PHOTONS that tick like a clock could allow atomic clocks to be synchronised with unheard-of accuracy. They could be used to help test Einstein鈥檚 theories of relativity, and lead to astonishingly precise satellite-based positioning systems.
Today鈥檚 atomic clocks are so precise that it is hard to get them to agree on the time. One way to synchronise them is to send a third clock from one to the other, carrying the time of the first so that the second can be reset. But sending bulky atomic clocks is impractical, especially if you鈥檙e trying to synchronise orbiting satellites.
One alternative might be to send a quantum clock. It is possible to put a photon of light into a quantum combination of two states that interfere, so that the photon oscillates between them like a pendulum. This 鈥渢icking photon鈥� can give some information about the time elapsed since it was sent.
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But the information is tricky to extract. A measurement of such a photon gives only an either/or answer: the photon鈥檚 quantum states are either in phase or out of phase. That鈥檚 like a clock with an hour hand that points only straight up or straight down.
Now quantum information theorists Stephen Bartlett of the University of Sydney and Mark de Burgh at the University of Queensland, both in Australia, have worked out how to get a more precise timing. In their scheme, a series of ticking photons are bounced back and forth between two clocks. The first few photons produce a rough estimate of the time elapsed since they were sent, then further readings gradually zero in on shorter time intervals. (Physical Review A, vol 72, p 0423010)
鈥淭he first photons give a rough estimate of the time elapsed, further photons zero in on shorter time intervals鈥�
The work pulls the rug from under claims that clocks can only be synchronised to this accuracy if the photons are in an exotic 鈥渆ntangled鈥� quantum state, in which two or more photons share the same existence. 鈥淲e鈥檝e shown that鈥檚 false,鈥� says Bartlett.
The finding should simplify the process of quantum synchronisation, since it is extremely difficult to prepare and send the large sets of entangled particles required by that scheme.
Modern fibre optics should allow the technique to be used in physics experiments for which accurate timing is essential, such as gravitational wave detectors and other tests of relativity. Eventually the method could be used to send signals through the atmosphere to souped-up GPS satellites, which depend on clock synchronisation for their accuracy. Such a system could pinpoint locations to within millimetres.
