Be still, my quivering atoms: Here's a new way to count a second

Paris Observatory enters the time standard fray

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Atoms trapped by light and measured with a CCD camera have the potential to outdo today's most accurate atomic clocks, and although it's early days, a pair of linked optical lattice clocks have yielded accuracy of a second every 300 million years.

In doing so, the group that performed the test, outlined here in Nature and published in full in Nature Communications say their experiment performed at the limit of experimental accuracy, outperforming three linked caesium-based atomic clocks.

Ever since 1967, the time standard has been governed by cesium clocks, in which the transition of atoms between high- and low-energy states is measured with microwaves. Their accuracy is the basis of an awful lot of science, as well as more familiar applications like GPS.

However, the microwave probes are one of the constraints on the accuracy of the devices: the standard clocks use frequencies around 10 GHz, but the atomic oscillations they measure happen trillions of times a second.

So in the competition to devise a more accurate atomic clock, it's no surprise that optics is one of the techniques being investigated, since optical frequencies are far, far higher than microwaves.

The Paris Observatory group hasn't actually broken the record-of-records for accuracy. There is, to pick one example, NIST's quantum logic clock which claims an accuracy of a second every 3.7 billion years.

What they have achieved is to demonstrate that their atomic lattice design has the potential to be the basis of much more accurate atomic clocks in the future. University of Western Australia associate professor and ARC fellow John McFerran, resident atomic clock maker at the institution, explained the significance of the work to The Register.

“They have shown that two optical atomic clocks (based on strontium) agree with each other to within the systematic plus statistical uncertainties, 1.5e-16,” Professor McFerran told Vulture South in an e-mail.

“This heralds a new era: optical clocks agreeing with each other at accuracies exceeding that of the best microwave clocks.”

The optical clock, Professor McFerran explained, has an important theoretical advantage over many other proposed approaches: rather than trying to observe a single atom, the Paris Observatory device works with thousands.

“One can probe tens of thousands of atoms at once, whereas in most ion clocks you are limited to one lonely quantum absorber. So the signal-noise ratio is far superior in lattice clocks, which means assessing the systematic frequency shifts can be carried out much more quickly. And this is a good recipe for perhaps making the best of tomorrow's clocks.”

The optical lattice has another really useful property: it constrains the movement of the atoms to within a half-wavelength of the light you're using for measurement, and constrains the direction of movement rather than randomly. With those constraints, Professor McFarren wrote, “The atoms move in a periodic fashion that is easy to characterise, hence all motional effects can be accounted for in the accuracy estimate.

“In previous neutral atom based clocks the atoms weren't constrained like this: they moved in a random fashion (very slowly though) … a related effect, the second order doppler effect, stopped you dead in your tracks trying make any improvements in accuracy.”

The optical lattice clock uses a variety of what – to the layperson anyhow – look like the more esoteric applications of lasers. One traps the strontium atoms on its half-wavelength nodes, while one or two more lasers use the trick of resonance to cool the atoms down. The cooling and trapping lasers are supplemented by magnetic fields.

Yet another laser – a very high stability, low noise laser – acts as the oscillator to provide the “ticks”.

All of this is held in the hardest vacuum the clock-builder can manage, a CCD camera to capture the signal, and a central computer to switch the magnetic fields and lasers on and off, trigger the camera, change the frequencies of the lasers if required, and record the data.

While a second-in-billions-of-years accuracy might seem an obsessive target, Professor McFarren told Vulture South there are applications both in the physics laboratory and outside it.

Greater accuracy helps test fundamental constants such as the “holier than holy” fine-structure constant, the quantum chromodynamics coupling parameter lambda, and the proton-to-electron mass ratio, he explained, along with helping refine our examinations of general relativity via tests of of Einstein's equivalence principle.

However, outside the lab, he said, linked clocks can help monitor changes in altitude at different points on the Earth's surface more accurately. “There are also prospects of studying hydrological flows using atomic clocks,” he noted, “since as the water moves, the strength of gravity changes, and this affects the ticking rate of the clocks.” ®

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