Quantum cryptographic systems
take advantage of Heisenberg's uncertainty principle, according to
which measuring a quantum
system in general disturbs it and yields incomplete
information about its state
before the measurement. Eavesdropping on a quantum
communication
channel therefore causes an unavoidable disturbance, alerting the
legitimate users. This yields a cryptographic system
for the distribution of a secret random cryptographic key between two parties
initially sharing no secret information that is secure against an
eavesdropper having at her disposal unlimited computing power. Once this secret key is established, it can be
used together with classical cryptographic techniques such as the
onetimepad to allow the parties to communicate meaningful information in
absolute secrecy.
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