IQIM Postdoctoral and Graduate Student Seminar

Abstract: Light-matter coupling rate is a critical parameter in a cavity quantum electrodynamics (QED) setup, where a material is placed inside an optical cavity so that the material electronic excitation couples to the cavity photons in a coherent manner. Thanks to the recent advances in quantum-engineered solids with large dipole moments and high-quality optical cavities, the light-matter coupling rate can be significantly scaled up, and when it becomes comparable with the bare cavity resonance and material transition frequency, the system achieves the ultrastrong coupling regime. Because light and matter mix to an extreme degree in this regime, possibilities of revealing novel quantum phenomena have been opened up. In this talk, I will discuss my experimental study of ultrastrong light-matter coupling in two contexts. In the first part, I will focus on my attempt to incorporate a high-mobility two-dimensional electron gas with a high-Q terahertz photonic cavity. In a perpendicular magnetic field, the cyclotron resonances of billions of electrons cooperatively couple to the cavity photon field, forming Landau polaritons in the ultrastrong coupling regime. Terahertz spectroscopic measurements on this system allow me to make clear observations of polariton frequency shifts due to high-order terms in the light-matter coupling Hamiltonian. In the second part, I will discuss a bizarre experiment where signatures of ultrastrong light-matter coupling show up even in the absence of any light-related component. In an ErFeO3 single crystal, I made the collective magnon field of antiferromagnetically ordered Fe3+ spins play the role of cavity light field in a canonical cavity QED setup, and demonstrated that the coupling rate of paramagnetic Er3+ ions with the magnon field not only is ultrastrong but also scales with the square root of the Er3+ density, evidencing Dicke cooperativity. Such a demonstration of "cavity QED experiment without light" opens up possibilities to understand interactions in solids using concepts and tools available in quantum optics.