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.’”