Resonance inversion in a superconducting cavity coupled to artificial atoms and a microwave background

Abstract

We demonstrate how heating of an environment can invert the line shape of a driven cavity. We consider a superconducting coplanar cavity coupled to multiple artificial atoms. The measured cavity transmission is characterized by Fano-type resonances with a shape that is continuously tunable by bias current through nearby (magnetic flux) control lines. In particular, the same dispersive shift of the microwave cavity can be observed as a peak or a dip. We find that this Fano-peak inversion is possible due to a tunable interference between a microwave transmission through a background, with reactive and dissipative properties, and through the cavity, affected by bias-current induced heating. The background transmission occurs due to crosstalk between the control and transmission lines. We show how such background can be accounted for by Jaynes-Cummings type models via modified boundary conditions between the cavity and transmission lines. We find generally that whereas resonance positions determine system energy levels, resonance shapes give information on system fluctuations and dissipation.

Probing the Tavis-Cummings Level Splitting with Intermediate-Scale Superconducting Circuits

“…However, the signal peak becomes narrower and more difficult to detect from the overall noise. This behavior of the signal can also be reproduced by masterequation simulations for a cavity coupled to eight qubits [20].…”

“…5(b). We estimate that the temperature at different bias points varies between 130 and 175 mK [20], and is maximal for N = 1. Furthermore, the broadening κ increases at N = 1, since an on-resonance qubit shares photons with the cavity, and thereby can also dissipate them (here with a decay rate much higher than γ ).…”

“…Here κ c is the effective coupling between the dressed resonator and the transmission line. Since the cavity equation of motion depends only weakly on [20], it follows that this solution is (up to a constant front factor) also the solution for an output without the presence of a background. The data before and after background removal is shown in Fig.…”

“…The coupling to background modes is understood to emerge from a crosstalk with the multiple local control lines, since it is absent in a single-qubit chip fabricated in the same run. This Fano interference effect leads to an inverted spectrum, but does not affect the energy-level spacing [20]. Furthermore, once the background transmission is characterized from an off-resonance transmission, its contribution on resonance can be subtracted from the transmission amplitude, resulting in the cavity spectrum only, see the Appendix.…”

“…As a result, the signal strength decays more slowly for N > 1 than between N = 0 and N = 1. However, when increasing N > 1, another effect comes into play, that instead tends to reduce the broadening κ: the number of qubits that cause broadening of the effective cavity frequency through off-resonance hopping (by inducing fluctuations to the dispersive shift of the cavity [20]) reduces. However, the signal peak becomes narrower and more difficult to detect from the overall noise.…”