Cryptography Reference
In-Depth Information
Polarization qubits
Time-bin qubits
(A)
(D)
t
t
Round-Trip
x
x
Alice
Bob
Alice
Bob
(B)
(E)
t
t
One-Way
x
x
Alice
Bob
Alice
Bob
(C)
t
(F)
t
3
6
Symmetric
2
4
2
5
1
3
1
4
x
x
Alice
Bob
Alice
Bob
Figure 10.1 Space-time diagrams of six noise-immune QKD schemes organized by
encoding (polarization or time-bin) and signal flow (round-trip, one-way, and sym-
metric). The dashed lines and curved arrows show how the advanced wave interpre-
tation relates the round-trip schemes [(A) and (B)] to the one-way schemes [(B) and
(C)], and the one-way schemes to the symmetric schemes [(C) and (F)]. The dotted
lines connecting photons indicate entanglement. The photon labels in (C) and (F) are
used later in this chapter.
Alice's bit setting. From Bob's point of view, the scheme is equivalent to
Bennett's two-state protocol [10], since he is attempting to distinguish prob-
abilistically between two nonorthogonal states. The noise-immune feature is
derived from the unique property of the Faraday rotator: whatever the po-
larization transformation along the line from Bob to Alice, the photon that
Alice reflects will arrive in Bob's laboratory in a polarization state orthogonal
to its original state [11]. For example, if Bob sent
, then either the first or
the second photon he receives from Alice will be in the state
|
V
|
H
. Thus if he
measures one photon in state
, he knows the
value of Alice's bit. Any other detection pattern is ambiguous, and Alice and
Bob discard these cases.
The AWI was originally conceived as a method for generating one-photon
experiments from two-photon experiments. However, we may reverse this
procedure and determine which two-photon state embodies the action of
|
V
and the other in state
|
H
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