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QUANTUM clocks could become the beating hearts of future hyper-accurate
satellite navigation systems. Researchers in Britain and at NASA have shown
that, unlike conventional clocks, quantum clocks can be perfectly synchronised,
no matter how far apart they are. The accuracy of satellite navigation
ultimately depends on how closely their clocks are synchronised, so the
technique could allow people to pinpoint their positions more accurately.

There are two ways to synchronise ordinary clocks. The first is to send a
time signal from one clock to the other. If the second clock knows the distance
between them and how fast the signal travelled, it can correct for the signal鈥檚
travel time and synchronise itself with the first clock. This is how the clocks
on board GPS satellites are updated. The snag is that the satellites鈥� positions
cannot be measured perfectly, and the speed of radio waves that carries the
synchronisation signal changes according to atmospheric conditions, limiting
accuracy.

The second method is to synchronise two clocks in the same place and then
move them apart. But to do this, the clocks have to be accelerated, and
relativity tells us that this makes clocks run slow. This relativistic effect is
enough to upset clocks on GPS satellites when they are launched into orbit at
great speed.

Now researchers from the University of Bristol and NASA鈥檚 Jet Propulsion
Laboratory in Pasadena, California, have come up with a third method. 鈥淚t
provides a way of synchronising clocks exactly, and that is a very important
problem for GPS,鈥� says Richard Jozsa of the Bristol team.

Quantum clocks start life as pairs of quantum particles鈥攕uch as photons
or atoms鈥攍inked by the strange phenomenon know as entanglement. Alter the
state of one entangled particle, and the state of the other instantly alters
too, no matter how far apart they may be. The researchers propose entangling two
caesium atoms, which normally oscillate at 9.191 631 770 gigahertz. They would
place them in clocks belonging to 鈥淎lice鈥� and 鈥淏ob鈥濃�攖he protagonists
physicists traditionally use when discussing quantum communication鈥攚ho
would then take their clocks to opposite sides of the Universe. 鈥淚n this
entangled state, the quantum clocks would be idling,鈥� says Jozsa.

Alice could start both atoms oscillating at exactly the same instant by
performing a measurement on her atom alone. This would destroy the entanglement,
acting as a trigger from which to start measuring time. With a few details from
Alice about how she did her measurement鈥攚hich reveals what state the atom
ended up in鈥擝ob can then tell the time. The result would be perfect
synchrony, no matter how far apart Alice and Bob might be.

However, David DiVicenzo, a quantum physicist at IBM鈥檚 research centre in
Yorktown Heights, New York, says that the scheme is far from straightforward.
Alice and Bob would need to agree on a common set of coordinates in which to
carry out their measurements鈥攂ut he wonders how they could do this over
distances so large that space-time is curved.