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Quantum 2.0 Conference and Exhibition

Characterizing the performance of QKD transceiver platforms in a hybrid quantum network testbed

Samuel Fritsch, Andre Olearain, Alexander Kaufman, Wm. Randall Babbitt, Krishna Rupavatharam

Published Jan 1, 2023• 3 citations
Publisher

Abstract

A hybrid quantum network using multicore fibers, that supports both classical and quantum communications is implemented. Commercially available Quantum Key Distribution (QKD) systems were tested for loss tolerance, vulnerability to direct injection attacks, and the ability to generate secret keys in the presence of co-propagating classical communications was investigated. The loss of visibility, increase in the quantum bit error rate, and failure to generate secret keys is observed when an externally injected optical power typically exceeds -26 dBm. The dependence of the QKD operational vulnerability on copropagating signal wavelengths and implications for hybrid communications are discussed.

DOI
10.1364/quantum.2023.qw2a.2

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A hybrid quantum network using multicore fibers, that supports both classical and quantum communications is implemented. Commercially available Quantum Key Distribution (QKD) systems were tested for loss tolerance, vulnerability to direct injection attacks, and the ability to generate secret keys in the presence of co-propagating classical communications was investigated. The loss of visibility, increase in the quantum bit error rate, and failure to generate secret keys is observed when an externally injected optical power typically exceeds -26 dBm. The dependence of the QKD operational vulnerability on copropagating signal wavelengths and implications for hybrid communications are discussed.
The feasibility of a quantum-classical hybrid network using multicore fiber is explored and key performance metrics of the quantum channel with respect to co-propagating classical light are examined. Multiple commercial time bin quantum key distribution systems from ID Quantique are characterized using direct injection techniques to determine optimal operational parameters for the classical optical channels in order to minimize the quantum bit error rate and maximize secret key generation. A phenomenological model that determines the expected quantum bit error rate for a wide range of injected input wavelengths and powers is utilized to evaluate the quantum key distribution system performance. Analysis shows that the performance limitations exhibited by the tested commercial quantum key distribution systems in a hybrid network based on multicore fiber could be overcome by a strategic choice of the classical channel parameters.
The feasibility of a quantum-classical hybrid network using multicore fiber is explored and key performance metrics of the quantum channel with respect to co-propagating classical light are examined. Multiple commercial time bin quantum key distribution systems from ID Quantique are characterized using direct injection techniques to determine optimal operational parameters for the classical optical channels in order to minimize the quantum bit error rate and maximize secret key generation. A phenomenological model that determines the expected quantum bit error rate for a wide range of injected input wavelengths and powers is utilized to evaluate the quantum key distribution system performance. Analysis shows that the performance limitations exhibited by the tested commercial quantum key distribution systems in a hybrid network based on multicore fiber could be overcome by a strategic choice of the classical channel parameters.
Quantum key distribution (QKD) is increasingly considered for deployment in realistic communication networks, where long distances, heterogeneous fiber infrastructure, and coexistence with classical traffic present substantial challenges. Here, we demonstrate trusted-node QKD between Linköping University and the Stockholm hub of the Swedish national quantum communication infrastructure over 270 km of deployed single-mode fiber, extended by a 33 km multi-core fiber (MCF) segment emulating a metropolitan access link, for a total distance of 303 km. The two sub-links use commercial QKD systems whose receivers are interfaced with external superconducting nanowire single-photon detectors, enabling operation at losses beyond those supported by standard internal gated-mode detectors. We operate the link while actively switching the QKD channel between two MCF cores, with co-propagating Ethernet traffic and injected broadband optical noise in the other cores. The results demonstrate the integration of commercial QKD into demanding, dynamically reconfigurable fiber infrastructure relevant to future hybrid quantum-classical networks. Finally, using the generated secret keys, we illustrate how limited and time-varying QKD throughput affects one-time-pad-protected image transmission: image fidelity depends strongly on the available QKD-generated key budget and the choice of compression algorithm, highlighting application-level challenges for QKD-based encryption in realistic scenarios.
Quantum key distribution (QKD) guarantees unconditional security and has thus been intensely studied both from a theoretical and an experimental point of view [1]. However, limitations due to transmission loss and noise are still detrimental for these systems performance in terms of reachable distance and key rates, having so far confined QKD implementations to the use of dedicated channels and hence reduced a successful integration with the classical network, with few exceptions [2–6]. In this work we demonstrate a step forward in the co-existence of QKD with classical bright signals, exploiting a 7.9-km long 37-core heterogeneous multicore fiber (MCF) [7], and a space division multiplexing (SDM) technique to achieve a total of 87 Mbit/s secret key generation rate (SKR) in the finite key scenario with a simultaneous co-propagating classical signal of 10 Gbit/s.
Multicore-fibres (MCF) have emerged as an attractive solution for capacity scaling in optical communication. Securing such links with Quantum Key Distribution (QKD) is critical with the advent of Quantum computers. For practical deployment, QKD is required to operate alongside high-speed classical communication over field-deployed fiber infrastructure. In this work, the performance of QKD is experimentally evaluated in coexistence with high-speed classical data transmission over field-deployed underground and aerial multicore fiber links. The QKD system is implemented with Coherent One-Way Slim protocol, with dedicated Quantum and classical channels for Quantum communication and QKD post-processing. The deployed 4-core MCF infrastructure at IIT Madras provides an effective transmission length of 48 km through loopback operation. QKD performance over the MCF is evaluated in terms of quantum bit error rate (QBER) under varying attenuation, controlled noise injection, and inter-core crosstalk over both singlecore fiber and MCF links. The experimental results show that inter-core crosstalk has a stronger impact in MCF, leading to increased excess noise at the quantum receiver. For a 32-GBaud PM 16-QAM classical modulation format, stable QKD operation is maintained when the spectral separation is approximately six to eight times the occupied bandwidth.
The feasibility of transmitting discrete-variable quantum key distribution channels with carrier-grade classical optical channels over multicore fibers is experimentally explored in terms of achievable quantum bit error rates, secret key rates as well as classical signal bit error rates. A coexistence transmission record of 11.2 Tb/s is achieved for the classical channels simultaneously with a DV-QKD channel over a 1 km-long 7-core multicore fiber. Coexistence over the same multicore fiber core is identified as a dominant factor for the performance of the quantum channel requiring optical bandpass filtering of 17 nm for the quantum channel to avoid the effect of Raman noise. Also, counter-propagation of classical channels and quantum channels probe more tolerance to noise proliferation than co-propagation. In addition, the performance of the quantum channel is maintained when more than three cores are used for the classical channels. Furthermore, by adding a second DV-QKD channel in the multicore fiber, the simultaneous transmission of classical channels as well as the generation of quantum-secured keys of two QKD channels is achieved with an operational range of 10 dB of launched power into the MCF.