Fast and high-fidelity dispersive readout of a spin qubit with squeezed microwave and resonator nonlinearity

Abstract Fast and high-fidelity qubit measurement is crucial for achieving quantum error correction, a fundamental element in the development of universal quantum computing. For electron spin qubits, fast readout stands out as a major obstacle in the pursuit of error correction. In this work, we explore the dispersive measurement of an individual spin in a semiconductor double quantum dot coupled to a nonlinear microwave resonator. By utilizing displaced squeezed vacuum states, we achieve rapid and high-fidelity readout for semiconductor spin qubits. Our findings reveal that introducing modest squeezing and mild nonlinearity can significantly improve both the signal-to-noise ratio (SNR) and the fidelity of qubit-state readout. By properly marching the phases of squeezing, the nonlinear strength, and the local oscillator, the optimal readout time can be reduced to the sub-microsecond range. With current technology parameters (κ≈ 2χₛ, χₛ/(2π)≈ 0.15 \:MHz), utilizing a displaced squeezed vacuum state with $30$ photons and a modest squeezing parameter r≈ 0.6, along with a nonlinear microwave resonator charactered by a strength of λ≈ -1.2 χₛ, a readout fidelity of $98%$ can be attained within a readout time of around 0.6\:. Intriguing, by using a positive nonlinear strength of λ≈ 1.2χₛ, it is possible to achieve an SNR of approximately $6$ and a readout fidelity of $99.99%$ at a slightly later time, around 0.9\:, while maintaining all other parameters at the same settings.