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

Within the framework of input-output theory, we analyze the dispersive readout of a single spin in a semiconductor double quantum dot coupled to a microwave resonator. We demonstrate fast and high-fidelity readout of semiconductor spin qubits by using squeezed coherent states and nonlinear resonators. We find that for fixed dispersive coupling strength $chi_s$ and leakage rate $kappa$, the presence of squeezing can enhance the signal-to-noise ratio as well as the fidelity of the qubit-state readout, whereas the introduction of nonlinearity into the resonator (e.g., via a SQUID) can substantially speed up the optimal readout time. With current technology ($kappaapprox 2pi imes 3.0:mbox{MHz}$, $chi_sapprox 0.15kappa$), using a squeezed coherent state with $50$ photons and a squeezing parameter $rapprox 3.5$, a readout fidelity $Fapprox 90\%$ is achievable within a readout time $tapprox 200:mbox{ns}$, which is one order of magnitude faster than the optimal readout time ($approx 2:mu$s) without squeezing. Under the same condition, $Fapprox 95\%$ is achievable within a readout time $tapprox 100:mbox{ns}$ with a squeezing parameter $rapprox 6$ and a nonlinear strength $lambdaapprox -0.7kappa$.