Cavity magnonics deals with the interaction of magnons - elementary excitations in magnetic materials - and confined electromagnetic fields. We introduce the basic physics and review the experimental and theoretical progress of this young field that
is gearing up for integration in future quantum technologies. Much of its appeal is derived from the strong magnon-photon coupling and the easily-reached nonlinear regime in microwave cavities. The interaction of magnons with light as detected by Brillouin light scattering is enhanced in magnetic optical resonators, which can be employed to manipulate magnon distributions. The cavity photon-mediated coupling of a magnon mode to a superconducting qubit enables measurements in the single magnon limit.
We report fast charge state readout of a double quantum dot in a CMOS split-gate silicon nanowire transistor via the large dispersive interaction with microwave photons in a lumped-element resonator formed by hybrid integration with a superconducting
inductor. We achieve a coupling rate $g_0/(2pi) = 204 pm 2$ MHz by exploiting the large interdot gate lever arm of an asymmetric split-gate device, $alpha=0.72$, and by inductively coupling to the resonator to increase its impedance, $Z_text{r}=560~Omega$. In the dispersive regime, the large coupling strength at the double quantum dot hybridisation point produces a frequency shift comparable to the resonator linewidth, the optimal setting for maximum state visibility. We exploit this regime to demonstrate rapid dispersive readout of the charge degree of freedom, with a SNR of 3.3 in 50 ns. In the resonant regime, the fast charge decoherence rate precludes reaching the strong coupling regime, but we show a clear route to spin-photon circuit quantum electrodynamics using hybrid CMOS systems.
Developing fast, accurate and scalable techniques for quantum state readout is an active area in semiconductor-based quantum computing. Here, we present results on dispersive sensing of silicon corner state quantum dots coupled to lumped-element elec
trical resonators via the gate. The gate capacitance of the quantum device is configured in parallel with a superconducting spiral inductor resulting in resonators with loaded Q-factors in the 400-800 range. For a resonator operating at 330 MHz, we achieve a charge sensitivity of 7.7 $mu$e$/sqrt{text{Hz}}$ and, when operating at 616 MHz, we get 1.3 $mu$e$/sqrt{text{Hz}}$. We perform a parametric study of the resonator to reveal its optimal operation points and perform a circuit analysis to determine the best resonator design. The results place gate-based sensing at par with the best reported radio-frequency single-electron transistor sensitivities while providing a fast and compact method for quantum state readout.
