To find the prime factors of a 2048 number
it would take a classical computer millions of years, a quantum computer could do it in
just minutes. And that is because a quantum computer is
built on qubits, these devices which take advantage of quantum super position to reduce
the number of steps required to complete the computation. But how do you actually make a qubit in practice
and how do you read and write information on it? I met up with researchers who are using the
outer most electron in a phosphorous atom as a qubit. This single phosphorous atom is embedded in
a silicon crystal right next to a tiny transistor. Now the electron has a magnetic dipole called
its spin. And it has two orientations, up or down, which
are like the classical one and zero. Now to differentiate the energy state of the
electron when it is in spin up and spin down, you need to apply a strong magnetic field. >> And to do that we use a super conducting
magnet. So super conducting magnet is a large solenoid. It is a coil of super conducting wire that
sits inside of that vessel that is full of liquid helium. >> So now the electron will line up with its
spin pointing down. That is its lowest energy state. And it would take some energy to put it into
the spin up state. But actually not that much energy and if it
were at room temperature the electron would have so much thermal energy that it would
be bouncing around from spin up to spin down and back. And so you need to cool down the whole apparatus
to only a few hundredths of a degree above absolute zero. That way you know that the electron will definitely
be spin down. >> There is not enough thermal energy in the
surroundings to flip it the other way. >> Now if you want to write information onto
the qubit, you can put the electron into the spin up state by hitting it with a pulse of
microwaves. But that pulse needs to be a very specific
frequency and that frequency depends on the magnetic field that the electron is sitting
in. >> So what you see here is the frequency that
is being produced by this microwave source and it is 45.021 gigahertz, which in the magnetic
field that we are applying now is the resonance frequency of the electron. >> So the electron is a little bit like a
radio that can only tune in to one station. And when that station is broadcasting, the
electron gets all excited and turns to the spin up state. >> But you can stop at any point. So if you just make a new tape and stop your
pulse and some specific point, what you have created is a special quantum super position
of the spin up and spin down states with a specific phase between the two super positions. >> And how do you read out the information? Well, you use the transistor that this phosphorous
atom is embedded next to. >> The spin down has the lower energy. And the spin up has the higher energy. Now in this transistor there is, in fact,
a little bundle of electrons. This bundle of electrons is filled up up to
a certain axis. This vertical axis here is energy. And here we have got all these electrons that
line up in energy just like the electrons on the shells of an atom. So now if the electron is pointing up, it
can jump into the transistor, right, because it has more energy than all the others. It leaves behind the bare nuclear charge of
the phosphorous, right? The phosphorous one more positive charge in
the nucleus as compared to silicon, but normally it is neutralized by the extra electron so
the two things cancel out. But if you take the electron away, then the
phosphorous has a positive charge. So it is as if you have a positive voltage,
a more positive voltage applied to this gate. It doesn’t come from the gate. It comes from the atom, but is the same. It is just a positive voltage. >> It is like the transistor has been switched
more on. And so you see a pulse of current and that
indicates that the electron was in the spin up state. >> In this measurement phase, if you find
one of these spikes of current, it is because you had an electron spin up. So it can play catching a spin up or a spin
down event. You use, there was no current here. That was a spin down event. And try again. Again a spin down electron. Spin up electron. >> Now these researchers have actually gone
further using the nucleus of the phosphorous atom as a qubit. Like an electron, the nucleus has a spin,
although it is 2000 times weaker than the spin of the electron. But you can still write to it the same way
using electromagnetic radiation, only it needs to be a longer wavelength and a longer pulse
in order to get the spin to flip. >> Because it is so small, so weakly magnetic
and so perfectly isolated from the rest of the world, it is a qubit that lives for a
very long time. >> But how do you read out the spin of the
nucleus? Well, you use the electron. Remember that the electron’s frequency that
it will respond to depends on the magnetic field that it is sitting in. >> So that magnetic field is the external
magnetic field that is produced by the super computing magnet, but there is also an internal
magnetic field coming from the nucleus. But that internal magnetic field can have
two directions. Right? The nucleus can be pointing up or down itself. So what it means is that there are two frequencies
at which the electron can respond, depending on the direction of the nucleus. >> So the nucleus actually acts as a little
selector. It tells the electron, basically, which radio
station it can listen to. >> So what you are looking at now is an experiment
where we actually flip the nucleus every five seconds. So for five seconds you will see that the
electron always responds, because the nucleus is always in the right direction to make the
electron respond to the frequency we are applying to the electron. And then for the other five seconds, the electron
will not respond, because we have flipped the nucleus the other way. So now watch. You see? >> So in this period of time the nucleus has
been flipped down. >> Yeah. >> And now after five seconds it will flip
up and then... >> Yeah, you see? >> And then the electron starts responding. >> Yeah. So you are watching on the oscilloscope screen
in real time the measurement of the direction of a single nucleus and our ability to flip
it at will every five seconds. >> The spin of the single nucleus. >> Nucleus. >> Now because all of this depends so sensitively
on magnetic fields, you need to make sure to eliminate all spin from the silicon crystal. But, unfortunately, natural silicon contains
about five percent the isotope silicon 29 and that does have a spin. >> But, in fact, the beauty of silicon is
that it has this isotope called silicon 28 that has no nuclear spin. The nuclear spin is zero. So it is a completely non magnetic atom. >> But where are you going to find a pure
crystal of silicon 28? Oh, wait. >> These isotopically purified silicon 28
crystals are being produced anyway for a purpose completely different from particle computing. They are being produced to redefine the kilogram
through the Avogadro project. >> So the off cuts from that silicon sphere
are actually being used as the home for qubits. That, I think, is incredible. There is no waste in this science. Hey, there. This episode of Veritasium was supported by
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