# Sub-ns timing accuracy for satellite quantum communications

## TL;DR

This paper demonstrates sub-nanosecond timing accuracy in satellite quantum communications by using laser ranging techniques and commercial detectors to detect faint signals from medium Earth orbit satellites, enhancing quantum communication precision.

## Contribution

The study introduces a method to achieve approximately 230 ps temporal accuracy in satellite quantum communication detection using existing technology.

## Key findings

- Achieved ~230 ps timing accuracy in satellite photon detection.
- Demonstrated detection of faint signals from satellites over 7500 km.
- Evaluated the Matera Laser Ranging Observatory's performance for quantum communication.

## Abstract

Satellite quantum communications have rapidly evolved in the past few years, culminating in the proposal, development, and deployment of satellite missions dedicated to quantum key distribution and the realization of fundamental tests of quantum mechanics in space. However, in comparison with the more mature technology based on fiber optics, several challenges are still open, such as the capability of detecting, with high temporal accuracy, single photons coming from orbiting terminals. Satellite laser ranging, commonly used to estimate satellite distance, could also be exploited to overcome this challenge. For example, high repetition rates and a low background noise can be obtained by determining the time-of-flight of faint laser pulses that are retro-reflected by geodynamics satellites and then detected on Earth at the single-photon level. Here we report an experiment with regard to achieving a temporal accuracy of approximately 230 ps in the detection of an optical signal of few photons per pulse reflected by satellites in medium Earth orbit, at a distance exceeding 7500 km, by using commercially available detectors. Lastly, the performance of the Matera Laser Ranging Observatory is evaluated in terms of the detection rate and the signal-to-noise ratio for satellite quantum communications.