Ideal Quantum Nondemolition Readout of a Flux Qubit without Purcell Limitations*

When performing large-scale quantum computation, a superconducting qubit is often read out by its dispersive coupling with an auxiliary cavity. The readout mechanism is based on Rabi model with large detuning. However, because of virtual excitation exchange, this approach is are usually significantly limited by the Purcell effects. By considering a superconducting qubit nonperturbative dispersive coupling (NPDC) to a frequency-tunable measurement resonator,we show [1] how to realize an ideal QND readout of a superconducting qubit.The NPDC-based readout mechanism is free of dipole-field interactions, and is not deteriorated by intracavity photons. Moreover, the NPDC can be conveniently turned on/off via an external control flux. Our proposal can be extended to a multiqubit architecture for a joint qubit readout. The work of R. Dassonneville et al. [2] describes a protocol very similar to ours, and can be viewed as an experimental realization of the QND measurement of qubits via the NPDC mechanism proposed by us much earlier in [1].
[1] X. Wang, A. Miranowicz, and F. Nori, Phys. Rev. Applied 12, 064037 (2019)
[2] R. Dassonneville et al., Phys. Rev. X 10, 011045 (2020)