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and bridge distances much larger than those achievable with purely ground-
based laboratories.
3.2.6.1.1 Free-Space Optical Links with Entangled Photons.
Our group
recently demonstrated how combine free-space optical technology with en-
tangled photon pairs. The first experiment took place over the Danube in
Vienna, where we could demonstrate the distribution of entangled photon
pairs over 600 m [87]. The second experiment [88] was set up over the city
of Vienna and achieved a distance of approximately 8 km, which exceeds the
atmospheric attenuation for satellite communication.
The schematic setup of the Danube experiment is shown in Figure 3.11.
The compact, portable down-conversion source (see Section 2.5) was placed
(b)
(a)
(c)
Figure 3.11
(a) Experimental schematic and communication diagram from [87]. The
upper figure shows the positioning of the source and receiver stations for the ex-
periment. The down-conversion source was positioned on the southwest bank of the
Danube River. One receiver station, named Alice, was located 500 m away on a rooftop
on the northeast side of the river, while the second receiver, Bob, was located on a sec-
ond rooftop 150 m away across a railroad and a highway. The inset shows a schematic
of the receiver telescope (the sender is the same with no polarizer). The lower figure
shows how data were communicated and shared for the experiment (see text). (b)
One of the transmitter telescopes during alignment. (c) Measured polarization corre-
lations between receivers. The data show the measured coincidence rate (per second)
between the two receivers as a function of the angle of the polarizer at receiver B
when the polarizer at receiver A was set to 0
◦
(solid circles, solid line) and to 45
◦
(open
circles, dotted line), respectively. The obviously noisy part in the data coincides with
the passing of a freight train underneath the link to receiver B. The visibilities of the
best fit curves are 88.2
±
5.7% and 89.0
±
3.3%.
