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Continuous-variable quantum key distribution with true local oscillator

Brian P. Williams, Bing Qi, Muneer Alshowkan, Philip G. Evans, Nicholas A. Peters

Published Jan 1, 2023• 2 citations
Publisher

Abstract

In continuous variable quantum key distribution (CV-QKD), using a truly local (not transmitted over the network) oscillator improves security. Here we report the first implementation of this CV-QKD scheme over a deployed optical fiber network.
DOI
10.1364/cleo_fs.2023.ff1a.2

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In quantum secure communication, continuous-variable quantum key distribution (CV-QKD) using a true local oscillator (LO) located at the receiver has been proposed to remove side-channel-attack vulnerabilities and reduce excess noise, but implementations have been confined to the lab. The authors demonstrate CV-QKD with a receiver-based true LO over a deployed fiber network, with coexistent classical communications. This represents a substantial technical and engineering advance over prior tabletop demonstrations.
Continuous-variable quantum key distribution (CV-QKD) using a true local (located at the receiver) oscillator (LO) has been proposed to remove any possibility of side-channel attacks associated with transmission of the LO as well as reduce the cross-pulse contamination. Here we report an implementation of true LO CV-QKD using "off-the-shelf" components and conduct QKD experiments using the fiber optical network at Oak Ridge National Laboratory. A phase reference and quantum signal are time multiplexed and then wavelength division multiplexed with the classical communications which "coexist" with each other on a single optical network fiber. This is the first demonstration of CV-QKD with a receiver-based true LO over a deployed fiber network, a crucial step for its application in real-world situations.
Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on the laws of physics. Continuous-variable (CV) QKD with coherent states and coherent detection integrates well with existing telecommunication networks. Thus far, long-distance CV-QKD has only been demonstrated using a highly complex scheme where the local oscillator is transmitted, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100-kilometer fiber channel with a total loss of 15.4 decibels. This record-breaking distance is achieved by controlling the phase noise-induced excess noise through a machine learning framework for carrier recovery and optimizing the modulation variance. We implement the full CV-QKD protocol and demonstrate the generation of keys secure against collective attacks in the finite-size regime. Our results mark a substantial milestone for realizing CV quantum access networks with a high loss budget and pave the way for large-scale deployment of secure QKD.
In this study, we investigate the feasibility of utilizing optical injection locking with phase-locked loop feedback to generate a true local oscillator for continuous-variable quantum key distribution (CV-QKD). We evaluate the noise and imperfections associated with this novel approach and compare our findings with existing CV-QKD schemes. Notably, our calculations demonstrate significantly lower noise levels compared to other locally generated local oscillator-based CV-QKD schemes. Furthermore, we assess the secure key rate and achievable distance under collective attack for our proposed scheme and compare it to other variants of CV-QKD protocols employing a true local oscillator. Our results align well with experimental data, highlighting the promise of our approach.
Continuous-variable quantum key distribution (CV-QKD) is realized with coherent detection and is therefore very suitable for a cost-efficient implementation. The major challenge in CV-QKD is mitigation of laser phase noise at a signal to noise ratio of much less than 0 dB. So far, this has been achieved with a remote local oscillator or with auxiliary signals. For the first time, we experimentally demonstrate that CV-QKD can be performed with a real local oscillator and without auxiliary signals which is achieved by applying Machine Learning methods. It is shown that, with the most established discrete modulation protocol, the experimental system works down to a quantum channel signal to noise ratio of -19.1 dB. The performance of the experimental system allows CV-QKD at a key rate of 9.2 Mbit/s over a fiber distance of 26 km. After remote local oscillator and auxiliary signal aided CV-QKD, this could mark a starting point for a third generation of CV-QKD systems that are even more attractive for a wide implementation because they are almost identical to standard coherent systems.
We propose and demonstrate a fully passive discrete-state continuous-variable quantum key distribution (CV-QKD), which can eliminate all modulator side channels on the source side, using a local local oscillator. Using specially designed phase rotation and discretization methods, the CV-QKD system achieves a maximum fiber transmission length of 100 km at a repetition rate of 1 GHz in asymptotic security, with the corresponding secret key bit rate being 70.7 kbps. In composable security, secret keys are distributed at 25 km of fiber with a secret key bit rate of 1.66 Mbps. Our protocol significantly simplifies the architecture of CV-QKD system by eliminating the need for optical modulators and random number generators as well as presents robust practical security and superior performance, which is comparable to that of its active discrete-state counterpart. Overall, this protocol is expected to play an important role in quantum metropolitan area networks and quantum access networks requiring high realistic security.