(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.