Measuring a superconducting qubit linearly decoupled from the readout resonator (1/2)*

Dispersive readout of superconducting qubit is now a standard measurement scheme in circuit quantum electrodynamics (cQED) systems. It is realized by linearly coupling a readout resonator to a qubit in the dispersive regime. However, undesired qubit state transition induced by readout photons (the so-called “T1 versus nbar” effect) tends to limit further improvement of readout performance. To overcome this obstacle, we introduce a superconducting artificial molecule that integrates a “dark” transmon qubit and a “bright” meter mode with quartic coupling between each other. A two-stage readout of the qubit state is mediated by the intra-cavity meter mode. The absence of linear coupling between the qubit and the readout cavity improves readout fidelity and speed.

Part 1 of this series of two talks introduces the circuit design and explains the operating scheme of a two-stage readout protocol.