Fast quantum crypto demoed

By Eric Smalley, Technology Research News

Researchers at Northwestern University are tapping the laws that govern subatomic matter and energy in order to securely encrypt data and transmit it at 250 megabits per second over fiber-optic lines.

Sending a secret message involves two steps: encrypting, or scrambling, the message so that only the right person can read it, and making sure that the key to unscramble the message gets to the intended recipient and no one else.

Most quantum cryptography research has focused on the second step: finding ways to securely exchange encryption keys, which are random strings of numbers used to encrypt and decrypt messages.

The approach uses individual photons to represent each bit of a string of random bits. Because individual photons cannot be observed without altering their quantum states, an eavesdropper cannot look at a message sent using single photons without tipping off the sender and receiver to his presence. If the sender and receiver are sure that no one spied on their bit string, they can use it as an encryption key.

The Northwestern researchers have addressed the first step with a way to use quantum physics in the encryption process itself, with the premise that the parties communicating have already exchanged an encryption key. "No one has been addressing actual data encryption," said Prem Kumar, a professor of electrical and computer engineering and physics at Northwestern University.

The researchers' technique uses the quantum noise present in ordinary lasers. There is always a certain amount of randomness at the level of atoms and subatomic particles because the quantum world is ruled by probabilities. This randomness is known as quantum noise.

Quantum noise in lasers is caused by random fluctuations in the number of photons generated over a given amount of time, said Kumar. For example, if a laser produces 10,000 photons a nanosecond, or billionth of a second, it would have a fluctuation proportional to the square root of 10,000, which is 100, he said. "There will be a spread around the mean value of 10,000 and that spread would have a standard deviation of 100 photons," he said. "So sometimes I will have 10,100 photons, sometimes 10,090 photons."

The researchers' scheme mixes the message and the encryption key with this random quantum noise to produce a pattern that appears to be random to anyone without the encryption key.

The researchers demonstrated the technique by sending an encrypted message at a speed of 250 megabits per second over 4 kilometers of optical fiber in the laboratory and across a 2-kilometer fiber-optic line between two buildings.

Several research teams have used the single-photon quantum key distribution method to demonstrate communications systems that are impervious to eavesdroppers. But this method has major drawbacks: it is exceedingly slow and is limited to short distances, said Kumar.

Because each bit of the key is used to encrypt one bit of the message, and each message requires a new key, messages can be transmitted only as fast as keys can be sent.

The light sources that generate single photons can only produce data at about one kilobit, or thousand bits, per second, said Kumar. This translates to about 40 words a second, or about five single-spaced pages of text per minute. It would take about 17 minutes to transmit a print-quality image that takes up about one megabit, or million bits, and about 70 hours to transmit 250 megabits. "In communications networks we talk about tens of gigabits per second," Kumar said. A gigabit is one billion bits.

And because photons can't be copied without changing their quantum states and thus destroying the information they represent, quantum cryptographic transmissions can't be sent through the repeaters used to boost signals over longer distances in ordinary communications lines.

By sidestepping the issue of secure key exchange, the researchers were able to send secure data using the faster, standard lasers used in optical communications rather than devices that emit just one photon at a time, said Kumar.

The signals can also pass through amplifiers unharmed, and thus traverse long distances, Kumar said. "The encrypted communications can theoretically span thousands of kilometers, he said.

There are many efforts under way to study implementations of quantum cryptography using common, practical equipment, rather than single-photon generators, said Nicolas Gisin, a professor of applied physics at the University of Geneva in Switzerland. Several research teams have been developing quantum key distribution systems based on ordinary laser beams, he said. "We are also working on this."

It is too early, however, to judge the impact of the work, Gisin said. "It is not even clear whether these new schemes are really secure," he added. Researchers have not been able to analyze how the new schemes would handle the most powerful eavesdropping attacks, he said.

The Northwestern University researchers' claim for security against eavesdropping has to be tested, said Anton Zeilinger, a professor of physics at the University of Vienna in Austria. "If it is valid then this is a very significant [development]," he said. "We are analyzing that claim in my group," he added.

The researchers next plan to boost the data rate of their system to 2.5 gigabits per second, and increase the distance it covers, said Kumar. They also plan in the next year or two to tackle the problem of using ordinary laser beams to distribute perfectly secure encryption keys, he said.

The researchers expect to demonstrate the feasibility of their data encryption scheme in the next couple of years, and practical applications could be possible in five years, said Kumar.

Kumar's research colleagues were Horace Yuen, Geraldo Barbosa, Eric Corndorf and Chuang Liang. The research was funded by the Defense Advanced Research Projects Agency (DARPA).

Timeline:   5 years
Funding:   Government
TRN Categories:   Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Secure communications using coherent states," scheduled to appear in the Proceedings of QCMC'02: Quantum Communication, Measurement, and Computing; "Continuous Variable Quantum Cryptography Using Coherent States," Grosshans and Grangier, Physical Review Letters, February 4, 2002


November 27/December 4, 2002

Page One

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Fast quantum crypto demoed

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