Plug-n-Play Three Pulse Twin Field QKD

TL;DR

The paper demonstrates a practical three-time-bin, phase-encoded Twin-Field QKD implemented in a Sagnac-based star-topology plug-and-play architecture, achieving intrinsic phase and polarization self-compensation without active stabilization. The scheme uses the first time-bin as a phase reference to monitor rapid fluctuations and applies a post-processing step to correct phase misalignments, enabling secure key generation over a 50 km asymmetric fiber link with a maximum visibility of 87% and a secure-key rate around 1.5 × 10^-5 bits per pulse. This approach shows robustness to environmental perturbations and supports scalable, multi-user quantum networking by embedding polarization self-compensation in the network topology. The results highlight the practicality and potential for real-world deployment of star-topology TF-QKD with minimal active stabilization and passive stability enhancements. Future work may extend the method to more users and further optimize phase-tracking and post-processing to maximize SKR under diverse channel conditions.

Abstract

We present the experimental implementation of a three-time-bin phase-encoded Twin-Field Quantum Key Distribution (TF-QKD) protocol using a Sagnac-based star-topology plug-and-play architecture. The proposed encoding method leverages the relative phases of three consecutive time bins to encode two bits per signal. The Sagnac loop configuration enables self-compensation for both phase and polarisation drifts, eliminating the need for active stabilisation. However, field deployments are subject to rapid phase fluctuations caused by external vibrations, which can degrade interference visibility. We used the first time bin for real-time phase-fluctuation monitoring. Although this monitoring reduces the effective key generation rate, the system achieved a secure key rate of approximately 1.5e-5 bits per pulse, with a corresponding visibility of up to 87% over a 50 km asymmetric optical fibre channel. These results demonstrate the practicality, stability, and scalability of the proposed three-time-bin TF-QKD protocol for real-world quantum communication networks.

Figures (8)

• Figure 1: Phase encodings achieved by synchronising the PM's RF signal with the optical pulse envelope.
• Figure 2: Interference of signals from Alice and Bob at Charlie. BS: Beam splitter, C: constructive port, D: destructive port. $\phi_{a1}, \phi_{a2}, \phi_{b1}, \phi_{b2}$ represent phases encoded by Alice and Bob in time bins 2 and 3, corresponding to bits $a1, a2, b1, b2$, respectively.
• Figure 3: Schematic of PnP twin field QKD setup with phase encoding.
• Figure 4: Schematic of modified Sagnac-based Twin field QKD setup
• Figure 5: Interference visibility at Charlie when Alice and Bob send a 0-0-$\pi$, for two different TF-QKD configurations.