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Phase noise characterisation of a 2-km Hollow-Core Nested Antiresonant Nodeless Fibre for Twin-Field Quantum Key Distribution

Mariella Minder, Sophie Albosh, Obada Alia, Radan Slavik, Rupesh Kumar, Francesco Poletti, George Kanellos, Marco Lucamarini

TL;DR

The paper addresses the challenge of maintaining phase coherence for Twin-Field QKD over long-distance channels and evaluates hollow-core NANF as a potentially superior medium. The authors perform two 2-km experiments: parallel asymmetric Mach-Zehnder interferometers to quantify phase-noise in HCF versus SMF, and a TF-QKD-like interferometer to test practical viability. They show that phase-noise performance of the NANF is comparable to SMF across a broad frequency range, with low-frequency thermal fluctuations and high-frequency laser-noise dominated behavior that can be mitigated. The results, including a high-visibility TF-QKD demonstration and a low QBER of about 1.75% with 1 GHz pulses, suggest NANF as a promising platform for phase-based QKD, offering polarization stability and low nonlinearity suitable for multiplexing with classical channels.

Abstract

The performance of quantum key distribution (QKD) is heavily dependent on the physical properties of the channel over which it is executed. Propagation losses and perturbations in the encoded photons' degrees of freedom, such as polarisation or phase, limit both the QKD range and key rate. The maintenance of phase coherence over optical fibres has lately received considerable attention as it enables QKD over long distances, e.g., through phase-based protocols like Twin-Field (TF) QKD. While optical single mode fibres (SMFs) are the current standard type of fibre, recent hollow core fibres (HCFs) could become a superior alternative in the future. Whereas the co-existence of quantum and classical signals in HCF has already been demonstrated, the phase noise resilience required for phase-based QKD protocols is yet to be established. This work explores the behaviour of HCF with respect to phase noise for the purpose of TF-QKD-like protocols. To achieve this, two experiments are performed. The first, is a set of concurrent measurements on 2 km of HCF and SMF in a double asymmetric Mach-Zehnder interferometer configuration. The second, uses a TF-QKD interferometer consisting of HCF and SMF channels. These initial results indicate that HCF is suitable for use in TF-QKD and other phase-based QKD protocols.

Phase noise characterisation of a 2-km Hollow-Core Nested Antiresonant Nodeless Fibre for Twin-Field Quantum Key Distribution

TL;DR

The paper addresses the challenge of maintaining phase coherence for Twin-Field QKD over long-distance channels and evaluates hollow-core NANF as a potentially superior medium. The authors perform two 2-km experiments: parallel asymmetric Mach-Zehnder interferometers to quantify phase-noise in HCF versus SMF, and a TF-QKD-like interferometer to test practical viability. They show that phase-noise performance of the NANF is comparable to SMF across a broad frequency range, with low-frequency thermal fluctuations and high-frequency laser-noise dominated behavior that can be mitigated. The results, including a high-visibility TF-QKD demonstration and a low QBER of about 1.75% with 1 GHz pulses, suggest NANF as a promising platform for phase-based QKD, offering polarization stability and low nonlinearity suitable for multiplexing with classical channels.

Abstract

The performance of quantum key distribution (QKD) is heavily dependent on the physical properties of the channel over which it is executed. Propagation losses and perturbations in the encoded photons' degrees of freedom, such as polarisation or phase, limit both the QKD range and key rate. The maintenance of phase coherence over optical fibres has lately received considerable attention as it enables QKD over long distances, e.g., through phase-based protocols like Twin-Field (TF) QKD. While optical single mode fibres (SMFs) are the current standard type of fibre, recent hollow core fibres (HCFs) could become a superior alternative in the future. Whereas the co-existence of quantum and classical signals in HCF has already been demonstrated, the phase noise resilience required for phase-based QKD protocols is yet to be established. This work explores the behaviour of HCF with respect to phase noise for the purpose of TF-QKD-like protocols. To achieve this, two experiments are performed. The first, is a set of concurrent measurements on 2 km of HCF and SMF in a double asymmetric Mach-Zehnder interferometer configuration. The second, uses a TF-QKD interferometer consisting of HCF and SMF channels. These initial results indicate that HCF is suitable for use in TF-QKD and other phase-based QKD protocols.

Paper Structure

This paper contains 6 sections, 5 equations, 3 figures.

Table of Contents

  1. INTRODUCTION
  2. METHODS and Results
  3. Phase Noise Investigation
  4. Proof-of-concept TF-QKD experiment
  5. Conclusion
  6. Calculation of the power spectral density

Figures (3)

  • Figure 1: Schematics of the experimental setup. (a) Parallel AMZIs built side by side for testing a nested anti-resonant nodeless hollow core fibre (NANF), indicated as 'HCF' in the figure and in the text, in comparison with an SMF. The core structure of the HCF is represented in the expanded diagram. The setup consists of a Rio Orion laser module, 50:50 polarisation maintaining beam splitters, two 2 km spools, one for the HCF and one for the SMF (Thorlabs SMF-28-1000), variable optical attenuators (Att.), polarisation controls (PC), linear polarisers (LP), power meters (Pow. M), fast photodetectors and a 36 GHz oscilloscope (Fast Osc.). (b) and (c) traces of the photodetector voltage at the output of the HCF and SMF AMZIs, recorded on the fast oscilloscope with a sampling frequency of 200 kS/s.
  • Figure 2: Experimental PSD characterisation. (a) The phase noise PSDs of the SMF and HCF AMZIs, calculated using Welch's method and a Hann window with a width $1\times 10^6$ samples. (b) The frequency noise PSDs of the SMF and HCF AMZIs calculated from the phase noise PSDs.
  • Figure 3: TF-QKD setup and interference. (a) The test TF-QKD experimental setup, consisting of CW laser, intensity modulator (IM), a phase modulator (PM) on the HCF path and a delay line (DL) on the SMF path, polarisation controls (PC), variable optical attenuators (VOA), power meters (Pow. M), polarisation beam splitters (PBS) and detectors (DT). (b) The count rate recorded over 250 ms on the SNSPD when no phase modulation is applied (upper panel); the derived phase drift (middle panel) and phase drift rate (lower panel) over the first 25 ms of the sample. (c) The count rate recorded over 20 ms when 0 and $\pi$ phase modulation is applied on the PM on the HCF arm.