Modern superconducting quantum hardware—whether it is a qubit processor, a microwave photon counter, or a cryogenic detector array—lives or dies by its readout chain. The signals of interest are extremely weak (often at the single-photon level), and the first amplifier in the chain largely sets the overall noise temperature and thus the achievable sensitivity. My research in this area focuses on Josephson-based parametric amplifiers, with particular emphasis on traveling-wave architectures, and on the nonlinear dynamics that ultimately determines their gain, bandwidth, stability, and usefulness in real experiments.
Traditional lumped-element Josephson parametric amplifiers can reach quantum-limited performance but are often bandwidth-limited and can saturate at relatively low powers. Traveling-wave Josephson parametric amplifiers (TWJPAs/JTWPAs) address these constraints by distributing the nonlinearity along a transmission line: instead of a single resonant element, amplification occurs continuously over an extended structure. In practice, this enables the combination of:
broadband gain,
high dynamic range,
and compatibility with multiplexed readout architectures.
A recurring goal of my work is to move from “a device that amplifies” to “a device that amplifies predictably”, i.e., an amplifier whose performance can be understood and engineered from microscopic junction physics up to circuit-level behavior.
Even though the basic parametric process can be described in terms of wave mixing, real TWJPAs are complex nonlinear systems. My activity addresses several tightly connected questions:
1) Phase matching and dispersion engineering
A parametric amplifier is only as good as its phase matching. In a traveling-wave device, dispersion can prevent the pump, signal, and idler from staying in the right phase relationship, reducing gain and introducing strong frequency dependence. I work on modeling and optimizing phase-matching strategies, including resonant phase matching schemes that use engineered resonant elements to tailor the dispersion while controlling spurious resonances and reflections. The aim is to deliver robust gain profiles that remain stable under realistic parameter spreads.
2) Non-sinusoidal current–phase relations (CPR) and higher harmonics
Many practical junction implementations deviate from the ideal sinusoidal CPR. In distributed amplifiers, higher harmonics in the CPR effectively introduce additional nonlinear channels that can reshape the amplification process—sometimes beneficially, sometimes catastrophically. I have investigated how a second-harmonic CPR component modifies gain, bandwidth, and pump dynamics, and how this can be exploited (or must be mitigated) depending on the intended operating regime.
3) The onset of strong nonlinearity: from saturation to chaos
A key message I like to convey to students is that “nonlinearity” is not only about saturation: it can produce qualitatively new dynamical regimes. When pushed hard (high pump power, strong mixing, imperfect phase matching), TWJPAs can enter regimes where the dynamics becomes irregular and eventually chaotic. Understanding where these regimes arise, how they appear in experimentally measurable quantities (gain ripples, intermodulation, noise bursts, instabilities), and how to avoid or harness them is a central part of my current activity. This is also where my broader background on stochastic/nonlinear dynamics becomes directly useful for microwave quantum hardware.
I am strongly interested in the “full stack” implication of amplifier physics: how a given amplifier architecture interacts with the rest of the cryogenic readout (attenuators, circulators/isolators, filtering, pump injection, impedance environment) and with the target experiment. This includes work in large collaborative efforts aimed at ultra-low-noise broadband readout, where TWJPAs are integrated and characterized at millikelvin temperatures, and where modeling is continuously cross-checked against measured gain, noise, and stability.
A practical outcome of this approach is a growing set of tools and design principles for:
predicting amplifier behavior in realistic environments,
assessing robustness to fabrication spread,
and identifying “safe” operating regions that preserve quantum-limited performance without triggering unwanted nonlinear dynamics.
My approach is deliberately multi-scale: I try to connect junction-level physics to circuit-level performance and, whenever possible, to the measurement observables that appear in a cryogenic lab.
Analytical modeling of parametric processes
I use coupled-mode / wave-mixing descriptions to capture gain, bandwidth, pump depletion, and idler generation, and to extract design-oriented scaling laws.
Distributed circuit and dispersion engineering
For traveling-wave devices, I model the transmission line as a nonlinear medium with engineered dispersion, including resonant phase-matching networks. The key outputs are phase-mismatch, gain ripple, stop-bands, and stability margins.
Nonlinear dynamics beyond the “small-signal” regime
I explicitly analyze the onset of strong nonlinearity—compression, bifurcations, intermodulation, and routes to chaos—because these regimes often set the practical operating boundaries of a TWJPA.
Microscopic inputs that matter at the system level
I incorporate non-idealities such as non-sinusoidal current–phase relations, junction parameter spread, and loss mechanisms, and I study how they propagate into amplifier-level figures of merit (gain flatness, saturation power, stability).
Bridging to experiments and readout chains
I aim for models that can be calibrated from standard measurements (gain/noise vs frequency, pump-power sweeps, S-parameters, intermodulation tests) and used to propose discriminating follow-up measurements or design iterations.
This topic is naturally suited for joint theory–experiment projects. Examples of well-scoped MSc/PhD thesis or collaboration directions:
From device parameters to performance maps
Build a fast modeling pipeline that turns geometry + junction CPR assumptions + dispersion engineering into predicted gain/noise/saturation maps, including uncertainty bands due to parameter spreads.
Nonlinear dynamics and stability boundaries
Identify and classify dynamical instabilities (period-doubling, chaos-onset, spurious resonances) and connect them to measurable microwave diagnostics; propose mitigation strategies.
Resonant phase matching design
Optimize resonant phase-matching networks to achieve target gain flatness over a specified band while controlling reflections and pump leakage; deliver design rules that are directly implementable.
Amplifiers for higher-frequency quantum sensing (e.g., X-band)
Explore scaling laws, loss mechanisms, and dispersion constraints when moving TWJPA concepts to higher-frequency platforms relevant for quantum sensing.
If you are developing a device and you have S-parameters, gain/noise curves, or time-series data from the readout chain, I can help connect those measurements to an underlying dynamical model and propose the most discriminating next experiments.
C. Guarcello, C. Barone, G. Carapella, G. Filatrella, A. Giachero, and S. Pagano, Properties of Josephson Traveling Wave Parametric Amplifier with Non-Sinusoidal Current-Phase Relation and Resonant Phase Matching, 36, 1700405 (2026)
C. Guarcello, C. Barone, G. Carapella, V. Granata, G. Filatrella, A. Giachero, S. Pagano, Driving a Josephson Traveling Wave Parametric Amplifier into chaos: effects of a non-sinusoidal current-phase relation, Chaos, Solitons & Fractals 189, 115598 (2024).
C. Guarcello, C. Barone, G. Carapella, G. Filatrella, A. Giachero, S. Pagano, Effect of second-harmonic current-phase relation on a behavior of a Josephson Traveling Wave Parametric Amplifier, Applied Physics Letters 126, 162602 (2025).
C. Guarcello, et al. (DARTWARS Collaboration), Nonlinear behavior of Josephson Traveling Wave Parametric Amplifiers, IEEE Transactions on Applied Superconductivity 34(3), 1701105 (2024).
C. Guarcello, et al. (DARTWARS Collaboration), Modeling of Josephson Traveling Wave Parametric Amplifiers, IEEE Transactions on Applied Superconductivity 33(1), 0600207 (2023).
M. Faverzani, et al. (DARTWARS Collaboration), Broadband Parametric Amplification in DARTWARS, Journal of Low Temperature Physics 216, 156 (2024).
F. Ahrens, et al. (DARTWARS Collaboration), Development of KI-TWPA amplifiers for the DARTWARS project, IEEE Transactions on Applied Superconductivity 34(3), 1700605 (2024).
V. Granata, et al. (DARTWARS Collaboration), Characterization of Traveling-Wave Josephson Parametric Amplifiers at T = 0.3 K, IEEE Transactions on Applied Superconductivity 33(1), 0500107 (2023).
A. Rettaroli, et al. (DARTWARS Collaboration), Ultra low noise readout with travelling wave parametric amplifiers: the DARTWARS project, Nuclear Instruments and Methods in Physics Research A 1046, 167679 (2023).
M. Borghesi, et al. (DARTWARS Collaboration), Progress in the development of a KITWPA for the DARTWARS project, Nuclear Instruments and Methods in Physics Research A 1047, 167745 (2023).