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arXiv:2505.24549 · Physical Review Applied

Strongly driven transmon as an incoherent noise source

Anonymous, Rémi Robin, Mazyar Mirrahimi, Alexandru Petrescu

Published Jun 12, 2026• 0 citations
arXivHTMLPDF

Abstract

Under strong drives, which are becoming necessary for fast high-fidelity operations, transmons can be structurally unstable. Due to chaotic effects, the computational manifold is no longer well separated from the remainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. We show here that these detrimental effects can further propagate to other degrees of freedom, for example to neighboring qubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise to another circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noise spectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certain limit, the interaction with the driven transmon can be modeled as a stochastic diffusive process driving the TLS.

DOI
10.1103/1zff-vp5q

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Under strong drives, which are becoming necessary for fast high-fidelity operations, transmons can be structurally unstable. Due to chaotic effects, the computational manifold is no longer well separated from the remainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. We show here that these detrimental effects can further propagate to other degrees of freedom, for example to neighboring qubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise to another circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noise spectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certain limit, the interaction with the driven transmon can be modeled as a stochastic diffusive process driving the TLS.
Scaling up quantum processors to run practical quantum algorithms remains a major challenge due to hardware constraintsand control limitations. This dissertation explores two complementary directions to address the scalability bottlenecksin superconducting quantum architectures: the detrimental impact of strong drives required for fast and high-fidelityoperations, and the integration of quantum memories into hybrid architectures to reduce the hardware overhead.In the first part, we investigate how strong drives can generate detrimental effects in the circuit. In this regime, transmonscan be structurally unstable. Due to chaotic effects, the computational manifold is no longer well separated from theremainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. Weshow here that these detrimental effects can further propagate to other degrees of freedom, for example to neighbouringqubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise toanother circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noisespectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certainlimit, the interaction with the driven transmon can be modelled as a stochastic diffusive process driving the TLS, therebyrevealing a fundamental source of drive-induced crosstalk in multi-qubit setups.In the second part, we turn to a modular approach for scalable quantum computation, which combines the processingunit with a storage module. We focus on spin-based quantum memories, where itinerant electromagnetic fields arestored in large ensembles of effective two-level systems embedded in a cavity. Using a mean-field framework, we modelthe ensemble as an effective spin communication channel and develop a cascaded quantum model to describe bothabsorption and emission processes. We derive optimal time-dependent modulations of the cavity linewidth that maximizestorage and retrieval efficiency for finite-duration wavepackets. Our analysis yields an upper bound on efficiency, whichcan be met in the narrow bandwidth regime. It also shows the existence of a critical bandwidth above which the efficiencyseverely decreases.
Transmon qubits are ubiquitously used in superconducting quantum information processor architectures.Strong drives are required to realize fast, high-fidelity, gates and measurements, including parametrically activated processes.Here, we show that even off-resonant drives, in regimes routinely used in experiments, can cause strong modifications to the structure of the transmon spectrum rendering a large part of it chaotic.Accounting for the full nonlinear dynamics of the transmon in a Floquet-Markov formalism, we find that these chaotic states, often neglected through the hypothesis that the anharmonicity is weak, strongly impact the lifetime of the transmon's computational states.In particular, we observe that chaos-assisted quantum phase slips greatly enhance band dispersions.In the presence of a measurement resonator, we find that approaching chaotic behavior correlates with strong transmon-resonator hybridization, and an average resonator response centered on the bare resonator frequency.These results lead to a photon-number threshold characterizing the appearance of chaos-induced quantum demolition effects during strong-drive operations, such as dispersive qubit readout.The phenomena described here are expected to be present in all circuits based on low-impedance Josephson junctions.
Achieving fast gates and high-fidelity readout in superconducting quantum circuits often benefits from the use of large amplitude microwave drives. As the strength increases, unwanted effects such as excitation out of the logical states also become more prevalent. In transmon qubits, coherent controls relying on sub-resonant drives are fundamentally limited by the eventual appearance of strong hybridization in the spectrum. Understanding how this threshold depends on intrinsic device parameters is vital for the design and operation of circuits utilizing the transmon as a source of nonlinearity. In this work, we present an experimental study of 14 transmons with a wide range of anharmonicities, and use comparison with Floquet branch analysis and semiclassical simulations to establish a straightforward relationship between the critical value of the effective drive amplitude ηmax⁡η_{\max}ηmax​ and the ratio γ=ωq/αγ=ω_q/αγ=ωq​/α, where ωqω_qωq​ is the transmon frequency and −α-α−α is the anharmonicity. For the case where the transmon is used as a parametrically driven coupler, these results further allow us to estimate maximum parametric interaction rates, which informs the circuit design process for optimizing gate rates and fidelities.
Materials improvements in transmon qubits have yielded remarkable advances in qubit lifetimes (T1)and coherence times (T2). However, state-of-the-art superconducting qubits experience temporal fluctuations in their performance, which limit gate fidelities and quantum processor performance. We observe correlated increases in qubit relaxation and dephasing rates that last for minutes to hours, which we call T1 and T2 excursions. Using interleaved lifetime and coherence time measurements, noise spectroscopy, and two-level-system (TLS) spectroscopy, we find that these excursions are caused by single or few strongly-coupled TLS defects that spectrally diffuse into resonance with the qubit frequency. Our measurement protocol provides a powerful tool for understanding the noise sources that cause temporal fluctuations in superconducting qubit performance.
Qubit measurement is an essential step in any quantum computation. In circuit quantum electrodynamics, a leading quantum computer architecture, qubit readout is commonly one of the longest and lowest-fidelity processes. The authors numerically explore the dynamics of a driven transmon-resonator system under strong, nearly resonant measurement drives to better understand this issue. They find clear signs of transmon ``ionization'', in which the qubit escapes its confining potential under the influence of the drive, and semiclassical methods then reveal the mechanism. This approach can be used to optimize circuit parameters, suppress these spurious effects, and increase readout fidelity.
Material defects give rise to parasitic two-level systems (TLS) which present a major source of decoherence in superconducting qubits. Here, we study a strongly coupled TLS that resides in the tunnel barrier of transmon qubit. We use multi-photon spectroscopy and TLS strain tuning to explore the rich spectrum of the interacting three-partite system consisting of TLS, qubit, and its readout resonator. This reveals a strong effective resonant coupling between the TLS and the qubit's readout resonator which dresses the resonator states and results in a resonance frequency shift that spoils the readout signal. Our finding presents yet another way how material defects can interfere with qubit operation and hinder the realization of solid-state quantum processors.