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Fast, high quality random numbers from irradiated Josephson junctions

Avery Thompson

Published Oct 14, 2025• 0 citations
Publisher

Abstract

High frequency radio waves applied to Josephson junctions produce chaotic noise that could improve cryptographic algorithms and Monte Carlo simulations.
DOI
10.1063/10.0039653

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Recently, a high-quality random-number generator has been highly expected. In obtaining high-quality random-numbers, the use of random physical phenomena is necessary. Among random-number generation methods, we focused on the chaos in a Josephson junction (JJ) under irradiation with a radio frequency (RF). In this study, the chaos in the intrinsic JJ for random-number generation was numerically investigated using the equivalent circuit of the RF-irradiated JJ. In particular, it is important for the random-number generation to obtain a high-quality white-noise output signal. We proposed that the quality of the white noise is quantitatively evaluated using the half-width of the autocorrelation peak. Furthermore, random-numbers were generated from the chaos in the RF-irradiated JJ at high-speed operation than the previous study. The high-quality random-numbers were successfully generated from this chaos, and their quality was verified through statistical verification.
The configuration of a robust, high-security system has become increasingly important because, as computer processing speeds continue to improve, the vulnerabilities of cryptographic systems that rely on conventional pseudo-random-numbers have raised concerns regarding potential leaks of confidential information. To establish such a secure system, high-quality and high-speed random numbers are essential. Here, high-quality random-numbers are defined as statistically unbiased, non-reproducible, and unpredictable values. One approach to generating such numbers is to exploit physical random phenomena, such as chaos. In particular, chaos in Josephson junctions (JJs) under radio-frequency (RF) irradiation has been identified as a suitable mechanism for random-number generation. In this study, we considered RF-biased intrinsic Josephson junctions (IJJs) in a mesa-type Bi2_{2}2​Sr2_{2}2​CaCu2_{2}2​O8+δ_{8+\delta }8+δ​single crystal. Since the interactions among stacked IJJs must be considered, it is insufficient to evaluate their properties using the conventional resistively and capacitively shunted junction (RCSJ) circuit model. Instead, we numerically investigated the chaotic voltage of an IJJ stack irradiated with higher frequencies than those typically used in conventional RF systems, employing the capacitively coupled JJ model with diffusion current, which incorporates interlayer interactions. Based on the time-series output voltage of the IJJ, we then generated random-numbers and evaluated their quality using statistical tests.
We report on the study of the phase dynamics of high-critical-temperature superconductor Josephson junctions. We realized YBa2{}_{2}2​Cu3{}_{3}3​O${}_{7\ensuremath{-}x}$ grain boundary biepitaxial junctions in the submicron scale using low-loss substrates and analyzed their dissipation by comparing the transport measurements with Monte Carlo simulations. The behavior of the junctions can be fitted using a model based on two quality factors, which results in a frequency-dependent damping. Moreover, our devices can be designed to have Josephson energy of the order of the Coulomb energy. In this unusual energy range, phase delocalization strongly influences the device's dynamics, promoting the transition to a quantum phase diffusion regime. We study the signatures of such a transition by combining the outcomes of Monte Carlo simulations with the analysis of the device's parameters, the critical current, and the temperature behavior of the low-voltage resistance R0{R}_{0}R0​.