Optical lattice clocks are a new type of optical atomic clock. They were proposed in 2001 by University of Tokyo Professor Hidetoshi Katori, and experimentally demonstrated in 2005. Recently, Professor Katori’s group accurately kept count of time to 17 decimal places, or a 100 quadrillionth of a second, with an averaging period of 15 minutes. This was the world’s first demonstration of an extremely stable optical lattice clock.

Conventional optical atomic clocks use single atoms, and measure time by taking averages over a long period. By contrast, optical lattice clocks can make measurements a million times faster, by observing a million atoms simultaneously.

“Optical lattice clocks were originally intended to read out time to 18 digits through one-second measurement. That kind of accuracy will enable us to see that our space is dominated by relativity.”

“In Dali’s famous painting The Persistence of Memory, which is said to have been inspired by Einstein’s theory of relativity, space is bent and curved. With an 18-digit clock, we could see that space is curved on our ordinary timescale. For example, if one clock is placed one centimeter higher than another clock, the higher clock is affected by less gravity, so it goes faster. That difference could be read out in the 18th decimal place of the clocks in one second averaging time. Until now, clocks have been thought of as tools for sharing a common time. But with clocks like this, conversely, we can understand that time passes at different speeds, depending on the time and place a clock is at.”

Professor Katori expects that using optical lattice clocks to demonstrate this kind of relativistic space-time, and to measure the distortion of space-time, will lead to new applications.

“When we make an atomic clock, we believe that the fundamental constants of physics are constant. If we believe they’re constant, then atomic frequencies are also constant, as they depend on the physical constants. So, based on those constants, we expect we can define a universal second. That’s our assumption when we make atomic clocks. But we can make clocks with very high performance using different atoms, and compare the second defined by one atom with the second defined by another atom. When we do that, if the physical constants are constant, the two clocks should show one second together. But if the fundamental constants of physics aren’t constant, if they’ve slowly changed since the Big Bang, the two clocks will each show “a different second.” Conversely, by comparing clocks like that, we can investigate the truly fundamental concepts of physics. Also, if we put the two clocks in different places, and they start to show one second differently, we can say that the gravitational force at one place may be different from that at another place. That phenomenon could be used to sense changes in geodesy, which might make it possible to predict earthquakes. And many other applications are likely to emerge in the future.”

Currently, GPS is based on atomic clocks accurate to 14 or 15 digits. But when people conceived of atomic clocks 50 years ago, no one imagined applications like GPS car navigation. When optical lattice clocks deliver 18-digit accuracy, advanced applications to match that performance are likely to be invented.

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