Vol. 22, No. 2, July 18, 2002

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.

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