Cryptography Reference
In-Depth Information
(a)
(b)
Figure 2.12
(a) Quantum teleportation as a quantum relay. (b) Fidelity of the trans-
mitted quantum state as a function of the distance for different configurations. Direct
transmission (
n
=
1), with an EPR source in the middle (
n
=
2), teleportation (
n
=
3),
and entanglement swapping (
n
4). We assume that the fidelity is only affected by the
detector's noise. The curves are plotted for a realistic dark count probability
D
=
10
−
4
=
per ns and a fiber attenuation of 0.25 db/km.
successful. Consequently, the bad chance of a dark count is reduced to the
cases where a photon is lost in the last trunk only. This intuitive idea can be
elaborated (see [68]) though one should keep in mind that this trick does not
improve the bit rate (quite the opposite actually; the poor efficiency of the
Bell measurement reduces the bit rate). But it provides a good motivation to
work on quantum teleportation. Furthermore, once quantum memories exist,
combining them with the quantum relays will provide a working quantum
repeater that will extend quantum cryptography to unlimited distances and
bit rates.
Recently we demonstrated quantum teleportation in optical fibers over
a significant distance. In a first experiment [80], the receiver was set in a lab
55 meters away and connected to the EPR source by 2 km of fiber on a spool.
In a second [81] experiment we extended this to a case of 3
2 km: three
trunks each of 2 km of fiber, the first one between Alice's source and the
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