Plug-n-play Twin Field Quantum Key Distribution Achieves 1.5e-5 Bit Secure Key Rate with 87% Efficiency

Quantum Key Distribution (QKD) promises unconditionally secure communication, but practical implementation requires systems robust enough for real-world deployment, and Anagha Gayathri, Aryan Bhardwaj, Nilesh Sharma, et al. now demonstrate a significant advance in this field. The team presents an experimental QKD system based on a novel three-time-bin encoding method within a Twin-Field protocol, achieving stable performance over a 50km fibre optic channel. By cleverly utilising a Sagnac-based architecture, the researchers eliminate the need for complex active stabilisation, as the system self-compensates for phase and polarisation drifts, a major hurdle in long-distance quantum communication. Importantly, the system incorporates real-time phase-fluctuation monitoring, and achieves a secure key rate of approximately 1. 5e-5 bits per pulse with up to 87% visibility, demonstrating the practicality and scalability of this approach for future quantum communication networks.

This research explores a method for securely distributing cryptographic keys using the principles of quantum mechanics, guaranteeing security based on the laws of physics rather than computational complexity. TF-QKD is an advanced protocol that significantly improves key rate and transmission distance compared to traditional methods by separating the signal and local oscillator fields, reducing the impact of detector imperfections and signal losses. The star topology, where all nodes connect to a central node, offers advantages for scalability and simplifies the compensation for signal polarisation changes.

The star topology simplifies the addition of new users and inherently compensates for signal polarisation drift, a major challenge in fibre optic quantum communication. This architecture supports multiple users through orthogonal polarisation states, eliminating the need for direct links between them. By combining TF-QKD with the star topology, the team aimed to increase the key rate and transmission distance of quantum communication systems while mitigating the effects of signal backscattering, a significant source of noise in fibre optic cables. This innovative approach encodes two bits of information per signal by leveraging the relative phases of three consecutive time bins, significantly enhancing data transmission efficiency. A key innovation lies in the Sagnac loop configuration, which inherently compensates for both phase and polarisation drifts, eliminating the need for complex and costly active stabilisation systems typically required in quantum communication setups. This self-compensation dramatically improves the stability and practicality of the system for real-world deployments.

To counteract the effects of external vibrations and resulting rapid phase fluctuations, the team employed the first time bin for real-time phase-fluctuation monitoring, a technique that actively tracks and mitigates signal degradation. Experiments were conducted over a 50km asymmetric optical fibre channel, where the system consistently maintained a visibility of up to 87%, showcasing its resilience over significant distances. The experimental setup utilises a star-topology architecture, enabling a plug-and-play configuration that simplifies system integration and maintenance. Scientists harnessed this architecture to delegate the measurement operation to a third party, mitigating potential detector side-channel attacks and bolstering the security of the key exchange. This approach, inspired by Measurement Device Independent QKD, ensures the protocol’s integrity even with untrusted or potentially compromised measurement devices. By encoding two bits of information within the relative phases of three consecutive time bins, the team achieved a secure key rate of approximately 1. 5x 10⁻⁵ bits per pulse over a 50km fibre optic channel, with a visibility reaching 87%. A key achievement lies in the system’s inherent stability, as the Sagnac loop configuration provides self-compensation for both phase and polarisation drifts, eliminating the need for complex active stabilisation.

The researchers addressed the challenge of phase fluctuations caused by external vibrations by utilising the first time bin for real-time monitoring. This approach confirms the feasibility of building stable and scalable quantum communication networks capable of operating in real-world conditions. The research team’s innovative use of the Sagnac interferometer provides intrinsic self-compensation for phase fluctuations, a critical advancement for long-distance quantum communication. This work represents a significant step towards practical quantum communication, offering a robust and efficient solution for secure data transmission.