arXiv subjects

Kyungwon An

Publications and source records attributed to Kyungwon An.

At least 19 recordsLinked to original sources

arXiv2024

Room-temperature amplified transduction of infrared to visible photons

Frequency transduction, which converts photons from one energy level to another, provides a way to bridge different quantum devices. The frequency transduction has been studied across various systems and frequency ranges, depending on the applications. In particular, infrared photons are ideal for long-distance communication, but their detection efficiency is often low. Converting infrared photons to visible light, where affordable detectors with high quantum efficiency are widely available, would offer significant advantages. Here, we report an experimental demonstration of transduction of 1500-nm photons to 553-nm photons at room temperature using barium atoms of a three-level $\Lambda$ system. In our experiment conducted in free space, we could amplify the visible photons, achieving an internal efficiency of 1.49, exceeding unity. We also observed that the minimum transduction bandwidth is determined by the total decay rate of the excited state in the $\Lambda$-type energy levels. Moreover, we propose ways to improve the internal efficiency by 200-fold and to implement polarization-sensitive transduction in our scheme to be applicable in quantum information. The present work is a step forward for the integration of quantum devices at different energy levels as well as for the development of efficient infrared-photon detectors.

quant-ph↗

arXiv2024

Numerical analysis of a superradiance-sideband-assisted laser with a zero frequency pulling and a narrow linewidth

Numerical simulations based on the quantum Langevin equations have been performed for a large number of two-level atoms in a beam interacting with a low-Q cavity with the atomic initial superposition states close to the north pole of the Bloch sphere. When the pump Rabi frequency was modulated at $\Delta_{pa}$ with zero pump-atom detuning for various cavity-atom detunings, we obtained a lasing peak at the atomic resonance and superradiant lasing peaks at $\pm\Delta_{pa}$ simultaneously while the central peak exhibiting a zero frequency pulling coefficient. The linewidth of the central peak was reduced beyond the gain narrowing as the mean number of atoms was increased, resulting in a minimum linewidth as small as a millionth of the atomic or cavity linewdith. A pump carrier detuning caused asymmetric heights for the side superradiant peaks, the height difference of which can be used to lock the pump laser to the atom within the linewidth of the central lasing peak. Our results may lead to development of a new type of ultra-stable active optical clocks for future frequency standards when applied to proper atomic systems.

quant-ph↗

arXiv2024

A photonic quantum engine driven by superradiance

Performance of nano- and micro-scale heat engines can be improved with a help from quantum mechanical phenomena. Recently, heat reservoirs with quantum coherence have been proposed to enhance engine performance beyond the Carnot limit even with a single reservoir. However, no physical realizations have been achieved so far. Here, we report the first proof-of-principle experimental demonstration of a photonic quantum engine driven by superradiance employing a single heat reservoir composed of atoms and photonic vacuum. Reservoir atoms prepared in a quantum coherent superposition state underwent superradiance while traversing the cavity. This led to about 40-fold increase of the effective engine temperature, resulting in a near-unity engine efficiency. Moreover, the observed engine output power grew quadratically with respect to the atomic injection rate. Our work can be utilized in quantum mechanical heat transfer as well as in boosting engine powers, opening a pathway to development of photomechanical devices that run on quantum coherence embedded in heat baths.

quant-ph↗

arXiv2024

Pushing single atoms near an optical cavity

Optical scattering force is used to reduce the loading time of single atoms to a cavity mode. Releasing a cold atomic ensemble above the resonator, we apply a push beam along the direction of gravity, offering fast atomic transport with narrow velocity distribution. We also observe in real time that, when the push beam is illuminated against gravity, single atoms slow down and even turn around in the mode, through the cavity-transmission measurement. Our method can be employed to make atom-cavity experiments more efficient.

quant-ph↗

arXiv2022

Entropic comparison of Landau-Zener and Demkov interactions in the phase space of a quadrupole billiard

We investigate two types of avoided crossings in a chaotic billiard within the framework of information theory. The Shannon entropy in the phase space for the Landau--Zener interaction increases as the center of the avoided crossing is approached. Meanwhile, that for the Demkov interaction decreases as the center of avoided crossing is passed by with an increase in the deformation parameter. This feature can provide a new indicator for scar formation. In addition, it is found that the Fisher information of the Landau--Zener interaction is significantly larger than that of the Demkov interaction.

nlin.CD↗

arXiv2022

Precise and extensive characterization of an optical resonator for cavity-based quantum networks

Cavity-based quantum node is a competitive platform for distributed quantum networks. Here, we characterize a high-finesse Fabry-Perot optical resonator for coupling single or few atomic quantum registers. Our cavity consists of two mirrors with different reflectivities: One has minimal optical loss, and the other high transmission loss where more than 90% of the intracavity photons would be emitted. Cavity finesse, birefringent effects, and mechanical resonances are measured using the lasers at 780, 782, and 795 nm. In order to obtain cavity geometric parameters, we drive the adjacent longitudinal or transverse modes with two lasers simultaneously, and measure those frequencies using a precision wavelength meter (WLM). A major novelty of this method is that the parameters' uncertainties are solely determined by the resolution of the WLM, eliminating all of the temporal environment fluctuations. Our scheme makes it possible to quantify the atom-cavity coupling constant up to four significant figures, the most precise and accurate estimation so far, which would become a key ingredient for benchmarking a cavity-based quantum node. Furthermore, the distortion of polarized photonic qubits would be minimized owing to the small birefringent splitting, below 4.9% of the cavity linewidth. Our system should operate in the intermediate atom-cavity coupling regime that would allow us to implement various quantum network protocols.

quant-ph↗

arXiv2021

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 $σ_{y}=10^{-12}$ 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. The frequency drift of the WLM is measured to be about 2.0(4) MHz over 36 hours. Utilizing the two lasers as a cooling and repumping field, we demonstrate a magneto-optical trap of $^{87}$Rb atoms near a high-finesse optical cavity. Our laser stabilization technique operates at broad wavelength range without a radio frequency element.

quant-ph↗

arXiv2019

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 3dB variance reduction in a setting of laser has not been possible. Here, we report sub-Poisson field lasing in a microlaser operating with hundreds of atoms with well-regulated atom-cavity coupling and interaction time. Its photon-number variance was 4dB below the standard quantum limit while the intracavity mean photon number scalable up to 600. The highly sub-Poisson photon statistics were not deteriorated by simultaneous interaction of a large number of atoms. Our finding suggests an effective pathway to widely scalable quasi-Fock-state lasing at the macroscopic scale.

quant-ph↗

arXiv2019

Realization of superabsorption by time reversal of superradiance

Emission and absorption of light lie at the heart of light-matter interaction. Although emission and absorption rates are regarded as intrinsic properties of atoms and molecules, various ways to modify these rates have been sought in applications such as quantum information processing, metrology and light-energy harvesting. One promising approach is to utilize collective behaviour of emitters in the same way as in superradiance5. Although superradiance has been observed in diverse systems, its conceptual counterpart in absorption has never been realized11 until now. Here we demonstrate enhanced cooperative absorption - superabsorption - by implementing a time-reversal process of superradiance. The observed superabsorption rate is much higher than that of ordinary absorption, with the number of absorbed photons scaling with the square of the number of atoms, exhibiting the cooperative nature of superabsorption. The present superabsorption - which performs beyond the limitations of conventional absorption - can facilitate weak-signal sensing, light-energy harvesting and light-matter quantum interfaces

quant-ph↗

arXiv2019

Behavior of Shannon entropy around an exceptional point in an open microcavity

We have investigated the Shannon entropy around an exceptional point (EP) in an open elliptical microcavity as a non-Hermitian system. The Shannon entropy had an extreme value at the EP in the parameter space. The Shannon entropies showed discontinuity across a specific line in the parameter space, directly related to the occurrence of exchange of the Shannon entropy as well as the mode patterns with that line as a boundary. This feature results in a nontrivial topological structure of the Shannon entropy surfaces.

quant-ph↗

arXiv2018

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

The relation between the Shannon entropy and avoided crossings is investigated in dielectric microcavities. The Shannon entropy of probability density for eigenfunctions in an open elliptic billiard as well as a closed quadrupole billiard increases as the center of avoided crossing is approached. These results are opposite to those of atomic physics for electrons. It is found that the collective Lamb shift of the open quantum system and the symmetry breaking in the closed chaotic quantum system give equivalent effects to the Shannon entropy.

quant-ph↗

arXiv2018

Coherent single-atom superradiance

Quantum effects, prevalent in the microscopic scale, generally elusive in macroscopic systems due to dissipation and decoherence. Quantum phenomena in large systems emerge only when particles are strongly correlated as in superconductors and superfluids. Cooperative interaction of correlated atoms with electromagnetic fields leads to superradiance, the enhanced quantum radiation phenomenon, exhibiting novel physics such as quantum Dicke phase and ultranarrow linewidth for optical clocks. Recent researches to imprint atomic correlation directly demonstrated controllable collective atom-field interactions. Here, we report cavity-mediated coherent single-atom superradiance. Single atoms with predefined correlation traverse a high-Q cavity one by one, emitting photons cooperatively with the atoms already gone through the cavity. Such collective behavior of time-separated atoms is mediated by the long-lived cavity field. As a result, a coherent field is generated in the steady state, whose intensity varies as the square of the number of traversing atoms during the cavity decay time, exhibiting more than ten-fold enhancement from noncollective cases. The correlation among single atoms is prepared with the aligned atomic phase achieved by nanometer-precision position control of atoms with a nanohole-array aperture. The present work deepens our understanding of the collective matter-light interaction and provides an advanced platform for phase-controlled atom-field interactions.

quant-ph↗

arXiv2017

Observation of a non-equilibrium steady state of cold atoms in a moving optical lattice

We investigated non-equilibrium atomic dynamics in a moving optical lattice via observation of atomic resonance fluorescence spectrum. A three-dimensional optical lattice was generated in a phase-stabilized magneto-optical trap (MOT) and the lattice was made to move by introducing a detuning between the counter-propagating trap lasers. A non-equilibrium steady states (NESS's) of atoms was then established in the hybrid of the moving optical lattice and the surrounding MOT. A part of atoms were localized and transported in the moving optical lattice and the rest were not localized in the lattice while trapped as a cold gas in the MOT. These motional states coexisted with continuous transition between them. As the speed of the lattice increased, the population of the non-localized state increased in a stepwise fashion due to the existence of bound states at the local minima of the lattice potential. A deterministic rate-equation model for atomic populations in those motional states was introduced in order to explain the experimental results. The model calculations then well reproduced the key features of the experimental observations, confirming the existence of an NESS in the cold atom system.

physics.atom-ph↗

arXiv2016

Indirect measurement of three-photon correlation in nonclassical light sources

We have observed the three-photon correlation in nonclassical light sources by using an indirect measurement scheme based on the dead time effect of photon-counting detectors. We first developed a general theory which enables us to extract the three-photon correlation from the two-photon correlation of an arbitrary light source measured with detectors with finite dead times. We then confirmed the validity of our measurement scheme in experiments done with the cavity-QED microlaser operating with a large intra-cavity mean photon number exhibiting both sub- and super-poissoniaq photon statistics. The experimental results were in a good agreement with the theoretical expectation. Our measurement scheme provides an alternative approach for N-photon correlation measurement employing (N-1) detectors and thus a reduced measurement time for a given signal-to-noise ratio, compared to the usual scheme requiring N detectors.

quant-ph↗

arXiv2016

Generation of an optical Schrödinger-cat-like state in a non-ideal cavity by injecting opposite-phase atomic dipoles

We propose a method to generate an optical Schrödinger-cat-like state in a cavity in a substantial decoherence regime. Even when the cavity decay rate is considerably large, a cat-like state can be generated in a laser-like setting if the gain for the field is larger than the loss. Under the condition that opposite-phase atomic dipoles repeatedly traverse the cavity, the cavity field converges to a squeezed vacuum state in a steady state. A Schrödinger-cat-like state is then generated when a single photon decay occurs. The phase-space distribution of the cat state can be revealed in homodyne detection by using the decaying photon as a herald event. Quantum trajectory simulation was used to identify the conditions for the Schrödinger-cat-like state formation as well as to analyze the properties of those states. Based on these simulations, possible experiments are proposed within the reach of the current technology.

quant-ph↗

arXiv2015

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 $0.41\dot{6}η^2$ for an octapolar component, nonlinearly induced by a quadrupolar one with a magnitude of $η$ much less than unity. No experimental confirmation on this prediction has been reported. Nonetheless, accurate determination of multipolar components is important as in optical fiber spinning, film blowing and recently in optofluidic microcavities for ray and wave chaos studies and photonics applications. Here, we report experimental verification of his theory. By using optical forward diffraction, we measured the cross-sectional boundary profiles at extreme positions of a surface-oscillating liquid column ejected from a deformed microscopic orifice. We obtained a coefficient of $0.42\pm0.08$ consistently under various experimental conditions. We also measured the resonance mode spectrum of a two-dimensional cavity formed by the cross-sectional segment of the liquid jet. The observed spectra agree well with wave calculations assuming a coefficient of $0.415\pm0.010$. Our measurements establish the first experimental observation of Bohr's hydrodynamic theory.

physics.optics↗

arXiv2015

Fluorescence spectra of atoms in a phase-stabilized magneto-optical trap as an optical lattice

We present a study on characteristics of a magneto-optical trap (MOT) as an optical lattice. Fluorescence spectra of atoms trapped in a MOT with a passively phase-stabilized beam configuration have been measured by means of the photon-counting heterodyne spectroscopy. We observe a narrow Rayleigh peak and well-resolved Raman sidebands in the fluorescence spectra which clearly show that the MOT itself behaves as a three-dimensional optical lattice. Optical-lattice-like properties of the phase-stabilized MOT such as vibrational frequencies and lineshapes of Rayleigh peak and Raman sidebands are investigated systematically for various trap conditions.

physics.atom-ph↗

arXiv2015

Correction to the detector-dead-time effect on the second-order correlation of stationary sub-Poissonian light in a two-detector configuration

Exact measurement of the second-order correlation function $g^{(2)}(t)$ of a light source is essential when investigating the photon statistics and the light generation process of the source. For a stationary single-mode light source, Mandel Q factor is directly related to $g^{(2)}(t)$. For a large mean photon number in the mode, the deviation of $g^{(2)}(t)$ from unity is so small that even a tiny error in measuring $g^{(2)}(t)$ would result in an inaccurate Mandel Q. In this work, we have found that detector dead time can induce a serious error in $g^{(2)}(t)$ and thus in Mandel Q in those cases even in a two-detector configuration. Our finding contradicts the conventional understanding that detector dead time would not affect $g^{(2)}(t)$ in two-detector configurations. Utilizing the cavity-QED microlaser, a well-established sub-Poissonian light source, we measured $g^{(2)}(t)$ with two different types of photodetectors with different dead time. We also introduced prolonged dead time by intentionally deleting the photodetection events following a preceding one within a specified time interval. We found that the observed Q of the cavity-QED microlaser was underestimated by 19% with respect to the dead-time-free Q when its mean photon number was about 600. We derived an analytic formula which well explains the behavior of the $g^{(2)}(t)$ as a function of the dead time.

physics.optics↗