The New Atomic Clock Will Be Accurate for 15 Billion Years

That's almost the age of the universe itself.

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Apr 22 2015, 2:25pm

Image: Ye group and Baxley/JILA

The world's most accurate optical atomic clock is now three times more precise than it was a year ago, thanks to engineers and physicists at the National Institute of Standards and Technology and the University of Colorado's JILA lab.

Said clock could keep time for 15 billion years, the approximate age of the universe, and be assured of never gaining or losing a second.

In terms of stability, or how close one tick matches the next tock, this is an improvement of about 50 percent over last year at this time, when the JILA clock set the previous world record. Now, the clock is capable of detecting shifts in gravitational effects—per relativity, gravity distorts time—that result from lifting it a mere two inches above the Earth's surface.

"Precise and accurate optical atomic clocks have the potential to transform global timekeeping, enabling orders-of-magnitude improvements in measurement precision and sensor resolution for a wide range of scientific and technological applications," the JILI/NIST researchers, led by CU optical physicist Jun Ye, write in a paper out Tuesday in Nature Communications. "The pursuit of better atomic clocks has also had strong impact on many fundamental research areas, providing improved quantum state control, deeper insights in quantum science, tighter limits on fundamental constant variation, and enhanced sensitivity for tests of relativity."

So, what exactly is an optical lattice clock? An optical lattice is basically a grid of intersecting laser beams meeting in the right places and at the right angles to create a pattern of dips or depressions along a flat plane. An atom, strontium in this case, can be confined to one of these depressions in the sense that escaping requires some amount of extra energy, like eggs in an egg carton. In the JILA clock, many thousands of atoms are confined in this way.

"JILA's strontium lattice atomic clock now performs better than ever because scientists literally take the temperature of the atoms' environment. Two specialized thermometers, calibrated by NIST researchers and visible in the center of the photo, are inserted into the vacuum chamber containing a cloud of ultracold strontium atoms confined by lasers." Credit: Marti/National Institute of Standards and Technology

Surrounding any atom are some number of electrons, which "orbit" around the atomic nucleus in different layers or shells corresponding to the varying energy levels of the electrons. In the innermost shell, we'd find the lowest energy electrons and in every shell moving outwards we'd find increasing electron energies. These shells are discrete (quantized), which means that there's no fuzziness in between. It's possible to excite an electron such that it jumps up to the next shell by firing some photons (electromagnetic radiation) at it, but, because there are no in-between energy levels, the only time electrons will actually absorb this radiation is if it's just the right amount to elevate it to the next one. There's no saving those photons for later.

In other words, the only energy that an electron in an atom can absorb is the difference between its energy level and the next one. In an optical lattice-type atomic clock, laser beams (different from the lattice confinement beams) are fired at the trapped atoms, which may or may not have the effect of causing an electron to jump to the next energy level, which is known as the "clock state." The closer the frequency of the beam is to the frequency required to make an electron take its leap, the better the odds are of the particle actually doing so.

So, laser beams are fired at all of these strontium atoms and whether or not their electrons made it to the clock state can be determined by observing whether or not they eventually give off a few photons, as would be expected when an excited electron gets bored and drops back down to a lower energy state (the electrons "relax"). The "time" in this setup is the excitation frequency of the strontium atom, which is determined through a succession of laser beam probes. Using many atoms, as in the optical lattice method, means the possibility of eliminating uncertainty (by taking averages of all of the atoms) and, thus, increasing the clock's precision. (A more conventional atomic clock would be set up as above, but with just a single atom instead.)

This is where we get our headline statistic: uncertainty. Take the systematic uncertainty of all of those atoms and divide it into one and then divide the number of seconds in a year (31.5 million) into that. This results in the minimum number of years to accumulate one second of error.

Once, timekeeping was an innovation that allowed many people in many places to share the same measurements of time. Thanks to the Unix Epoch Clock, we can be assured that our computers are keeping the same time, for one example. But, as Hidetoshi Katori, the physicist who originally cooked up the optical lattice clock scheme in 2001, notes in the video above, optical atomic clocks offer something even more special: the possibility of telling differences in time. Time changes everywhere because mass changes everywhere, but it takes subatomic physics to capture its finest details.