Measurement-induced transmon ionization

Short answer: Measurement-induced state transitions, also called transmon ionization, are events where a strong readout drive kicks a superconducting qubit out of its computational states, which degrades readout fidelity. A 2022 Physical Review Applied paper numerically finds clear signatures of the effect and traces it to resonances at specific resonator photon populations, and a 2024 Physical Review X paper builds a framework that explains the origin and estimates the photon number at which it occurs. Both are theory and simulation, compared against earlier experiments.

The problem

The 2022 abstract says qubit measurement and control in circuit quantum electrodynamics rely on microwave drives, and higher drive amplitudes ideally mean faster processes. Yet degradation in qubit coherence time and readout fidelity has been observed even at moderate drive amplitudes, corresponding to a few photons in the measurement resonator. The 2024 abstract says dispersive readout is plagued by a loss of its quantum nondemolition character and a drop in fidelity as measurement strength rises, even though high measurement fidelity can now be reached.

2022: numerical signatures

The authors numerically explore a driven transmon-resonator system under strong, nearly resonant measurement drives. They find clear signatures of transmon ionization, where the qubit escapes out of its cosine potential. Using a semiclassical model, they interpret it as resulting from resonances at specific resonator photon populations. They find that the populations where these spurious transitions occur depend strongly on parameters, and can occur at low photon population, which they say may explain the experimentally observed loss of measurement fidelity.

2024: a framework

The 2024 paper elucidates the nature of the process with three complementary descriptions: a fully quantized transmon-resonator model, a semiclassical model where the resonator acts as a classical drive, and a fully classical model. All three keep the full cosine potential of the transmon and give similar predictions. The framework identifies the multiphoton resonances responsible, and lets the authors compute numerical estimates of the photon number threshold for ionization, which they say agree remarkably well with recent experimental results. They expect the tools to become an integral part of the theory behind circuit QED experiments, a statement of expectation.

Limits

Both abstracts describe models and simulations, and the 2022 authors say their explanation "may" account for the observed fidelity loss. Parameter dependence means no single threshold applies to all devices. The two papers cover the same effect, and no mitigation method is reported in the abstracts used here.

FAQ

What is transmon ionization?

The qubit escaping out of its cosine potential under a strong measurement drive, per the 2022 paper.

Why does it matter for readout?

It breaks the quantum nondemolition character of dispersive readout and lowers fidelity as measurement strength increases, per the 2024 abstract.

What causes it?

Multiphoton resonances at specific resonator photon populations, according to both papers.

Can it be predicted?

The 2024 authors compute photon-number thresholds in agreement with recent experiments.

Does it happen at low photon numbers?

The 2022 authors report it can occur at low photon population for some parameters.