Published in Butsuri

Published on arXiv with Keiju Murata, et al.

Published on arXiv with Nicolas Marquez Peraca, et al.

Published in Physical Review Research in collaboration with Andrey Baydin, et al.

Published in Communications Physics with Takayuki Kurihara, et al.

Published in Scientific Reports with Kenji Hayashida, et al.

Published in Nano Letters in collaboration with Riad Yahiaoui, et al.

Published on arXiv in collaboration with Andrey Baydin, et al.

Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems. 

Full Text

Andrey Baydin, Kenji Hayashida, Takuma Makihara, Fuyang Tay, Xiaoxuan Ma, Wei Ren, Guohong Ma, G. Timothy Noe II, Ikufumi Katayama, Jun Takeda, Hiroyuki Nojiri, Shixun Cao, Motoaki Bamba, Junichiro Kono

Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons


Published in Scientific Reports in collaboration with Fumiki Yoshihara, et al.

We derive the Hamiltonian of a superconducting circuit that comprises a single-Josephson-junction flux qubit inductively coupled to an LC oscillator, and we compare the derived circuit Hamiltonian with the quantum Rabi Hamiltonian, which describes a two-level system coupled to a harmonic oscillator. We show that there is a simple, intuitive correspondence between the circuit Hamiltonian and the quantum Rabi Hamiltonian. While there is an overall shift of the entire spectrum, the energy level structure of the circuit Hamiltonian up to the seventh excited states can still be fitted well by the quantum Rabi Hamiltonian even in the case where the coupling strength is larger than the frequencies of the qubit and the oscillator, i.e., when the qubit-oscillator circuit is in the deep–strong-coupling regime. We also show that although the circuit Hamiltonian can be transformed via a unitary transformation to a Hamiltonian containing a capacitive coupling term, the resulting circuit Hamiltonian cannot be approximated by the variant of the quantum Rabi Hamiltonian that is obtained using an analogous procedure for mapping the circuit variables onto Pauli and harmonic oscillator operators, even for relatively weak coupling. This difference between the flux and charge gauges follows from the properties of the qubit Hamiltonian eigenstates.

Full Text

Fumiki Yoshihara, Sahel Ashhab, Tomoko Fuse, Motoaki Bamba, Kouichi Semba

Hamiltonian of a flux qubit-LC oscillator circuit in the deep–strong-coupling regime Scientific Reports 12, 6764 (2022)

Published on arXiv in collaboration with Takayuki Kurihara, et al.

We excite the spin precession in rare-earth orthoferrite YFeO3 by the magnetic field of intense terahertz pulse and probe its dynamics by transient absorption change in the near infrared. The observed waveforms contain quasi-ferromagnetic-mode magnon oscillation and its second harmonics with a comparably strong amplitude. The result can be explained by dielectric function derived from magnetorefractive Hamiltonian. We reveal that the strong second harmonic signal microscopically originates from novel dynamics of the quasi-ferromagnetic mode magnon at nonlinear regime, wherein spin canting angle periodically oscillates. 

Full Text

Takayuki Kurihara, Motoaki Bamba, Hiroshi Watanabe, Makoto Nakajima, Tohru Suemoto

Spin canting in nonlinear terahertz magnon dynamics revealed by magnetorefractive probing in orthoferrite