Cross-encoded quantum key distribution exploiting time-bin and polarization states with qubit-based synchronization

TL;DR

This paper introduces a cross-encoded quantum key distribution scheme combining polarization and time-bin encoding, achieving stable, high-accuracy transmission over 50 km fiber with qubit-based synchronization, suitable for hybrid quantum networks.

Contribution

It presents a novel hybrid encoding and synchronization method that enhances stability and compatibility for quantum key distribution over fiber-optic channels.

Findings

Successful transmission over 50 km fiber with low error rates

Stable performance demonstrated over 12 hours of operation

Effective qubit-based synchronization without extra hardware

Abstract

Robust implementation of quantum key distribution requires precise state generation and measurements, as well as a transmission that is resistant to channel disturbances. However, the choice of the optimal encoding scheme is not trivial and depends on external factors such as the quantum channel. In fact, stable and low-error encoders are available for polarization encoding, suitable for free-space channels, whereas time-bin encoding represent a good candidate for fiber-optic channels, as birefingence does not perturb this kind of states. Here we present a cross-encoded scheme where high accuracy quantum states are prepared through a self-compensating, calibration-free polarization modulator and transmitted using a polarization-to-time-bin converter. A hybrid receiver performs both time-of-arrival and polarization measurements to decode the quantum states and successfully leaded to a transmission over 50 km fiber spool without disturbances. Temporal synchronization between the two parties is performed with a qubit-based method that does not require additional hardware to share a clock reference. The system was tested in a 12 hour run and demonstrated good and stable performance in terms of key and quantum bit error rates. The flexibility of our approach represents an important step towards the development of hybrid networks with both fiber-optic and free-space links.