Jun Ye: “The reason we get very excited
talking about clocks is not just really making time, but really about exploring the frontier
problems of quantum physics. Clock, is, I feel, one part of the human endeavor. You can actually turn that into a quantum
physics playground.” Inside this basement lab a team of physicists
are wrangling atoms at super high speeds and suspending them in optical traps to measure
atomic ticks. “When you walk into our lab, the first thing
comes to your mind is like, "Oh man, that's crazy.” On this table top, Jun Ye and his team at
the University of Colorado have built the world’s most precise atomic clock. And it gets its ticks from the vibrations
of 10,000 atoms. Time is a universal constant in our lives. GPS navigation, power grids, financial networks,
whether you get to work before your boss... all of this depends on reliable timekeeping. But have you ever stopped to think about what
time actually is? It's a very precise measurement of ticks,
and thanks to the march of technological progress, that “tick” has gone from the movement
of the sun, to a pendulum swing, to the vibrations of a quartz crystal. And ever since the 1960s, we’ve been on
atomic time. “Inside the atom, electron is moving around
nucleus and that has very periodic oscillation, and we say we want to use that as our fundamental
unit of time. We want to measure those energy level structure
extremely precisely because that's a constant of nature. If you can measure extremely precisely, it
should be a universal value and that's what the atomic clock is all about.” This is the NIST F-2, an atomic clock at the
National Institute of Standards and Technology in Colorado. It’s one of the world’s master clocks
and is designed to measure the very specific oscillations of a silvery atom on the periodic
table: cesium. Inside it, a gas of cesium atoms enter the
clock's vacuum chamber, microwave laser beams push the cesium atoms together into a ball,
the lasers toss the ball up, then it falls back down, emitting photons. The time it takes for the cesium ball to move
between two different energy levels is 9,192,631,770, and that is the definition of a second. That sounds pretty precise, but a new generation
of atomic clocks are on the horizon, which use laser light instead of microwaves to divide
time into even finer slices. “The reason why we want to move from microwave
to optical frequency is given by one simple fact is, the light frequency oscillates much
faster than a microwave. In a blink of an eye, you can have a million,
billion cycles go by if you're dealing with optical frequencies, while if you're dealing
with microwave frequencies, you might be dealing with only one billion cycles per second. The more cycles you can measure per fixed
time, the less fractional mistakes you will be making.” Creating an optical clock is an incredible
change. And while this looks like a labyrinth of wires,
everything has a purpose. “It's like if you're little, if we can shrink
your size down by a factor of 10 and you walk along those mirrors, it would be like...Complete
black forest of mirrors. Every single mirror on that table has its
sole purpose which is allowing us to steer all kinds of colorful lasers to interact with
the strontium atom.” Instead of cesium, Ye and other teams at the
NIST are building optical clocks based on other elements, like strontium and ytterbium,
that can tick at higher frequencies. “Strontium sits at the second column of
the periodic table and it's characterized by two valence electrons. When you have one electron, it's very volatile. When you have two, it’s much less volatile
compare to cesium atoms. The strontium atom, when you liberate it,
they're moving at the speed of 300 meters per second which is essentially like a bullet
train. So if I ask, "What time is it strontium atom? I wouldn't be able to tell you the time. The first thing we need to do is slowing them
down so they're standing still in front of you. So we need a bunch of lasers//we take a few
tens of milliseconds to finally prepare them to very low temperatures, and we load them
into an optical trap. How we do that is by using another laser coming
in, it's almost like a tweezer made of light. So this laser light coming from outside the
vacuum window focuses its light down to a little focus spot, and polarizes the atom
and hold them in the middle of the vacuum chamber so you can actually look at the atom.” Ye's team was able to cool the strontium atoms
to below a microkelvin, turning it into a quantum gas that allowed the atoms to spread
out and organize into an optical lattice. “Once the atoms cool down and trapped, then
you need to turn on the clock laser, finally try to match your laser color to the transition
of the atom you're trying to interrogate as a clock signal.” The tick for this 3D gas clock is the exact
frequency that prompts the strontium atoms to switch energy levels, which is 430 trillion
cycles per second. It’s so precise that it can keep time without
losing or gaining a second for 15 billion years. “We all know that these optical atomic clocks
are now performing hundred times better than the microwave clock. Time is something that's been discussed very
actively right now. When will be the good time to replace the
current cesium clock with the strontium clock, or some other atom. But defining time is a human enterprise. It requires international cooperation. It requires a universal time where every country
should agree upon. And this is really, a very serious matter.” But Ye’s pursuit in making an ultra-precise
atomic clock isn’t just about refining the standard of time. Because atomic clocks are measuring the interplay
between electrons and elementary particles, they are a unique tool to investigate why
our universe is the way it is. “We are building more precise and more sensitive
scientific instruments to be allowing ourselves to be able to detect gravitational waves or
detect the presence of dark matter, because in the end, if you are the master of the space-time
fabric, you got to be able to figure out that dark matter is bending the space-time a little
bit, and the one way to figure this out is just to measure time. There's a symphony going on, and time, remember,
is not something unique. Time is related to space, so if there are
all kinds of bodies moving around, merging, separating, rebirth, and so on, the time's
changing everywhere. And so that's at the very tiny scale. If you build clocks so well, eventually you
get to the point where you will not help but hear all these microscopic noise that's going
on in the universe. I'm optimistic, within the next 30 years we
might get to the point where we can measure the gravitational effect on quantum physics
and maybe just keep going to the point where the universe says ‘guys, all the times are
different and here's the final limit.’”
