Quantum secrets ride phone lines

By Eric Smalley, Technology Research News

The ability to safeguard secret messages using the quirks of quantum physics has been thoroughly demonstrated in the laboratory. Now field tests of quantum cryptography are showing that the technology can withstand the rigors of real-world communications.

Researchers in Switzerland have used this type of cryptography, which represents bits of information using single photons, to send theoretically perfectly secure messages between the cities of Geneva and Lausanne, which are 67 kilometers apart.

Quantum cryptography provides perfect security because it allows users to tell for sure whether the key they are using to encrypt and decrypt a message has been compromised.

Researchers at Los Alamos National Laboratory previously proved that a quantum signal could travel 50 kilometers. But that was over a spooled fiber-optic line contained in a laboratory, said Nicolas Gisin, a physics professor at the University of Geneva. "In our case the two end points were really spatially separated," he said.

More importantly, the Swiss experiment used existing fiber-optic phone lines. The fibers were "nothing special," said Gisin. They were not in commercial use during the experiment, but were part of a cable containing many fibers that were, he said.

Key encryption schemes use a unique mathematical key to mask each message. The sender and intended recipient use the key to encrypt a message, send it over unsecured channels, then decrypt it. The trick to keeping the message secret is making sure no one but the sender and receiver have access to the key.

The quantum cryptography scheme sends encryption keys over fiber-optic lines in a perfectly secure way by representing each bit with only one photon. Using two or more photons per bit makes it possible for an eavesdropper to siphon off some extra photons in order to peek at the key without being detected. Using only one photon per bit means that an eavesdropper would have to replace the photons she intercepted, but it is impossible to replicate all of the photons correctly.

This is because any given photon, or particle of light, can have one or more attributes, including polarization, which has to do with how the photon vibrates, and wave phase.

The researchers' quantum cryptography scheme generates photons in one of four states based on their wave phases. The system splits each photon, sends the halves down short pieces of fiber of slightly different lengths, and then joins the two halves. Because the halves travel different distances, their waves are out of phase, meaning the crests and troughs are out of sync by a particular amount.

The photons' four phase states come in two types: those whose waves match or are exactly opposite, and those whose waves are half way out of phase with one wave ahead of the other. Each type can be used to represent the 1s and 0s of digital information.

It is a quirk of quantum physics -- the Heisenberg uncertainty principle -- that makes the scheme perfectly secure: you can't look for both of the pairs of states at the same time, and you only get one look before the photon disappears. If you measure a photon to see if it is a 1 or 0 based on one pair of states, but it was generated in one of the other two states, you're out of luck. Your measuring device has absorbed the photon during your first look so you will never know whether it represented a 1 or 0.

This means an eavesdropper would only be able to correctly measure half of the photons he intercepts and would have to guess at the other half to produce substitutes. And he would only get about half the missing half right by chance, meaning one quarter of the substitute bits would be wrong.

The sender and receiver can check the error rate and so detect the eavesdropper by comparing a few bits. If the key has been compromised, they can throw it out and send another until they get an uncompromised key to encrypt their data. To form a key, the receiver measures the photons by randomly picking one of the two sets of states. Then they compare notes and the sender tells the receiver which photons he measured correctly. They then use those bits as the key.

The researchers' quantum key distribution system can only be used across relatively short distances because its performance drops off as the distance increases. At 10 kilometers the system can transmit quantum keys at 4,000 bits per second. At 20 kilometers the bit rate drops to 1,500 per second, and at 50 kilometers it drops to 100 bits per second. An ordinary modem transmits 56,000 bits per second. Once the users have an uncompromised key, however, the encrypted data can be sent over fast communications lines that include repeaters.

Today's fiber-optic communication systems compensate for diminishing signal strength -- and thus span great distances -- by using repeaters, which copy and retransmit fading light pulses. Repeaters can't be used to send quantum keys because they would intercept photons in the same manner as an eavesdropper.

The company id Quantique in Geneva, a spinoff from Gisin's laboratory, is marketing the quantum key distribution system. It consists of a pair of 18-inch-wide boxes that connect to personal computers via USB ports, and to each other over a fiber-optic line.

Gisin's research colleagues were Damien Stucki and Hugo Zbinden of the University of Geneva, and the Olivier Guinnard and Grégoire Ribordy of id Quantique SA. They published the research in the July 12, 2002 issue of the journal New Journal of Physics. The research was funded by the European Union.

Timeline:   Now
Funding:   Government
TRN Categories:   Quantum Computing and Communications; Cryptography and Security
Story Type:   News
Related Elements:  Technical paper, "Quantum Key distribution over 67 km with a plug & play system," New Journal of Physics, July 12, 2002


August 7/14, 2002

Page One

Ultimate memory demoed

Programming tool makes bugs sing

Nanotubes grown in place

Quantum secrets ride phone lines

Chip keeps atoms in line


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