quantum crypto demoed
Technology Research News
There has been tremendous progress in quantum
cryptography in recent years, and one system is already available commercially.
But there's a long way to go before the technology matches its promise,
and one of the biggest issues is coming up with devices that reliably
generate and detect single photons at high speeds.
Using single photons isn't the only path to perfectly secure communications,
however. Working out how to use only standard telecommunications gear
to transmit cryptographic keys could dramatically improve quantum cryptography's
Cryptographic keys are numbers used to mathematically scramble and unscramble
secret messages. The trick to using secret keys to encrypt messages is
making sure they get to the intended recipient and no one else.
A group of researchers from the Institute of Optics in France and the
Free University of Brussels (ULB) in Belgium has demonstrated a quantum
cryptographic system that uses ordinary, weak laser pulses of several
hundred photons each rather than single-photon pulses that entail special
equipment. "Producing and detecting single photon pulses is not easy,"
said Phillippe Grangier, research director at the Institute of Optics.
"Our scheme avoids these difficulties by using a completely different
technique [that is] closer to standard optical telecommunications," he
The multi-photon method taps the nature of pulses instead of individual
photons to guard against eavesdropping,
Initial results from the researchers' table-top prototype show that the
multi-photon scheme should allow for faster information transmission than
single-photon schemes over distances shorter than ten kilometers, said
Grangier. In simulations of transmissions over short distances, the researchers
were able to transmit secret keys as fast as 1.7 megabits, or millions
bits, per second, according to Grangier. Today's single-photon quantum
cryptographic systems transmit secret keys at speeds of a few hundred
to a few thousand bits per second.
At the equivalent of just over 15 kilometers, the system transmitted data
at about 75 kilobits per second, said Grangier. "Our system should allow
much higher secret bit rates over short distances, but might be more sensitive
to losses [and thus] may not work well over large distances," he said.
Single-photon systems use the polarizations of individual photons to represent
each bit of a random string of 1s and 0s. Photons can be polarized in
one of four paired orientations: horizontal and vertical and the two diagonals.
The horizontal orientations can represent 0 and the vertical orientation
can represent 1, for example.
The Heisenberg uncertainty principle explains why the system provides
perfect security: a photon cannot be measured for both pairs of orientations.
In order to eavesdrop on single photons, an eavesdropper -- Eve -- would
have to replace the photons she intercepts, but because she would only
be able to correctly measure half of them she would have to guess at the
other half. The sender and receiver could compare a few of their photons,
and if a quarter of them failed to match they would be tipped off to Eve's
presence. They would discard those photons and try again until they got
an unobserved string of photons. Over an ordinary communications line,
Alice tells Bob which photons he measured correctly and they use those
corresponding bits as a key.
The researchers' method uses weak laser pulses that contain a few hundred
photons each, but taps the quantum nature of the pulses' amplitude and
phase to secure the information they carry, said Gangier. Quantum physics
describes the rules for individual atoms, photons and other particles.
Lasers are quantum devices because their photons flow in lockstep, and
the state of a laser pulse can only be measured according to quantum rules,
including the Heisenberg uncertainty principle, said Grangier.
Instead of using the polarization of single photons to represent the bits
of a key, a sender -- Alice -- would randomly choose an amplitude and
phase for each weak laser pulse she sends to Bob, Grangier said. The method
works because amplitude and phase are non-commuting quantum variables,
which means they, like the pairs of polarizations, cannot be measured
simultaneously. "This can be used to hide the secret key," he said.
The researchers' scheme also contains a twist. Its process of comparing
bits reverses Alice's and Bob's usual roles. "The trick of reverse reconciliation
is that the correct key is not what was sent by Alice, but rather what
was received by Bob," said Grangier.
Reversing the process incorporates transmission errors into the key, said
Grangier. This is more secure because Eve can only intercept messages
before they get to Bob. "Eve always knows less about what was received
by Bob than about what was sent by Alice," he said. This gives Alice and
Bob a larger advantage over Eve compared to standard quantum key distribution
systems, he added.
The difficult part of putting together the scheme was evaluating how much
information is shared by the sender, receiver and eavesdropper, said Gangier.
"While this is simple and well-known for photon counting, it was not clearly
done for [multi-photon systems] before our work," he said.
The researchers have proved the system is secure in some situations, but
have work to do to prove that it is perfectly secure. The initial results
look good, however, said Grangier. "Our feeling is that the protocol is
also robust against more general attacks, but this remains to be proven."
The researchers are not the first to propose a multi-photon quantum key
distribution system, but the other methods generally rely on unusual,
hard-to-produce light beams.
The work is "undoubtedly... the most advanced [multi-photon] quantum crypto
demonstration," said Nicolas Gisin, a professor of applied physics at
the University of Geneva in Switzerland.
The distance the scheme can be used over is limited, however, said Gisin.
"It is not clear how realistic the proposal is for significantly longer
distances." More work also needs to be done before the security of the
system is clear, he added.
The researchers are working on testing the protocol at the wavelengths
used by today's telecommunications systems, said Grangier. "The first
challenge [is] using telecom integrated modulators and optical fibers
at [a wavelength of] 1,550 nanometers instead of our present bulk optics
set-up at 780 nanometers," he said.
They are also looking into quantum repeaters, he said. "We're also interested
in... quantum repeaters that would do for quantum key distribution what
optical repeaters can do for usual telecommunications, i.e., reaching
arbitrarily large distances [using] optical fibers," he said.
How close the scheme is to practical depends on exactly how it will be
used, said Grangier. "Since quantum key distribution setups are now on
the market, the problem is no [longer] only technological," said Gangier.
"One should decide what is ultimately needed: high bit [rates] over short
distances [or] smaller bits rates over larger distances; fiber systems
or free space systems?"
Gangier's research colleagues were Frédéric Grosshans, Jérôme Wenger and
Rosa Brouri from the Institute of Optics, and Gilles Van Assche and Nicolas
J. Cerf from the Free University of Brussels in Belgium. They published
the research in the January 16, 2003 issue of the journal Nature. The
research was funded by the European Union Information Society Technologies
TRN Categories: Quantum Computing and Communications; Cryptography
Story Type: News
Related Elements: Technical paper, "Quantum Key Distribution
Using Gaussian-Modulated Coherent States," Nature, January 16, 2003.
29/February 5, 2003
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