QKD Transmitter Unit of the EAGLE-1 Satellite

Abstract

EAGLE-1 is a European project by ESA and European Commission for Quantum Key Distribution (QKD) over satellite with space segment, user segment and ground segment. The EAGLE-1 satellite distributes the quantum keys from a LEO orbit via laser-link to telescopes on ground based on the prepare-and-measure approach. The DLR Institute of Communications and Navigation contributes to the space segment with the design of a transmitter unit as part of the QKD payload of the satellite.

The QKD transmitter unit is a technical system that converts key and protocol information into an optical signal. The transmitter input is protocol data with key data based on information from a quantum random number generator (QRNG). The transmitter output is transmitted via laser terminal to ground. The protocol is based on BB84 and uses double pulses with phase encoding and a key generation rate of 2.25Gbit/s. The desired output of the transmitter is a single photon per key bit.

The main elements of the transmitter unit are: pulse generation, pulse amplification, pulse attenuation and a hacking protection: The pulse generation part creates the optical signal with a C-band laser and modulates the key and header information according to the EAGLE-1 QKD protocol. The pulse amplification is needed for strong optical signals with protocol header information that is required for identification and synchronization of a QKD receiver. The pulse attenuation part controls the transmitter output to single photon levels per key bit. The transmitter unit requires also an automatic calibration of the temperature sensitive system. Finally, the hacking protection contains countermeasures for external laser attacks.