Cryptography Reference
In-Depth Information
D1
D2
D3
(1)
BS
T
1
3
4
2
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
EPR
EPR
Classical limit
D1
D2
D3
(2)
Theory
BS
T
3
4
1
2
EPR
EPR
0
5
10
15
20
Relative Intensity
(a)
(b)
Figure 3.2 (a) (1) Original qubit teleportation scheme using polarization-entangled
photon pairs based on spontaneous parametric down-conversion (EPR sources); (2)
Freely propagating teleported qubits. (b) Conditional fidelities (squares) and noncon-
ditional fidelities (circles) obtained in 45 teleportation for different attenuation 1
.
With increasing attenuation of mode 1, an increase of nonconditional fidelities is ob-
served while the conditional ones remain constant. For
γ =
0
.
05, the classical limit 2/3
is clearly overcome.
successful teleportation. This will also lead to threefold coincidences of T-
D1-D2, but in this case no teleportation occurs, as mode 3 is simply empty. To
ensure a successful teleportation, it has been necessary to confirm the presence
of photon 3 by actually detecting it (Figure 3.2). For this reason, the original
Innsbruck experiment [9], the first experimental demonstration of quantum
teleportation, has been called a “postselected” one, while probably the word
“conditional” would be more appropriate, as detection of photon 3 does not
depend on its state. In the new protocol, an unbalanced two-photon interfer-
ometer is used to make a detection at Bob's side obsolete and therefore allow
for a free propagation of the teleported qubits. The number of unwanted D1-
D2 coincidence counts is reduced by attenuating beam 1 by a factor of
γ
while
leaving the modes 2-3 unchanged. Then a threefold coincidence D1-D2-T will
occur with probability p 2
owing to successful teleportation, and with signif-
icantly lower probability
p 2 there will be a spurious coincidence. Thus the
probabilty for a successful teleportation event (conditioned on D1-D2-T) will
scale with 1
γ
it will not be necessary any-
more to detect the teleported photon 3; a freely propagating teleported beam
of qubits emerges.
To demonstrate experimentally nonconditional teleportation, we inserted
a series of neutral density filters in mode 1 and showed that the probability
of having a successful teleportation conditioned on a threefold coincidence
/(
1
+ γ)
, and for sufficiently low
γ
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