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