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Published in Nature Photonics in collaboration with Weilu Gao, et al.

Continuous transition between weak and ultra-strong coupling through exceptional points in carbon nanotube micro-cavity exciton polaritons Non-perturbative coupling of photons and excitons produces hybrid particles, exciton–polaritons, which have exhibited a variety of many-body phenomena in various microcavity systems. However, the vacuum Rabi splitting (VRS), which defines the strength of photon–exciton coupling, is usually a single constant for a given system. Here, we have developed a unique architecture in which excitons in an aligned single-chirality carbon nanotube film interact with cavity photons in polarization-dependent manners. The system reveals ultrastrong coupling (VRS up to 329 meV or a coupling-strength-to-transition-energy ratio of 13.3%) for polarization parallel to the nanotube axis, whereas VRS is absent for perpendicular polarization. Between these two extremes, VRS is continuously tunable through polarization rotation with exceptional points separating crossing and anticrossing. The points between exceptional points form equienergy arcs onto which the upper and lower polaritons coalesce. The demonstrated on-demand ultrastrong coupling provides ways to explore topological properties of polaritons…
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`Exceptional’ research points way toward quantum discoveries

Rice University scientists make tunable light-matter couplings in nanotube films Rice University scientists are known for exceptional research, but a new paper led by physicist Junichiro Kono makes that point most literally. Full text News Release by Rice University Original article Weilu Gao, Xinwei Li, Motoaki Bamba, and Junichiro Kono Continuous transition between weak and ultra-strong coupling through exceptional points in carbon nanotube micro-cavity exciton polaritons Nature Photonics, published online (2018)

Published in Nature Photonics in collaboration with Xinwei Li, et al.

Vacuum Bloch-Siegert shift in Landau polaritons with ultra-high cooperativity A two-level system resonantly interacting with an a.c. magnetic or electric field constitutes the physical basis of diverse phenomena and technologies. However, Schrödinger’s equation for this seemingly simple system can be solved exactly only under the rotating-wave approximation, which neglects the counter-rotating field component. When the a.c. field is sufficiently strong, this approximation fails, leading to a resonance-frequency shift known as the Bloch–Siegert shift. Here, we report the vacuum Bloch–Siegert shift, which is induced by the ultra-strong coupling of matter with the counter-rotating component of the vacuum fluctuation field in a cavity. Specifically, an ultra-high-mobility two-dimensional electron gas inside a high-Q terahertz cavity in a quantizing magnetic field revealed ultra-narrow Landau polaritons, which exhibited a vacuum Bloch–Siegert shift up to 40 GHz. This shift, clearly distinguishable from the photon-field self-interaction effect, represents a unique manifestation of a strong-field phenomenon without a strong field. Full text Xinwei Li, Motoaki Bamba, Qi Zhang, Saeed Fallahi,…
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Quantum shift shows itself in coupled light and matter

Rice University scientists corral, quantify subtle movement in condensed matter system A team led by Rice University scientists used a unique combination of techniques to observe, for the first time, a condensed matter phenomenon about which others have only speculated. The research could aid in the development of quantum computers. Full text News Release by Rice University Original article Xinwei Li, Motoaki Bamba, Qi Zhang, Saeed Fallahi, Geoff C. Gardner, Weilu Gao, Minhan Lou, Katsumasa Yoshioka, Michael J. Manfra, and Junichiro Kono Vacuum Bloch-Siegert shift in Landau polaritons with ultra-high cooperativity Nature Photonics, published online (2018)

Published in Physical Review A

Circuit configurations which may or may not show superradiant phase transitions Several superconducting circuit configurations are examined on the existence of superradiant phase transitions (SRPTs) in thermal equilibrium. For some configurations consisting of artificial atoms, whose circuit diagrams are however not specified, and an LC resonator or a transmission line, we confirm the absence of SRPTs in the thermal equilibrium following the similar analysis as the no-go theorem for atomic systems. We also show some other configurations where the absence of SRPTs cannot be confirmed. Full text Motoaki Bamba and Nobuyuki Imoto Circuit configurations which may or may not show superradiant phase transitions Physical Review A 96, 053857 (2017) arXiv:1703.03533 [quant-ph]

Published in Physical Review Letters

Superradiant Phase Transition in a Superconducting Circuit in Thermal Equilibrium We propose a superconducting circuit that shows a superradiant phase transition (SRPT) in thermal equilibrium. The existence of the SRPT is confirmed analytically in the limit of an infinite number of artificial atoms. We also perform a numerical diagonalization of the Hamiltonian with a finite number of atoms and observe an asymptotic behavior approaching the infinite limit as the number of atoms increases. The SRPT can also be interpreted intuitively in a classical analysis. Full text Motoaki Bamba, Kunihiro Inomata, and Yasunobu Nakamura Superradiant Phase Transition in a Superconducting Circuit in Thermal Equilibrium Physical Review Letters 117, 173601 (2016) arXiv:1605.01124 [quant-ph]