mathematical statement but there is evidence indicating that it is correct and that's very exciting because it means that anything that can occur in nature in principle because we're only interested in things that can happen efficiently in nature because the things that take uh you know a zillion years we don't really care about yeah you know those will always be simulate with quantum computers although if it turns out that the quantum version of the extended Church touring thesis is incorrect that's really exciting too because it means that we haven't yet captured fully with our current concept of a quantum computer what nature is capable of computationally yeah no the statement that it's a fascinating statement saying that a quantum computer can basically in principle and we'll get to principle versus practice answer any any compute any physical process but implicit in that is the statement though that a classical commuter cannot I think that's an important thing I mean there's the positive aspect but there's a negative aspect which says that it is not true that a classical computer can simulate any any physical process because the world is quantum mechanical damage yeah but to be honest we can't prove that as a mathematical statement uh though we have good reason to think it's true the best reason probably just that people have tried very hard to come up with ways of simulating complex Quantum systems using classical computers and nevertheless the best algorithms that we have are very inefficient that requires a Time on the classical computer which grows exponentially with the size of the quantum problem well okay now I want to step back and parse some of the because you gave a great summary of this but I want to parse this little more carefully I want to take people to the to the a little bit of the nuts and bolts of of of of quantum computers to understand why why some of the statements you made and I made are work are true and to understand that I will we need to describe the difference between a qubit and a and a bit and and although people can hear that a lot over the internet I I I will ask you to basically uh give the difference now if you wouldn't mind okay well actually can I just give a few examples yeah that's even that's even that's right that's that's fine with me examples are always good yeah um so well I think the concept of a bit is familiar to most people um a number which is either zero one or zero or one a switch um in a chip uh but the transistor which can be said either on or off that's the concept of a bit um an example of a qubit is what um physicists call uh Spin and so how is a a spin or a qubit different from an ordinary bit well one way of saying what the difference is is that there's just one way to look at a bit no matter how you look at it it's either going to be a definitely a zero or definitely one but in the case of a qubit we have different complementary ways of looking at it and so one way of describing that in kind of a geometrical language and which is why we use the word spin as you can think of the spin as pointing in some direction in space and it might be up or down along the vertical axis yeah but there's another way we can measure it which we can look on a horizontal axis and ask whether it points left or right and if we just know what happens when we look on the vertical axis to see whether it points up or down we don't know everything about the qubit it has a richer structure because we have these alternative ways of looking at it and one consequence of that is that the correlations among qubits are richer and more complex than correlations among bits because if I have two qubits I can ask okay if I look at both of them along the a vertical axis you know are they pointing in the same direction or opposite directions but there's a different question you can ask which is what if you look at both of them along some horizontal direction will they both be pointing left or I don't know one be pointing left and one be pointing right and because of this richer structure as you increase the number of qubits the correlations become more and more complex and harder and harder to encode in terms of ordinary classical information in terms of bits and that's really the secret of the qubit I would say that it has there are different possible ways of looking at it and that makes correlations very different let me let me elaborate a little bit um on that because uh just so we we get everyone clear the um the strange the question is you know how to encapulate the straight weirdness of quantum mechanics and as you pointed out if you if you if you measure a spin you know a particle Spin and you measure whether it's pointing spinning up or down um and then that doesn't tell you anything if you make a measurement of of whether of of of the spin in the in the horizontal Direction you'll find out that it might be spinning left or right and you might say okay well now I know what it's doing but then when you go back having made that second measurement and after the horizontal you say okay but I knew it was spinning up at the beginning when you go back you find out may not be spinning up and and that and that that process of of of of these as we say non-communitive but the the the process of saying well when I know just because I know what the particle is doing I measured it here I really can't say it was doing anything specific in the X Direction and and part of the proof is when I measure in the X Direction I suddenly find out I can't say anything specific about what was doing in the Z Direction anymore and that's really strange that's part of the the the part of the long chain of arguments that suggest that that it's that this uncertainty in quantum mechanics doesn't come from just not knowing enough not having made enough measurements but there's something intrinsic you can't there's no classical way in which you could picture what just happened in that by saying that the particle was in some definite State before you made the measurement yeah that that's a very good point and it highlights uh something that we should emphasize which is the difference between qubits and ordinary bids one can look at a bit and ascertain whether it's a zero or one without disturbing the bid in any way so it might be a switch on a transistor and I can shine light on it and see whether the switch is opened or closed and that's not going to change it from open to close or vice versa qubits are more delicate if we acquire information about the state of the qubit if we observe it then that will typically disturb it in some uncontrollable and unpredictable way and that's part of the challenge of quantum Computing that's quite a wonderful yeah we can do it and uh that's part of what yeah so hard to do yeah an important point that you made also which I would like to uh reiterate is that there's the way probabilities arise when we talk about ordinary bits and when we talk about qubit is uh very different like if I have an ordinary coin which is a bit I might flip it and it lands on the table and I cover it up and now we know it's either heads or tails but we don't know which and so we might say Well it has probability one half of being heads and probability one-half being Tails because I don't have any a priori information about which is true um but it really is either heads or tails it's just that we don't know whereas with quantum computers or or with qubits when we observe them it's not that the qubit already is determined to be pointing up or pointing down along the vertical axis um the probability is really intrinsic even if we have the most complete description that nature will allow us to have of that qubit we are still powerless to predict uh whether we'll see it pointing up or down along the vertical axis exactly whereas in some sense if you with the with the head or tail if you had all the information you could possibly have about the motion and speed with which you you you flipped it and the air resistance and everything else you might be able to calculate uh right exactly the probability probability enters because of our ignorance not because of some fundamental exactly the other the other thing the other aspect I want to elaborate on is actually one I guess I learned from you from reading at least learned how to emphasize from reading some of your work it's a really important difference between and this is really relevant it has to do with with this phenomenon of entanglement the fact that that a Quantum system has correlations between many different parts of it that are just absent classical you can think of if atoms were billiard balls you could think of them as each billiard ball doing its thing but in but but you can't separate a a an atomic system of a bunch of molecules that may look like billiard balls into a separate set of billion balls they're they're they're all there are correlations between them which are quantum mechanical and intrinsic and the way that that manifests itself which is probably also another way of thinking about why classical computers can't mimic or understand Quantum systems so much and and why Quantum says is so much richer it's a statement that if I have and I again I learned this from you that if you have a system a large system made up of a lot of entangled particles if I know the state of that whole system if it's classical if I know the state of the whole system then I know the state of every one of the particles that every every object within it you know if I know a whole bunch of bits if I know the state whether it's one zero one zero zero zero then I know where each bit is in but knowing the state of an entangled um a complete description of that of that entangled state does not allow you even in principle to know what the separate components are to describe the separate components so you don't have enough information when you understand the whole state to understand the the the state of every single particle within that system and that's a really important difference and I think you emphasize that and that that leads to the intrinsic of course complexity of quantum systems and also the fact that clearly there can be much more the information stored in such systems then you then you can access just by measuring the whole system itself well it's a bit debatable whether it's there if we can't access it yeah in some sense it is because we don't know how to simulate what's going on uh before we observe it in an efficient way using just a classical machine yeah and but that but as you point out that there there so so that this gives some sort of heuristic understanding of the power once again these systems are doing many things at the same time and if you can manipulate them appropriately you can you can effectively perform many calculations at the same time which which you could never do classically but but but you but you point out you can only do this this these quantum mechanical correlations are so strange to us why don't we understand quantum mechanics and why did Feynman hope that a quantum computer would teach in quantum mechanics because we're classical because these weird correlations just vanish in the world we live in we don't see them particle billiard balls behave like billiard balls and and you know and taking a cue ball and doing something it so it doesn't affect the eight ball over the other end of the table but um and so it's so strange because those quantum mechanical correlations vanish and yet the whole point of quantum computers is to ensure a system which is macroscopic in some sense but which the correlations don't vanish and that means it's it's really difficult and uh I there's a quote from from my one of my former colleagues when I taught at Yale Sergio Roche was at Yale at the time I was there and now in back in France he won the Nobel Prize for the working Dylan measuring Atomic systems the quantum mechanics I think search hirosh and Ramon said that you know these quantum algorithms these quantum computers are a computer scientists dream and experimentalist Nightmare and and maybe that's so true although um you know hirosh was not alone when he uh said that in that article with Ramona was uh maybe 1996 something so it was a couple years after Shore's algorithm where there was a lot of interest in Quantum Computing that had been ignited by Peter Shore's Discovery and interest shared by a growing theoretical community and also many experimentalists but there was skepticism which was reasonable skepticism about whether quantum computers could ever really be built and operated to solve really hard problems the essential difficulty being you know what you were highlighting and what hirosh was one of the world's great experts on what we call decoherence this comes back to the observation we spoke of a few minutes ago that you can't observe a qubit or a Quantum system without disturbing it in some uncontrollable way and in the case of a quantum computer even if we're not looking at the content of a Quantum memory ourselves it's always interacting with the environment and in some sense the environment is observing the system information about the state of the system is leaking to the environment now that can happen for a classical computer too but it's fine uh it might be that the environment is affected differently by a bit which is a zero and then a bit which is a one but it's still a zero or one yeah but in the case of a cubit when information about the state of the qubit leaks to the outside the qubit is damaged so in some sense if we want a quantum computer to really operate without errors we have to keep it perfectly isolated from the outside world and that's extremely difficult if not impossible so what was very important which followed after Shore's initial discovery of his algorithm just by a couple of years is the idea we call Quantum air correction which is a way of protecting a complex Quantum system from damage and the key is to encode the information that we want to protect in a sufficiently clever way that it's very hard for the environment to find out what the state of the quantum memory is and so in effect it becomes very well isolated from the outside so we can manipulate it and do a reliable computation and that's how we expect eventually uh sufficiently large-scale quantum computers will operate and give answers to very hard problems that we can solve classically they will make use of this idea of quantum error correction yeah we'll get then Quantum error correction is very important I have to say as the Eternal um I was going to say cynic I'm skeptic I remember I was skeptical I didn't think the gravitational wave detector would ever work and and I when I first heard about quantum computers I said well it's nice but they're just it's the the the problems are so immense that I mean people talk about and you know back early on people were saying oh we'd have quantum computers so do this or that quickly and I oh and I was skeptical for this clear problem that quantum mechanical systems are are beautiful for precisely the reasons that we never see them as as being quantum mechanical because they because they uh uh I did you get did you did you lose me there for a second yeah yeah I did but you're back okay you're back I saw you the whole time but I could see that you were but but the fact that quantum mechanical systems are are um are beautiful for the same reasons that we never see them that that that that this coherence is is is a very special um um um uh property of uh of um of of quantum mechanical systems that are isolated from the environment and and isolating them from the environment I always thought would just be impractical enough and of course as usual I underestimated experimentalists and theorists for being able to think of ways to get around this problem of of decoherence and and and isolating for the environment I want to talk about that a little bit uh for a second I want to go back though and ask you I I was trying to think about how when I was reading your work about your statements about you know the but the key fact about a quantum mechanical system is that measuring its whole state doesn't give you the state of the par of each of the objects and I was trying to think of a simple example and and I and I I it tell me if this simple example Works um it's the simplest example of entanglement in general if I if I prepare a system of two particles with their spins opposite so we say that this they basically cancel out and the time the total spin of the system is zero we can we can Define that system completely by saying it's a spin zero system but then you know and these particles can separate and it's still a spin zero system if it doesn't interact environment but having set told said that that doesn't tell us anything about the spin directions of the individual particles so knowing that it's in a total spin zero system which is all you need to know to describe that quantum mechanical system it doesn't tell you anything about that about about what you're going to measure when you measure the spin directions of the individual particles is that a reasonable example of the simplest example one can think of that of that effect that you were talking about yeah that's the the prototypical example the two qubit yeah I'd say but the thing I would emphasize is that in the case of qubits you know we have more than one way of observing them and what that's been single state that you described as the interesting property that if we look at the two spins along say the vertical axis they'll be correlated in a certain way namely they'll be opposite but the same is true if we look along a different axis like a horizontal position and that's something very different from correlations in a classical system where we have just one way of looking at them uh it could be that somebody uh decided to prepare two coins where one is heads and one is Tails and gave one of those coins to me and another one to you and then when we uncover the coins we would say hey we have opposite uh you have heads I have tails um but uh that's first of all because we didn't have the most complete description if you really had we would have known uh who had heads and who had tails but also we just have that one way of looking at things that's what makes the entanglement of qubits a lot more interesting than the correlations of bits yeah absolutely if I look at my heads I may know in advance you have tails even if you're an Alpha Centauri but there's no communication between us but but um but you know in the qubit that's fine I may know one thing but if I measure if you make a different measurement I won't know anything about that measurement of that of that particular qubit if you might decide to measure it not in the vertical Direction but the horizontal Direction I Would nothing I about my measurement will be able to tell you anything about the one you made and it's a bit subtle and it often causes confusion because if this is not a mechanism for instantaneous communication yeah between uh Prince Edward Island where you are and uh a distant Galaxy uh it's just that the correlations have a characteristic structure that is different from correlations among bits yeah the way I think of it when P you know I get asked this question a lot about Community about quantum teleportation and why it doesn't give instantaneous why doesn't violate Einstein or anything and my answer is usually that we're just thinking about the system wrong I mean the we're thinking about this system classically we're thinking about it as if they're two separate objects but they're not two separate objects they're the same object they're part of the same object and and you're making two measurements of the same object and and so it's just we we're just think as always the quantum paradoxes occur because we're thinking classically when um when our old friend and in some case Mentor Sydney Coleman would have said we gotta it's you know quantum mechanics in your face we've got to think about it quantum mechanically and and and the real thing is people get hung up on the classical interpretation of quantum mechanics and as he would say that's backwards the world is quantum mechanical you shouldn't try and talk about how to interpret it in terms of this classical cluge that happens to be something we're used to we really should talk about the interpretation of classical mechanics in terms of the underlying Theory and so much of the weirdness of quantum mechanics when expressed classically is just a property of the fact that we're expressing it badly that's right in the case of two entangled qubits are classical reasoning will just lead us to incorrect conclusions because that's not the way nature works it's Quantum not classical okay well now I want to get to the okay so so we've outlined the problems I want to talk about what's really happening I mean and and so you know 20 years ago or not more than 20 years ago um uh uh Yeah Boy a lot more than 20 years ago shorter did his his stuff and and people you know and people have been talking about quantum computers for a long time and then people say yeah but all we have is you know I think we have 50 qubits 75 Q I don't know what the number what the record is now I know the the one that achieved the term that I think you created right called Quantum Supremacy um which is the point where a quantum computer can do something in a finite time than a classical computer would do in an on either longer than the age of the universe or an unfeasonably long time when that happens when there's a calculation that quantum computer can do that it has achieved Quantum Supremacy that was announced I think a year or two ago right by Google or um um and involved a 53 qubit system am I right yeah that was in 2019. two already three years ago four years ago now soon um wow yeah we're 2023 now um but uh so there are challenges but you can do things so I would like you to talk about the systems that are being used to two examples maybe ions and maybe super conductor systems that are being used to try and and capture and and and physically become quantum computers and also the techniques that are being used to try and recognize that you need to do error correction that that that that that quantum computers don't give answers they give answers um that because of errors um you have to repeat the calculation many times and and see where the the answer is tending rather than than what the answer is and so could you uh could you go over those those areas because those are the real logistical practical aspects of what are making quantum computers useful or not useful and the ones which will eventually uh determine whether they ever become all that is promised these things take time don't they yeah they do yeah you mentioned uh gravitational wave detection I guess I I have a skeptical nature also yeah and uh you know when I first came to Caltech in 1983 uh they were already constructing the 40 meter prototype which was 1 100th scale of ligo and the idea that we'd actually be able to build the big thing and make it work it seemed uh you know quite a reach a lot of smart people worked very hard and and made it happen in the case of quantum Computing look it's it's 40 years now since Feynman first suggested the idea uh 26 years well a little bit longer than that almost 28 years Insurance yeah um and so so where are we we're just getting to the point where quantum computers are arguably capable of doing interesting things uh what are the different Hardware approaches I think was one of your yeah questions uh well one is to use individual atoms as qubits uh you know it's interesting that around the time that Shore's algorithm was discovered it happened that for rather different reasons with different motivation physicists were developing the tools to manipulate single Quantum systems like individual atoms Sergio roshu you mentioned was one of the early Heroes and uh that was that was very timely in fact people had developed um trapped ion technology where we trap charged atoms with electromagnetic fields by the mid-1990s for the purpose of making better clocks it turned out a lot of the technology that you need to make the world's best clock is also very relevant to Quantum Computing because what they had learned to do was to manipulate individual atoms with lasers and it sounds like it would be very hard to see a little atom but it's actually not so hard uh if the qubit could be encoded in either the atom being in its lowest energy State it's ground state or some excited state and if you shine a laser on the atom with the right frequency it will either absorb and re-emit the light so it will fluoresce and glow and you can see a little spot of light or it might not interact with the light at all and stay dark and that's a way of reading out whether it's a zero or one of course we need to get the qubits to interact with one another that's the hardest part of any Quantum Computing technology and we need to do that in a very well controlled way and in the case of ions in a trap you can take advantage of the fact that they vibrate that the Trap is like a a potential that confines the motion of the ions and they Rock back and forth and you can excite the vibrations of the ion in the Trap in a way that allows the state of one ion to be manipulated conditioned on the state of another ion and that's the sort of the fundamental operation in a quantum computer it enables you to entangle two atoms and you put together many of those two qubit entangling gates and and you're doing a Quantum computation then reading out the ions at the end the way I set the current state of the yard is you know there are ion traps with uh say 32 ions they can do pretty good Gates um but not really great gates in a device with many ions typically every time you do one of these two qubit entangling gates uh you make an error about one time out of a hundred yeah now that's sort of the state of the art though for other Technologies as well and other competing technology is to use superconducting electrical circuits circuits at very low temperature which conduct electricity without resistance and in that case the the detailed physical setting is different but it's it's kind of an artificial atom um it's kind of amazing actually because the superconducting circuit involves the collective motion of billions of pairs of electrons but we we learned how to make it behave as though it were a single atom and uh to manipulate it uh in that case not with visible light with lasers like we do with the ions but with microwave light a much uh you know lower frequency type of electromagnetic radiation but again we can we can manipulate uh those superconducting qubits and we can entangle them and the devices now are up to over 100 qubits but again the problem is that those entangling Gates just aren't good enough they have a probability of error of about one percent every time you do a gate and so that's a limitation on how large a computation you can do if you try to do too many gates you'll just get random junk yeah that I want to I want there's a number you quoted there but I wanna before I get there I want to ask so you talk about how you can entangle the atoms in a trap um how do you entangle this superconducting circuits well interesting question so in the case of the superconducting qubits um the key to the technology is what we call a Joseph's injunction it's a kind of quantum mechanical device in a superconducting circuit and what it crucially does is it makes the system behave non-linearly uh which means it doesn't depend well it doesn't behave just like uh you know light that doesn't interact with other light but it makes potential for interactions and these uh superconducting devices people call them transmones are coupled to a microwave resonator this is kind of the analog of the atom being coupled to its vibrations in the Trap and so I can have two of these uh transmons these superconducting devices with a microwave resonator uh coupling them and that makes it possible for Quantum information to uh be transmitted back and forth between them or actually I could say it more simply than that in the case of the ions the key thing is the ions are charged so they um interact with one another that you know just the coulomb uh repulsion of the ions is the key thing and that means the different vibrational modes you know will uh give rise to normal moments that we can manipulate and in the case of the superconducting circuits well they're circuits and so you know you can put inductors and capacitors there's the electric Fields can couple or the magnetic fields can couple and that allows two trans months to talk to one another and and but you hit the key point which is the gates the the things that basically do the entangling in in ways that you determine in advance so you can do the computation you want to have happen the difference between classical Gates and quantum mechanical Gates Again pretty clear to state if you have a one or a zero you know you can you can you can have a gate that takes a one to a zero or a zero to one but in quantum mechanics you have these things called unitary Transformations but basically you have a Continuum which is part of the reason that the quantum mechanical system is so much richer and if you have a Continuum then then if you're doing an experimental thing where you want to let's say think of a Continuum as an angle and you want to turn it by 27 degrees well you're going to turn it by maybe 26 or 28 degrees and because all experiments have an intrinsic uncertainty and if you can only do it to one percent and you keep multiplying that error as you pointed out if you want a a if you had a hundred a hundred Gates you'd have to have something like a hundred thousand qubits if you had a one percent a point one percent error which is ten times better than you can even try and achieve achieve now you have a half a hundred thousand cubits before you could get a reliable before you could overcome that error that's a number you quote so I'm assuming it's right um well uh when I spoke of a hundred thousand cupits I was probably talking about using Quantum error correction yeah which is another story but having more qubits if we don't do Quantum error correction just makes things worse right yeah it means more things that can fail um so so so you say one you have an error of one percent but let's talk about that Quantum error correction because that seems to me to be the great hope I mean that's what if anything is going to overcome my original uh skepticism about whether these things can work I'm still convinced you know you can't keep these things coherent for a long time or maybe maybe you can but it's going to be hard but but if you can overcome some of these problems by air correction then maybe my concerns are not so great so why don't you talk for a few minutes about Quantum error correction and maybe well the idea of the idea of quantum mayor correction is as I noted earlier if we want to manipulate Quantum information accurately we have to prevent it from interacting with the environment and our Hardware isn't perfectly isolated from the environment so what we do is we make use of entanglement we encode the information that we want to protect in a highly entangled state and what that means is and you mentioned this earlier if we look at the parts of the system one at a time we don't see the hidden information we don't see the encoded information but that's how the environment typically interacts with this system in a way that's spatially local and we spread out the quantum information in the form of entanglement involving many qubits and that makes it possible to protect it and we've learned how to efficiently manipulate or process information that's encoded in that very entangled way and that's how Quantum error correction will work in the long run but it's expensive because if our gates have one percent is just a little too high but let's say and this will probably happen reasonably soon the hardware improves to the point where the probability of error Brigade is about one in a thousand instead of one in a hundred then in principle Quantum error correction will work but we'll need a lot of extra qubits to have enough redundancy to protect the information well and that's probably where that hundred thousand number came from if I wanted to run shower's algorithm uh to factor a number which is cryptographically relevant like to break codes that people use today to protect their privacy um that would require um a few thousand uh protected qubits and the number of physical qubits that will we would need is in the tens of millions if the error rate per gate is 10 to the minus three so that's that's a long way from where we are now because the hardware now you know is more at the level of 100 qubits and we're probably going to want to have millions and that's going to that's going to take some time in the meantime though the quantum Computing technology I think is already scientifically interesting because it does give us an opportunity to study the behavior of many highly entangled Quantum particles in a way that's never been experimentally accessible before and I anticipate that we'll be learning things about Quantum Dynamics from experiments with quantum computers and Quantum simulators in the next few years and arguably that's that's starting to happen already so for me as a physicist who's interested in understanding nature better I think uh Quantum Computing has reached a very interesting stage in terms of economic impact uh applications of broad interest in the business Community those are probably still uh considerably further off because this might be wrong but as far as we can currently tell we'll probably need Quantum mayor correction for that and that's a big leap from the current state of the technology we'll get there but it's going to take time that this is perfect because it's it segues to to the last thing I want to talk about but but what you're talking about I don't know if you invented this term nisq um you want to it which is basically says these systems are workable enough now to do interesting things what does nisq stand for right I pronounce it nisk yeah as though the Q were okay yeah yeah and uh it's an acronym it stands for noisy intermediate scale Quantum and what intermediate scale means is that we now have devices with say a border 100 cubits or 50 to 100 cubits which are of a scale that we can by Brute Force simulate with an ordinary computer exactly what the quantum device is doing it's just too hard but noisy reminds us that these are not error corrected devices and the noise is a limitation on their computational power a limitation on how many gates we can do and still read out a useful result so in this experiment that you mentioned that was announced with some Fanfare by Google in late 2019 they had a 53 qubit device and they had a great air rate around one percent and they performed uh computations my dog isn't a huge fan of arrogates one okay what's your dog's name uh well we have two of them Levi is my my dog and then my my inherited my mother's dog her name is Tasha and they once one starts to bark the other can't help it yeah it's a Jewish dog a Jewish dog Levi exactly yeah that's right okay good um yeah so in that case uh because there were hundreds of entangling gates when they when they read out the result in the end about one time in 500 they were able to get a valid result and 499 times out of 500 they just got random junk so they had to repeat the computation millions of times and then they were able to extract a statistically useful signal so that's kind of the state of the art now but the point is but the point is if if if if repeating if you can do it if it takes a few seconds or minutes you can repeat it thousands or millions of times and that's okay if the classical calculation would take thousands or millions of years you don't mind repeating a Quantum calculation thousand times or a million times if you're doing many of them per second I guess yeah well that's right and in fact to do that do millions of repetitions did take only a few minutes an interesting thing that happened is that the um classical uh Team uh was inspired by this experiment and other related ones to come up with better methods for simulating what computer does those methods are better now than they were in 2019 so the gap between how long it would take for a classical computer to imitate what the quantum computer does for this particular experiment has narrowed a lot but the essential point is that as you increase the number of qubits the difficulty of doing that classical simulation the number of steps that it takes grows exponentially with the number of qubits so if the Gap is still not so wide for 53 qubits if we go up to say 70 cubits the quantum computer will really be left in this far ahead of the classical yeah that's the thing I was going to get it's so really important is that if you have 53 qubits and it takes some amount of time 100 cubits it's not going to just double things it's it's it's exponential and so yeah if you're if they're close to each other now all you if you know that's the great hope of computers is you're just because it improves exponentially as you get a few more you'll be away from the domain where classical computers could compete but it is an interesting fact you're absolutely right within a few days that and that does bring I can't help but when I think of the object lessons that comes from from our talk uh We've mentioned a few for graduate students but the other thing that is interesting I found in my career as a scientist in my own career I've written papers that I could have written 10 years earlier but until the experiment was done I guess I never thought it took it seriously enough to think about the details I mean and it's all it's amazing how actually doing something inspires people to think about the implications a lot more carefully than they would have you know go knocking experiments work to some extent but seeing but but really having the data is an inspiration that for theorists that in that you might not imagine it would be and and you know so sometimes people say the theorists take their ideas too seriously but when I was a student Steve Weinberg told me he thought the opposite was true that the theorists don't take their ideas seriously enough because it's just so hard to believe that the you know the scribbles we make on a piece of paper are really going to correspond to the way nature behaves at a fundamental level it is it is intimidating it's intimidating and almost um frightening to think when you're doing something uh that that nature may actually behave that way it is a really it's a it's exhilarating if it's right but it's terrifying it's hard to believe you're absolutely true and we don't and and and I yes on my own career and I think all of us you know we uh well and you know often I've written papers saying well you know I remember there were I haven't written papers saying well this would be interesting but they'll never be able to do this so I won't write down the paper well that's a mistake because uh because uh experimentalists are actually uh hard working and can do amazing things and So speaking of that and if we're at the point where now we've got sort of this we're able to do useful stuff and you talk about it's true and I think it's really important to point out that you know I've been in meetings with recently a meeting of crypto people talking about they need new Quantum error correction codes that you know we'll we'll preserve the world's banking systems the world's banking systems are not threatened immediately and um and um and unlikely in the near future would you agree well yes and no um I think it's true which I think is what you're suggesting that we will not most likely have quantum computers that are capable of breaking the crypto systems that we currently use um in the next say 20 years but on the other hand okay it's urgent to replace the crypto systems which will eventually become vulnerable to attack by quantum computers with new crypto systems which we think are resistant to Quantum attacks because first of all it takes a long time to implement that that new uh key infrastructure and secondly you want a crypto system that you're using to keep information secure for some time after you begin using it you know it's always possible for an adversary to capture traffic which can't be encrypted right away but when technology is more advanced later on we'll be able to encrypt it so you have to ask yourself not just how long will it be before people can run things like shorts algorithm but also how long will it take to implement Alternatives and how long do we want to protect the information I don't think anybody can promise you that we won't have quantum computers that break the crypto systems we're using today in 25 years um so you know it's time to start worrying about it trying to start worrying about it but one should also add that quantum mechanics is double-edged sword it it brings not just the threat but Solutions as well quantum mechanics by the very same kinds of entanglement um allows one to to actually know whether messages that have been sent have been eavesdropped on and that and so it may one may use similar technology that one uses to build quantum computers to build Technologies to ensure that one's protected from eavesdropping are disturbances so it's a nice kind of complementarity there in quantum mechanics it's another manifestation of what we said earlier about Quantum information having the feature that you can't observe it without disturbing it in some detectable way and that's the fundamental physics principle that makes Quantum cryptography um but well in principle um yeah so in the future to to come to the end now the two things one is scalability and and and I mean you've talked about these Technologies and I've been involved at various times in proposing experiments to look for things like dark matter which you know we're fine when you have grammar size detectors but if you need 10 ton detectors it's a different story are all that are that the technologies that you've discussed scalable or will one need new technologies in order to scale up to go from 50 to 5000 qubits well certainly needs some new technologies I think we're not at the stage where you know it's we can hand it over to the engineers and uh I think we still have a lot of basic science to do I think as of right now there are really two fundamental questions about the future of quantum Computing both of which I regard as largely open one is how are we going to use these things what will be the most important applications for powerful Quantum Computing technology when we have it and the other is how are we going to scale up from the relatively small quantum computers we have now to much bigger ones which are capable of solving very hard problems and we don't know for sure what the answer is to either one of those questions okay great I mean not knowing as I've my whole new book my most recent book is about the importance of not knowing or at least recognizing them what that one doesn't know because it gives one an invitation to learn um the the uh and I don't like asking people to make predictions because people always ask me to make predictions and I say I don't not unless they're 10 trillion years in the future that I'm happy to predict um but so rather than say asking you where Quantum Computing will be 25 or 50 years because who the heck knows let me ask you what's what excites you the most for the next 10 or 20 years what what what what are your what are you what do you expect and what do you hope for well one thing I expect on a shorter time scale than you mentioned uh is that we will see significant progress towards realizing Quantum mayor correction not at the scale that we'll eventually need but you know up to now we haven't reached the Milestone of showing that if we use a Quantum air correcting code we can make a computation much more reliable and continue to do so as we scale up to larger and larger nodes theoretically but we believe that's the case but we'd really like to see that demonstrated in Hardware because the theory is based on certain assumptions about the noise which we won't really be able to validate until we try it in different Hardware platforms I think we'll see a lot of progress on that in on a five-year time scale um as far as the applications to physics are concerned I think we're going to learn things on a scale of five to ten years about how Quantum chaos Works about what happens when a Quantum system has many strongly interacting particles you know there was kind of a revolution in classical physics sure back in the 60s and 70s when people started to simulate using their conventional computers the behavior of classical non-linear dynamical systems and that led to a lot of insights into the types of chaos that can arise we know relatively little about chaos in a Quantum setting because we can't simulate those systems with our conventional computers and that's part of what our Quantum devices will be capable of and even relatively noisy ones are going to teach us interesting things in the longer term I think we can anticipate applications of quantum Computing to the problems that Feynman originally had in mind to understanding chemistry and materials more deeply we have to keep in mind that the classical algorithms are not so bad even though they don't scale well and so and they'll continue to improve in fact they have improved a lot in the last 10 years so exactly one we'll see quantum computers surpassing uh our best conventional computers running the best algorithms for problems in in chemistry and material materials well we don't really know that it will happen eventually and I think that will be one of the ways in which Quantum Computing will eventually uh have a big practical impact on the world if you had to ask one question you'll come on computer and get an answer what question would it be well it doesn't have oh yeah you know this reminds me of something that you asked me when you were writing uh you know the sequel to the physics of Star Trek yeah I I was supposed to uh ask uh not a quantum computer but uh some all-knowing Oracle uh uh question and and I I asked it is physics and environmental science you know in other words uh you know are the laws of physics really determined or are there a role of the dice um anyway uh so I don't think since then have you asked me a similarly profound question um well the answer that comes to mind is ever since I first got into the subject in the 1990s I have been interested in what Quantum Computing might teach us about quantum gravity and we touched on that briefly before but one way in which ideas from Quantum information have had a significant impact on our understanding of fundamental physics is the people who do quantum gravity for a living the people with string theory backgrounds and so on they use a rather different language now than they did 10 years ago if you go to a conference you'll hear people talking about Quantum air correction and Quantum complexity and so on in the context of quantum gravity and part of what's Driven that is the realization that we can think of space itself as an emergent property in which the underlying mechanism is quantum entanglement you know in a sense what's holding space together is quantum entanglement and what I would like to see uh when I'm still around to enjoy it is uh insights into how space can emerge from a highly entangled system coming from simulations run on quantum computers great well you know I was going to ask you a more leading question I was hoping you'd go there so I'm glad that's where you went but in a in a way this talk and our discussion has been full of poetry because you're really appalled even though you don't know it um the the Poetry of many aspects of your life and I just it's kind of interesting you mentioned that question I asked you a long time ago about whether physics is an environmental science in some sense since String Theory or since quantum gravity May one of the things one or every may tell us is that physics is an environmental science in the sense it may tell us that the Universe we live in is is an accident of our circumstances um it may answer that same question that you wanted to know way back then so I think it's kind of poetic in a way that maybe in the long run when we learn about whether space is an emergent property we may learn about whether our space um whether it can emerge in many different ways and uh and that would be interesting and so uh I certainly hope you're around to uh keep asking profound questions and keep pushing an important field and and also I hope you continue to always try your graduate predilection of understanding everything uh because it helps to understand something in your case and I really do appreciate you're taking this incredibly generous time to to talk to me about about your time as a physicist and about this incredibly New Field which where where a lot is said and one often has to parse it carefully to see what's accurate and it's nice just nice to go to the horse's mouth not not saying that you're a horse but but nevertheless I appreciate it it's been a lot of fun Lawrence foreign [Music] I hope you enjoyed today's conversation this podcast is produced by the origins project Foundation a non-profit organization whose goal is to enrich your perspective of your place in the cosmos by providing access to the people who are driving the future of society in the 21st century and to the ideas that are changing our understanding of ourselves and our world to learn more please visit originsprojectfoundation.org

Channel: The Origins Podcast

Views: 79,684

Keywords: The Origins Podcast, Lawrence Krauss, The Origins Podcast with Lawrence Krauss, The Origins Project, Science, Podcast, Culture, Physicist, Video Podcast, Physics

Id: Y6ilk3WA9Zw

Length: 134min 21sec (8061 seconds)

Published: Sat Feb 18 2023