Preparing for a new second

A bit techie, this one, but I liked the story. It’s a news piece for Nature.

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A new type of atomic clock could transform the way we measure time.

The international definition of a second of time could be heading for a change, thanks to the demonstration by researchers in France that a new type of ‘atomic clock’ has the required precision and stability.

Jérôme Lodewyck of the Observatoire de Paris and his colleagues have shown that two so-called optical lattice clocks (OLCs) can remain as perfectly in step as the experimental precision can establish [1]. They say that this test of consistency is essential if OLCs are to be used to redefine the second, currently defined according to a different sort of atomic clock.

This is “very beautiful and careful work, which gives grounds for confidence in the optical lattice clock and in optical clocks generally”, says Christopher Oates, a specialist in atomic-clock time standards at the National Institute of Standards and Technology (NIST) in Boulder, Colorado.

Defining the unit of time according to the frequency of electromagnetic radiation emitted from atoms has the attraction that this frequency is fixed by the laws of quantum physics, which dictate the energy states of the atom and thus the energy and frequency of photons of light emitted when the atom switches from one state to the other.

Since 1967, one second has been defined as the duration of 9,192,631,770 oscillations of the microwave radiation absorbed or emitted when a caesium atom jumps between two particular energy states.

The most accurate way to measure this frequency at present is in an atomic fountain, in which a laser beam is used to propel caesium atoms in a gas upwards. Emission from the atoms is probed as they pass twice through a microwave beam – once on the way up, once as they fall back down under gravity.

The time standard for the United States is defined using a caesium atomic-fountain clock called NIST-F1 at NIST. Similar clocks are used for time standards elsewhere in the world, including the Observatoire de Paris.

The caesium fountain clock has an accuracy of about 3x10**-16, meaning that it will keep time to within one second over 100 million years. But some newer atomic clocks can do even better. Monitoring emission from individual ionized atoms trapped by an electromagnetic field can supply an accuracy of about 10**-17.

The clocks studied by Lodewyck and colleagues are newer still – first demonstrated under a decade ago [2]. And although they can’t yet beat the accuracy of trapped-ion clocks, they have already been shown to be comparable to caesium fountain clocks, and some researchers suspect that they’ll ultimately be the best of the lot.

That’s for two reasons. First, like trapped-ion clocks, they measure the frequency of visible light, with a frequency tens of thousands of times higher than microwaves. “Roughly speaking, this means that optical clocks divide a second into many more time intervals than microwave caesium clocks, and so can measure time with a higher precision,” Lodewyck explains.

Secondly, they measure the average emission frequency from several thousand trapped atoms rather than just one, and so the counting statistics are better. The atoms are trapped in a so-called optical lattice, rather like an electromagnetic eggbox for holding atoms.

If OLCs are to succeed, however, it’s essential to show that they are reliable: that one such clock ticks at exactly the same rate as another prepared in an identical way. This is what Lodewyck and colleagues have now shown for the first time. They prepared optical lattices each holding about 10,000 atoms of the strontium isotope strontium-87, and have shown that the two clocks stay in synchrony to within a precision of at least 1.5x10**-16, which is the detection limit of the experiment.

But if the definition of a second is to be switched from the caesium standard to the OLC standard, it’s also necessary to check that two types of clock are in synchrony. The French team have done that too. They found that their strontium OLCs will keep pace with all three of the caesium clocks in the Observatoire, to an accuracy limited only by the fundamental limit on the caesium clocks themselves.

“These sorts of comparisons have historically been critical in laying the groundwork for redefinitions of fundamental units”, says Oates.

Accurate timing is crucial to satellite positioning systems such as GPS, which is why GPS satellites have onboard atomic clocks. But their accuracy is currently limited more by other factors, such as air turbulence, than by the performance of their clocks. There are, however, other good reasons for going beyond the already astonishing accuracy of caesium clocks.

For example, in astronomy, if the arrival times of light from space could be compared extremely accurately for different places on the Earth’s surface, this could allow the position of the light’s source to be pinpointed very precisely – with a resolution that, as with current interferometric radio telescope networks, is “equivalent to a continent-sized telescope”, says Lodewyck.

Better time measurement would also enable high-precision experiments in fundamental physics: for example, to see if some of nature’s fundamental constants change over time, as some speculative theories beyond the Standard Model of physics predict.

Before switching to a new standard second, says Lodewyck, there are more hurdles to be jumped. Optical clocks are needed that can run constantly, and there must be better ways to compare the clocks operating in different institutes.

“This measurement is a significant advance towards a new definition of the second”, says Uwe Sterr of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, which also operates an atomic-clock standard. “But to agree on a new standard for time the pros and cons of the different candidates that are in the play needs to be evaluated in more detail”, he adds.

“It’s not yet decided which atomic species nor which kind of optical clocks will be chosen as the next definition of the SI second”, Lodewyck concurs. “But we believe that strontium OLCs are a strong contender.”

References
1. Le Targat, R. et al., Nature Communications 4, 2109 (2013).
2. Takamoto, M., Hong, F. -L., Higashi, R. & Katori, H. Nature 435, 321–324 (2005).