Time-frequency Quantum Key Distribution over a Free-Space Optical Link

TL;DR

This paper demonstrates a time-frequency quantum key distribution system using standard telecom components over a free-space optical link, achieving daylight secure communication with precise coupling and optical tracking.

Contribution

It introduces a practical implementation of TF-QKD with modulation techniques and optical tracking, enabling daylight free-space QKD over 388 meters.

Findings

Successful free-space QKD over 388 meters

Daylight QKD achieved with spectral and spatial filtering

Implementation of optical tracking system for precise coupling

Abstract

We present an implementation of the time-frequency (TF) quantum key distribution (QKD) protocol realized mainly with standard telecommunication components at 1550 nm. TF-QKD is implemented with modulations in time and frequency, namely pulse position modulation (PPM) and frequency shift keying (FSK). The time-frequency uncertainty relation ensures the security of the protocol. We further demonstrate free-space optical QKD transmission over a 388-m distance. Since the QKD setup is single-mode fiber (SMF) -based, precise coupling in and out of the SMFs is crucial. Thus, we implemented an optical tracking system. The QKD signal is wavelength-multiplexed with a strong beacon for optical tracking. Strong spectral filtering is necessary to separate beacon and QKD signal. Together with spatial filtering caused by the SMF most background light is filtered, enabling daylight QKD transmission.