[Paper Review] 600 km repeater-like quantum communications with dual-band stabilisation

Mirko Pittaluga, Mariella Minder+6 more|arXiv (Cornell University)|Dec 30, 2020

Quantum Information and Cryptography4 citations

TL;DR

This paper presents a dual-band phase stabilisation scheme for twin-field quantum key distribution (TF-QKD), using two distinct wavelengths to separate stabilisation and quantum signals. By eliminating Rayleigh scattering noise from high-power reference pulses, the method enables 605 km and 555 km secure key rates in asymptotic and finite-size regimes, respectively, achieving repeater-like performance with a 100-fold improvement in key rate over previous single-wavelength approaches at long distances.

ABSTRACT

Twin-field (TF) quantum key distribution (QKD) fundamentally alters the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the new opportunities for secure long-distance communications allowed by TF-QKD, formidable challenges remain to unlock its true potential. Previous demonstrations have required intense stabilisation signals at the same wavelength as the quantum signals, thereby unavoidably generating Rayleigh scattering noise that limits the distance and bit rate. Here, we introduce a novel dual-band stabilisation scheme that overcomes past limitations and can be adapted to other phase-sensitive single-photon applications. Using two different optical wavelengths multiplexed together for channel stabilisation and protocol encoding, we develop a setup that provides repeater-like key rates over record communication distances of 555 km and 605 km in the finite-size and asymptotic regimes respectively, and increases the secure key rate at long distance by two orders of magnitude to values of practical significance.

Motivation & Objective

• To overcome the fundamental limit of Rayleigh scattering noise in long-distance twin-field QKD, which arises when high-power stabilisation and quantum signals share the same wavelength.
• To enable practical, long-distance quantum key distribution by stabilising phase drift over 600 km of fibre without compromising signal-to-noise ratio.
• To develop a dual-wavelength stabilisation technique that decouples high-power reference signals from low-power quantum signals, thereby improving system performance and detector dynamic range.
• To demonstrate repeater-like key rates over 555 km (finite-size) and 605 km (asymptotic) in real-world optical fibre conditions.

Proposed method

• A dual-band stabilisation scheme multiplexes two optical wavelengths on a single fibre: one for high-power phase reference signals and another for quantum signals.
• The high-power reference signal stabilises the phase drift in real time, reducing residual drift by over 1000×, allowing low-power quantum signals to be used for final phase compensation.
• The two wavelengths are generated by independent lasers and separated using wavelength-division multiplexing, preventing Rayleigh backscattering from contaminating the quantum signal.
• The system uses a two-way classical communication (TWCC) protocol to correct errors and eliminate the quantum bit error rate floor, enabling long-distance key distribution.
• Phase matching is achieved through active feedback using the reference signal, while quantum signal detection occurs at the same wavelength as the stabilisation signal but at much lower intensity.
• The method is compatible with existing TF-QKD protocols, particularly the sending-or-not-sending (SNS) variant, and can be extended to other phase-sensitive single-photon applications.

Experimental results

Research questions

• RQ1 Can dual-wavelength stabilisation eliminate Rayleigh scattering noise in long-haul TF-QKD systems?
• RQ2 What is the maximum secure key rate achievable over 600 km of fibre using a dual-band stabilisation approach?
• RQ3 How does dual-band stabilisation compare to single-wavelength schemes in terms of key rate and error rate at long distances?
• RQ4 To what extent can the dual-band method extend the range of practical quantum key distribution beyond the current 509 km record?
• RQ5 Can the dual-band approach maintain high key rates while operating within the finite-size regime of practical QKD systems?

Key findings

• The system achieved a secure key rate of 1.777 bit/s over 555.172 km of quantum fibre, representing a 100-fold improvement in key rate compared to previous single-wavelength schemes at similar distances.
• In the asymptotic regime, the system demonstrated a secure key rate of 14.68 bit/s over 605 km, exceeding the PLOB bound by a factor of 63.4.
• The dual-band method reduced the phase drift by over 1000× compared to single-wavelength stabilisation, enabling stable operation over 600 km of fibre.
• The system achieved a Z-error rate of 1.22% after two-way classical communication, significantly below the 9.98% before correction, demonstrating effective error mitigation.
• The ratio of actual secure key rate to the PLOB bound reached 63.4 in the asymptotic regime, indicating near-optimal performance.
• The method enabled 605 km transmission with a key rate of 14.68 bit/s, setting a new record for repeater-like performance in TF-QKD.