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. 2024 Jan 5;10(1):eadi9474.
doi: 10.1126/sciadv.adi9474. Epub 2024 Jan 3.

Long-distance continuous-variable quantum key distribution over 100-km fiber with local local oscillator

Adnan A E Hajomer  1 , Ivan Derkach  1   2 , Nitin Jain  1 , Hou-Man Chin  1   3 , Ulrik L Andersen  1 , Tobias Gehring  1
Affiliations

Long-distance continuous-variable quantum key distribution over 100-km fiber with local local oscillator

Adnan A E Hajomer et al. Sci Adv. .

Abstract

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.

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Figures

Fig. 1.
Fig. 1.. Long-distance continuous-variable–quantum key distribution (CV-QKD) system.
Alice’s station consists of a continuous-wave (CW) laser operating at 1550 nm, an in-phase and quadrature (IQ) modulator with an automatic bias controller (ABC) for producing coherent states at sideband frequencies. A digital-to-analog converter (DAC) with a resolution of 16 bits and a sampling rate of 1 gigasample/s was used to drive the IQ modulator. A variable optical attenuator (VOA) was used after the IQ modulator to adjust the modulation variance of the quantum signal. A Faraday isolator (FI), whose forward direction is indicated by the arrow, is used before a 100-km ultralow-loss fiber channel that constitutes the quantum channel. Bob’s station consists of a polarization controller (PC) to adjust the polarization of the incoming signal and a balanced beam splitter to overlap this signal with a LO generated from another CW laser (unlocked/free-running with respect to Alice’s laser). The signal was detected and digitized using a balanced detector (BD), followed by an analog-to-digital converter (ADC) with a sampling rate of 1 gigasample/s. EC, electric connection; SMF, single-mode fiber; PMF, polarization-maintaining fiber.
Fig. 2.
Fig. 2.. DSP routines of the long-distance local local oscillator (LLO) continuous-variable–quantum key distribution (CV-QKD) system.
See Materials and Methods for the details. a.u., arbitrary units.
Fig. 3.
Fig. 3.. Modulation variance optimization.
Experimentally obtained reconciliation efficiency β, frame error rate (FER), and asymptotic secret key rate (SKR) against the modulation variance Vmod.
Fig. 4.
Fig. 4.. Performance of long-distance continuous-variable–quantum key distribution (CV-QKD).
(A) The secret key rate (SKR) versus fiber channel length with an attenuation factor of 0.146 dB/km (taking into account additional coupling loss of 82%) in asymptotic (dashed) and finite-size (solid) regimes. Points correspond to experimentally achieved results. (B) Cumulative excess noise as a function of the number of acquired frames.
See this image and copyright information in PMC

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