Thresholdless coherence in a superradiant laser.

Lasing threshold in the conventional lasers is the minimum input power required to initiate laser oscillation. It has been widely accepted that the conventional laser threshold occurring around a unity intracavity photon number can be eliminated in the input-output curve by making the so-called β parameter approach unity. The recent experiments, however, have revealed that even in this case the photon statistics still undergo a transition from coherent to thermal statistics when the intracavity mean photon number is decreased below unity.

Frequency pushing enhanced by an exceptional point in an atom-cavity coupled system.

We observed the frequency pushing of the cavity resonance as a result of the coupling of the cavity field with the ground state Ba in a high-Q cavity. A weak probe laser propagated along the axis of a Fabry-Pérot cavity while ground-state barium atoms traversed the cavity mode perpendicularly. By operating the atom-cavity composite in the vicinity of an exceptional point, we could observe a greatly enhanced frequency shift of the cavity transmission peak, which was pushed away from the atomic resonance, resulting in up to 41 ± 7 kHz frequency shift per atom from the empty cavity resonance.

Observation of a half-illuminated mode in an open Penrose cavity.

The illumination problem in mathematics questions the existence of a bounded region in which light rays from a point light source do not illuminate the whole region. Since Penrose disproved the illumination problem with elliptical reflective boundaries, the interest has mostly remained in ray optics mainly because there can be no completely dark region for light waves due to diffraction. Here, in a two-dimensional Penrose cavity with elliptical boundaries, we report experimental observation of a symmetry-broken mode in the long-wavelength regime with the half of the cavity region with reflection symmetry almost unilluminated in the steady state.

Simultaneous determination of the shape and refractive index of a deformed microjet cavity from its resonances.

Measuring the boundary shape of a deformed liquid microjet is of great importance for using it as an optical resonator for various applications. However, there have been technical challenges due to transparency and uncertainty in the refractive index of the liquid. In this study, we have developed a spectroscopic technique that enables simultaneous determination of the boundary shape and the refractive index of a liquid deformed microjet.

Locking Multi-Laser Frequencies to a Precision Wavelength Meter: Application to Cold Atoms.

We herein report a simultaneous frequency stabilization of two 780-nm external cavity diode lasers using a precision wavelength meter (WLM). The laser lock performance is characterized by the Allan deviation measurement in which we find σy=10-12 at an averaging time of 1000 s. We also obtain spectral profiles through a heterodyne spectroscopy, identifying the contribution of white and flicker noises to the laser linewidth.

Hyperradiance by a stream of phase-correlated atomic dipole pairs traversing a high-Q cavity.

Hyperradiance in which radiation rate exceeds that of superradiance has been theoretically investigated in various coherently-coupled emitter-field systems. In most cases, either proposed setups were experimentally challenging or the mean photon number in a cavity was limited. In this paper, with numerical simulations and analytic calculations, we demonstrate that significant hyperradiance with a large mean photon number can occur in a microlaser system, where pairs of two-level atoms prepared in quantum superposition states traverse a high-Q cavity in the presence of a pump field intersecting the cavity mode.

Practical lineshape of a laser operating near an exceptional point.

We present a practical laser linewidth broadening phenomenon in the viewpoint of high sensitivity of an exceptional point (EP). A stochastic simulation model is implemented to describe the fluctuations in the cavity resonance frequencies. The linewidth originated from external noises are maximized at the EP.

Schlieren-style stroboscopic nonscan imaging of the field-amplitudes of acoustic whispering gallery modes.

We report a schlieren-style stroboscopic phase-contrast field-amplitude imaging of two-dimensional acoustic whispering gallery modes in a circular shell cavity immersed in liquid. A schlieren signal is combined with a presplit reference beam to enable nonscan field-amplitude imaging. Excitation mechanisms of standing and traveling eigenmodes, respectively, are analyzed with acoustic ray simulations presented in a Poincaré surface of sections.

Maximal Shannon entropy in the vicinity of an exceptional point in an open microcavity.

The Shannon entropy as a measure of information contents is investigated around an exceptional point (EP) in an open elliptical microcavity as a non-Hermitian system. The Shannon entropy is maximized near the EP in the parameter space for two interacting modes, but the exact maximum position is slightly off the EP toward the weak interaction region while the slopes of the Shannon entropies diverge at the EP. The Shannon entropies also show discontinuity across a specific line in the parameter space, directly related to the exchange of the Shannon entropy as well as the mode patterns with that line as a boundary.

Observation of scalable sub-Poissonian-field lasing in a microlaser.

Sub-Poisson field with much reduced fluctuations in a cavity can boost quantum precision measurements via cavity-enhanced light-matter interactions. Strong coupling between an atom and a cavity mode has been utilized to generate highly sub-Poisson fields. However, a macroscopic number of optical intracavity photons with more than 3 dB variance reduction has not been possible.

Shannon entropy and avoided crossings in closed and open quantum billiards.

The relation between Shannon entropy and avoided crossings is investigated in dielectric microcavities. The Shannon entropy of the probability density for eigenfunctions in an open elliptic billiard as well as a closed quadrupole billiard increases as the center of the avoided crossing is approached. These results are opposite to those of atomic physics for electrons.

Coherent single-atom superradiance.

Superradiance is a quantum phenomenon emerging in macroscopic systems whereby correlated single atoms cooperatively emit photons. Demonstration of controlled collective atom-field interactions has resulted from the ability to directly imprint correlations with an atomic ensemble. Here we report cavity-mediated coherent single-atom superradiance: Single atoms with predefined correlation traverse a high-quality factor cavity one by one, emitting photons cooperatively with the atoms that have already gone through the cavity ( represents the number of atoms).

Observation of an exceptional point in a two-dimensional ultrasonic cavity of concentric circular shells.

We report observation of an exceptional point in circular shell ultrasonic cavities in both theory and experiment. In our theoretical analysis we first observe two interacting mode groups, fluid- and solid-based modes, in the acoustic cavities and then show the existence of an EP of these mode groups exhibiting a branch-point topological structure of eigenfrequencies around the EP. We then confirm the mode patterns as well as eigenfrequency structure around the EP in experiments employing the schlieren method, thereby demonstrating utility of ultrasound cavities as experimental platform for investigating non-Hermitian physics.

Experimental Observation of Bohr's Nonlinear Fluidic Surface Oscillation.

Niels Bohr in the early stage of his career developed a nonlinear theory of fluidic surface oscillation in order to study surface tension of liquids. His theory includes the nonlinear interaction between multipolar surface oscillation modes, surpassing the linear theory of Rayleigh and Lamb. It predicts a specific normalized magnitude of 0.

Nonlinear resonance-assisted tunneling induced by microcavity deformation.

Noncircular two-dimensional microcavities support directional output and strong confinement of light, making them suitable for various photonics applications. It is now of primary interest to control the interactions among the cavity modes since novel functionality and enhanced light-matter coupling can be realized through intermode interactions. However, the interaction Hamiltonian induced by cavity deformation is basically unknown, limiting practical utilization of intermode interactions.

Three-dimensional imaging of cavity vacuum with single atoms localized by a nanohole array.

Zero-point electromagnetic fields were first introduced to explain the origin of atomic spontaneous emission. Vacuum fluctuations associated with the zero-point energy in cavities are now utilized in quantum devices such as single-photon sources, quantum memories, switches and network nodes. Here we present three-dimensional (3D) imaging of vacuum fluctuations in a high-Q cavity based on the measurement of position-dependent emission of single atoms.

Controlled generation of single photons in a coupled atom-cavity system at a fast repetition-rate.

We have demonstrated high-speed controlled generation of single photons in a coupled atom-cavity system. A single 85Rb atom, pumped with a nanosecond-pulse laser, generates a single photon into the cavity mode, and the photon is then emitted out the cavity rapidly. By employing cavity parameters for a moderate coupling regime, the single-photon emission process was optimized for both high efficiency and fast bit rates up to 10 MHz.

Tunneling-induced spectral broadening of a single atom in a three-dimensional optical lattice.

We have investigated the spectral broadening in the near-resonance fluorescence spectrum of a single rubidium atom trapped in a three-dimensional (3D) optical lattice in a strong Lamb-Dicke regime. Besides the strong Rayleigh peak, the spectrum exhibited weak Stokes and anti-Stokes Raman sidebands. The line width of the Rayleigh peak for low potential depths was well explained by matter-wave tunneling between the first-two lowest vibrational states of 3D anisotropic harmonic potentials of adjacent local minima of the optical lattice.

Observation of resonance effects in the pump transmission of a chaotic microcavity.

We observed resonance effects on the transmission of a pump beam in a chaotic microcavity in an optimal free-space optical-pumping configuration. The far-field pattern of cavity transmission was significantly modified when the pump laser was resonant with a scar mode. From the difference between the non-resonant and on-resonance transmission patterns, we obtained the efficiency of the pump coupling into the scar mode to be as high as 45%, which is consistent with the recent excitation spectroscopy results of Yang et al.

Pump-induced dynamical tunneling in a deformed microcavity laser.

Pump-induced dynamical tunneling has been observed in free-space resonant optical pumping of a deformed microcavity by employing excitation spectroscopy. A focused-pump beam was injected into the cavity by refraction and then coupled to a high-Q cavity mode via dynamical tunneling. Pump-coupling efficiency as high as 50% and an effective coupling constant responsible for the tunneling were obtained from the observed pumping efficiency with a mode-mode coupling model.

Continuous control of the coupling constant in an atom-cavity system by using elliptic polarization and magnetic sublevels.

Atom-cavity coupling constant is a key parameter in cavity quantum electrodynamics for describing the interaction between an atom and a quantized electromagnetic field in a cavity. This paper reports a novel way to tune the coupling constant continuously by inducing an averaging of the atomic dipole moment over degenerate magnetic sublevels with elliptic polarization of the cavity field. We present an analytic solution of the stationary-state density matrix for this system with consideration of F -> F +1 hyperfine transition under a weak excitation condition.

Quasieigenstate coalescence in an atom-cavity quantum composite.

We report the first direct observation of an exceptional point (EP) in an open quantum composite of a single atom and a high-Q cavity mode. The atom-cavity coupling constant was made a continuous variable by utilizing the multisublevel nature of a single rubidium atom when it is optimally coupled to the cavity mode. The spectroscopic properties of quasieigenstates of the atom-cavity composite were experimentally investigated near the EP.

Rigorous criterion for characterizing correlated multiphoton emissions.

Strong correlation of photons, particularly in the single-photon regime, has recently been exploited for various applications in quantum information processing. Existing correlation measurements, however, do not fully characterize multi-photon correlation in a relevant context and may pose limitations in practical situations. We propose a conceptually rigorous, but easy-to-implement, criterion for detecting correlated multi-photon emission out of a quantum optical system, drawn from the context of wavefunction collapse.

Observation of an exceptional point in a chaotic optical microcavity.

We present spectroscopic observation of an exceptional point or the transition point between mode crossing and avoided mode crossing of neighboring quasieigenmodes in a chaotic optical microcavity of a large size parameter. The transition to the avoided mode crossing was impeded until the degree of deformation exceeded a threshold deformation owing to the system's openness also enhanced by the shape deformation. As a result, a singular topology was observed around the exceptional point on the eigenfrequency surfaces, resulting in fundamental inconsistency in mode labeling.

Atomic Solc filter: multi-resonant photoemission via periodic poling of atom-cavity coupling constant.

This paper describes a novel atom-cavity interaction induced by periodically poled atom-cavity coupling constant which leads to multiple narrow photoemission bands for an initially inverted two-level atom under the strong coupling condition. The emission bandpass narrowing has a close analogy with the folded Solc filter in the context of quasi-phase matching by periodic poling. We present a closed form solution of the emission probability at the end of interaction and deduce the multiple phase matching condition for this system which is programmable by the interaction time.