Investigating unwanted transitions in dispersive qubit measurement at high readout power

In the cQED architecture, the Josephson parametric amplifier (JPA) improves the signal to noise ratio (SNR) and enables high fidelity measurement. Improvement in the SNR is also possible by increasing the readout power and integration time, but unwanted transitions within and outside the computational subspace limit the fidelity. We investigate a multimodal circuit [1], nicknamed the “quantromon”, with cross-Kerr coupling between the modes: a split transmon qubit and a linear cavity. We measured a fidelity of 97.63% for 1.2 us integration time without using a JPA while the best fidelity obtained with a JPA was 99.37% for a 170 ns integration time. The fidelity in both cases was limited by unwanted transitions. Numerical simulations suggest that at high readout power, these transitions are suppressed for cross-Kerr coupling when compared with a transmon transversely (σ_xσ_x) coupled to a readout cavity. In the quantromon, the asymmetry between the two Josephson junctions introduces a small transverse coupling between the qubit mode and cavity mode. We theoretically and experimentally investigate the effect of this residual transverse coupling on readout fidelity with the help of a quantromon device with tunable asymmetry.

Reference:
1. R. Dassonneville et. al. PRX 10, 011045 (2020)