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Conference on Lasers and Electro-Optics

Directly intensity-modulated quantum key distribution

George L. Roberts, Marco Lucamarini, J. F. Dynes, Seb J. Savory, Zhiliang L. Yuan, Andrew J. Shields

Published Jan 1, 2017• 0 citations
Publisher

Abstract

The coherent one-way (COW) protocol is implemented using direct laser modulation, with security enabled by optical injection locking. This method generates secure keys at rates above 1 Mbit/s with interference visibilities over 98 %.
DOI
10.1364/cleo_qels.2017.ftu4f.7

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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.
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The performance of quantum key distribution (QKD) is severely limited by multiphoton pulses emitted by laser sources due to the photon-number splitting attack. Coherent-one-way (COW) QKD has been introduced as a promising solution to overcome this limitation, and thus extend the achievable distance of practical QKD. Indeed, thanks to its experimental simplicity, the COW protocol is already used in commercial applications. Here, we derive simple upper security bounds on its secret key rate, which demonstrate that it scales at most quadratically with the system's transmittance, thus solving a long-standing problem. That is, in contrast to what has been claimed, this approach is inappropriate for long-distance QKD transmission. Remarkably, our findings imply that all implementations of the COW protocol performed so far are insecure.
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.
Due to the rapid advancement of quantum technology, the traditional established classical cryptographic protocols are no longer secure. To make the world quantum safe, different quantum protocols have been taken into account. Quantum Key Distribution (QKD) protocols are one of them where two legitimate parties can securely communicate by upholding the quantum principles. Out of various QKD protocols, Coherent One Way (COW) protocol is one of the most famous protocol because of its ease of hardware deployment, and resilience nature towards PNS attack. In this initiative, we have implemented the original version of two pulses COW QKD protocol and a very recent variant of it, three pulses COW (Phys. Rev. Applied 18, 064053 Published 19 December 2022), in Optisystem v21.1. We demonstrate the encoding as well as decoding portions of the protocols under both noisy and noiseless scenario considering different weather conditions. Finally, we report a comparative study amongst the protocols under wired (optical fibre) and wireless (free space) environments to check the proper integrity of transmission. The simulation results provide us an overview regarding the practical implementation of the protocols under different weather conditions.