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Journal of the Physical Society of Japan

Synchronization Phenomena Originating from Quantum Effects of Photon Fields

Riku Sezaki, Kiyoshi Kobayashi, Kensuke Miyajima, Akira Ishikawa

Published Feb 7, 2022• 1 citation
Publisher

Abstract

Synchronization phenomena in multiple polarizations through quantum effects of photon fields have been observed as superradiance. These phenomena are often observed in macroscopic and classical systems. In contrast, superradiance is cooperative spontaneous emission, that is, a synchronization phenomenon in microscopic and quantum systems. In this study, we consider two emitters in a free photon field and investigate their exact phase dynamics. By analogy with the Kuramoto model, we illustrate clearly that the interaction of synchronization in superradiance originates from spontaneous emission owing to the quantum effects of photon fields. In contrast to superradiance in-phase, subradiance in antiphase is also possible. We illustrate the parameter diagram that determines whether in-phase or antiphase oscillation occurs. In addition, we find that a temporal transition occurs from the in-phase to antiphase synchronization owing to the energy dissipation. The results of this study will stimulate further research on synchronization phenomena in microscopic and quantum systems.

DOI
10.7566/jpsj.91.034401

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Synchronization phenomena in multiple polarizations through quantum effects of photon fields have been observed as superradiance. These phenomena are often observed in macroscopic and classical systems. In contrast, superradiance is cooperative spontaneous emission, that is, a synchronization phenomenon in microscopic and quantum systems. In this study, we consider two emitters in a free photon field and investigate their exact phase dynamics. By analogy with the Kuramoto model, we illustrate clearly that the interaction of synchronization in superradiance originates from spontaneous emission owing to the quantum effects of photon fields. In contrast to superradiance in-phase, subradiance in antiphase is also possible. We illustrate the parameter diagram that determines whether in-phase or antiphase oscillation occurs. In addition, we find that a temporal transition occurs from the in-phase to antiphase synchronization owing to the energy dissipation. The results of this study will stimulate further research on synchronization phenomena in microscopic and quantum systems.
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We study the relationship between the collective phenomena of super- and subradiance and spontaneous synchronization of quantum systems. To this aim we revisit the case of two detuned qubits interacting through a pure dissipative bosonic environment, which contains the minimal ingredients for our analysis. By using the Liouville formalism, we are able to find analytically the ultimate connection between these phenomena. We find that dynamical synchronization is due to the presence of longstanding coherence between the ground state of the system and the subradiant state. We finally show that, under pure dissipation, the emergence of spontaneous synchronization and of subradiant emission occur on the same time scale. This reciprocity is broken in the presence of dephasing noise.
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The present state of theoretical and experimental research on collective spontaneous emission (superradiance) in a system of radiators (atoms, molecules, or nuclei) is reviewed. The distinction between superradiance and the amplification of spontaneous emission is discussed. There is also a discussion of conditions under which the effect can be observed and of various theoretical methods for describing superradiance: the quantum single-mode and multimode models and the semiclassical approach. Theoretical papers on superradiance in systems with dimensions smaller than the wavelength of the radiation and also in extended systems are reviewed. It is shown that superradiance may occur in weakly amplifying media. A situation in which the superradiance is oscillatory is described. The possible use of superradiance to generate coherent emission in the x-ray and ? ranges is discussed. The superradiance accompanying Raman scattering of light in atomic and molecular media is studied. The theoretical results are compared with experimental observations of superradiance in the optical range. In an appendix, the nature of the phase transition in a system of radiators interacting through an electromagnetic field is discussed.
When atoms are coupled to a common electromagnetic environment, the exchange of photons through dipole-dipole interactions leads to the emergence of cooperative effects. As a particular example, superradiance arises from spontaneous emission when this exchange leads to constructive interference of the emitted photons. Here, we introduce an integrated method for studying cooperative radiation in many-body systems. This method, which allows to study extended systems with arbitrarily large number of particles, can be formulated by an effective, nonlinear, two-atom master equation that describes the dynamics using a closed form which treats single- and many-body terms on an equal footing. We apply this method to a homogeneous gas of initially inverted two-level atoms, and demonstrate the appearance of both superradiance and subradiance, identifying a many-body coherence term as the source of these cooperative effects. We describe the many-body induced broadening -- which is analytically found to scale with the optical depth of the system -- and light shifts, and distinguish spontaneous effects from induced ones. In addition, we theoretically predict the time-dependence of subradiance, and the phase change of the radiated field during the cooperative decay.
Fully-excited two-level atoms separated by less than the transition wavelength cooperatively emit light in a short burst, a phenomenon called superradiance by R. Dicke in 1954. The burst is characterized by a maximum intensity scaling with the square of the number of atoms NNN and a temporal width reduced by NNN compared to the single atom spontaneous decay time. Both effects are usually attributed to a synchronization of the electric dipole moments of the atoms occurring during the process of light emission. Contrary to this explanation, it was recently shown by use of a quantum path description that the peak intensity results from the quantum correlations among the atoms when occupying symmetric Dicke states. Here we investigate from this perspective the temporal evolution of the ensemble, starting in the small sample limit, i.e., when the atoms have mutual separations much smaller than the transition wavelength λλλ and pass down the ladder of symmetric Dicke states. In addition, we explore the temporal evolution for the case of non-interacting atoms with mutual separations much larger than λλλ. We show that in this case a similar superradiant burst of the emitted radiation is observed if the quantum correlations of the atoms are generated by conditional photon measurements retaining the atomic ensemble within or close to the symmetric subspace.