Dispersive-plus-Dissipative Coupling in Niobium Photon-Pressure Circuits

Photon-pressure (PP) circuits are the circuitQED equivalent of cavity optomechanics, but without any mechanical components, and they are discussed e.g. for dark matter axion detection and quantum technologies. In PP systems, a low-frequency (LF) LC resonator is typically coupled to a high-frequency (HF) LC resonator via an optomechanical-like dispersive interaction. The MHz current in the LF circuit generates a magnetic field, which is detected by a SQUID integrated into the GHz HF circuit as a flux, a mechanism that alters the HF inductance and hence its resonance frequency. Here, we report the realization of niobium-based photon-pressure circuits, their characterization in the thermal regime at 4.2 K, and the observation of not only the well-known dispersive interaction, but a significant contribution from dissipative PP. Dissipative PP appears, when the LF circuit modulates the decay rate of the HF mode instead of (or in addition to) its resonance frequency. In our experiment, we find that the dissipative contribution is roughly twice as strong as the dispersive coupling. This paves the way for studies of a generalized optomechanical interaction in PP circuitQED and may open up opportunities for ground-state cooling and squeezing in the unresolved-sideband regime.