Generating the local oscillator "locally" in continuous-variable quantum key distribution based on coherent detection

Abstract: Continuous-variable quantum key distribution (CV-QKD) protocols based on coherent detection have been studied extensively in both theory and experiment. In all the existing implementations of CV-QKD, both the quantum signal and the local oscillator (LO) are generated from the same laser and propagate through the insecure quantum channel. This arrangement may open security loopholes and also limit the potential applications of CV-QKD. In this paper, we propose and demonstrate a pilot-aided feedforward data recovery scheme which enables reliable coherent detection using a "locally" generated LO. Using two independent commercial laser sources and a spool of 25 km optical fiber, we construct a coherent communication system. The variance of the phase noise introduced by the proposed scheme is measured to be 0.04 (rad^2), which is small enough to enable secure key distribution. This technology also opens the door for other quantum communication protocols, such as the recently proposed measurement-device-independent (MDI) CV-QKD where independent light sources are employed by different users.

• The paper introduces a pilot-aided feedforward scheme that enables secure local oscillator generation with minimal phase noise.
• Using two independent commercial lasers over a 25 km fiber, the experiment achieved a phase noise variance of 0.04 rad², validating system viability.
• The technique mitigates security risks by eliminating the need to transmit the local oscillator, offering a practical upgrade to standard CV-QKD implementations.

Generating Local Oscillators Locally in Continuous-Variable Quantum Key Distribution

The paper "Generating the local oscillator 'locally' in continuous-variable quantum key distribution based on coherent detection" addresses a significant longstanding issue in continuous-variable quantum key distribution (CV-QKD): the generation of the local oscillator (LO) at the receiver's side rather than transmitting it through an insecure channel. Traditional CV-QKD systems transmit both the quantum signal and the LO from the sender (Alice) to the receiver (Bob) through the same channel, which can introduce security vulnerabilities. The authors propose a novel pilot-aided feedforward data recovery scheme that enables the generation of the LO locally at Bob's end using an independent laser source, while still permitting reliable coherent detection.

Numerical Results and Methodology

Utilizing two independent commercial laser sources and a 25 km optical fiber, the authors implemented a coherent communication system as a demonstration. The phase noise variance introduced by their scheme was recorded as 0.04 (rad$^2$), demonstrating that this variance is sufficiently small to maintain secure key distribution under real-world operating conditions. The robust performance observed in this experimental setup underlines the feasibility of local LO generation without compromising security.

Security and Implementation Implications

By allowing the LO to be generated locally, the proposed method alleviates several limitations of traditional CV-QKD systems. Key among these is the risk mitigation of an adversary manipulating the LO, an issue highlighted in recent literature. Furthermore, eliminating the need to transmit a strong LO through a lossy channel enhances efficiency, as it sidesteps the high photon number requirements that are typically necessitated at the receiver's end.

The authors convincingly argue that their phase recovery scheme is operationally equivalent to conventional CV-QKD, with established security proofs remaining applicable. This equivalence is elucidated through a detailed security analysis showing that the phase information recovery via the proposed scheme incurs minimal excess noise, stepping well within the tolerable bounds for secure communication.

Potential Future Developments

The research presented here could guide future iterations of CV-QKD systems, particularly aspiring to integrate with existing communication infrastructures where signal and LO multiplexing pose constraints. Importantly, by demonstrating the utility of the feedforward technique and local LO generation, this work may pave the way for the practical realization of other quantum protocols, such as measurement-device-independent (MDI) CV-QKD.

In sum, the authors provide a conceptually elegant and experimentally validated solution to a core issue in the field of CV-QKD. Their use of commercially viable components and demonstration on lengths typical in existing optical infrastructures underscore the practicality of their approach. While further refinements, particularly in reducing phase noise, could improve system performance, this study represents a meaningful advancement in the secure and efficient deployment of quantum communications technology. Future innovations in laser technology, such as reduced linewidth lasers, could further enhance the durability and applicability of this protocol, with significant implications for the burgeoning field of secure quantum communications.