At the fourth photon, the time will be...
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.
At the fourth photon, the time
will be...
• 05 November
2005
• NewScientist.com news
service
• Stephen
Battersby
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's theories of relativity, and lead to astonishingly precise
satellite-based positioning systems.
Today's 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're 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
"ticking photon" can give some information about the time elapsed since it was
sent.
But the information is tricky to extract. A
measurement of such a photon gives only an either/or answer: the photon's
quantum states are either in phase or out of phase. That's 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)
“The first
photons give a rough estimate of the time elapsed, further photons zero in on
shorter time intervals”
The work pulls the rug from under claims that clocks
can only be synchronised to this accuracy if the photons are in an exotic
"entangled" quantum state, in which two or more photons share the same
existence. "We've shown that's false," 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.
From issue 2524 of
New Scientist magazine, 05 November 2005, page 9
Posted: Sat
- November 19, 2005 at 09:11 PM
