Cryptography Reference
In-Depth Information
Figure 9.2
Transmitter light source with spatial filter removed. The pulse driver with
the computer interface is attached on the back side of the unit. The size is small
compared to a telescope needed for focusing the beam to a distant location.
quantum cryptography system. The close thermal coupling of the laser diodes
also ensures a fixed wavelength relation between the individual laser diodes,
keeping the laser frequency side channel for a possible eavesdropping attack
closed.
The beam from the filter transmitting only one spatial mode is trans-
formed in a Galilean telescope to a collimated beam with a diameter of
∼
40 mm FWHM (Figure 9.3). Together with the alignment laser and the
single-photon detector, the whole system is mounted on a 25
50 cm bread-
board, attached to a microradian-sensitive pointing stage on a sturdy tripod.
The lasers are randomly driven from a computer via a digital output card at
a 10 MHz repetition rate using subnanosecond duration pulses. This creates
∼
×
500 ps duration optical pulses randomly polarized in 0
◦
,90
◦
,45
◦
,or135
◦
directions. The computer uses a prestored random number to choose the po-
larization for the present set of experiments. Alternatively, nearly real-time
generation was possible. A sequence of bits produced by a quantum random
number generator running at 20 MHz could be fed to the Alice computer
seconds before the transmission.
Receiver
The receiver system (Figure 9.4) consists of a 25-cm-diameter
commercial telescope (Meade LX200) with computer controlled pointing ca-
pability. Unfortunately, the resolution of the mechanics of this system was
the limiting factor for the alignment of the receiver and was also difficult to
handle in the harsh outdoor conditions. Yet the stability of the system was
very convincing.
A compact four-detector photon-counting module [27,28] was coupled
to the back of the telescope after an RG780 long-pass filter to block out
short-wavelength background. The module consists of a polarization-in-
sensitive beam splitter passing two beams to polarizing beam splitters that
