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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)
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References in periodicals archive ?
While Micius and the ground-based QKD networks give China the edge--for now--in secure quantum communications, it still trails the US in quantum computing.
As the first commercial QKD solution in the U.S., Quantum Xchange exclusively owns the distance enhancing Trusted Node technology developed by Battelle which can extend the QKD range indefinitely using 100-mile multiples, making large-scale QKD over long distances possible and practical.
In this QKD protocol, the real randomness of the key is guaranteed by the essential properties of the quantum: uncertainty principle.
History of developments of QKD. Because the raw keys are not necessarily secure when the channel has noise or two photons are transmitted, many studies have been done to find a way to guarantee security when the communication device has such imperfections.
It should be noted that the presented simulation data hold for the QKD protocol and not necessarily for its implementations; the latter must adjust to changes to their environment and parameter values that are variable over time and never fully known.
It is interesting to compare the key rates from QKD networks which were constructed in previous years.
QKD is already a reality, although limited in capability.
But the beauty of the QKD protocol is that if anyone was to attempt to intercept a quantum key, it would alter the quantum states of the photons and alert users that the key is under attack.
The quantum secure direct communication (QSDC) protocol differs from the QKD protocols used to distribute private keys and has been proposed [9] for directly transmitting secret messages, without having to share a private key between two legitimate users before hand.
Researchers at the National Institute of Standards and Technology (NIST), Gaithersburg, Md., have taken a step into the quantum realm in cryoptography with their High-Speed Fiber Quantum Key Distribution System, (QKD).
Achieving a completely secure, noninterceptable operational environment requires the secure transfer of information using channels dominated by quantum effects--that is, quantum key distribution (QKD) (fig.