Investigating readout-induced state transitions in a multimode Josephson circuit

Dispersive readout in superconducting circuits is in principle a quantum non-demolition (QND) measurement, in which the post-readout qubit state should remain in the measured eigenstate. However, unwanted state transitions are generally observed in experiments on superconducting qubits submitted to a strong readout probe tone, limiting the readout performance. Such state transition events can be classified into two types, the readout-induced energy relaxation into spurious two-level systems [1] and leakage out of qubit manifold due to non-linear transitions [2]. Both effects have been widely reported in transmon readout experiments.

We extend these investigations into a superconducting artificial molecule consisting of two non-linear modes [3]. One of the modes has purely non-linear coupling to the readout resonator, serving as a transmon with intrinsic Purcell protection and direct readout access. The other mode mediates this non-linear coupling. We will present experimental results and preliminary theoretical understanding of the readout-induced state transitions of both types. We explain how our analysis applies to a wide choice of parameters for such multimode Josephson circuits.

[1] Thorbeck et al. arXiv:2305.10508 (2023)

[2] Sank et al. PRL 117, 190503 (2016)

[3] Gambetta et al., PRL 106, 030502 (2011)