Submicrosecond high-fidelity dispersive readout of a spin qubit with squeezed photons

Fast and high-fidelity qubit measurement is essential for realizing quantum error correction, a key ingredient to universal quantum computing. For electron spin qubits, fast readout is one of the significant challenges toward error correction. Here we examine the dispersive readout of a single spin in a semiconductor double quantum dot coupled to a microwave resonator. We show that using displaced squeezed vacuum states for the probing photons can improve the qubit readout fidelity and speed. With proper phase matching, moderate squeezing can enhance both the signal-to-noise ratio and the fidelity of the qubit readout, and the optimal readout time can be shortened to the submicrosecond range with above $97\%$ fidelity. These enhancements are achieved at low probing microwave intensity, ensuring nondemolition qubit measurement.

Figure 1

Schematic of the setup for dispersive detection of spin qubits.

Figure 2

SNR and readout fidelity with respect to different squeezing parameters and coupling rates. Here, $\alpha =\sqrt{30}, {\chi }_s/2\pi \equiv 0.15\text{MHz}, {\theta }_{\xi }=\pi$, and $T_1=3\text{ms}$ [50]. The coupling rates are (a), (b) $\kappa ={\chi }_s$ and (c), (d) $\kappa =2{\chi }_s$. The black (lower) lines represent the coherent state inputs.

Figure 3

SNR and readout fidelity with respect to different phase mismatches and squeezing parameters at a fixed measurement time $t\approx 0.714\mu \mathrm{s}$. Here, $\left|\alpha \right|=10, \kappa =2{\chi }_s, {\chi }_s/2\pi \equiv 0.15\text{MHz}$, and $T_1=3\text{ms}$. (a), (c) A squeezing parameter of $r=0.74$ is chosen, and the black dashed lines represent the coherent state inputs. (b), (d) The phase-matching condition is given by $\mathrm{\Delta }\theta \equiv \varphi -{\theta }_{\xi }/2=0$.