Quantum key distribution (QKD) enables private communication with information theoretic security. Free-space optical communication allows one to implement QKD without the limitations imposed by fiber networks such as the exponential scaling of transmission losses in optical fibers. Therefore, free-space QKD via satellite links is a promising technology to provide long-distance quantum communication connections. In free-space QKD systems, background light is the main source of noise, which has to be suppressed by means of spectral, spatial, and temporal filtering to reach a sufficiently low quantum bit error rate (QBER). Only then a quantum key can be exchanged successfully. To be able to define the requirements for a free-space QKD system, the background light must be examined more closely. Current considerations concentrate on cloud-free skies and rural environments. Free-space QKD will also take place when the sky is partly clouded and most likely also in urban environments. Here, an overview of physical causes of background light for downlink scenarios is given. Furthermore, the relation between QBER and background light is derived for a decoy-state BB84 protocol with polarization-encoded qubits to give an example of the dependency. Moreover, a setup to experimentally investigate the background light is shown. Measurement data were taken with this setup in Oberpfaffenhofen near Munich (Germany) in C-band. The measurement data are used to verify a background light simulation tool. The outcome underlines that simulation tools are sufficient for clear sky scenarios.
Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest are ever increasing. Same is true for satellite free-space optical communications (FSOC). In order to enable a robust channel for transmission it is indispensable to account for static and dynamically changing misalignments between the transmitter and receiver pair. This work will focus on the transmitter terminal (Alice) and the design and verification process of the active beam steering system. The novelty is a recently developed variable reluctance fine steering mirror (FSM) including eddy current sensors (ECS) to measure its tip and tilt. A cascaded architecture was chosen in order to combine the optical stabilization objective with the dynamics of the mirror platform. The inner control loop makes use of an observer model whose estimated output is fed into a state controller allowing for an increased responsiveness. While high gains increase the closed loop bandwidth the eigenfrequency of the system introduces a pole to the plant which has to be avoided by the controller output. A digital notch filter was introduced to reject the excitation of the critical frequency band which gets obsolete in a system with high frequency sampling capabilities. The outer loop is engaged when a valid optical signal is received and a transition from a closed loop pointing to a closed loop tracking mode is performed. A proportional-integral (PI) controller keeps the received beam at the 4-quadrant-diode (4QD) whose center is used as the main reference through prior calibration with the transmit beam launching on the same path. The presented cascaded control scheme allows improvements in system performance and reliability.
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