cross-resonance-based readout scheme of a superconducting flux qubit*

We propose a cross-resonance-based readout scheme of a superconducting flux qubit, in which a flux qubit is coupled to a resonator, and a microwave flux pulse tuned to the resonator is applied to the flux qubit. At the optimal flux bias, the persistent current of the flux qubit is an increasing or decreasing function of the flux bias, depending on the state of the qubit. When a microwave flux drive is applied to the qubit, the phase of the induced microwave signal felt by the resonator depends on the state of the qubit, and the difference between the two values is 180 degrees. Since this qubit-state-dependent phase difference is larger than that of dispersive-interaction schemes [1], the proposed cross-resonance-based readout scheme has the potential to be faster. The proposed scheme takes advantage of the large contrast of the flux-bias dependence of the persistent current, and, hence, faster readout is expected compared to the alternative readout schemes [2, 3], which are mainly for transmon qubits.
[1] A. Blais et al., PRA 69, 062320 (2004). [2] N. Didier et al., PRL 115, 203601 (2015). [3] S. Touzard et al., arXiv:1809.06964.