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Fast and high-fidelity qubit measurement is crucial for achieving quantumerror correction, a fundamental element in the development of universal quantumcomputing. For electron spin qubits, fast readout stands out as a majorobstacle in the pursuit of error correction. In this work, we explore thedispersive measurement of an individual spin in a semiconductor double quantumdot coupled to a nonlinear microwave resonator. By utilizing displaced squeezedvacuum states, we achieve rapid and high-fidelity readout for semiconductorspin qubits. Our findings reveal that introducing modest squeezing and mildnonlinearity can significantly improve both the signal-to-noise ratio (SNR) andthe fidelity of qubit-state readout. By properly marching the phases ofsqueezing, the nonlinear strength, and the local oscillator, the optimalreadout time can be reduced to the sub-microsecond range. With currenttechnology parameters ($\kappa\approx 2\chi_s$, $\chi_s\approx 2\pi\times 0.15\:\mbox{MHz}$), utilizing a displaced squeezed vacuum state with $30$ photonsand a modest squeezing parameter $r\approx 0.6$, along with a nonlinearmicrowave resonator charactered by a strength of $\lambda\approx -1.2 \chi_s$,a readout fidelity of $98\%$ can be attained within a readout time of around$0.6\:\mu\mbox{s}$. Intriguing, by using a positive nonlinear strength of$\lambda\approx 1.2\chi_s$, it is possible to achieve an SNR of approximately$6$ and a readout fidelity of $99.99\%$ at a slightly later time, around$0.9\:\mu\mbox{s}$, while maintaining all other parameters at the samesettings.

Fast and high-fidelity dispersive readout of a spin qubit via squeezing and resonator nonlinearity