Quantum-limited measurements of optical signals from a satellite in geostationary earth orbit

Abstract

Summary form only given. Quantum key distribution (QKD) has raised increased attention over the past years as one of the most attractive quantum technologies for practical implementation. QKD has already been implemented in intra-city networks all around the world. But up to now, bridging global distances with quantum communication remains an outstanding challenge. A promising candidate to provide this link is via optical satellite communication. As space-to-ground communication is already well developed for classical applications, one can make use of the already existing technology for QKD, i.e. modern Laser Communication Terminals (LCTs) may be adapted for quantum communication. An important first step to achieve this goal is a precise characterization of the system and the channel with regard to their quantum noise behaviour. We present quantum-limited measurements of signals from an LCT located on the satellite Alphasat in geostationary Earth orbit. The LCT generates binary phase shift keyed (BPSK) coherent states originally used for classical data communication. Our receiver is a Transportable Adaptive Optical Ground Station (T-AOGS) located at the Teide observatory on Tenerife. It corrects for phase front distortions in order to interfere the incoming signal with a local oscillator. This allows for heterodyne detection or, by phase-locking the local oscillator, for homodyne detection. The detector itself is quantum noise limited such that it can resolve the quantum uncertainty of the detected states (see Fig. 1). Evaluating the signal variance for homodyne detection, we are able to show that the detected states are quantum limited coherent states. Thus, we conclude that the coherence of the quantum states is preserved after propagation over a distance of 38600 km including the passage through turbulent atmosphere and we give an upper bound for any excess noise that the states might have acquired [1]. Moreover, we extent our analysis to heterodyne detection, characterizing arbitrary field quadratures. Our results indicate that satellite quantum communication is feasible in principle in such a scenario.