Fast, high fidelity, quantum non-demolition readout with an intrinsically Purcell protected qubit*

Fast, high fidelity, quantum-non demolition readout is a key resource for a wide range of applications in quantum processing units, including qubit initialization, quantum sensing, measurement-based entanglement generation and quantum error correction. This is achieved in superconducting circuits by an approximate dispersive Hamiltonian between the qubit defined in the two lowest levels of a transmon and its readout resonator. The speed and fidelity of such readout techniques depends on the linewidth of the readout resonator, the strength of the dispersive interaction and the power of the readout drive. At the same time, it is important to suppress the radiative Purcell decay of the qubit via the resonator. Typically, dedicated Purcell filters are coupled to readout resonators to suppress such unwanted effects. 

Recently, it has been shown [1] that reducing the qubit-readout detuning is largely beneficial to circumvent the detrimental nonlinear state transitions caused by the readout tone. However, designing effective Purcell filters for such small detuning following the conventional approach is particularly challenging. We demonstrate an intrinsically Purcell protected qubit realized in a multimodal circuit [2], while retaining strong measurability through a mediated dispersive interaction. We explain the properties of such a circuit and demonstrate a state-of-the-art fast, high fidelity, quantum non-demolition qubit readout in our experimental realization of the device.

[1] Xu Xiao et al. arXiv:2304.13656 (2023)

[2] Gambetta et al., PRL 106, 030502 (2011)