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Physical Review A

Coherent states of photonic dimers

Qihang Liu, Yao Zhou, Jung‐Tsung Shen

Published Nov 7, 2023• 2 citations
Publisher

Abstract

The concept of coherent states of single photons is generalized to the scenario wherein the fundamental building blocks are entangled two-photon states rather than single-photon states. To provide a concrete discussion, the case of photonic dimers (two-photon bound states) is explored. The coherent states of photonic dimers are defined. The correlation functions are calculated and expressed in terms of series expansion which is suitable for computational investigations. The quadrature properties and the squeezing effects are computed and compared with the experimental results at suitable regimes. Finally, the statistical properties of the coherent states of photonic dimers in the BEC and BCS limits are investigated. The correlation functions are shown to exhibit a BEC-BCS crossover behavior when the two-photon pairing correlations (i.e., the size of photonic dimers) are comparable to the average spacing between the dimer pairs.

DOI
10.1103/physreva.108.053705

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The concept of coherent states of single photons is generalized to the scenario wherein the fundamental building blocks are entangled two-photon states rather than single-photon states. To provide a concrete discussion, the case of photonic dimers (two-photon bound states) is explored. The coherent states of photonic dimers are defined. The correlation functions are calculated and expressed in terms of series expansion which is suitable for computational investigations. The quadrature properties and the squeezing effects are computed and compared with the experimental results at suitable regimes. Finally, the statistical properties of the coherent states of photonic dimers in the BEC and BCS limits are investigated. The correlation functions are shown to exhibit a BEC-BCS crossover behavior when the two-photon pairing correlations (i.e., the size of photonic dimers) are comparable to the average spacing between the dimer pairs.
Photon-photon correlations assume a pivotal significance in optical coherence. Recently, a new, to the best of our knowledge, type of quantum photonic states, the coherent state of photonic dimers, has been introduced, wherein the fundamental building blocks are two-photon bound states, instead of individual photons as in conventional lasers. In this Letter, we investigate the first-order coherence properties of the photonic-dimer coherent states, as well as the interference patterns in a double-slit interferometer setup, and compare with the coherence properties of other optical light sources, e.g., the conventional laser and the thermal light.
Coherent states are the quantum states, which give the closest description to classical states. Since their superpositions show quantum properties, research on these states has been of great interest. In addition, having nonclassical properties is necessary for quantum correlations. In this paper, we focus on two-mode entangled coherent states which are out of phase, and study the nonclassical properties such as squeezing of quadrature operators, antibunching and oscillatory photon statistics. Then we discuss about their entanglement, which is a quantum correlation in different conditions and compare the results.
We investigate the ground-state properties of a two-component attractive Fermi gas confined to a spherical surface, demonstrating how curvature and finite-size effects lead to qualitatively different behavior compared to the planar limit. In the strongly attractive regime, the system forms a Bose-Einstein condensate (BEC) of tightly bound dimers, yielding a gap and chemical potential that approach flat-space results with leading-order geometric corrections. However, as the attraction decreases, the system crosses over into a Bardeen-Cooper-Schrieffer (BCS) regime where the Cooper pair size becomes constrained by the sphere radius, producing pronounced deviations from flat-space behavior. We identify strong shell effects at magic particle numbers set by the single-particle spectrum, revealing the geometry-driven suppression of fermionic pairing at weak interactions. Our results highlight a fundamental interplay among geometry, quantum statistics, and interactions, motivating future experiments with shell-trapped ultracold fermions and inspiring geometry-controlled condensed matter devices.
Photonic Molecule, also called Photonic Dimer, is a quantum bound state of two photons. A photonic bound state has a specific entanglement of a Lorentzian anti-correlation in frequency space and two photons are in proximity due to its binding nature.Such signatures of a photonic molecule illuminates a potential tool that increases the two-photon microscopy efficiency to orders of magnitude higher. Here we numerically and analytically demonstrate the two-photon excitation efficiency between photonic molecules, long uncorrelated light pulses and ultrashort light pulses. The high excitation efficiency of a photonic molecule enables a saturation of fluorophores, such that the linear dependence of two-photon excitation crosssection does not necessarily hold. Also, we exhibits two possible methods to obtain the photonic molecules, as a fundamental possibility for a continuous photonic molecule source.
We calculate two-photon Raman spectra for fermionic atoms with interactions described by a single-mode mean-field BCS-BEC crossover theory. We compare calculated spectra of interacting and noninteracting systems and find that interactions lead to the appearance of correlated atomic pair signal due to Cooper pairs; splitting of peaks in the spectroscopic signal due to the gap in fermionic dispersion; and attenuation of signal due to the partial conversion of fermions into the corresponding single-mode dimer. By exploring the behavior of these effects, one can obtain quantitative estimates of the BCS parameters from the spectra.
Understanding the effects of interactions on the ground-state of a paired degenerate Fermi gas throughout the BEC-BCS crossover has been a long standing physical quest for which countless papers have been written. The crossover describes the smooth transition of the physics of a BEC of tightly bound dimers to that of a paired BCS superfluid. A Feshbach resonance is used to tune the interactions necessary to study the crossover. Right around resonance the interactions are expected to be parameterized by a single universal parameter beta [1--3]. This thesis describes a measurement of the axial size of paired, 6Li clouds in the BEC-BCS crossover and provides a comparison with theory. In the BEC regime, absolute measurements of the molecular scattering length are compared with the atomic scattering length and the ratio is compared with theory. Finally, a measurement of beta is made and compared with theory.