Cryptography Reference
In-Depth Information
(a)
(b)
fs-Pulse UV Laser
BOB
VICTOR
Bell-State
Measurement
Alice
D3+
Pump
Laser
532nm@10W
Bob
D0-
D0+
D3-
Logic
0
12
1
2
3
Ψ
PBS
Ti:Sapphire
Laser
Modelocked
789@1,6W
PBS
Pol Control
EPR Source 1
EPR Source 2
Pol Control
(c)
3
CCD
f=4cm
0
Dichroic
Mirror
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
LBO
Crystal
f
CYL
=15cm
Motoried.
Translation
Stage
Mirror
UV Laser Pulse
f
CYL
=10cm
1
BBO
Crystal
Dichroic
Mirror
Filter
Dichroic
Mirror
2
Bell Inequality Violation Limit
Classical Limit
ALICE
Picomotor
Mirror Mount
Pol Control
Picomotor
Mirror Mount
Fidelity
Fidelity with Delayed Choice
BS
0.0
22.5
45.0
67.5
90.0
Power(
λ
)
D2
Polarizer Orientation
φ
0
=
φ
3
[°]
D1
Spectrometer
Logic
Figure 3.1
(a) Scheme for entanglement swapping, i.e., the teleportation of entangle-
ment. Two entangled pairs 0, 1 and 2, 3 are produced by two entangled photon sources
(EPR). One particle from each of the pairs is sent to two separated observers; say 0 is
sent to Alice and 3 to Bob. 1 and 2 become entangled through a Bell state measurement,
by which 0 and 3 also become entangled. This requires the entangled qubits 0 and 3 nei-
ther to come from a common source nor to have interacted in the past. (b) Experimental
setup for the demonstration of teleportation and entanglement swapping using pairs
of polarization entangled photons. The two entangled photon pairs were produced by
down-conversion in barium borate (BBO), pumped by femtosecond UV laser pulses
traveling through the crystal in opposite directions. After spectral filtering, all photons
were collected in single-mode optical fibers for further analysis and detection. Single-
mode fibers offer the benefit that the photons remain in a perfectly defined spatial
mode allowing high-fidelity interference. In order to optimize the temporal overlap
between photon 1 and 2 in the beam splitter, the UV mirror was mounted on a mo-
torized translation stage. Photons 0 and 3 were sent to Bob's two-channel polarizing
beamsplitters for analysis, and the required orientation of the analyzers was set with
polarization controllers in each arm. All photons were detected with silicon avalanche
photodiodes, with a detection efficiency of about 40%. Alice's logic circuit detected
coincidences between detectors D1 and D2. (c) Experimental violation of Bell's inequal-
ities from particles that never interacted with each other obtained through correlation
measurements between photons 0 and 3, which is a lower bound for the fidelity of the
teleportation procedure (from [16]).
φ
0
(
φ
3
) is the setting of the polarization analyzer
for photon 0 (photon 3) and
φ
0
=
φ
3
. The minimum fidelity of 0.84 is well above the
classical limit of 2
/
3 and also above the limit of 0.79 necessary for violating Bell's
inequality.
Photon 4 acts as a trigger to indicate the presence of photon 1. If one pair
of photons is emitted in each of the pairs of modes 1-4 and 2-3, a threefold
coincidence of T-D1-D2 is sufficient to guarantee a successful teleportation.
However, owing to the probabilistic nature of SPDC, two photon pairs are
both
p
2
emitted
into
modes
1-4
with
the
same
probability
as
for
a
