Cryptography Reference
In-Depth Information
Alice
Bob
laser
a)
α
β
µ << 1
2hν
b)
α
β
µ >1
2h
ν
c)
α
β
µ
>1
α
β
2hν
d)
µ >1
α′
β′
Figure 2.7 Single-photon based quantum cryptography using (a) a faint-pulse source,
(b) a two-photon source, (c) entanglement-based quantum cryptography with active,
and (d) with passive choice of bases. 2h
ν
denotes the photon-pair source,
µ
the mean
number of photons per pulse, and the parameters
α
and
β
the settings of the qubit-
creation and analyzing devices, respectively.
It is possible to use the full quantum correlation of entangled pairs to generate
identical keys at Alice's and Bob's and to test the presence of an eavesdropper
via a test of a Bell inequality (see Figure 2.7c). This beautiful application of
tests of Bell inequalities has been pointed out by Ekert in 1991 [33] — without
knowing about the “discovery” of quantum cryptography by Bennett and
Brassard seven years earlier. The setup is similar to the one used to test Bell
inequalities, with the exception that Alice and Bob each have to choose from
three and not only two different bases. Depending on the basis chosen for
each specific photon pair, the measured data is used to establish the sifted
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