Dipole-dipole Interaction Strength Research Articles

Dense dipole-dipole-coupled two-level systems in a thermal bath

The quantum dynamics of a dense and dipole-dipole-coupled ensemble of two-level emitters interacting via their environmental thermostat is investigated. The static dipole-dipole interaction strengths are being considered strong enough but smaller than the transition frequency. Therefore, the established thermal equilibrium of ensemble's quantum dynamics is described with respect to the dipole-dipole-coupling strengths. We have demonstrated the quantum nature of the spontaneously scattered light field in this process for weaker thermal baths as well as non-negligible dipole-dipole couplings compared to the emitter's transition frequency. Furthermore, the collectively emitted photon intensity suppresses or enhances depending on the environmental thermal baths' intensities. Published by the American Physical Society 2024

Scaling Law for Kasha's Rule in Photoexcited Molecular Aggregates.

We study the photophysics of molecular aggregates from a quantum optics perspective, with emphasis on deriving scaling laws for the fast nonradiative relaxation of collective electronic excitations, referred to as Kasha's rule. Aggregates exhibit an energetically broad manifold of collective states with delocalized electronic excitations originating from near-field dipole-dipole exchanges between neighboring monomers. Photoexcitation at optical wavelengths, much larger than the monomer-monomer average separation, addresses almost exclusively symmetric collective states, which for an arrangement known as H-aggregate show an upward hypsochromic shift. The extremely fast subsequent nonradiative relaxation via intramolecular vibrational modes populates lower energy, subradiant states, resulting in effective inhibition of fluorescence. Our analytical treatment allows for the derivation of an approximate scaling law of this relaxation process, linear in the number of available low-energy vibrational modes and directly proportional to the dipole-dipole interaction strength between neighboring monomers.

Vibrational Dipole-Dipole Coupling and Long-Range Forces between Macromolecules.

Long-range interactions between biomacromolecules are considered important for directing intracellular processes. Recent studies have posited that interactions between oscillating dipoles are well-suited to mediating long-range forces because they are weakly screened by a dielectric environment. Here, we extend these studies and present a quantum electrodynamic mechanism for resonant interactions between vibrational transition dipole moments of molecules. We explicitly consider the molecular charge density oscillations as IR transition dipoles. This gives a physical, molecular assignment to the idea of oscillating dipoles and allows us to develop explicit expressions for the interactions that can be quantified using parameters known from experiment. Moreover, in the same framework, we can describe van der Waals forces. We use numerical calculations to estimate the strength of resonant vibrational dipole-dipole interactions over long distances and compare these estimates to the van der Waals interaction. We find that the resonant vibrational dipole-dipole interactions dominate over the long range.

Multiphoton blockade and antibunching in an optical cavity coupled with dipole-dipole-interacting Λ -type atoms

We study multiphoton blockade effects in a single-mode cavity interacting with two three-level atoms in $\Lambda$-configuration having position-dependent atom-field coupling. We consider the effects of dipole-dipole interaction (DDI) between the three-level atoms and show how the presence of DDI strongly influences the multiphoton blockade. For symmetric coupling of the atoms with the field, the DDI induces an asymmetry in the emission spectra as a function of pump field detuning. At positive detuning, the single-photon blockade gets stronger as a function of DDI strength, leading to photon antibunching. However, it becomes weaker at negative detuning and can also completely vanish. We show that this vanishing single-photon blockade is associated with a strong two-photon blockade, leading to two-photon bunching. Therefore, by just tuning the frequency of the pump field, we can achieve two very distinct features. We also study the effects of DDI when the atoms are asymmetrically coupled with the field and show that the proposed system exhibits two-photon bunching. We believe our results are important for the experimental realization of such systems where DDI may be present.

Topological defects of dipolar bose-einstein condensates with dresselhaus spin-orbit coupling in an anharmonic trap

We consider the topological defects and spin structures of binary Bose-Einstein condensates (BECs) with Dresselhaus spin-orbit coupling (D-SOC) and dipole-dipole interaction (DDI) in an anharmonic trap. The combined effects of D-SOC, DDI and anharmonic trap on the ground-state phases of the system are analyzed. Our results show various structural phase transitions can be achieved by adjusting the magnitudes of the D-SOC and DDI. Meantime, a ground-state phase diagram is given as a function of the D-SOC and DDI strengths. In addition, we find that tuning the D-SOC and the DDI can derive novel rich topological configurations, including ghost vortex, half-quantum vortex, skyrmion pair, vertical skyrmion string and horizontal skyrmion string.

Quantum correlations of localized atomic excitations in a disordered atomic chain

The atom-waveguide interface mediates significant and long-range light-matter interactions through guided modes. In this one-dimensional system, we theoretically investigate the excitation localization of multiple atomic excitations under strong position disorder. Deep in the localization side, we obtain the time evolutions of quantum correlations via Kubo cumulant expansions, which arise initially and become finite and leveled afterward, overtaking those without disorder. This indicates two distinct regimes in time: before the onset of excitation localization, the disorder engage the disturbance of quantum correlations, which is followed by disorder-assisted buildup of quantum correlations that maintain at a later stage owing to the absence of excitations diffusion. The crossing of distinct regimes is pushed further in time for longer-range correlations, which indicates a characteristic timescale needed for disorder to sustain them. We also explore the effect of directionality of couplings and resonant dipole-dipole interactions, which can drive the system toward the delocalized side when it is under chiral couplings or large dipole-dipole interaction strengths. The time-evolved quantum correlations can give insights into the studies of few-body localization phenomenon and nonequilibrium dynamics in open quantum systems.

Quantum phases and dynamics of dipolar spin-1 ferromagnetic Bose–Einstein condensates with spin–orbit coupling in a double-well potential

We consider a spin-1 ferromagnetic Bose–Einstein condensate with Rashba spin–orbit coupling (SOC) and dipole–dipole interaction (DDI) in a double-well potential. For the nonrotating case, the ground state of the system is obtained by using the imaginary time propagation method, and a ground-state phase diagram is given with respect to the SOC strength and the DDI strength. It is shown that the nonrotating system sustains rich exotic ground-state phases including the ox-horn phase with hidden vortices and antivortices, density droplet phase with vortex–antivortex pairs, modulated stripe phase with vortex–antivortex chains, checkerboard phase with visible vortex–antivortex chains and hidden vortex–antivortex pairs, and the triangular vortex–antivortex lattice phase. For the rotating case, the dynamics of the system is investigated by using a phenomenological dissipation model. The effects of SOC, DDI and rotation on the unique dynamic behaviors of the rotating system and the final steady-state structures are revealed and analyzed. In particular, the rotating system supports fascinating novel spin textures and skyrmion excitations, and experiences a series of topological structure transitions such as the transitions from an interlaced skyrmion–antiskyrmion lattice pair to a skyrmion–meron lattice pair and then to a meron lattice pair.

Dynamical effective field model for interacting ferrofluids: II. The proper relaxation time and effects of dynamic correlations

The recently proposed dynamical effective field model (DEFM) is quantitatively accurate for ferrofluid dynamics. It is derived in paper I within the framework of dynamical density functional theory (DDFT) along with a phenomenological description of nonadiabatic effects. However, it remains to clarify how the characteristic rotational relaxation time of a dressed particle, denoted by τ r , is quantitatively related to that of a bare particle, denoted by . By building macro-micro connections via two different routes, I reveal that under some gentle assumptions τ r can be identified with the mean time characterizing long-time rotational self-diffusion. I further introduce two simple but useful integrated correlation factors, describing the effects of quasi-static (adiabatic) and dynamic (nonadiabatic) inter-particle correlations, respectively. In terms of both the dynamic magnetic susceptibility is expressed in an illuminating and elegant form. Remarkably, it shows that the macro-micro connection is established via two successive steps: a dynamical coarse-graining with nonadiabatic effects accounted for by the dynamic factor, followed by equilibrium ensemble averaging captured by the static factor. By analyzing data from Brownian dynamics simulations on monodisperse interacting ferrofluids, I find is, somehow unexpectedly, insensitive to changes of particle volume fraction. A physical picture is proposed to explain it. Furthermore, an empirical formula is proposed to characterize the dependence of on dipole-dipole interaction strength. The DEFM supplemented with this formula leads to parameter-free predictions in good agreement with results from Brownian dynamics simulations. The theoretical developments presented in this paper may have important consequences to studies of ferrofluid dynamics in particular and other systems modeled by DDFTs in general.

Designing gate operations for single-ion quantum computing in rare-earth-ion-doped crystals

Quantum computers based on rare-earth-ion-doped crystals show promising properties in terms of scalability and connectivity if single ions can be used as qubits. Through simulations, we investigate gate operations on such qubits. We discuss how gate and system parameters affect gate errors, the required frequency bandwidth per qubit, and the risk for instantaneous spectral diffusion (ISD). Furthermore, we examine how uncertainties in the system parameters affect the gate errors, and how precisely the system needs to be known. We find gate errors for arbitrary single-qubit gates of $2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ when ISD is not considered and $3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ when we take heed to minimize it. Additionally, we construct two-qubit gates with errors ranging from $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\ensuremath{\rightarrow}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ over a broad range of dipole-dipole interaction strengths.

A feasible quantum heat engine driven by dipole-dipole interaction

We propose and investigate the implementable quantum heat engine cycle via tuning the dipole-dipole interaction strength in NMR-like setups with the low magnetic field. The nuclear spins of interacting H1 and C13 isotopes are considered as a realistic model. The cycle control procedure is provided by the finite-time driving the angle between vector connected two spins and the external static magnetic field in quantum adiabatic strokes. We observe that the almost frictionless region where the cycle operates as a heat engine with maximum power can be created in the considered realistic parameter range. Lastly, we make an estimation of thermalization times in the quantum isochoric strokes due to dipole-dipole relaxation.

Ground state of the polaron in an ultracold dipolar Fermi gas

An impurity atom immersed in an ultracold atomic Fermi gas can form a quasiparticle, so-called Fermi polaron, due to impurity-fermion interaction. We consider a three-dimensional homogeneous dipolar Fermi gas as a medium, where the interatomic dipole-dipole interaction (DDI) makes the Fermi surface deformed into a spheroidal shape, and, using a Chevy-type variational method, investigate the ground-state properties of the Fermi polaron: the effective mass, the momentum distribution of a particle-hole excitation, the drag parameter, and the medium-density modification around the impurity. These quantities are shown to exhibit spatial anisotropies in such a way as to reflect the momentum anisotropy of the background dipolar Fermi gas. We also give numerical results for the polaron properties at the unitarity limit of the impurity-fermion interaction in the case in which the impurity and fermion masses are equal. It has been found that the transverse effective mass and the transverse momentum drag parameter of the polaron both tend to decrease by $\ensuremath{\sim}10%$ when the DDI strength is raised from 0 up to around its critical value, while the longitudinal ones exhibit a very weak dependence on the DDI.

Quantum hydrodynamic theory of quantum fluctuations in dipolar Bose–Einstein condensate

Traditional quantum hydrodynamics of Bose-Einstein condensates (BECs) is restricted by the continuity and Euler equations. The quantum Bohm potential (the quantum part of the momentum flux) has a nontrivial part that can evolve under quantum fluctuations. The quantum fluctuations are the effect of the appearance of particles in the excited states during the evolution of BEC mainly consisting of the particles in the quantum state with the lowest energy. To cover this phenomenon in terms of hydrodynamic methods, we need to derive equations for the momentum flux and the current of the momentum flux. The current of the momentum flux evolution equation contains the interaction leading to the quantum fluctuations. In the dipolar BECs, we deal with the long-range interaction. Its contribution is proportional to the average macroscopic potential of the dipole-dipole interaction (DDI) appearing in the mean-field regime. The current of the momentum flux evolution equation contains the third derivative of this potential. It is responsible for the dipolar part of quantum fluctuations. Higher derivatives correspond to the small scale contributions of the DDI. The quantum fluctuations lead to the existence of the second wave solution. The quantum fluctuations introduce the instability of the BECs. If the dipole-dipole interaction is attractive, but being smaller than the repulsive short-range interaction presented by the first interaction constant, there is the long-wavelength instability. There is a more complex picture for the repulsive DDI. There is the small area with the long-wavelength instability that transits into a stability interval where two waves exist.

Disorder-assisted excitation localization in chirally coupled quantum emitters

One-dimensional quantum emitters with chiral couplings can exhibit nonreciprocal decay channels, along with light-induced dipole-dipole interactions mediated via an atom-waveguide interface. When the position disorders are introduced to such atomic array, we are able to identify the dynamical phase transition from excitation delocalization to localization, with an interplay between the directionality of decay rates and the strength of light-induced dipole-dipole interactions. Deep in the localization phase, its characteristic length decreases and saturates toward a reciprocal coupling regime, leading to a system dynamics whose ergodicity is strongly broken. We also find an interaction-driven re-entrant behavior of the localization phase and a reduction of level repulsion under strong disorder. The former coincides with a drop in the exponent of power-law decaying von Neumann entropy, which gives insights to a close relation between the preservation of entanglement and nonequilibrium dynamics in open quantum systems, while the latter presents a distinct narrow distribution of gap ratios in this particular disordered system.

Ground-state phases and spin textures of spin–orbit-coupled dipolar Bose–Einstein condensates in a rotating toroidal trap* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11475144 and 11047033), the Natural Science Foundation of Hebei Province, China (Grant Nos. A2019203049 and A2015203037), the University Science and Technology Foundation of Hebei Provincial Department of Education, China (Grant No. Z2017056), Science

We investigate the ground-state phases and spin textures of spin–orbit-coupled dipolar pseudo-spin-1/2 Bose–Einstein condensates in a rotating two-dimensional toroidal potential. The combined effects of dipole–dipole interaction (DDI), spin–orbit coupling (SOC), rotation, and interatomic interactions on the ground-state structures and topological defects of the system are analyzed systematically. For fixed SOC strength and rotation frequency, we provide a set of phase diagrams as a function of the DDI strength and the ratio between inter- and intra-species interactions. The system can show rich quantum phases including a half-quantum vortex, symmetrical (asymmetrical) phase with quantum droplets (QDs), asymmetrical segregated phase with hidden vortices (ASH phase), annular condensates with giant vortices, triangular (square) vortex lattice with QDs, and criss-cross vortex string lattice, depending on the competition between DDI and contact interaction. For given DDI strength and rotation frequency, the increase of the SOC strength leads to a structural phase transition from an ASH phase to a tetragonal vortex lattice then to a pentagonal vortex lattice and finally to a vortex necklace, which is also demonstrated by the momentum distributions. Without rotation, the interplay of DDI and SOC may result in the formation of a unique trumpet-shaped Bloch domain wall. In addition, the rotation effect is discussed. Furthermore, the system supports exotic topological excitations, such as a half-skyrmion (meron) string, triangular skyrmion lattice, skyrmion–half-skyrmion lattice, skyrmion–meron cluster, skyrmion–meron layered necklace, skyrmion–giant-skyrmion necklace lattice, and half-skyrmion–half-antiskyrmion necklace.

Topological defects in rotating spin–orbit-coupled dipolar spin-1 Bose–Einstein condensates

We consider the topological defects and spin structures of spin-1 Bose–Einstein condensates with spin–orbit coupling (SOC) and dipole–dipole interaction (DDI) in a rotating harmonic plus quartic trap. The combined effects of SOC, DDI and rotation on the ground-state phases of the system are analyzed. Our results show that for fixed rotation frequency structural phase transitions can be achieved by adjusting the magnitudes of the SOC and DDI. A ground-state phase diagram is given as a function of the SOC and DDI strengths. It is shown that the system exhibits rich quantum phases including vortex string phase with isolated density peaks (DPs), triangular (square) vortex lattice phase with DPs, checkerboard phase, and stripe phase with hidden vortices and antivortices. For given SOC and DDI strengths, the system can display pentagonal vortex lattice with DPs, vortex necklace with DPs, and exotic topological structure composed of multi-layer visible vortex necklaces, a hidden giant vortex and hidden vortex necklaces, depending on the rotation frequency. In addition, the system sustains fascinating novel spin textures and skyrmion excitations, such as an antiskyrmion pair, antiskyrmion-half-antiskyrmion (antiskyrmion–antimeron) cluster, skyrmion–antiskyrmion lattice, skyrmion–antiskyrmion cluster, skyrmion–antiskyrmion–meron–antimeron lattice, double-layer half-antiskyrmion necklaces, and composite giant-antiskyrmion–antimeron necklaces.

Controlling the Dipole Blockade and Ionization Rate of Rydberg Atoms in Strong Electric Fields.

We study a hitherto unexplored regime of the Rydberg excitation blockade using highly Stark-shifted, yet long-living, states of Rb atoms subject to electric fields above the classical ionization limit. Such states allow tuning the dipole-dipole interaction strength while their ionization rate can be changed over 2 orders of magnitude by small variations of the electric field. We demonstrate laser excitation of the interacting Rydberg states followed by their detection using controlled ionization and magnified imaging with high spatial and temporal resolution. Our work reveals new possibilities to engineer the interaction strength and dynamically control the ionization and detection of Rydberg atoms, which can be useful for realizing and assessing quantum simulators that vary in space and time.

Generation of density waves in dipolar quantum gases by time-periodic modulation of atomic interactions

We study the emergence of density waves in dipolar Bose-Einstein condensates (BEC) when the strength of dipole-dipole atomic interactions is periodically varied in time. The proposed theoretical model, based on the evolution of small perturbations of the background density, allows to compute the growth rate of instability (gain factor) for arbitrary set of input parameters, thus to identify the regions of instability against density waves. We find that among other modes of the system the roton mode is most effectively excited due to the contribution of sub-harmonics of the excitation frequency. The frequency of temporal oscillations of emerging density waves coincides with the half of the driving frequency, this being the hallmark of the parametric resonance, is characteristic to Faraday waves. The possibility to create density waves in dipolar BECs, which can persist after the emergence, has been demonstrated. The existence of a stationary spatially periodic solution of the nonlocal Gross-Pitaevskii equation has been discussed. The effect of three-body atomic interactions, which is relevant to condensates with increased density, upon the properties of emerging waves has been analyzed too. Significant modification of the condensate's excitation spectrum owing to three-body effects is shown.

Controlling photon bunching and antibunching of two quantum emitters near a core-shell sphere

The collective spontaneous emission of two point-dipole emitters near a plasmonic core-shell nanosphere is theoretically investigated. Based on the expansion of mode functions in vector spherical harmonics, we derive closed analytical expressions for both the cooperative decay rate and the dipole-dipole interaction strength associated with two point dipoles close to a sphere. Considering a plasmonic nanoshell containing a linearly amplifying medium inside the core, the second-order correlation function for the two emitters shows that it is possible to tune the photon emission, selecting either photon bunching or antibunching as a function of the polarization and position of the sphere. This result opens vistas to applications involving tunable single-photon sources in engineered artificial media.

Anisotropic diffusion of 2D superparamagnetic dusty plasma liquids

Diffusion of two-dimensional (2D) superparamagnetic dust grains interacting via both Yukawa and magnetic dipole-dipole interactions is investigated based on the Langevin dynamics simulation. The magnetic dipole moment, induced by the external magnetic field, is tilted at angle α relative to the 2D layer. It is demonstrated that the system in the liquidlike state behaves in anisotropic diffusion when α is larger than the agglomeration threshold, and the anisotropic diffusions are identified as the normal type. The anisotropy degree depends on the strength of magnetic dipole-dipole interaction and tilt angle of the magnetic dipole moment. An empirical law describing the anisotropy degree as a function of α is given.

Geometric Rydberg quantum gate with shortcuts to adiabaticity.

We propose a controlled-PHASE gate for neutral atoms in which one of the qubit state components adiabatically evolves along the multiple-atom eigenstate formed by the chirped laser pulse coupling to Rydberg states and intrinsic dipole-dipole exchange interactions and, consequently, accumulates an interaction-induced geometric phase. The geometric Rydberg gate is not limited by an adiabatic condition, which is sped up by shortcuts to adiabaticity (STA). Analyses show that an STA scheme is more robust than a non-adiabatic case against the variations of control parameters and faster than an adiabatic case, which protects from the decay of Rydberg states. Furthermore, an intermediate value of dipole-dipole interaction strength is enough for our scheme.