improve quantum crypto
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
"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,
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,"
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,
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
TRN Categories: Cryptography and Security; Quantum Computing
Story Type: News
Related Elements: Technical paper, "Single photon quantum
cryptography," European Quantum Information Processing and Communications
workshop in Dublin, September, 2002
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