Last Mile Free Space QAN
Developed by the Group of Prof. Weinfurter from the university of
Munich (LMU), this free space QKD system bridges the last mile between
the fibre node ERD and the building in which the final presentation
takes place (see figure 1). It employes the BB84 protocol using
polarisation encoded attenuated laser pulses over a distance of 80
m. Decoy states are used to ensure key security even with faint
pulses. The system is capable of working night and day using excessive
filtering to suppress background light.
In figure 2 the transmitter (Alice) and in figure 3 the receiver (Bob)
are shown. Both setups are housed in aluminium boxes to protect them
from environmental influences as for example rain and extreme
temperatures. The transmitter box hosts the preparation of the
photonic states as well as some calibration equipment. The receiver
mainly consists of the spatial filter and the polarisation analyses
module with the single photon detectors. On each side small telescopes
are the only part visible from the outside, both constantly being
automatically aligned with the help of stepper motors. Providing an
overall Alice to Bob transmittance of approximately -10 dB the system
is able to provide us with a secret key rate of up to 20 kbit/s
starting from a repetition rate of 10 MHz.
Figure 1: Bridging the last mile, the LMU system is installed on the
buildings where the final presentation takes place (bottom) and in
which the fibre node ERD is located (top).
Figure 2: Transmitter Alice
Figure 3: Receiver Bob
