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Overcoming the Phase Diffusion Limit in a Semiconductor Laser-based QRNG

Adam Brzosko, Yuen San Lo, P. R. Smith, Robert I. Woodward, Davide G. Marangon, J. F. Dynes, S. Juárez, Taofiq K. Paraïso, R. M. Stevenson, A. J. Shields

Published Jan 1, 2026• 0 citations
Publisher

Abstract

We report a novel technique of extracting randomness from the phase of optical pulses coming from a gain-switched Distributed Feedback (DFB) laser cavity beyond the phase diffusion limit, with a demonstration at 10 GHz.
DOI
10.1364/cleo_fs.2026.fm3a.2

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Gain-switching of 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 the time required for 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 substantially suppresses interpulse phase correlations, restoring 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.
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.