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  • Code System Employs Laser Pulses

Photonics Spectra
Mar 2003
Daniel S. Burgess

A team of researchers at Centre National de la Recherche Scientifique (CNRS) in Orsay, France, and at Université Libre de Bruxelles in Brussels, Belgium, has demonstrated a quantum cryptography system that encodes its information on the electric field of weak laser pulses. The new method promises high transmission rates and technical simplicity.

Using laser pulses offers significant advantages over existing techniques that employ single photons. "We don't have to produce and detect single photons, which is the main technological limita-tion of photon-counting quantum key distribution," said Philippe Grangier of CNRS. Moreover, the approach should be more efficient because homodyne detection is faster than photon counting, he said. Quantum cryptography systems update the classic one-time-pad cipher, in which the communicating parties, conventionally known as Alice and Bob, share a unique key with which they can encode and decode messages. The quantum approaches fundamentally link the act of receiving the information for building a key that comes over the communications channel with the information that is received. This ensures that a third party, known as Eve, cannot also learn the key as it is distributed because intercepting a piece of the key influences the information that Bob receives. Alice and Bob immediately know whether the channel is secure by publicly comparing a segment of what each thinks the key should be.

A new quantum cryptography system encodes the key information on the electric field of laser pulses, and the information is recovered using homodyne detection. In a test of the system with an introduced loss of 3.1 dB, the communicating parties, conventionally known as Alice and Bob, shared key data at rates as high as 75 kb/s. Courtesy of CNRS Institut d'Optique.

In the new technique, Alice encodes the key in both quadrature components of the electric field of the pulse, and Bob chooses which he will measure. He then discusses his choices with Alice so they know which results should be correlated. The uncertainty principle applies because the phase and the amplitude of the laser pulses are quantum-noise-limited, Grangier said, so that if Eve spies on the transmission, Alice and Bob will detect errors in a segment that they share publicly.

1.7-Mb/s transmission

The proof-of-principle system featured a 780-nm diode laser as the photon source. The CW output passed through an acousto-optic modulator to yield 120-ns pulses at a repetition rate of 800 kHz. The sender's station used an electro-optic modulator to encode the key information in the amplitude of the pulses, which were measured on the receiving end with standard silicon PIN photodiodes.

To explore the robustness of the system, the researchers introduced line losses with a variable optical attenuator. With a loss of 3.1 dB, the system's overall homodyne detection efficiency was 0.84, enabling them to transmit key data at rates of more than 55 kb/s and as high as 75 kb/s. With no loss, the transmission rate was 1.7 Mb/s.

Although detecting the information in the phase or amplitude of the pulses creates its own challenges, Grangier said that it should be easy to address these issues at telecommunications wavelengths, and the researchers plan to construct such a system.

"We are working at 780 nm, but there are much better laser diodes, fibers, modulators and detectors at 1550 nm," he said. "So using faster modulators, lowering the electronic noise of the detectors, improving the homodyne detection efficiency, etc., is technically possible, and this would certainly improve the performance."

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