Josephson junctions are among the cleanest—and most versatile—nonlinear systems available in solid-state physics. At the same time, they are inherently exposed to fluctuations: thermal noise, electromagnetic noise from the environment, and sometimes strongly non-Gaussian disturbances. In this research line, I use Josephson devices as a quantitative laboratory for nonequilibrium stochastic physics, focusing on how rare events (switching, escape, phase slips, soliton nucleation) emerge, how they can be predicted, and how they can be exploited as diagnostic and sensing mechanisms.
A unifying perspective is that many relevant observables are not well described by averages. The physically meaningful information often sits in the full probability distributions—their widths, asymmetries, and especially their tails. Those tails encode the physics of extreme fluctuations and determine device reliability, false-trigger rates in detectors, and the stability boundaries of superconducting circuits operating near thresholds.
In the resistively and capacitively shunted junction (RCSJ) picture, the phase behaves like a particle in a tilted washboard potential. A junction biased close to a metastable state can remain trapped for a long time and then suddenly escape, producing a measurable voltage event. This “escape problem” is a cornerstone of my work, but with two twists:
I study escape and switching statistics not only for thermal (Gaussian) activation, but also for colored noise and non-Gaussian sources.
I treat switching observables as tools: by looking at switching-time distributions, switching-current histograms, and residence-time statistics, one can infer properties of the noise source and of the underlying dynamics.
This viewpoint is very powerful for students: it turns a seemingly technical question (“why does it switch?”) into an experimentally accessible route to learn about stochastic processes in a strongly nonlinear system.
A major thrust of my activity concerns Lévy and α-stable noise, where rare, large fluctuations dominate the dynamics. In these regimes, the usual intuition from Gaussian noise breaks down: the “typical” fluctuation is no longer the one that drives activation. This has deep consequences for:
the shape of switching distributions (especially the tails),
the scaling of mean escape times with bias and noise intensity,
and the optimal observables for detection and inference.
Beyond the fundamental interest, Lévy-driven switching suggests concrete device concepts: Josephson junctions can behave as threshold detectors for heavy-tailed fluctuations, with signatures that can be extracted from voltage-drop statistics or switching histograms. This connection between stochastic theory and device-oriented diagnostics is a recurring theme across my publications in this area.
When the junction is long, the phase becomes a field and the dynamics is governed by (stochastic) sine-Gordon physics. This opens a rich playground where noise interacts with spatially extended nonlinear excitations:
fluxons/solitons (kinks) whose motion and stability are noise-sensitive,
breathers, i.e., localized oscillatory modes that can be generated, stabilized, locked, and transported under driving and dissipation,
and spatially heterogeneous noise environments, where the “where” of fluctuations matters as much as the “how much.”
Here, I focus on mechanisms and statistics: how do breathers emerge under realistic forcing, what sets their lifetime, how does correlated noise reshape their stability landscape, and which experimentally measurable signatures reveal their presence. These questions also provide natural bridges to caloritronics and to circuit applications, where dynamical states can become controllable operational modes.
While noise is commonly seen as detrimental, nonlinear systems can display counterintuitive behavior such as noise-enhanced stability and noise-assisted metastability. Part of my work investigates when and why fluctuations can increase lifetimes or stabilize specific dynamical states, and how this can be leveraged to:
enlarge safe operating regions of superconducting devices,
create robust dynamical modes (e.g., breathers) under realistic dissipation,
and improve discrimination strategies in threshold-based detection.
This “useful noise” mindset is also relevant whenever one aims at sensing extremely weak signals on top of uncontrolled backgrounds: understanding the statistics is what allows you to set decision thresholds intelligently.
The escape-time toolbox naturally connects to sensing and detection. If a weak signal slightly reshapes the effective barrier or the phase dynamics, it can produce a measurable change in switching statistics. This logic underpins several applications, including rare-event strategies for weak-signal searches (where one cares about tail events and false-alarm control) and resonant-activation ideas in superconducting platforms.
Methodologically, I combine:
Analytical stochastic modeling (escape rates, effective potentials/quasi-potentials, colored vs. non-Gaussian noise, rare-event scaling);
Numerical simulations of stochastic differential equations (RCSJ models) and stochastic field equations (sine-Gordon dynamics in long junctions);
Data-driven statistical analysis (distributions, tails, extreme events, parameter inference), often with a direct eye to experimental observables (switching histograms, lifetime distributions, voltage traces).
I’m happy to discuss collaborations with theorists and experimentalists working on superconducting electronics, noise spectroscopy, and nonlinear dynamics. Here are some concrete project formats that work well for an MSc/PhD thesis or a collaboration:
Inferring non-Gaussian noise from switching statistics
Build an inference pipeline that discriminates thermal/colored vs. Lévy-like noise from measured switching-current or switching-time datasets.
Noise-assisted control of breathers in long junctions
Map stability “phase diagrams” (drive, damping, noise color/intensity) for breather generation and locking; propose experimental protocols for robust initialization and readout.
Rare-event detection metrics for weak signals
Compare decision rules (escape-time thresholding, likelihood-ratio tests on full distributions, tail-based estimators) and optimize signal-to-noise in realistic noise backgrounds.
Spatially structured noise in extended Josephson systems
Explore how spatial correlations or inhomogeneous noise sources reshape soliton/breather nucleation and transport, and identify robust signatures for experiments.
B. Spagnolo, A. Bayat, C. Guarcello, Editorial: Special Issue on Nonequilibrium statistical mechanics: Methods, applications and new trends, Chaos, Solitons & Fractals 202, 117604 (2026).
D. De Santis, B. Spagnolo, G. Di Fresco, A. Carollo, C. Guarcello, Noisy sine-Gordon breather dynamics: A short review, Chaos, Solitons & Fractals 199, 116641 (2025).
A. A. Dubkov, C. Guarcello, B. Spagnolo, Enhancement of stability of metastable states in the presence of Lévy noise, SciPost Phys. 18, 006 (2025).
G. Di Fresco, D. De Santis, C. Guarcello, B. Spagnolo, A. Carollo, D. Valenti, Effects of correlated noise on the excitation of robust breathers in an ac-driven, lossy sine-Gordon system, Chaos, Solitons & Fractals 189, 115678 (2024).
C. Guarcello, G. Filatrella, D. De Santis, B. Spagnolo, D. Valenti, Lévy noise-induced effects in a long Josephson junction in the presence of two spatial noise distributions, Chaos, Solitons & Fractals 187, 115421 (2024).
D. De Santis, C. Guarcello, B. Spagnolo, A. Carollo, D. Valenti, Noise-induced, ac-stabilized sine-Gordon breathers: Emergence and statistics, Commun. Nonlinear Sci. Numer. Simul. 131, 107796 (2024).
D. De Santis, C. Guarcello, B. Spagnolo, A. Carollo, D. Valenti, ac-locking of thermally induced sine-Gordon breathers, Chaos, Solitons & Fractals 170, 113382 (2023).
D. De Santis, C. Guarcello, B. Spagnolo, A. Carollo, D. Valenti, Breather dynamics in a stochastic sine-Gordon equation: evidence of noise-enhanced stability, Chaos, Solitons & Fractals 168, 113115 (2023).
D. De Santis, C. Guarcello, B. Spagnolo, A. Carollo, D. Valenti, Generation of traveling sine-Gordon breathers in noisy long Josephson junctions, Chaos, Solitons & Fractals 158, 112039 (2022).
C. Guarcello, Lévy noise effects on Josephson junctions, Chaos, Solitons & Fractals 153, 111531 (2021).
C. Guarcello, G. Filatrella, B. Spagnolo, V. Pierro, D. Valenti, Voltage drop analysis on Josephson junctions for Lévy noise detection, Phys. Rev. Research 2, 043332 (2020).
C. Guarcello, D. Valenti, B. Spagnolo, V. Pierro, G. Filatrella, Josephson-based threshold detector for Lévy distributed current fluctuations, Phys. Rev. Applied 11, 044078 (2019).
C. Guarcello, D. Valenti, A. Carollo, B. Spagnolo, Effects of Lévy noise on the dynamics of sine-Gordon solitons in long Josephson junctions, J. Stat. Mech.: Theory Exp. 054012 (2016).
C. Guarcello, D. Valenti, B. Spagnolo, Stabilization effects of dichotomous noise on the lifetime of the superconducting state in a long Josephson junction, Entropy 17, 2862 (2015).
D. Valenti, C. Guarcello, B. Spagnolo, Switching times in long-overlap Josephson junctions subject to thermal fluctuations and non-Gaussian noise sources, Phys. Rev. B 89, 214510 (2014).