But if there is an eavesdropper on the line, the receiver will see an error rate in the photon values and no key will be generated. In the absence of that error rate, the security of the channel is assured, Ribordy says.
However, since security can only be assured after the fact -- when the error rate is measured, which happens immediately -- the channel should be used to send only the keys, not actual messages, he notes.
The other limitation of the system is its range, which currently doesn't exceed 100 kilometers (62 miles), although the company has achieved 250 kilometers in the lab. The theoretical maximum is 400 kilometers, Ribordy says. Going beyond that would require the development of a quantum repeater -- which would presumably use the same technology as a quantum computer.
QKD security isn't cheap: An emitter-receiver pair costs about $97,000, Ribordy says.
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