Light years ahead: keeping time with single ion optical clocks

Light years ahead The NPL femtosecond comb is used to measure the frequency of the strontium ion optical clock.

Professor Patrick Gill, Dr Helen Margolis, Dr Geoffrey Barwood, Dr Hugh Klein, Dr Stephen Lea, Dr Stephen Webster and Dr Kazu Hosaka.
National Physical Laboratory.

Extraordinarily accurate optical clocks under development at the National Physical Laboratory (NPL) may lead to a new definition of the second. The clocks, based on optical frequencies, are now almost as accurate as the current international standard clocks based on microwave frequencies but have the potential to exceed this level of accuracy 1000-fold.

Such new extremes of accuracy could offer improved satellite navigation, internet synchronisation and enable more stringent tests of the laws of physics. A clear example is Global Positioning Systems (GPS) that use satellites and hand-held devices to pinpoint your position. 'At best, you can locate your position to within a few metres', says Patrick Gill of NPL. 'But this margin of error increases when you are moving, so for aeroplanes it presents quite a problem'. Improved timing accuracy would mean that in the future GPS alone could land a plane with extreme accuracy and safety. Patrick also envisages more down-to-earth uses. 'If you could remotely pinpoint locations to well below a metre it would be extremely useful for tracking lost or stolen objects like mobile phones'.

'The key to developing more accurate clocks is dividing time into smaller and smaller parts', says Patrick. 'The more subdivisions or ticks you can count per second, the more accurate the clock'. In both the current microwave and new optical clocks the 'ticks' are the oscillations, or rise and fall, of the microwaves or light waves the equivalent of the swinging pendulum in a grandfather clock. Light waves oscillate one million billion times (1015)a second and therefore have the potential to make for extreme accuracy. Any clock also requires a reference by which to calibrate the ticks: in a traditional clock this is the rotation of the earth or the period of one day; in microwave clocks the reference is an absorption in a cloud of atoms; and in the case of the NPL opt