杏吧原创

PHYSICISTS have spent decades trying to snare a gravitational wave. These
ripples in the fabric of space-time are thought to be created by violent events
in space, such as the merging of two black holes, but they are so weak that no
one has yet built a detector sensitive enough to pick one up. While a new
generation of huge detectors are being built, physicists from Portsmouth
University now think they have found an alternative and rather unexpected way of
snaring a gravity wave. Their idea is to use the quantum encryption technology
now being devised for sending secure messages.

Gravitational waves stretch and squeeze space as they pass. Researchers have
set up huge metal bars in isolation tanks in the hope that a passing wave would
set it ringing, but to no avail. A new generation of large-scale interferometers
are now under development, including LIGO in the US, and VIRGO and Geo-600 in
Europe. With arms as long as several kilometres, these use laser beams as
鈥渞ulers鈥� to measure the distortion of space caused by a passing wave.

The Portsmouth researchers point out that this distortion could also affect a
quantum encryption system. Fabrizio Tamburini, Bruce Bassett and Carlo Ungarelli
considered a system in which two people who wish to communicate securely,
traditionally named Alice and Bob, each receive a secret 鈥渒ey鈥� with which to
encode and decode their messages. The key is a string of photons whose
polarisation state, for instance, represents the 0s and 1s of digital code. It
is sent from a central source to both Alice and Bob.

Crucially, the source creates entangled pairs of photons, with one photon
from each pair being sent to Alice and the other to Bob. The peculiar property
of entangled photons is that they behave as a single particle even if far apart.
When a measurement is made on one of them, the pair adopts a state in which
knowledge about the polarisation of one tells you about the polarisation of the
other. This means that if an eavesdropper intercepts a photon destined for
Alice, destroying its polarisation, the photon for Bob also loses its
polarisation. 鈥淏oth Alice and Bob notice a distortion in the string of bits of
the key and conclude there is an eavesdropper,鈥� says Tamburini.

What the Portsmouth team point out is that a gravitational wave acts like an
eavesdropper. It makes the distance between the photon source and Alice
marginally different from the distance between the source and Bob. 鈥淭he change
in the arrival time is like a small gap in the string of bits,鈥� says Ungarelli.
鈥淚t would be noticed as a variation in the correlation between photons arriving
at Alice and Bob.鈥�

Such a detector would yield different information about a source of gravity
waves and could be cheaper than interferometers. 鈥淗aving a second method to
cross-check results would very useful,鈥� says Ungarelli. Jim Hough of the
University of Glasgow agrees: 鈥淎 new technique for detecting g-waves, if it held
up, would be very good news鈥�.