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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Optical Injection-Induced Timing Jitter Reduction in Gain-Switched 1550-nm Discrete Mode Semiconductor Lasers

María Duque Gijón, Ana Quirce, Jordi Tiana‐Alsina, Angel Valle A, Cristina Masoller

Published Jun 26, 2023• 0 citations
Publisher

Abstract

Advanced laser modulation techniques have enabled the development of versatile and efficient light sources for quantum communications [1]. In fact, semiconductor lasers are normally used as single-photon sources in most commercial and research quantum key distribution (QKD) systems [2]. In particular, modulated single-mode semiconductor lasers, utilizing gain-switching, optical injection locking and direct phase modulations have been used to implement a great variety of QKD protocols [1]. In recently introduced protocols, like the measurement-device-independent one (MDI) based on the interference of pulses generated by two remote semiconductor laser sources, timing-jitter in gain-switched lasers affects the temporal overlap of the pulses producing a dramatic reduction in the interference visibility [1]. Very recently MDI-QKD systems using optical injection from a semiconductor laser into a gain-switched semiconductor laser have been demonstrated [1]. A reduction of timing-jitter induced by optical injection is then desirable to improve the interference visibility in this type of MDI-QKD systems.

DOI
10.1109/cleo/europe-eqec57999.2023.10232500

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Advanced laser modulation techniques have enabled the development of versatile and efficient light sources for quantum communications [1]. In fact, semiconductor lasers are normally used as single-photon sources in most commercial and research quantum key distribution (QKD) systems [2]. In particular, modulated single-mode semiconductor lasers, utilizing gain-switching, optical injection locking and direct phase modulations have been used to implement a great variety of QKD protocols [1]. In recently introduced protocols, like the measurement-device-independent one (MDI) based on the interference of pulses generated by two remote semiconductor laser sources, timing-jitter in gain-switched lasers affects the temporal overlap of the pulses producing a dramatic reduction in the interference visibility [1]. Very recently MDI-QKD systems using optical injection from a semiconductor laser into a gain-switched semiconductor laser have been demonstrated [1]. A reduction of timing-jitter induced by optical injection is then desirable to improve the interference visibility in this type of MDI-QKD systems.
Quantum key distribution (QKD) allows two users to communicate with theoretically provable secrecy [1]. This is vitally important to secure the confidential data of governments, businesses and individuals. As the technology is adopted by a wider audience, a quantum network will become necessary for multi-party communication, as in the classical communication networks in use today. Unfortunately, a number of phase-encoded QKD protocols based on weak coherent pulses have been developed. Whilst the first protocol, proposed by Bennett and Brassard in 1984 (BB84), is still commonly used, other protocols such as differential phase shift [2] or coherent one way QKD [3] are also adopted. Each protocol has its benefits; however all would require a different transmitter and receiver, increasing the complexity and cost of quantum networks. In this work we demonstrate a multi-protocol transmitter [4-6] that also has the benefits of small footprint, low power consumption and low complexity. We use this transmitter to give the first experimental demonstration of an improved version of the BB84 protocol, known as the differential quadrature phase shift protocol. We have achieved megabit per second secure key rates at short distances, and have shown secure key rates that are, on average, 2.71 times higher than the standard BB84 protocol. This enhanced performance over such a commonly adopted protocol, at no expense to experimental complexity, could lead to a widespread migration to the new protocol. The security of the BB84 protocol relies on each signal and reference pulse pair being globally phase randomised with respect to all other pulse pairs. In the DQPS protocol, blocks with a length L ≥ 2 are used and each block has a globally random phase with respect to all other blocks. Implementing this protocol would ordinarily require a high-speed random number generator and a phase modulator. As well as increasing device complexity, it would also require an unrealistic continuous source of electrical modulation signals for complete security. The transmitter we use injects light from a master laser diode into a 2 GHz gain-switched slave laser diode. The principal of optical injection locking means that the slave laser inherits the phase of the master laser. We apply modulations to the master laser current within a block to control the phase of the slave laser output pulses, and then drive the master laser below threshold for a short period of time when phase randomisation is required. This ensures the lasing comes from below threshold, thus the phase adopted by the slave laser pulse is completely random. We perform an autocorrelation measurement on the blocks to show their randomness. [1] N. Gisin et al. Rev. Mod. Phys. 74, 145 (2002). [2] K. Inoue et al. Phys. Rev. Lett. 89, 037902 (2002). [3] D. Stucki et al. Appl. Phys. Lett. 87 194108 (2005). [4] Z. Yuan et al. Phys. Rev. X. 6, 031044 (2016). [5] G. L. Roberts et al. Laser Phot. Rev. 11, 1700067 (2017). [6] G. L. Roberts et al. arXiv:1709.04214 [quant-ph] (2017).
Abstract Measurement-device-independent quantum key distribution (MDI-QKD) is a technique for quantum-secured communication that eliminates all detector side-channels, although is currently limited by implementation complexity and low secure key rates. Here, we introduce a simple and compact MDI-QKD system design at gigahertz clock rates with enhanced resilience to laser fluctuations—thus enabling free-running semiconductor laser sources to be employed without spectral or phase feedback. This is achieved using direct laser modulation, carefully exploiting gain-switching and injection-locking laser dynamics to encode phase-modulated time-bin bits. Our design enables secure key rates that improve upon the state of the art by an order of magnitude, up to 8 bps at 54 dB channel loss and 2 kbps in the finite-size regime for 30 dB channel loss. This greatly simplified MDI-QKD system design and proof-of-principle demonstration shows that MDI-QKD is a practical, high-performance solution for future quantum communication networks.
Since the introduction of the decoy-state technique, phase-randomized weak coherent light pulses have been the key to increase the practicality of quantum-based communications. Their ultrafast generation was accomplished via compact gain-switched (GS) lasers, leading to high key rates in quantum key distribution (QKD). Recently, the question arose whether the same laser could be employed to achieve high-speed measurement-device-independent QKD, a scheme that promises long-haul quantum communications immune to all detector attacks. For that, a challenging high-visibility interference between independent picosecond optical pulses is required. Here, we answer the above question in the affirmative by demonstrating high-visibility interference from two independent GS lasers triggered at 1 GHz. The result is obtained through a careful characterization of the laser frequency chirp and time jitter. By relating these quantities to the interference visibility, we obtain a parameter-free verification of the experimental data and a numerical simulation of the achievable key rates. These findings are beneficial to other applications making use of GS lasers, including random number generation and standard QKD.
Direct phase modulation via optical injection is a newly developed method for coding the phase of a gain-switched laser, which meets high requirements placed on transmitters for quantum key distribution: compactness, low losses, compatibility with CMOS technologies, and the absence of undesirable effects leading to the side-channel information leakage. Despite the successful implementation and good prospects for the further development of this system, there is still a lack of theoretical investigations of this scheme in the literature. Here, for the first time, we perform its theoretical analysis. We study the influence of the spontaneous emission noise, examine the role of the gain non-linearity, and consider the temperature drift effect. The results obtained reveal that these phenomena significantly affect system performance. We have tried to formulate here practical instructions, which will help to take these features into account when elaborating and employing the optical-injection-based phase modulator.