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SBFoton International Optics and Photonics Conference (SBFoton IOPC)

Implementation of a Fiber-Based Quantum Key Distribution (QKD) Testbed Using Attenuated Laser Pulses

Bruno Souza, Franciele Renata Henrique, Marcio Daldin Teodoro, Celso J. Villas-Bôas, Filippo Giovanni Ghiglieno, Paulo Henrique Dias Ferreira

Published Sep 21, 2025• 0 citations
Publisher

Abstract

We report on the early-stage development of a fiber-based quantum key distribution (QKD) testbed using attenuated laser pulses designed to operate under the Coherent-One-Way protocol with decoy-state preparation, reaching photon levels$(\mu)$in the range of 0.1 to 1.0 photons/pulse through intensity modulation. In Coherent One Way (COW) protocol, logical bits are encoded in the temporal position of weak coherent pulses, and quantum coherence between successive pulses is monitored to detect eavesdropping. Single-photon detection is achieved with InGaAs SPADs, and calibrated attenuation ensures sub-photon regimes. This setup enables the study of channel loss, error rates, and secure key generation under realistic conditions.

DOI
10.1109/sbfotoniopc66433.2025.11218470

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We report on the early-stage development of a fiber-based quantum key distribution (QKD) testbed using attenuated laser pulses designed to operate under the Coherent-One-Way protocol with decoy-state preparation, reaching photon levels(μ)(\mu)(μ)in the range of 0.1 to 1.0 photons/pulse through intensity modulation. In Coherent One Way (COW) protocol, logical bits are encoded in the temporal position of weak coherent pulses, and quantum coherence between successive pulses is monitored to detect eavesdropping. Single-photon detection is achieved with InGaAs SPADs, and calibrated attenuation ensures sub-photon regimes. This setup enables the study of channel loss, error rates, and secure key generation under realistic conditions.
Abstract It’s well known that many of the traditional cryptographic schemes (like RSA and DH) will be vulnerable when scalable quantum computers will be available. However, quantum cryptographic protocols can provide unconditional security. One such protocol for unconditionally secure quantum key distribution (QKD) is coherent one way (COW) protocol. In the present work, we report experimental realization of an optical fiber based COW protocol for QKD in the telecom wavelength (1550 nm) where the attenuation in the optical fiber is minimum. A laser of 1550 nm wavelength, attenuator and intensity modulator is used for the generation of pulses having average photon number 0.5 and repetition rate of 500 MHz. The experiment is performed over 40 km, 80 km and 120 km of optical fiber and several experimental parameters like disclose rate, compression ratio, dead time and excess bias voltage of the detector are varied for all the cases (i.e., for 40 km, 80 km and 120 km distances) to observe their impact on the final key rate. Specifically, It is observed that there is a linear increase in the key rate as we decrease compression ratio or disclose rate. The key rate obtains its maximum value for least permitted values of disclose rate, compression ratio and dead time. It seems to remain stable for various values of excess bias voltage. While changing various parameters, we have maintained the quantum bit error rate (QBER) below 6% (5%) for distance ≥ 120 km (≤ 120 km). The key rate obtained is also found to remain stable over time. Experimental results obtained here are also compared with the earlier realizations of the COW QKD protocol. Further, to emulate key rate at intermediate distances and at a distance larger than 120 km, an attenuator of 5 dB loss is used which can be treated as equivalent to ∼ 25 km of the optical fiber used in the present implementation. This has made the present implementation equivalent to the realization of COW QKD upto ∼ 145 km.
Coherent One-Way (COW) is a simple and widely utilized quantum key distribution (QKD) protocol, favored in both experimental and commercial applications. In this paper, we compare the information that an eavesdropper, Eve, can acquire in both the 2-decoy and 1-decoy COW protocols. Our findings demonstrate that the 2-decoy protocol offers enhanced security compared to the original 1-decoy COW, all while preserving the simplicity of implementation. Furthermore, we have experimentally implemented the improved security COW-QKD (2-decoy protocol), achieving a secure key rate exceeding 0.5 kbps over a distance of 110 km (22 dB loss) in optical fiber, with an interference visibility greater than 80%. We also emphasize that monitoring the click rates of the data line detector for each state individually serves as a powerful method for detecting zero-error attacks.
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.
The imperfect components of a quantum key distribution (QKD) system will degrade its security. The highly attenuated weak coherent laser source used in today's QKD system is vulnerable to PNS attacks due to multi-photon optical pulses. Decoy State QKD, which is capable of beating PNS attacks and providing unconditional security, has been an attractive scheme recently. We have used a one way Faraday-Michelson phase modulated QKD system to implement 123km decoy state QKD in the lab. Furthermore a field decoy state QKD experiment of 32km dark fiber in Beijing is fulfilled. The optical pulse repetition rate is 1MHz and the final secure key is 9.59×10-7 (6.784×10-6) bit per pulse for 123km (32km). The experiment result indicates that the decoy state scheme can be successfully used in real-life metropolitan QKD sessions.
The decoy-state method could effectively enhance the performance of quantum key distribution (QKD) with a practical phase randomized weak coherent source. Although active modulation of the source intensity is effective and has been implemented in many experiments, passive preparation of decoy states is also an important addition to the family of decoy-state QKD protocols. In this paper, following the theory of Curty et al. [Phys. Rev. A 81, 022310 (2010)], we experimentally demonstrate the phase-encoding passive-decoy-state QKD with only linear optical setups and threshold single-photon detectors. In our experiment, two homemade independent pulsed lasers, with visibility of Hong-Ou-Mandel interference $0.53(\ifmmode\pm\else\textpm\fi{}0.003)$, have been implemented and used to passively generate the different decoy states. Finally, a secret key rate of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$/pulse is obtained with 10-km commercial fiber between Alice and Bob.
With the increasing importance of securing quantum communication networks, practical and robust entity authentication is a critical requirement. Accordingly, we propose and experimentally validate a quantum entity authentication (QEA) protocol specifically designed for integration with BB84-type quantum key distribution (QKD) workflows and existing terminal architectures. We analyze the protocol's security against intercept-resend man-in-the-middle (MitM) impersonation, showing that an unauthenticated adversary induces a characteristic 25% correlation error and that the rejection probability approaches unity as the number of detected authentication events increases. For practical realization, the protocol is deployed using weak coherent pulses (WCPs) with decoy-state estimation to bound single-photon contributions and mitigate photon-number-splitting (PNS)-enabled leakage. The system is demonstrated over a field-deployed fiber link of approximately 20 km with ~8 dB optical loss using signal/decoy intensities of ~0.5/~0.15 and sending probabilities 0.88/0.10/0.02 (signal/decoy/vacuum). Across both verification directions, stable operation is observed with quantum bit error rate (QBER) typically fluctuating between 1% and 4% while the sifted key rate remains constant over time. These results provide an experimental basis for integrating physical-layer entity authentication into deployed quantum communication networks.