Strongly Coupled Magnons and Cavity Microwave Photons

We realize a cavity magnon-microwave photon system in which a magnetic dipole interaction mediates strong coupling between the collective motion of a large number of spins in a ferrimagnet and the microwave field in a three-dimensional cavity. By scaling down the cavity size and increasing the number of spins, an ultrastrong coupling regime is achieved with a cooperativity reaching 12 600. Interesting dynamic features including classical Rabi-like oscillation, magnetically induced transparency, and the Purcell effect are demonstrated in this highly versatile platform, highlighting its great potential for coherent information processing.

Figure 1

(a) Top: Simulated microwave cavity resonance TE101 mode distribution. The red arrows and colors indicate the magnetic field directions and amplitudes, respectively. Bottom: Device image showing half of the microwave cavity with a YIG sphere inside. (b) Measured normal mode splitting spectrum as a function of the bias magnetic field. (c) The evolution of the cavity energy after a pulse excitation at the varying bias magnetic field. (d) The measured Rabi-like oscillation signal at the zero detuned bias magnetic field. Red circles: Measurement results. Solid blue line: Theoretical calculation using parameters obtained from the normal mode splitting spectrum.

Figure 2

(a) Schematic of the linearly coupled magnon ($m$) and photon ($a$) system. $g,{\kappa }_m,{\kappa }_a$ are the coupling strength and dissipation rates of magnon and microwave cavity modes, respectively. (b) Different coupling regimes separated by the relative strength between the coupling rate and dissipation rates of the photon and magnon subsystems. (c) MIT spectrum at various bias magnetic fields. (d) Measured resonance spectrum (symbols) showing MIT at the zero detuning magnetic field. The solid lines are the theoretical fitting using Eq. (3). (e) Reflection spectrum showing the Purcell effect. (f) Cavity energy ringdown curves at two different bias magnetic fields $B_1$ (blue squares) and $B_2$ (red circles). Solid lines are the theoretical fittings. Inset: Resonance reflection spectra at $B_1$ (blue line) and $B_2$ (red line).

Figure 3

(a) Coupling strength as a function of modal frequency, ${\omega }_{\mathrm{eff}}$. Solid line is the theoretical prediction. The red star indicates a device reaching the ultrastrong coupling. (b) Ultrastrong coupling spectrum in the $K_a$ band. Red arrows show the ultrastrongly coupled magnon-microwave photon mode.