Characterization of a qubit–amplifier artificial molecule in 3D circuit QED architecture

In circuit quantum electrodynamics (cQED) systems, high-efficiency qubit readout has been realized by employing microwave quantum-limited parametric amplifiers, which are able to suppress the total added noise of the measurement circuit close to the level of vacuum fluctuations. To further improve measurement efficiency, one is obliged to alleviate the impact of parasitic dissipation between the readout resonator and the external parametric amplifier. To this effect, we introduce a superconducting artificial molecule that integrates a transmon qubit and a Josephson amplifier on the same chip, the whole being enclosed into a 3D superconducting readout cavity. In this artificial molecule, the electric dipoles of the qubit and the on-chip amplifier are perpendicular to each other such that the former is linearly decoupled from the electromagnetic field in the readout cavity. The combination of a "dark" qubit, a flux-loop-free layout, and a two-stage readout scheme is intended to simultaneously achieve long qubit coherence times, high readout efficiency, and low measurement back-action. We will present the experimental characterization of this artificial molecule, demonstrate the qubit readout mechanism, and discuss further improvements.