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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 ( 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,

quantum cryptography

An encryption method that can detect eavesdropping. Using optical transmission to send a secret key to the other side, quantum cryptography draws on the inherent properties of photons, which become slightly altered if they are observed by an intruder. When an alteration is detected, the receiver knows the sender's key has been compromised. See cryptography.

Polarized and Entangled Photons
One method relies on the polarization of the photons, which will be altered if observed (see QKD). Another method uses photon pairs that exhibit a correlation between them. Any observation along the way weakens the correlation, which can be detected.

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