Cryptography Reference
In-Depth Information
(a)
(b)
0.905
1.0
0.9
ψ+
ψ-
0.963
Alice logic
Classical channel
Bob
logic
ψ+/ψ-
ψ+
0.863
ψ-
PBS
0.846
600 m
0.8
0.7
0.6
BS
PBS
EOM
Alice
PC
PC
Bob
River Danube
Input
P
bc
d
0.590
UV-pulse
Trigger
Source
0.5
a
0.547
BBO
Quantum channel
Figure 3.3
(a) Sketch of the two laboratories located on either side of the Danube
River (from [23]). The laboratories were located in two sewage water system buildings
owned by the city of Vienna. The faster classical channel (microwave) and the slower
quantum channel (fiber) are shown above and underneath the Danube. The vertical
separation of the two channels is about 40 m. (b) Fidelity of the teleported states with
and without active switching.
of D1, D2, and T increases with decreasing
. The corresponding fidelities for
conditional (fourfold detection) and nonconditional (threefold detection D1-
D2-T) teleportation are shown in Figure 3.2b. We were able to demonstrate
the preparation of a freely propagating teleported quantum state with high
(nonconditional) fidelity of 0
γ
02, i.e., well above the classical limit.
∗
The possibility of letting the teleported qubit travel freely in space, together
with the high experimental visibility obtained, is a fundamental step in the
direction of the realization of long-distance quantum communication. Further
protocols, such as entanglement purification, are then needed to overcome
decoherence in long-distance quantum channels (see Section 3.2.2).
.
85
±
0
.
3.2.1.3 Long-Distance Quantum Teleportation
Teleportation is the basis for the quantum repeater [15], which allows dis-
tributing quantum entanglement over long distances. Also, quantum telepor-
tation over longer distances will be needed for realizing quantum network
schemes involving several parties [22] and naturally for interconnecting de-
vices utilizing quantum computational algorithms. As a next step toward a
full-scale implementation of a quantum repeater, we have realized a large-
scale implementation of teleportation [23] of photon qubits in an outdoor
environment. The two laboratories involved, Alice and Bob, are separated by
600 m across the Danube River in Vienna (see Figure 3.3). Additionally, while
it has been shown that systems based on linear optical elements can only de-
termine two of the four Bell states perfectly [24,7,25], our system achieves the
∗
Other nonconditional quantum teleportation experiments have been performed with
continuous variables [19] and, only recently, with ions [20,21]. However, the most
suitable systems for long-distance transmission are currently photons.
