Cryptography Reference
In-Depth Information
(a)
Alice
Bob
Pair 1
a3
b3
PBS
PBS
a1
b1
Pair 2
a4
b4
a2
b2
+/-
+/-
Classical
communication
(b)
a
b
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
H/H
H/V
V/H
V/V
+/+
+/-
-/+
-/-
c
d
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
+/+
+/-
-/+
-/-
H/H
H/V
V/H
V/V
Figure 3.4
(a) Scheme for entanglement purification of polarization-entangled qubits
[30]. Two shared pairs of an ensemble of equally mixed entangled states
ρ
AB
are fed
into the input ports of polarizing beam splitters that substitute the bilateral CNOT
operation necessary for a successful purification step. Alice and Bob keep only those
cases where there is exactly one photon in each output mode. This can happen only
if no bit-flip error occurs over the channel. Finally, to obtain a larger fraction of the
desired pure (Bell) state they perform a polarization measurement in the
basis in
modes
a
4
and
b
4
. Depending on the results, Alice performs a specific operation on the
photon in mode
a
3
. After this procedure, the remaining pair in modes
a
3
and
b
3
will
have a higher degree of entanglement than the two original pairs. (b) Experimental
results.
a
and
b
show the experimentally measured fractions both in the
H
|±
/
V
and in
the
bases for the original mixed state.
c
and
d
show the measured fractions of the
purified state in the modes a
3
and b
3
both in the
H
+
/
−
bases. Compared
with the fractions in
a
and
b
, the experimental results shown in
c
and
d
both together
confirm the success of entanglement purification.
/
V
and in the
+
/
−
