Improved qubit bifurcation readout in the straddling regime of circuit QED

TL;DR

This paper demonstrates that operating a nonlinear resonator in the straddling regime enhances qubit readout fidelity in circuit QED, enabling high-fidelity measurements with lower coupling and reducing spectral crowding.

Contribution

The study introduces the use of the straddling regime for bifurcation measurement, showing improved fidelity and reduced coupling requirements compared to traditional methods.

Findings

Up to threefold improvement in readout fidelity.

High-fidelity measurements achievable at smaller couplings.

Reduced Purcell decay rate in the straddling regime.

Abstract

We study bifurcation measurement of a multi-level superconducting qubit using a nonlinear resonator biased in the straddling regime, where the resonator frequency sits between two qubit transition frequencies. We find that high-fidelity bifurcation measurements are possible because of the enhanced qubit-state-dependent pull of the resonator frequency, the behavior of qubit-induced nonlinearities and the reduced Purcell decay rate of the qubit that can be realized in this regime. Numerical simulations find up to a threefold improvement in qubit readout fidelity when operating in, rather than outside of, the straddling regime. High-fidelity measurements can be obtained at much smaller qubit-resonator couplings than current typical experimental realizations, reducing spectral crowding and potentially simplifying the implementation of multi-qubit devices.