New and Improved Atomic Clocks Mean It's Time to Update Time

The latest system trims timekeeping errors to less 200 picoseconds over 25 days.

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May 25 2016, 2:00pm

Caesium fountain clock. Image: UK National Physical Laboratory

Physicists from the the National Metrology Institute of Germany have developed a new variant of atomic clock boasting up to 100 times the accuracy of today's traditional microwave-based atomic clocks.

The technology, which is described in a paper published this week in the journal Optica, could force us to once again redefine what a second is by virtue of both its accuracy, and, crucially, practicality.

Today, humanity's master clock is represented by Universal Coordinated Time (UTC), which is based on International Atomic Time (TAI, for the French name Temps Atomique International). TAI is determined by averaging 400 satellite-connected atomic clocks from around world, with each one keeping time by counting the natural oscillations of the element caesium. 9,192,631,770 of these cycles define a second, according to the International System of Units (SI).

That frequency puts the caesium atoms' oscillations within the microwave band of the electromagnetic spectrum. These frequencies are quite a bit lower than those that make up visible light (and still lower than X-rays and gamma rays).

The clock developed by the German group uses strontium atoms instead of caesium, which are able to oscillate at frequencies of up to 100,000 times faster than that of their caesium kin—that is, within the range of visible light. The higher-frequency strontium is able to keep time much more precisely in a caesium clock, which can accumulate a whole whopping 1 nanosecond of error over a month's time. Less "ticks" means less precision.

That optical atomic clocks keep time more precisely than microwave atomic clocks is itself old news. But adoption of the technology has been hampered by reliability. Simply, optical atomic clocks crash. A lot. This is unacceptable.

"Timescales provide us with coordinates for the position of events in time, much like a coordinate system does for the positioning in space," the Optica paper notes. "In particular, it needs to be realized without interruption to provide a continuous coordinate or to measure time intervals and to synchronize distant events."

Image: Grebing et al

The optical clock system described in the paper doesn't so much get rid of downtime as it does bridge it. This is accomplished using a microwave laser beam (a "maser"). The basic idea is that the oscillating strontium drives the oscillations of the maser in such a way that it's able to represent or simulate the higher frequency for any down periods that may arise. The metaphor used by the physicists of a flywheel storing energy in a mechanical system as rotational energy.

Simulating the higher-frequencies is a matter of taking the more widely spaced ticks of the microwave frequency and subdividing them further into sort of virtualized optical frequency ticks. This fudge works pretty well, even when the optical clock is operational less than half of the total time. Over 25 days, the system picked up less than 200 picoseconds of error, an improvement of one order of magnitude.

"The distortion of the scale unit due to the use of the flywheel while the clock is offline causes the dominant contribution to the achieved residual time error," the paper explains. "Yet, this shows for the first time that already today optical frequency standards with limited availability can actually serve as atomic references to support a local timescale over extended periods, yielding a long-term performance better than of the ones referenced to the best current microwave clocks even if they are operated free of interruptions."

Actually redefining the second is likely at least a decade away, the researchers caution. The SI timekeeping system is big and complex, and it remains undetermined which variety of optical atomic clock will prove to be the best one. The ultra-high frequency traders of the world will have to continue dealing with the errant nanosecond for some time to come.

An open-access version of the paper can be accessed at the arXiv pre-print server.