Time on the Move

EDITORIAL Vol.1 No.5 May 2007 www.nature.com/naturephotonics Time on the move Optical clocks promise new standards in the measurement of time. To ensure accuracy, however, different clocks must be compared, even if they are on opposite sides of the planet. On 1 April 2007, the source of the an advanced technology with a vast radio-frequency signal that has ensured network already available. The difficulty that the United Kingdom has ticked is that the operation wavelengths of to the same clock for the past 80 years most optical clocks are incompatible was moved from central England to with the transparency window of optical a new home on the northwest coast. fibres. What is necessary is to transfer Before railway networks spread across the stability to a wavelength suitable for the country, variations in local time transmission. The system constructed were made largely irrelevant by long and tested by the NIST team consists of a travelling times. However, during the number of phase-stabilized continuous- 1840s and 1850s, as they expanded, the wave lasers, a 750-m closed-ring network train companies carried the time in of erbium fibre and frequency combs London to local stations, and this was to shift the stability of the reference eventually adopted by public clocks. frequency to a second wavelength Legislation formally imposed Greenwich GPS. The stability of atomic clocks scales between 500 nm and 2 μm. The set- Mean Time (GMT) across the UK with oscillator frequency, and this has led up is just a prototype at present, and in 1880. Today’s source of GMT, the researchers towards optical-frequency the team admit that the principle must MSF, a 15-kW radio signal emitting at atomic clocks. These have seen a rapid be extended to longer transmission a frequency of 60 kHz, had, until last development over recent years and now distances. They suggest that distances in month, been broadcast from Rugby in rival microwave atomic clocks in terms of excess of 50 km could be possible using Warwickshire to act as a reference for the accuracy, with very real potential for vast so-called coherent repeaters. nation’s clocks, maintaining the accuracy improvement, possibly to the precision of The usefulness of a transferable, of a wide range of electronic devices to one second over a billion years. stable, optical signal goes well beyond within one millisecond. However, just like in Britain global positioning. Helen Margolis Scientists at the National Physical 150 years ago, the ability to transfer this from NPL highlights another possible Laboratory (NPL) keep the timing of time standard to far-flung locations while application on page 258 of this issue: the MSF, or ‘The Time from NPL’ as it maintaining the high level of stability and answering the intriguing question of is now known, up to date using atomic accuracy is of vital importance — not whether fundamental constants vary clocks. Just like in a conventional clock least to ensure that the standards in with time. Even the smallest variation, in or digital watch, atomic clocks rely different laboratories around the world the fine structure constant for example, on an oscillator; however, instead of a agree. On page 283 of this issue, scientists could be detected by a discrepancy in pendulum or a quartz crystal, they use at the National Institute of Standards two distant optical clock standards, as an electromagnetic wave that is stabilized and Technology (NIST) in Boulder long as this difference is larger than by reference to a resonant transition in Colorado detail how this could be any errors introduced while comparing between two energy states in an atom. achieved with photonics, and prove them. The long-term goal, of course, is At present, caesium-fountain clocks set their idea with a 750-m-long prototype to create a newer, more precise definition the standard with an atomic transition optical network. As the team points out, of the second. For this, it is crucial that at microwave frequencies, remaining the requirements for such a system are different clocks can be compared. Just as accurate to within one second over rigorous: the frequency of the oscillator long journey times in pre-railway Britain 14 many millions of years. Such a precise (approximately 2 × 10 Hz) should vary made the synchronization of clocks clock might seem to be of only academic by no more than 100 mHz over a period pointless, the phenomenal accuracy of interest, but in fact it is vital for global of one second and the timing jitter optical clocks is made far more useful positioning systems (GPS) and even should be below one femtosecond. When when it can be carried around the globe. further improvements are required in it comes to transmitting the standard, The work at NIST shows just how this order to increase the spatial accuracy of optical fibres seem to have the solution: could be done. nature photonics | VOL 1 | MAY 2007 | www.nature.com/naturephotonics 241.

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