# Motoaki BambaKyoto University | Kyodai · Department of Physics I

Motoaki Bamba

PhD in Science

Students and theoretical or experimental researchers to collaborate are always welcome.

## About

68

Publications

8,036

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1,169

Citations

Citations since 2016

Introduction

Among the subjects I am currently engaged in, I am most interested in the phenomenon called super-radiant phase transition. Although super-radiant phase transition was theoretically proposed in 1973, it has not been observed despite approximately 50 years of research.
However, in 2018 and 2022, in collaboration with experimental researchers, we discovered that a magnetic material shows the super-radiant phase transition if spin waves in that material are captured as light.

Additional affiliations

January 2021 - present

January 2021 - present

April 2019 - December 2020

## Publications

Publications (68)

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 co...

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 corresp...

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 explaine...

In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. However, such an equilibrium SRPT is forbidden in the case of charge-based light–matter coupling, known as no-go theorems. Here, we show that the low-temperature phase transition...

We investigate theoretically how the ground state of a qubit–resonator (Q–R) system in the deep-strong coupling (DSC) regime is affected by the coupling to an environment. We employ as a variational ansatz for the ground state of the Q–R–environment system a superposition of coherent states displaced in qubit-state-dependent directions. We show tha...

A system of N two-level atoms cooperatively interacting with a photonic field can be described as a single giant atom coupled to the field with interaction strength ~N^0.5. This enhancement, known as Dicke cooperativity in quantum optics, has recently become an indispensable element in quantum information technology based on strong light-matter cou...

Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant...

The ground state of the photon-matter coupled system described by the Dicke model is found to be perfectly squeezed at the quantum critical point of the superradiant phase transition (SRPT). In the presence of the counter-rotating photon-atom coupling, the ground state is analytically expressed as a two-mode squeezed vacuum in the basis of photons...

Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics (QED) systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations. The source of such phenomena are antiresonant terms in the Hamiltonian, yet antiresonant interactions are typic...

We derive the Hamiltonian of a superconducting circuit that comprises a single-Josephson-junction flux qubit and an LC oscillator. If we keep the qubit's lowest two energy levels, the derived circuit Hamiltonian takes the form of the quantum Rabi Hamiltonian, which describes a two-level system coupled to a harmonic oscillator, regardless of the cou...

We show that the low-temperature phase transition in ErFeO3 that occurs at a critical temperature of ~ 4 K can be described as a magnonic version of the superradiant phase transition (SRPT). The role of photons in the quantum-optical SRPT is played by Fe magnons, while that of two-level atoms is played by Er spins. Our spin model, which is reduced...

We investigate theoretically how the ground state of a qubit-resonator system in the deep-strong coupling (DSC) regime is affected by the coupling to an environment. We employ a superposition of coherent states displaced in the qubit-state-dependent directions as a variational ansatz for the ground state of the qubit-resonator-environment system. W...

We studied magnon-magnon ultrastrong coupling in YFeO3 using terahertz magnetospectroscopy in magnetic fields up to 30 T, which led to an extreme breakdown of the rotating-wave approximation where the counter-rotating term dominates the co-rotating term.

We excited terahertz magnons in Er*Yi_*FeO3 as a function of temperature, magnetic field, and yttrium composition (x), providing insight into the role of cooperative Er ³⁺ -Fe ³⁺ exchange interactions in the low-temperature phase transition.

Light and matter can strongly mix together to form hybrid particles called polaritons. In recent years, polaritons in the so-called ultrastrong coupling (USC) regime have attracted much attention from both fundamental and applied points of view. A variety of nonintuitive phenomena and novel ground states with exotic properties have been predicted f...

Traditionally, strong-field physics explores phenomena in matter (atoms, molecules, and solids) driven by an extremely strong laser field nonperturbatively. However, even in the complete absence of an external electromagnetic field, strong-field phenomena can arise when matter strongly couples with the zero-point field of the quantum vacuum state,...

Traditionally, strong-field physics explores phenomena in matter (atoms, molecules, and solids) driven by an extremely strong laser field nonperturbatively. However, even in the complete absence of an external electromagnetic field, strong-field phenomena can arise when matter strongly couples with the zero-point field of the quantum vacuum state,...

Terahertz time-domain spectroscopy measurements performed in pulsed, high magnetic fields reveal ultrastrong coupling between quasi-antiferromagnetic and quasi-ferromagnetic magnon modes in the canted antiferromagnet, YFeO3.

We have studied the low-temperature (~4.5 K) phase transition of Er1–xYxFeO3 as a function of temperature, magnetic field, and Y composition (x) through terahertz time-domain spectroscopy and created an experimental phase diagram.

Cooperative quantum magnetism
One of the earliest and most intensively studied problems in quantum optics is the interaction of a two-level system (an atom) with a single photon. This simple system provides a rich platform for exploring exotic light-matter interactions and the emergence of more complex phenomena such as superradiance, which is a co...

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....

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 develop...

光が物質によって屈折や反射するのと同じ原理に基づいて，ある温度を下回ると．光を構成する電場や磁場がどこからともなく現れたりする．この現象は超放射相転移と呼ばれ，４０年以上前に提唱されたが，いまだ観測されたことのない幻の現象である．本研究では，超伝導物質で構成された回路であれば，超放射相転移の類似物を人工的に起こせることを理論的に発見した．本研究を推進していくことで，将来的に，熱から光への画期的なエネルギー変換技術などの開発を目指している．

We demonstrate the existence of exceptional points in the dispersion of microcavity exciton polaritons, originating from the polarization-controlled ultrastrong coupling of 1D excitons in aligned single-chirality single-wall carbon nanotubes with microcavity photons.

Terahertz spectroscopy of Landau polaritons reveals a shift induced by the ultrastrong coupling of cyclotron-orbiting electrons with the counter-rotating component of the vacuum fluctuation field, evidencing the breakdown of the rotating wave approximation.

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 equilibriu...

The full text is uploaded from Parity magazine (November 2017) by the permission of Maruzen Publishing.

Thanks to an ultra-strong light-matter interaction, a transverse electromagnetic field gets a static coherent amplitude spontaneously in thermal equilibrium. While this phase transition has not been observed in atomic systems, it was recently found that a similar phase transition exists in a superconducting circuit.

Several superconducting circuit configurations are examined on the existence of super-radiant 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 equilibri...

We propose a superconducting circuit that shows the super-radiant phase transition (SRPT) in the thermal equilibrium. The existence of the SRPT is confirmed analytically in the limit of an infinite number of artificial atoms. We also perform numerical diagonalization of the Hamiltonian with a finite number of atoms and observe an asymptotic behavio...

We investigate theoretically the light amplification by stimulated emission of radiation (laser) in the ultrastrong light-matter interaction regime under the two-level and single-mode approximations. The conventional picture of the laser is broken under the ultrastrong interaction. Instead, we must explicitly discuss the dynamics of the electric fi...

From the Hamiltonian connecting the inside and outside of an Fabry-Perot cavity, which is derived from the Maxwell boundary conditions at a mirror of the cavity, a master equation of a non-Lindblad form is derived when the cavity embeds matters, although we can transform it to the Lindblad form by performing the rotating-wave approximation to the c...

When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process because there is no exciton level at that energy. However, according to the recent time-resolved measurements of P emiss...

The possibility of the superradiant phase transition in polarizable materials described by the minimal-coupling Hamiltonian with the longitudinal dipole-dipole interaction is examined. We try to reduce the Hamiltonian into the Dicke one in the homogeneous and infinite case, and discuss the stability of normal ground state by the formalism of the Gr...

In ultrastrong light-matter coupling regime, it is found theoretically that
lasing accompanies odd-order harmonics of radiation field both inside and
outside the cavity and even-order harmonics of atomic population. This
qualitative difference from the normal laser is generally obtained independent
of whether we choose the Coulomb gauge or the elec...

We have performed time-resolved photoluminescence (PL) spectroscopy for ZnO thin films with thicknesses of 90, 460, and 2800 nm under intense excitation condition. We clearly observed the P emission due to inelastic exciton-exciton scattering. It was found that, in the 460- and 2800-nm thick samples, the decay time of the P emission considerably de...

The generation of squeezed and entangled light fields is a crucial ingredient for the implementation of quantum information protocols. In this context, semiconductor materials offer a strong potential for the implementation of on-chip devices operating at the quantum level. Here we demonstrate a novel source of continuous variable squeezed light in...

When the light interacts with matters in a lossy cavity, in the standard
cavity quantum electrodynamics, the dissipation of the cavity fields is
characterized simply by the strengths of the two couplings: the light-matter
interaction and the system-environment coupling through the cavity mirror.
However, in the ultrastrong light-matter interaction...

We have investigated a coherent spectral change of exciton-polaritons in a ZnSe epitaxial layer. Using spectrally resolved four-wave mixing spectroscopy, we have observed the dramatically changed spectra, which exhibit an exchange of the dominant peak position between the different polariton branches. This result reflects the initial creation proce...

Intersubband polaritons are light-matter excitations originating from the strong coupling between an intersubband quantum well electronic transition and a microcavity photon mode. In this paper, we study how the Coulomb electron-electron interaction and the Pauli saturation of the electronic transitions affect the physics of intersubband polaritons...

In the standard theory of cavity quantum electrodynamics (QED), coupling
between photons inside and outside a cavity (cavity system and photonic
reservoir) is given conserving the total number of photons. However, when the
cavity photons (ultra)strongly interact with atoms or excitations in matters,
the system-reservoir coupling must be determined...

We have observed a coherent spectral change of exciton-polaritons in a ZnSe epitaxial layer through spectrally resolved four-wave mixing spectroscopy. The spectra exhibit an exchange of the dominant peak position between the different polariton branches depending on the delay time of the second pulse. This result reflects the initial creation proce...

Intersubband polaritons are light-matter excitations originating from the
strong coupling between an intersubband quantum well electronic transition and
a microcavity photon mode. In this paper we study how the Coulomb
electron-electron interaction and the Pauli saturation of the electronic
transitions affect the physics of intersubband polaritons....

We have investigated theoretically a dissipative polariton system in the
ultrastrong light-matter coupling regime without using the rotating-wave
approximation on system-reservoir coupling. Photons in a cavity and excitations
in matter respectively couple two large ensembles of harmonic oscillators
(photonic and excitonic reservoirs). Inheriting th...

Single photons are usually generated by non-resonant excitation of
single (artificial) atoms or by resonant excitation of Kerr systems with
a giant nonlinear interaction much larger than the losses of the system
(standard photon blockade). Here, we present a general class of
destructive quantum interference effects [1,2], which provide a robust
pro...

We propose a scheme for the resonant generation of counter-polarized single
photons in double asymmetric cavities with a small Kerr optical nonlinearity
(as that created by a semiconductor quantum well) compared to the mode
broadening. Due to the interplay between spatial intercavity tunneling and
polarization coupling, by weakly exciting with circ...

We have constructed a theoretical framework of biexciton-resonant hyperparametric scattering for pursuit of high-power and high-quality generation of entangled-photon pairs. Our framework is applicable to the nano-to-bulk crossover regime where the center-of-mass motions of excitons and biexcitons are confined and material surroundings and the pola...

We have constructed a theoretical framework of the biexciton-resonant
hyperparametric scattering for the pursuit of high-power and
high-quality generation of entangled photon pairs. Our framework is
applicable to nano-to-bulk crossover regime where the center-of-mass
motion of excitons and biexcitons is confined. Material surroundings and
the polar...

We have theoretically investigated the generation of entangled photons from biexcitons in a semiconductor film with a thickness in the nano-to-bulk crossover regime. In contrast with the cases of quantum dots and bulk materials, we can highly control the generated state of entangled photons through the design of a peculiar energy structure of excit...

We demonstrate parametric oscillation of discrete polariton states in a single squared GaAs/GaAlAs micropillar cavity. Resonantly exciting a selected polariton mode with a continuous wave pump laser, parametric oscillation is evidenced on the two neighbored modes (signal and idler). Abrupt switch-off of the device is observed under high excitation....

In a recent work [T. C. H. Liew and V. Savona, Phys. Rev. Lett. {\bf104},
183601 (2010)] it was numerically shown that in a photonic 'molecule'
consisting of two coupled cavities, near-resonant coherent excitation could
give rise to strong photon antibunching with a surprisingly weak nonlinearity.
Here, we show that a subtle quantum interference ef...

We investigate theoretically the nonlinear dynamics induced by an intense pump field in a disordered planar microcavity. Through a self-consistent theory, we show how the generation of quantum optical noise squeezing is affected by the breaking of the in-plane translational invariance and the occurrence of photon localization. We find that the gene...

An exciton and a photon behave as a polariton in macroscopic materials while they can be treated almost independently in nanostructured ones. We have theoretically investigated the crossover of exciton-photon coupled modes, each of which is characterized with a resonance frequency and a radiative decay rate, in a semiconductor film by continuously...

According to Fermi's golden rule, the radiative decay rate of excitons in a thin film becomes higher with increasing film thickness because of the increment of the interaction volume between excitons and photons. However, for a thick film, the decay rate is inversely proportional to the film thickness, because the excitons behave as polaritons and...

An electron and a hole attract each other due to the Coulomb interaction, and they behave as a quasi-particle called exciton. An exciton can be created by a photon which is incident into the material, and the created exciton collapses into a photon with a finite life time. After that, the emitted photon can also create another exciton at other posi...

We have theoretically studied the exciton superradiance which occurs under a phase-match between the exciton center-of-mass motion and the radiation field in a film with thickness over a light wavelength. With continuously increasing the film thickness, the radiative decay rates of excitons are calculated from extreme values of exciton correlation...

We have constructed QED theory for excitons with microscopic nonlocality or nonlocal susceptibility originating from their center-of-mass motion. The nonlocality must be properly considered in excitons' weak (center-of-mass) confinement regime where the long wavelength approximation is breakdown. This theory keeps good correspondences with its unde...

Anomalous exciton–radiation coupling in the sample-size regime beyond the long-wavelength approximation (LWA) is discussed. In high-quality samples fabricated using the latest technologies, the long-range excitonic coherence and non-LWA effect cause a peculiar interplay between the spatial structures of the radiation and excitonic waves, which emer...

We have constructed a complete quantum theory for an optical process of excitons with nonlocal susceptibility originating from their center-of-mass motion. This theory provides a practical calculation method for arbitrary-structured nano-to-macro scale dielectrics where excitons are weakly confined. We obtain good correspondences with underlying th...

We have constructed quantum electrodynamics theory for excitons whose center-of-mass motion is confined in arbitrary-structured dielectrics with considering their nonlocal optical susceptibility. We systematically investigate entangled-photon generation via biexcitons in nano-to-macro scale materials by this theory. It is revealed that nano-structu...

We construct full quantum theory for optical response of weakly confined excitonic systems with considering exciton dispersion and dielectric structures. It is revealed that confined exciton-polariton makes remarkable influences to entangled-photon generation.

We give a general method to calculate photonic band structure in the form of wave number k as a function of frequency omega, which is required whenever we want to calculate signal intensity related with photonic band structure. This method is based on the fact that the elements of the coefficient matrix for the plane wave expansion of the Maxwell e...

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