Cryptography Reference
In-Depth Information
optimal teleportation efficiency of 50% when using linear optics alone. This is
realized by an active feed-forward technique, namely by detecting two of the
four Bell states on the transmitter site, Alice, and correspondingly switching
the unitary transformation for the receiver photon at the receiver site, Bob,
with a fast electro optic modulator (EOM).
An important feature of our experiment was the implementation of an
optimized Bell state analyzer (BSA) capable of detecting two of the four Bell
states on Alice's side, and the implementation of an “actively switched” uni-
tary transformation on Bob's side triggered by the outcome of Alice's Bell state
measurement (BSM). Alice's BSM result was sent to Bob via a microwave link
(2.4 GHz) above the Danube River. The classical channel transmitted one bit
of information about Alice's BSM outcome (
+
). This signal traveled
the distance of 600 m almost at the vacuum speed of light, which took about
2 µs. Additionally, delays in the detectors and in several signal stages of the
transceivers introduced an extra delay of 0
−
or
.
6 µs. However, since the speed
of light in a fiber is approximately 2
3 of the vacuum speed of light, the en-
tangled photon
d
traveled the 800 m fiber from Alice to Bob in about 4 µs.
Therefore, the information on Alice's BSA outcome arrived at Bob's labora-
tory approximately 1
/
4 µs before the arrival of photon
d
. This provided Bob
with sufficient time to set an EOM to apply the birefringent phase shift of 0 or
π
.
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between the
H
and
V
optical light modes on the received photon
d
. When
Alice's BSM result was a
−
, then Bob left the EOM at the idle voltage (i.e., the
EOM introduces no phase shift and the teleported state remains unchanged),
and when it was a
+
, then Bob applied the activation voltage (i.e., the EOM
introduced a
phase shift between the horizontal and vertical polarization)
just before the photon passed through the EOM. In both cases, Bob eventually
obtains an exact replica of the initial teleportee state (see Equation (3.1)).
The EOM was a KDP Pockels cell, which achieved the phase shift of
π
π
with an accuracy of 1:200 at a voltage of 3.4 kV. The timing information of the
received classical signal was delayed with a digital delay generator, and when
a
+
was received, then the EOM was triggered with a 100 ns pulse with a rise
time of 20 ns. Additionally, Bob used a logic circuit to count only those detec-
tions of his photons that arrived at the expected times within a coincidence
window of
10 ns. Note that without operation of the EOM, Bob observes
only a completely mixed polarization for a
±
45
◦
and a circular polarization
input state. When teleporting polarizations along
H
or
V
, the transformation
is irrelevant, since the EOM phase shift does not affect these states. The full
teleportation protocol was demonstrated by teleporting distinct linear polar-
ization states and circular polarization states (see Figure 3.4b). The classical
fidelity limit of 2/3 [26] is clearly surpassed by our observed fidelities of
around 0.85. This is a step toward a full-scale implementation of a quantum
repeater to achieve shared pure entanglement between arbitrarily separated
quantum communication partners. Such a quantum repeater requires quan-
tum teleportation, purification of entanglement, and quantum memories.
Recent results [27,28] indicate the advance of quantum memory, which might
be suitable for future quantum communication networks.
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