Airborne reflectors for satellite-based quantum entanglement and key distribution.

Satellite-based quantum communication faces challenges due to link intensity loss over long distances caused by various factors such as geometric loss and atmospheric attenuation. These effects can lead to lower entanglement distribution rates, secret key rates, and increased quantum bit error rates, especially in direct satellite-to-ground communication. This paper proposes a practical solution: the design of an airborne gold-coated parabolic reflector to be placed in the stratosphere directly above the ground station. This reflector effectively acts as a second virtual transmitter, a novel concept introduced in this work. The proposed method brings about a substantial increase in the distributed entanglement rate, boosting it by up to 25 times at zenith compared to direct satellite-to-ground communication. It also reduces the minimum elevation angle for secure communication, approximately from [Formula: see text] to [Formula: see text] for BB84 and from [Formula: see text] to [Formula: see text] for E91 when using the proposed reflector method compared to direct satellite-to-ground communication. Furthermore, the proposed reflector method extends the communication time window by [Formula: see text] for the BB84 protocol and [Formula: see text] for the E91 protocol. These enhancements underscore the potential of our approach to significantly extend the duration of secure communication and improve performance, particularly at lower elevation angles where direct satellite-to-ground communication is not feasible.