Torque-induced dispersive readout in a weakly coupled hybrid system

We propose a quantum state readout mechanism of a weakly coupled qubit in a dispersive regime. The hybrid system consists of a ferromagnetic insulator and a weakly coupled quantum system, e.g., single electron spin, in a microwave cavity. The enhancement of the measurement sensitivity is achieved by exerting torque on the ferromagnetic insulator magnetization, which compensates the damping of the system leading to an exceptional point. The proposed mechanism allows us to measure the qubit state either via the transmission of the cavity or the FMR signal of the magnetic material.

Figure 1

The schematic picture of the system. YIG film and a quantum system reside near the antinode of the magnetic field and electric field, respectively. $g_m$ and $g_q$ denote magnon-photon and qubit-photon couplings, respectively. The two ports on both sides of the cavity are for photon input and output. One of the strip lines (blue in the picture) patterned on the YIG film is used to exert torque on the YIG. Another strip line (red in the picture) is used to measure spin pumping voltage.

Figure 2

(a) The left and right labels show the real (black dashed line) and imaginary (red dotted line) component of $\mathrm{\Delta }{\omega }_{mc}$ as a function of $\mathrm{\Delta }\omega ,$ respectively. (b) The same as in (a) for $\mathrm{\Delta }\omega .$ (c) and (d) The density plot of spin voltage and transmission amplitude, respectively. The dashed lines are the corresponding spectra. (e) and (f) The spin current and transmission amplitude evolution, respectively, as a function of frequency $\mathrm{\Delta }\omega$ at the value of $\mathrm{\Delta }{\omega }_{mc}$ shown in the picture. The dashed red lines are in the absence of the qubit ($〈{\sigma }_z〉=0$), while the red and blue curves are for $〈{\sigma }_z〉=\pm 1.$ All pictures are in the absence of the torque on YIG magnetization (${\alpha }_T=0$). The frequencies are presented in units of $g_q.$

Figure 3

The same as in Fig. 2 for the critical value of the spin torque ${\alpha }_T={\alpha }_{T,\mathrm{EP}}$.