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

We investigate dispersive measurement of an individual electron spin in a semiconductor double quantum dot that is coupled to a nonlinear microwave resonator. By employing displaced squeezed vacuum states, we achieve rapid and high-fidelity readout for semiconductor spin qubits. Our findings reveal that the introduction of modest squeezing and mild nonlinearity can significantly improve both the signal-to-noise ratio (SNR) and the fidelity of the qubit-state readout. By properly matching the phases of squeezing, the nonlinear coupling coefficient, and the local oscillator, the optimal readout time can be reduced to the sub-microsecond range. For example, with currently accessible parameters ($kappaapprox 2chi_s$, $chi_sapprox 2pi imes 0.15 :mbox{MHz}$), utilizing a displaced squeezed vacuum state with $30$ photons and a moderate squeezing parameter $rapprox 0.85$, along with a nonlinearity strength of $lambda approx -1.2 chi_s$, a readout fidelity of $98\%$ can be attained within a readout time of around $0.6:mumbox{s}$.