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(Quantum Key Distribution) A highly secure cryptographic method for transmitting secret keys from sender to receiver. Derived from random numbers, the key is sent one photon (one bit) at a time in a polarized state. If intercepted by an eavesdropper, the state changes, and an error is detected at the receiving side. The keys may be transmitted on a different line than the data and can be sent at a much slower rate. For example, 1,000 bps is fast enough to send and change the key one or more times per second. See quantum cryptography.

First Commercial System
In 2003, MagiQ Technologies, New York (www.magiqtech.com) introduced the first commercial QKD system. The QPN transmitter attenuates (reduces) a photonic signal to one photon that is polarized to a 0 or 1 in one of two polarization angles known as "basis i" or "basis j." Thus, each bit can be in one of four states: 0 or 1 in basis i or 0 or 1 in basis j.

When the receiver gets a bit, a random number generator acts like a coin toss and sends back over a clear channel which side of the coin is up (basis i or j). If the toss was correct and that happened to be the basis for that bit, the receiver uses the 0 or 1 as received. If not, it is discarded.

MagiQ Technologies QPN
In 2003, MagiQ Technologies was the first to ship a commercial QKD system. MagiQ QPN transceivers sit at both ends of the optical network. The sending side generates random numbers and transmits secret keys to the receiver using QKD. (Image courtesy of MagiQ Technologies, www.magiqtech.com)
References in periodicals archive ?
Based on this analytical result, the ERATO project made an experimental demonstration of QKD with weak coherent pulses on a real optical fiber, whose security is quantitatively guaranteed in the Gaussian approximation.
As initially proposed by DARPA [1] and later applied in SECOQC [5] and Tokyo, QKD links in QKD networks are organized on the following way: both endpoints of corresponding QKD link have a storage key reservoir and these reservoirs are gradually filled with the final key material.
The measure of QKD protocols security is Shannon's mutual information between legitimate users (Alice and Bob) and eavesdropper (Eve): [I.
Today's high-speed QKD technology has become sophisticated, but hasn't made it out of the laboratory yet.
Until now, however, most QKD systems have used attenuated laser light as a pseudo single-photon emitter.
A great deal of work has gone into QKD in recent years, and a number of companies even offer off-the-shelf systems that promise quantum security - providing you can install a fibre-optic link between the sender and receiver.
With QKD, a secure link can be established while remaining 100 metres underwater, as submarines will be able to transmit photons of laser light to satellites, for retransmission to base.
Polten in Lower Austria, the QKD demonstration involved secure telephone communication and video-conference as well as a rerouting experiment which demonstrated the functionality of the SEcure COmmunication network based on Quantum Cryptography (SECOQC).