PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 765 February14, 2006 by Phillip F. Schewe, Ben
Stein, and
Davide Castelvecchi
ATTACK OF THE TELECLONES: Should quantum cryptographers
begin to
worry? In contrast with everyday matter, quantum systems
such as
photons cannot be copied, at least not perfectly, according
to the
"no-cloning theorem." Nonetheless, imperfect
cloning is permitted,
so long as Heisenberg's Uncertainty Principle remains
inviolate.
According to Heisenberg, measuring the position of a
particle
disturbs it, and limits the accuracy to which its
complementary
property (momentum) can be determined, making it impossible
to
reliably replicate the particle's complete set of
properties.
Now, quantum cloning has been combined with quantum
teleportation in
the first full experimental demonstration of
"telecloning" by
scientists at the University of Tokyo, the Japan Science and
Technology Agency, and the University of York (contact Sam
Braunstein, schmuel cs.york.ac.uk and Akira Furusawa,
akirafap.t.u-tokyo.ac.jp). In ideal teleportation, the
original is
destroyed and its exact properties are transmitted to a
second,
remote particle (Heisenberg does not apply because no
definitive
measurements are made on the original particle). In
telecloning,
the original is destroyed, and its properties are sent to
not one
but two remote particles, with the original's properties
reconstructed to a maximum accuracy (fidelity) of less than
100%.
(Heisenberg limits the ability to make clones as otherwise
researchers could keep making copies of the original
particle and
learn everything about its state.)
In their experiment, the researchers didn't just teleclone
a single
particle, but rather an entire beam of laser light. They
transmitted
the beam's electric field, specifically its amplitude and
phase (but
not its polarization) to two nearly identical beams at a
remote
location with 58% accuracy or fidelity (out of a theoretical
limit
of 66%). This remarkable feature of telecloning stems from
the very
magic of quantum mechanics: quantum entanglement.
Telecloning
stands apart from local cloning and from teleportation in
requiring
"multipartite" entanglement, a form of
entanglement in which
stricter correlations are required between the quantum
particles or
systems, in this case three beams of light. (An example of
a
multipartite entanglement is the GHZ state between three
particles
that was featured in Update 414.)
In addition to representing a new quantum-information tool,
telecloning may have an exotic application: tapping quantum
cryptographic channels. Quantum cryptographic protocols are
so
secure that they may discover tapping. Nonetheless, with
telecloning, the identity and location of the eavesdropper
could be
guaranteed uncompromised. (Koike et al., Physical Review
Letters, 17
February 2006; for an earlier partial demonstration of
telecloning,
between an original photon and one clone at a remote
location and
another clone local to it, see Zhao et al., Phys Rev Lett,
13 July
2005)
[...]
***********
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