Oct 5, 2017

# Researchers just built the world's most precise clock

Two years ago, physicist Jun Ye and his colleagues at the National Institute of Standards and Technology built an atomic clock so precise it doesn't lose or gain a second over the entire age of the universe. Today, they report they've made one ten times as precise using a new technique.

Why it matters: Similar atomic clocks are accurate enough to measure the effects of gravity on time by moving them just a few centimeters up and down. Beyond improving GPS and other time-dependent technologies, atomic clocks could be used to better understand what is below Earth's surface. That could help them model the flow of water, better predict weather or detect magma moving inside a volcano.

The goal: These clocks aren't just for ticking. Their accuracy and precision are allowing scientists to begin to test the predictions of Einstein's general relativity. They hope to study the grand question of physics: what is the connection between quantum theory and gravity — the laws governing our universe's smallest and largest realms?

"This will be a trendsetter for the whole community," says Jan Thomsen, who researches atomic clocks and is head of the Niels Bohr Institute in Copenhagen. "It's mind-blowing."

How it works: Like the pendulum on your grandfather's clock, an electron's movement back and forth between two energy states is the counter in an atomic clock. That movement is caused by hitting an atom with light or radiation.

Today, a second is measured by exciting electrons in a cesium atom with a microwave at a particular frequency (9,192,631,770 Hz). In other words, the second can be thought of as the amount of time it takes for electrons in the element cesium to oscillate 9,192,631,770 times.

The problem: An atomic clock's performance is based on:

1. accuracy (how close a measured second is to its true value)
2. precision (any variation when you measure a second repeatedly)
3. stability (in this case, a measure of how much the ticking rate changes over time).

Researchers want to use atoms that oscillate at a higher frequency to improve accuracy and more atoms to make timekeeping more precise and stable. But more atoms jammed into a space, the more likely they are to interact with one another, which can quickly throw off the entire clock. They've built clocks that use different atoms and approaches to try to address this tradeoff, with varying success.

Ye and his colleagues report that a new scheme solves that problem with 100,000 times more "ticks" per second. "Nothing is perfect, but this approach comes closer than before," says NIST's William Phillips.

How it works: The researchers used lasers to cool and suspend 3,000 atoms of the element strontium in the shape of a cube. (Imagine a grid of light with an atom in each pocket of it.) Another laser, tuned to the frequency to make the electrons jump, hits the atoms and causes the oscillations. Those jumps are counted, and when 430 trillion occur, a second is considered passed.

What's new: In existing one-dimensional quantum clocks, each atom behaves as an individual and is measured independently. Ye created a quantum gas that allows the atoms to be held in a 3D matrix, which minimizes their interactions.

"We essentially created a system where you can continually expand the number of atoms without worrying about the interaction. We removed the compromise," says Ye, who says he hopes to build clocks like this with a million atoms.

What it means: This clock isn't for measuring time right now. Ye is quick to point out that while the precision of this new clock is improved, they haven't yet evaluated how accurately it counts a second.

The real advance, says Ye, is the method they used for cooling the atoms to create a gas in which quantum interactions between the atoms dominate. That allowed them to trap the atoms in a 3D grid while avoiding the deleterious effects of multiple atoms being close together, because at these near absolute zero temperatures, the atoms become aware of their neighbors and essentially synchronize. He hopes this quantum "playground" could one day be used to study the link between gravity and quantum mechanical effects.