Applications for Atomic Clocks原子钟

NIST's 2013 ytterbium lattice atomic clock. Credit: Burrus/NIST

Humans can measure time more accurately than any other quantity in the universe. These record-setting clocks are known as atomic clocks, and they tick according to cycles of extremely stable laser light, whose frequency is set to a quantum mechanical property of an atom, as opposed to a pendulum. In multiple sessions at 2020 SPIE Photonics West on Saturday and Sunday, researchers presented about the latest developments in timekeeping and its applications for physics, astronomy, and geoscience.

Precise clocks have far-ranging applications well beyond one's social calendar. Because the presence of gravity affects the rate of time passing, clocks closer to sea level actually tick slower than one on Mount Everest-which means that physicists can use these clocks to monitor the shape of our planet, a scientific field known as geodesy. Some researchers think that this capability could help them more accurately monitor the change in sea level.

To increase measurement accuracy, some researchers have started to synchronize facilities to a single precise time standard. Mario Siciliani de Cumis of the Italian Space Agency described the construction of a 1800-kilometer fiber optic network in Italy synchronized to one time standard that could potentially be used for geodesy. This synchronization could result in improvements in an astronomical imaging known as Very Long Baseline Interferometry (VLBI), a method that involves multiple observatories working together to image an object otherwise impossible to resolve with a single telescope. Astronomers used VLBI to capture the first image of a black hole earlier this year, for example. Better synchronization via this optical fiber could allow for even higher-resolution imaging.

Another avenue of research is launching atomic clocks in space. In 2019, NASA engineers launched an atomic clock in orbit. Researchers want to put more atomic clocks on satellites and space missions, Zachary Warren of the Aerospace Corporation said. These clocks would help the spacecraft orient themselves and navigate autonomously, for example. To meet the size and weight constraints of spacecraft, Warren is developing compact atomic clocks.

Researchers also think they can discover new physics by playing around with the internal hardware of the clocks, whose precise engineering also allows physicists to run experiments not directly related to timekeeping. For example, Shimon Kolkowitz of University of Wisconsin-Madison is developing a gravity experiment with a type of atomic clock called an optical lattice clock. Optical lattice clocks use atoms, lined up by lasers in an orderly grid, as a frequency reference. Kolkowitz wants to build a clock containing two optical lattices, and allow one optical lattice to fall while accelerating the other horizontally at the same rate. By doing this, he will test Einstein's equivalence principle from general relativity, which says that gravity is indistinguishable from non-gravitational acceleration.

The most precise clock in the world would have to tick 33 billion years before losing or gaining a second. And now, researchers are figuring out how to harness that precision to do science and explore the universe.