Phase randomness in a semiconductor laser: Issue of quantum random-number generation

TL;DR

This paper investigates the conditions under which phase randomization in gain-switched semiconductor lasers is maintained or violated, providing theoretical and experimental methods to estimate quantum noise contributions in quantum random number generation.

Contribution

It introduces new methods to estimate phase randomization and quantum noise in gain-switched lasers, especially under high repetition rates and near-threshold bias conditions.

Findings

Interference signals remain quantum with sufficient phase diffusion.

Phase randomization can be compromised at high pulse repetition rates.

A new experimental method analyzes statistical interference fringes for better insights.

Abstract

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.