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arXiv:2601.04031 · APL Photonics

Phase-Randomized Laser Pulse Generation at 10 GHz for Quantum Photonic Applications

Yuen San Lo, Adam H. Brzosko, Peter R. Smith, Robert I. Woodward, Davide G. Marangon, James F. Dynes, Sergio Juárez, Taofiq K. Paraïso, R. Mark Stevenson, Andrew J. Shields

quant-phSubmitted Jan 7, 2026• Announced Jan 8, 2026• 1 citation

v1 · Jan 7, 2026 v2 · Jan 8, 2026 v3 · Aug 20, 2026

Abstract

Gain-switching laser diodes is a well-established technique for generating optical pulses with random phases, where the quantum randomness arises naturally from spontaneous emission. However, the maximum switching rate is limited by phase diffusion: at high repetition rates, residual photons in the cavity seed subsequent pulses, leading to phase correlations, which degrade randomness. We present a method to overcome this limitation by employing an external source of spontaneous emission in conjunction with the laser. Our results show that this approach effectively removes interpulse phase correlations and restores phase randomization at repetition rates as high as 10 GHz. This technique opens new opportunities for high-rate quantum key distribution and quantum random number generation.

Journal reference: APL Photonics 1 August 2026; 11 (8): 086111
DOI: 10.1063/5.0321309
Comments: 16 pages, 5 figures. The following article has been submitted to APL Photonics. Copyright 2026 Authors. This article is distributed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) License. https://creativecommons.org/licenses/by-nc/4.0/

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Phase randomization is essential for the security of practical decoy-state quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources which are either actively phase-randomized, or gain-switched. However, at high repetition rates these show correlations, which can ultimately compromise security and performance. We present a 1.25 GHz phase-randomized QKD source based on a super-luminescent light emitting diode (SLED) operating in the C-band as a compact and cost-effective alternative. The source generates

\sim100

ps optical pulses with

400

ps pulse-to-pulse separation, compatible with high-speed time-bin encoding. Interferometric measurements demonstrate

>99\%

visibility between adjacent time bins, confirming strong first-order coherence within the same quantum signals, while the spontaneous-emission-driven nature of the SLED ensures intrinsic global phase randomization between adjacent signals. This work establishes a scalable SLED-based platform for high-speed prepare-and-measure QKD systems.

Gain-switched lasers are in demand in numerous quantum applications, particularly in systems of quantum key distribution and in various optical quantum random number generators. The reason for this popularity is natural phase randomization between gain-switched laser pulses. The idea of such randomization has become so familiar that most authors use it without regard to the features of the laser operation mode they use. However, at high repetition rates of laser pulses or when pulses are generated at a bias current close to the threshold, the phase randomization condition may be violated. This paper describes theoretical and experimental methods for estimating the degree of phase randomization in a gain-switched laser. We consider in detail different situations of laser pulse interference and show that the interference signal remains quantum in nature even in the presence of classical phase drift in the interferometer provided that the phase diffusion in a laser is efficient enough. Moreover, we formulate the relationship between the previously introduced quantum reduction factor and the leftover hash lemma. Using this relationship, we develop a method to estimate the quantum noise contribution to the interference signal in the presence of phase correlations. Finally, we introduce a simple experimental method based on the analysis of statistical interference fringes, providing more detailed information about the probabilistic properties of laser pulse interference.