Figure 2

Strong magnon–photon coupling achieved at cryogenic temperature. In (a–d), the transmission spectrum of the rectangular 3D cavity with a small YIG sphere is measured as a function of the static magnetic field at 22 mK in a dilution refrigerator. The input microwave power is (a) −130 dBm with different frequencies around TE₁₀₁, (b) −130 dBm with different frequencies around TE₁₀₂, (c) −100 dBm with different frequencies around TE₁₀₁, and (d) −100 dBm with different frequencies around TE₁₀₂. The horizontal dashed lines denote the resonant frequencies of the cavity and the tilted dashed lines show the magnetic-field dependence of the frequencies of the FMR (left) and MS (right) modes. The green solid curves correspond to the energy levels of the magnon–polariton obtained by diagonalizing the Hamiltonian in equation (3).