Ultra-strong light-matter interactions and super-radiant phase transitions by Prof. Motoaki Bamba, Osaka Univ

Ultra-strong light-matter interactions and super-radiant phase transitions

Interaction strengths between electromagnetic fields and matters (absorption or emission rates per photon) can be comparable to or larger than transition frequencies of excitations in matters. Such a regime is called the ultra-strong light-matter interaction one. It has been realized recently in a variety of systems [1], e.g., transitions in dye molecules (visible wavelength), inter-subband transitions in semiconductor quantum wells (THz), cyclotron transitions of two-dimensional electron gas (2DEG) (THz), optical phonons (THz), magnons (microwave), and artificial atoms in superconducting circuits (microwave).

One of remarkable phenomena in the ultra-strong regime is an existence of photons (called virtual photons) in the ground states of ultra-strongly interacting systems. Such virtual photons originate from counter-rotating contributions of the light-matter interactions, i.e., from a violation of rotating-wave approximation. In our recent experiment, we successfully evaluated the counter-rotating contributions for the first time by the use of an ultrahigh-mobility 2DEG, a high-quality-factor cavity, and circularly polarized THz radiation [2].

Another remarkable phenomenon owing to the ultra-strong light-matter interactions is phase transitions of electromagnetic fields in thermal equilibriums, which are called the super-radiant phase transition (SRPT). Since its first proposal in 1973, the SRPT has never been realized experimentally, while a non-equilibrium analogue was demonstrated in a system of cold atoms [3]. Recently, we theoretically found a thermal-equilibrium analogue of the SRPT in a superconducting circuit [4], where a persistent current appears spontaneously instead of the electromagnetic fields in the original picture. Further, we found experimentally a signature of another analogue in ErFeO¬3 [5], i.e., an evidence of long-range cooperative interactions of Er3+ spins mediated by magnons of ordered Fe3+ spins. This result is in contrast to the standard picture of magnets, where magnetic phase transitions appear basically owing to short-range interactions, e.g., exchange interactions between nearest neighbor spins.

These works are in collaborations with K. Inomata, Y. Nakamura, X. Li, J. Kono, and many other experimentalists.

[1]        See a review by P. Forn-Díaz, L. Lamata, E. Rico, J. Kono, and E. Solano, arXiv:1804.09275 [quant-ph].

[2]        X. Li, M. Bamba, Q. Zhang, S. Fallahi, G. C. Gardner, W. Gao, M. Lou, K. Yoshioka, M. J. Manfra, and J. Kono, Nat. Photonics 12, 324 (2018).

[3]        K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger, Nature 464, 1301 (2010).

[4]        M. Bamba, K. Inomata, and Y. Nakamura, Phys. Rev. Lett. 117, 173601 (2016).

[5]        X. Li, M. Bamba, N. Yuan, Q. Zhang, Y. Zhao, M. Xiang, K. Xu, Z. Jin, W. Ren, G. Ma, S. Cao, D. Turchinovich, and J. Kono, Science (accepted).

Bio: Motoaki Bamba is an Associate Professor specially appointed by Interactive Materials Science Cadet Program, Osaka University, and a PRESTO Researcher, JST. He got a Ph.D. in Science from Osaka University in 2009, worked as a post-doc in University Paris 7 during 2009 - 2012, and in Osaka University during 2012 - 2015. He became the Specially Pointed Associated Professor in 2015 and the PRESTO Researcher in 2017. He got two Young Scientist Awards from the Physical Society of Japan in 2017 and a Research Award from Research Foundation for Opto-Science and Technology in 2018.