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Long-haul high rate quantum key distribution

J. E. Nordholt

Published Jan 23, 2023• 0 citations
OpenAlex

Abstract

Multiple bit values can be encoded on a single photon in a quantum key distribution (QKD) system using a plurality of sidebands of an optical carrier frequency. Computational and conjugate bases can be defined, and photons decoded based on a selected state from either basis. If n sidebands are available, as many as log.sub.2n bits can be encoded on a single photon. Errors in detected bit values due to selection of an incorrect basis state or other errors can be at least partially corrected by bit distillation to identity bit strings for which a transmitter and a receiver record the same values, without insecure transmission of these values.

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Multiple bit values can be encoded on a single photon in a quantum key distribution (QKD) system using a plurality of sidebands of an optical carrier frequency. Computational and conjugate bases can be defined, and photons decoded based on a selected state from either basis. If n sidebands are available, as many as log.sub.2n bits can be encoded on a single photon. Errors in detected bit values due to selection of an incorrect basis state or other errors can be at least partially corrected by bit distillation to identity bit strings for which a transmitter and a receiver record the same values, without insecure transmission of these values.
In this work, a fully functional polarization-encoding quantum key distribution system (QKD) is demonstrated. At the transmitter side, the polarization states are prepared using an algorithm to determine the voltages to be applied on the electronic polarization controller (EPC), to generate up to four polarization states. At the receiver, a second EPC is used to perform the basis alignment. The symbol synchronization is achieved using a reference optical signal that is wavelength division multiplexed with the quantum signal which carries the polarization-encoded qubit. Frame synchronization is achieved at a protocol level. The presented QKD system works at a repetition rate of 500 Hz, with an average quantum bit error rate of 1.35%.
A novel, user-friendly quantum key distribution (QKD) system operating at a wavelength of 1550nm and at a clock rate of 10MHz was constructed to explore the compatibility of this emerging technology with the optical fiber network environment. Custom circuit boards providing the low-level control and sensing functions for both the transmitter and receiver were developed, allowing software-based system reconfiguration via USB interface to personal computers. The computer control allowed the user to change operating parameters such as detector bias voltages and pulse delays and also allowed for self-tuning of the system. Epitaxx avalanche photodiodes, operated in Geiger mode, were used to detect the single photons. A complete QKD protocol stack incorporating the sifting, reconciliation, privacy amplification, authentication and key confirmation functions was implemented in software. The system was tested over twenty five kilometers of dark underground fiber, producing 18.6 million sifted bits, with a sifted bit error rate of 4.9% at an average number of photons per pulse of 0.2, during a continuous 12-hour period of self-sustaining operation: a small portion of the secret bits distilled from each session's sifted bits were used to authenticate the next session. A total of 6.8 million shared secret bits were produced.
The binary (one-bit-per-photon) encoding that most existing quantum key distribution (QKD) protocols employ puts a fundamental limit on their achievable key rates, especially under high channel loss conditions associated with long-distance fiber-optic or satellite-to-ground links. Inspired by the pulse-position-modulation (PPM) approach to photon-starved classical communications, we design and demonstrate the first PPM-QKD, whose security against collective attacks is established through continuous-variable entanglement measurements that also enable a novel decoy-state protocol performed conveniently in post processing. We achieve a throughput of 8.0 Mbit/s (2.5 Mbit/s for loss equivalent to 25 km of fiber) and secret-key capacity up to 4.0 bits per detected photon, thus demonstrating the significant enhancement afforded by high-dimensional encoding. These results point to a new avenue for realizing high-throughput satellite-based or long-haul fiber-optic quantum communications beyond their photon-reception-rate limits.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.