Diamonds improve quantum crypto

By Eric Smalley, Technology Research News

Scientists have thoroughly demonstrated that the quirks of quantum physics can secure secret messages, and one company is already selling a commercial quantum cryptography system.

But there's still plenty of room for improvement. Prototype quantum key distribution systems are slow and only work over relatively short distances. The main challenge is coming up with a light source that reliably fires off one and only one photon per pulse.

Today's quantum cryptography prototypes use lasers that are so heavily filtered that most of the pulses contain no photons, a few contain a single photon and fewer still contain two photons. Sending a cryptographic key means waiting for enough single photon pulses to be generated, and compensating for pulses that contain too many photons or none at all.

Researchers from the French National Scientific Research Center (CNRS) and Ecole Polytechnic in France have bettered the usual weak laser pulse method with a deliberately dirtied microscopic diamond: a 40-nanometer diamond nanocrystal with a nitrogen atom embedded next to an atom-size gap in the center. A nanometer is one millionth of a millimeter, and an atom measures about one tenth of a nanometer.

The nanocrystal emits light by fluorescence. When hit by a laser, the nanocrystal absorbs energy, then gives it off in the form of a single photon.

"We have developed an efficient, stable, all solid-state, room temperature single-photon source [and] we have used this single-photon source in a quantum cryptography setup," said Alexios Beveratos, a researcher at CNRS.

When the researchers used the setup as a light source to transmit quantum encryption keys through the open air, they were able to transmit 9,000 secure bits per second over a distance of 50 meters. "The limiting factor for the distance is that we didn't have a longer corridor. We should be able to span larger distances," said Beveratos.

An encryption key is a string of numbers used to lock and unlock encrypted messages sent over unsecure communications lines.

The researchers' goal is to communicate perfectly secure keys between the Earth and satellites. Researchers at Los Alamos National Laboratory aiming for the same goal have demonstrated a quantum cryptographic system that spans 10 kilometers using weak lasers, which is roughly equivalent to sending photons up through the thinner upper atmosphere to reach satellites hundreds of kilometers above the Earth. The next challenge is being able to aim single photons precisely enough to hit satellites.

The efficiency of single-photon detectors limits the distance that quantum cryptographic systems can operate over. Detector efficiency is affected by thermal noise and so detectors are usually cooled. Noise produces false positives, or signals when no photon is present.

The researchers' diamond-based device emits a single photon about two percent of the time it is stimulated, and can deliver as many as 116,000 single-photon pulses per second. Weak lasers can also generate 116,000 single-photon pulses per second but the diamond only generates 90 two-photon pulses during that time compared to 1,300 for the weak laser, said Beveratos. Two-photon pulses compromise security, and minimizing the risk they pose lowers the efficiency of the device.

Because two-photon pulses are inevitable, quantum cryptographic schemes use privacy amplification, which reduces a string of bits that includes some that have been exposed to an eavesdropper to a smaller string of secret bits. Privacy amplification converts two or more of the original bits into a single, new bit. Even if an eavesdropper knows some of the original bits, she is highly unlikely to be able to figure out the new bit. The more two-photon pulses a light source emits, the more original bits have to be used to make one secret bit.

Quantum cryptography involves a trade-off between data rate and distance, meaning the further apart hypothetical correspondents Alice and Bob are, the fewer secure bits they can send to each other. Because it generates fewer two-photon pulses than weak lasers, the researchers' light source requires fewer pulses to make secret bits and so can span longer distances, said Beveratos.

Quantum cryptography allows users to tell for sure whether the encryption key they are using to encrypt and decrypt a message has been compromised.

Quantum cryptography schemes send encryption keys by representing each bit with only one photon. If there were two or more photons per bit, an eavesdropper could siphon off extra photons in order to copy the key without being detected. Using only one photon per bit means that an eavesdropper would have to replace the photons she intercepted, but the laws of physics make it impossible to replicate all of the photons correctly.

In the race to develop reliable single-photon light sources, several possibilities have surfaced. Certain molecules work well for a time, but "after having emitted a certain amount of photons, they photobleach, which means that they are not optically active anymore and do not emit any more photons," said Beveratos.

Quantum dots, which are microscopic specks of semiconductor that trap one or a few electrons, don't bleach, but they only emit single photons at very low temperatures, which requires cumbersome and expensive cryogenic equipment, said Beveratos.

The researchers' device, with its reduced multiple-photon rate, is the first to show a better secret bit rate than weak laser pulses, said Richard Hughes, a physicist at Los Alamos National Laboratory who has built a quantum key distribution prototype spanning 10 kilometers. "This type of light source and other similar ones will lead to improvement in the efficiency with which quantum key distribution [systems] can generate secret sharing keys," he said.

In order to reach satellites, the researchers will need to improve the device's efficiency from two percent to ten percent, said Beveratos. The researchers plan to test their system over longer distances and outdoors, he said.

Developing the quantum cryptographic systems for practical satellite communications will take at least five years, said Beveratos. The device would need to be miniaturized to fit on a satellite, he said. It will take less time to ready the device for use between two points on Earth, he said.

Beveratos' research colleagues were Rosa Brouri, André Villing, Jean-Philippe Poizat and Philippe Grangier of CNRS, and Thierry Gacoin of Ecole Polytechnique. The research was accepted for publication in the journal Physical Review Letters. The research was funded by the European Union.

Timeline:   5-6 years
Funding:   Government
TRN Categories:   Cryptography and Security; Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Single photon quantum cryptography," European Quantum Information Processing and Communications workshop in Dublin, September, 2002


September 18/25, 2002

Page One

Molecule chip demoed

Diamond electronics on deck

Huge lasers could spark fusion

Diamonds improve quantum crypto

Software agents ask for help


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