Abstract
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.
- Received 9 November 2022
- Accepted 21 March 2023
DOI:https://doi.org/10.1103/PRXQuantum.4.020312
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Transmon qubits are central building blocks of quantum information processor architectures based on superconducting circuits. An important roadblock for this technology is the observed reduction in lifetime of superconducting qubits under the strong drives that are crucial for fast gates and readout. Understanding these effects is widely recognized as one of the most important theoretical questions of the field. Up to now, theoretical attempts have relied on perturbation theory assuming weak drive amplitude or weak qubit anharmonicity.
Here, we follow a drastically different approach by considering the full nonlinear dynamics of a driven transmon. Our study reveals that chaotic effects, which are reminiscent of those of the classical driven pendulum, can be present at the moderate to strong drive amplitudes used in current experiments. The presence of chaotic states can lead to a phenomenon known as transmon ionization, where the population of the transmon’s computational states leaks into highly excited states resulting in gate and readout errors. This prediction is in agreement with recent experiments and exact numerical simulations. Remarkably, we show that spurious qubit transitions can occur even below the ionization threshold. These effects originate from the strong nonlinearity of the transmon and are in striking contrast with the predictions obtained in the usual weakly anharmonic model of the transmon.
By showing when these effects occur and how to avoid them, our work provides useful tools for the design of robust superconducting quantum processors mitigating the effect of these instabilities.
