Cryptography Reference
In-Depth Information
Figure 9.5
The Alice breadboard is in the foreground. Karwendelspitze is in the
background.
low-power green laser (3 mW) from Karwendelspitze that was clearly visible
from Zugspitze. This was followed by a fine alignment using an approxi-
mately 500 µW beam at 850 nm passing through the same spatial filter as the
faint light pulses in the Alice setup.
The collimation in the Alice system was adjusted to minimize the diameter
at 23.4 km, resulting in a beam 1-2 meters in diameter (depending on air
turbulence). This led to lumped optical losses in the transmission path of
about 18 to 20 dB. With a receiver efficiency of around 16% and using faint
pulses containing 0.1 photons per bit, the detected bit rate at Bob was about
1.5 to 2 kilobits per second.
Several night trials were carried out at various times from September 2001
to January 2002. In the final trial, several keys were exchanged over a period
of three days at a selection of pulse intensities ranging from 0.4 to 0.08 photons
per bit. Representative data are shown in Table 9.1. Total gated photon rates
