SPICE Workshop on Quantum Spinoptics, June 13th - 15th 2023
Junichiro Kono
Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter [1]. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA) [2]. We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field [3]. The electron cyclotron resonance peak exhibited a polariton splitting with a magnitude that is proportional to the square-root of the electron density, a hallmark of Dicke cooperativity. Additionally, we have obtained definitive evidence for the vacuum Bloch-Siegert shift [4], a direct signature of the breakdown of the RWA.
Furthermore, we have shown that cooperative USC also occurs in a magnetic solid in the form of matter-matter interaction, i.e., spin-magnon [6] and magnon-magnon [7] interactions in rare earth orthoferrites [8]. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition [9,10]. The original Dicke model describes the cooperative interaction of an ensemble of two-level atoms with a single-mode photonic field with no interatomic interactions. Extending this model by incorporating short-range atom-atom interactions makes the problem intractable but is expected to produce new phases. We have recently quantum-simulated such an extended Dicke model using a crystal of ErFeO3, where the role of atoms (photons) is played by Er3+ spins (Fe3+ magnons) [10]. Through magnetocaloric effect and terahertz magnetospectroscopy measurements, we demonstrated the existence of a novel atomically ordered phase in addition to the superradiant and normal phases that are expected from the standard Dicke model. These results provide a route for understanding, controlling, and predicting novel phases of condensed matter using concepts and tools available in quantum optics.
[2] For a review, see P. Forn-Díaz et al., “Ultrastrong coupling regimes of light-matter interaction,” Reviews of Modern Physics 91, 025005 (2019).
[3] Q. Zhang et al., Nature Physics 12, 1005 (2016).
[4] X. Li et al., Nature Photonics 12, 324 (2018).
[5] W. Gao et al., Nature Photonics 12, 362 (2018).
[6] X. Li et al., Science 361, 794 (2018).
[7] T. Makihara et al., Nature Communications 12, 3115 (2021).
[8] X. Li, D. Kim, Y. Liu, and J. Kono, Photonics Insights 1, R05 (2022).
[9] M. Bamba et al., Communications Physics 5, 3 (2022).
[10] N. Marquez Peraca et al., arXiv:2302.06028.