Editorial Feature

How do Optical Clocks Work?

Optical Clocks

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From grandfather clocks to sundials, every single clock is based on a steady oscillator. In an optical clock, the stable oscillator is a laser governed by the quantum oscillations of atoms.

Optical clocks are considered the most accurate type of clock. The lasers they are based on have frequencies of 100 s of terahertz, meaning that these clocks tick approximately a quadrillion (1 million billion) times each second.

An optical clock has three predominant components. The first is an extremely steady “reference” frequency supplied by a slim optical absorption line in an atom or ion. The second component is a feedback system that "binds" the output of a laser to the reference frequency. The third key element of an optical clock offers a very accurate means to measure the frequency of the laser, typically a “femtosecond comb”.

The most crucial aspect of an optical clock is the atom or ion that absorbs light over an incredibly narrow and very stable frequency range. To make certain this absorption occurs, the atom or ion is sequestered in a vacuum chamber and cooled to almost absolute zero using laser beams; to the point light is absorbed and reemitted in manner that reduces its kinetic energy.

A More Accurate Way to Keep Time

All clocks function by counting an oscillating event with a recognized frequency. Traditional atomic clocks use the natural oscillation of the caesium atom, which has a frequency in the microwave area of the electromagnetic spectrum. A second has been defined as the time that passes during 9,192,631,770 cycles of the caesium microwave signal.

Atomic clocks are incredibly precise because they use normal and universal atomic vibrations. However, even the most refined atomic clocks can still acquire an error of around 1 nanosecond over the course of a month.

Optical clocks operate in a fashion comparable to microwave clocks but use atoms or ions that oscillate approximately 100,000 times higher than microwave frequencies, in the visible portion of the electromagnetic spectrum. These greater frequencies mean that optical clocks “tick” more rapidly that atomic clocks, and this plays a part in their greater precision and stability over time. However, optical clocks do experience noteworthy downtimes due to their greater technical complexity.

In 2016, scientists presented an approach to use optical clocks for more precise timekeeping than is achievable with atomic clocks. The scientists also assessed an optical clock’s frequency - or it’s “ticking” - with unparalleled precision.

The implementation of a more precise timekeeping system could be massive. For instance, a more precise global system would permit financial networks to use more accurate time stamps and therefore take care of even more transactions in shorter quantities of time. It would also permit GPS and other satellite-based navigation systems to offer even more accurate location data.

Despite the fact that optical clocks have been more precise than microwave clocks for quite a while, their intricacy and resulting long downtimes held back their use as official timekeepers.

Another Use for Optical Clocks

In addition to keeping time more precisely, optical clock technology could also be used to hunt for proof of dark matter and dark energy. These clocks can also be used to create a gravitational wave telescope. Gravitational waves moving through a area of space modify the frequency of light waves passing through that same space, although the change is extremely slight. If light is transmitted from an optical clock on one satellite to a different optical clock on another satellite in close proximity, the clocks can recognize the slight change in light frequency that is a tell-tale sign of a gravitational wave.

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