Cryptography Reference
In-Depth Information
output 1
output 2
coupler
lens
crystal
filter
laser
Figure 2.9
Photo of our entangled photon pair source used in the first long-distance
field test of quantum correlation. Note that the whole source including temperature
and power control of the diode laser fits into a box of only 40
15 cm
3
.
×
45
×
the familiar entangled time-bin qubit state (Equation (2.2)). As long as the
coherence time of the pump laser is large compared to the travel-time differ-
ence
t
introduced in the interferometer, i.e., as long as pair emission with
time difference
t
is coherent, the coincidence count rate shows a sinusoidal
dependence on the sum of the phase in both interferometers.
After having realized in 1999 that it is possible to render “continuous”
energy-time entanglement “discrete” by replacing the high coherent pump by
a succession of a finite number of short, i.e., well localized pump pulses [46],
we performed a couple of experiments based on time-bin entangled photons.
For instance, in 2002, we demonstrated the robustness of maximally as well as
partially entangled qubits over 11 km of fiber on a spool [42]. Finally, in 2003,
we could extend the separation of the analyzing interferometers to more than
50 km, again taking advantage of fiber on a spool [44]. These investigations
showed that energy-time as well as time-bin entanglement is well suited for
QKD via optical fibers over long distances.
To conclude this section, let us briefly mention further directions of re-
search that emerged from the developments addressed above. First, the pos-
sibility of distributing entanglement over long distances enabled it to falsify
relativistic nonlocality (or multisimultaneity) [49-51] and made it possible to
