Six-state protocol offers advantages for quantum cryptography
James
E. Kloeppel, Physical Sciences Editor
(217) 244-1073; kloeppel@illinois.edu
7/11/02
CHAMPAIGN, Ill. —
As telecommunications and information systems become commonplace in society,
a more secure means of encrypting and transmitting data is required. Underlying
nearly all forms of encryption is the necessity for a truly secret key, which
can be distributed without the threat of an undetected eavesdropper. Several
protocols have demonstrated the potential effectiveness of quantum cryptography
in meeting this need.
Now, researchers at the University of Illinois at Urbana-Champaign and the Los
Alamos National Laboratory have implemented a six-state protocol using polarization-entangled
photons that could enhance the versatility of quantum cryptography.
Quantum cryptography uses quantum states of photons to transfer cryptographic
key material. In a typical protocol, the sender "Alice" uses single
photons (or entangled photons) to transmit secret random bits to the receiver
"Bob." Alice encodes each random bit value using one of several polarization
states. Bob randomly measures each photon’s polarization and records the
results. Then, by conventional communications, Alice and Bob reveal their basis
choice for each bit, and sift out the set for which they used the same basis.
If an eavesdropper were present, detectable errors would be introduced into
the key.
"Although the six-state protocol can make an eavesdropper substantially
more visible, the protocol is technically harder to perform, and more data is
lost," said Paul Kwiat, the John Bardeen Professor of Electrical and Computer
Engineering and Physics at Illinois. "Despite these drawbacks, the new
protocol could prove useful in certain applications."
To investigate the six-state protocol, Kwiat and his Los Alamos colleagues –
Daphna Enzer (now at the Jet Propulsion Laboratory), Phillip Hadley, Richard
Hughes and Charles Peterson – created pairs of polarization-entangled photons
by passing a laser pulse through two adjacent nonlinear crystals.
The photons were directed to Alice and Bob, who analyzed them in one of three
randomly chosen bases: horizontal or vertical, diagonal or anti-diagonal, and
right or left circularly polarized. Whenever Alice and Bob chose the same basis,
they obtained correlated results, which comprised their sifted cryptographic
key material. The researchers also simulated the effects of different eavesdropping
strategies.
"While the six-state protocol has enhanced eavesdropper sensitivity, it
significantly reduces the number of key-producing events," Kwiat said.
"For systems with low error rates – less than about 8 percent –
the efficiency for secret key generation is higher when using a simpler protocol.
However, as the error rate increases, the six-state protocol becomes beneficial.
While the six-state protocol is currently most useful for systems that have
a lot of noise or high error rates, that will change when quantum storage devices
become operational.
"With a quantum memory, Bob would store the photon until he hears from
Alice how he should measure it," Kwiat said. "In that case, the six-state
protocol would always yield a greater number of useful bits, and an eavesdropper
would also be much easier to spot."
Entangled photons offer several advantages over single-photon techniques, Kwiat
said. "Reliable single-photon sources don’t exist yet, so you have
to send a faint, attenuated pulse instead. An eavesdropper could pick off part
of the pulse and go undetected."
With their enhanced signal-to-noise ratio, entangled photons should permit secure
key distribution over longer distances – particularly in fiber-based systems,
which have significant attenuation and noisy detectors.
Entangled photons also allow automatic source verification, Kwiat said. "Any
tampering of the source would be readily detected, which is not always the case
with single-photon sources."
The researchers report their findings in the New Journal of Physics, a peer-reviewed,
all-electronic journal published by the Institute of Physics. The issue –
devoted to quantum cryptography with Kwiat as guest editor – will be available
July 12.