Cryptography Reference
In-Depth Information
Figure 9.4
The receiver (Bob) consists of a 25 cm aperture Schmidt-Cassegrainian
telescope. The miniature detector module is attached to the rear mounting of the tele-
scope. It consists of a nonpolarizing beam splitter (BS) followed by two polarizing
beam splitters (PBS). Single-photon detectors (D1-4) receive the output of the polariz-
ers. In the D1/D3 arm, a half-wave plate rotates the analyzed polarization to the 45
◦
basis. The module incorporated high-voltage supplies and discriminator circuitry to
produce standard NIM pulses at the output. The detector outputs D3, D4 are com-
bined with the D1, D2 outputs with a delay of 5 ns and input into the two-channel
timing card in the PC. A flip mirror allows a CCD camera to view the incoming light
for alignment purposes.
than l ns using a software-based phase-locked loop driven by the received
photodetections. The photodetections thus can be gated in two 1.4-ns-wide
time windows separated by 5 ns. Pulses outside these timing gates are ig-
nored. The error rate due to dark and background counts is thus suppressed
by a factor of
1/35. The random polarization pulses are sent in 700 ms blocks
preceded by a series of predetermined pseudorandom data sets lasting 110 ms
to determine uniquely the start time of each block. Following the transmis-
sion of the block, a settling time of
∼
300 ms allows the computers to verify
a successful transmission. Gross block length is thus just over 1.1 s. Sifting
and error correction of the 700 ms data blocks were then performed over the
telephone link using software developed in our 1.9 km experiment [11].
∼
9.3.2 The Long-Range Trial
To avoid air turbulence effects, the long-range experiment was carried out
over an elevated path with the receiver on Karwendelspitze and the trans-
mitter located at a small experimental facility of the Max-Planck-Institute for
Extraterrestrial Physics on the summit of Zugspitze, southern Germany. Ini-
tial alignment of both transmitter and receiver was achieved by shining a
