Identical Atoms Research Articles

Quantum engineering of superdark excited states in arrays of atoms

We suggest a regular method of achieving an extremely long lifetime of a collective singly excited state in a generic small-size ensemble of N identical atoms. The decay rate \Gamma_N of such a `superdark' state can be as small as \Gamma_N \propto \Gamma(r/\lambda)^{2(N-1)} (\Gamma is the radiative decay rate of an individual atom, r and \lambda are the system size and the wavelength of the radiation, respectively), i.e., considerably smaller than in any of the systems suggested up to now. The method is based on a special fine tuning of the atomic Hamiltonian: namely, on a proper position-dependent adjustment of atomic transition frequencies. So chosen set of the control parameters is sufficient to ensure the minimum of the spontaneous decay rate of the engineered state in a generic ensemble of atoms (`qubits').

Tripartite Entanglement Dynamics in the System Consisting of Three Damping Jaynes-Cummings Models

In this paper we consider the situation that three identical two-level atoms resonantly interact with three distant single-mode optical cavities via a one-photon hopping separately, and there exist the phenomena of atom-decay and cavity-decay. Under Jaynes-Cummings model the time evolution of the system is given. The tripartite negativity is used to quantify the degree of tripartite entanglement. The tripartite entanglement dynamics among atoms and among cavities are studied. The influences of the atom-decay and the cavity-decay on tripartite entanglement are discussed. The results obtained using the numerical method show that the tripartite entanglement among atoms and that among cavities all display damping oscillation behavior. On the other hand, as the atomic decay rate increases, their decays are accelerated, and so do they with the increase of cavity decay rate.

Joint effects of entanglement and symmetrization: Physical properties and exclusion

Entanglement and symmetrization lead to non-separable states that can modify physical properties. Using the example of atomic absorption we compare both types of effects when they are relevant at once. The presence of multi-particle superpositions largely alters the absorption rates of identical atoms, even inhibiting the dependence on overlapping for fermions. We also identify a set of non-standard excluded states related to multi-fermion superposition that naturally emerge in this context. We propose an arrangement based on the dissociation of molecules to test these ideas.

Theoretical Investigations of Rate Coefficients for H + O3 and HO2 + O Reactions on a Full-Dimensional Potential Energy Surface.

In this work, a machine learning method is used to construct a high-fidelity multichannel global reactive potential energy surface (PES) for the HO3 system from 21452 high-level ab initio calculations at the explicitly correlated multireference configuration interaction (MRCI-F12) level of theory. The permutation invariance of the PES with respect to the three identical oxygen atoms is enforced using permutation invariant polynomials (PIPs) in the input layer of a neural network (NN). This PIP-NN representation is highly faithful to the ab initio points, with a root-mean-square error of 0.20 kcal/mol. Using this PES, the kinetics of H + O3 → OH + O2 (R1) and HO2 + O → OH + O2 (R2) reactions were investigated using a quasi-classical trajectory method over a wide temperature range (200-2000 K). It was found that the calculated thermal rate coefficients of R1 and R2, exhibiting positive and negative temperature dependences, respectively, are in reasonably good agreement with most experimental measured values. These temperature dependences can be attributed to the presence and absence of an entrance channel potential barrier.

Spontaneous formation of a macroscopically extended coherent state

It is a straightforward result of electromagnetism that dipole oscillators radiate more strongly when they are synchronized, and that if there are $N$ dipoles, the overall emitted intensity scales with $N^2$. In atomic physics, such an enhanced radiative property appears when coherence among two-level identical atoms is established, and is well-known as "superradiance" \cite{Dicke:1954aa}. In superfluorescence (SF), atomic coherence develops via a self-organisation process stemming from the common radiated field, starting from a incoherently prepared population inversion \cite{Bonifacio:1975aa}. First demonstrated in a gas \cite{Skribanowitz:1973} and later in condensed matter systems \cite{Florian:1984}, its potential is currently being investigated in the fields of ultranarrow linewidth laser development for fundamental tests in physics \cite{Meiser:2009,Meiser:2010,Bohnet:2012aa,Norcia:2016, Norcia:2018}, and for the development of devices enabling entangled multi-photon quantum light sources \cite{Raino:2018aa,Angerer:2018aa}. A barely developed aspect in superradiance is related to the properties of the dipole array that generates the pulsed radiation field. In this work we establish the experimental conditions for formation of a macroscopic dipole via superfluorescence, involving the remarkable number of $4\times10^{12}$ atoms. Even though rapidly evolving in time, it represents a flexible test-bed in quantum optics. Self-driven atom dynamics, without the mediation of cavity QED nor quantum dots or quantum well structures, is observed in a cryogenically-cooled rare-earth doped material. We present clear evidence of a decay rate that is enhanced by more than 1-million times compared to that of independently emitting atoms. We thoroughly resolve the dynamics by directly measuring the intensity of the emitted radiation as a function of time.

Phase diagram of a pure substance?

We discuss a two-dimensional model that leads to a phase diagram which reproduces at least qualitatively that of a pure substance. It is known that a spring between two atoms is due to a bond created by the interaction between the electrons of both atoms. Consequently a variation of the quantum state of the electrons involved in the bond can modify the elastic force constant of the spring. That is a kind of atom-phonon coupling. We consider a square lattice of N identical atoms linked by springs between atoms first and second nearest neighbors. We assume that the elastic force constants of all the springs can vary. The system is studied by using a variational method. First order phase transitions are obtained. The phase diagram of the system displays the features observed in the phase diagram of a pure substance: three thermodynamic phases, three coexistence curves, one triple point and one critical point. Applied pressure can be introduced in the model.

Intramolecular sp2-sp3 Disequalization of Chemically Identical Sulfonamide Nitrogen Atoms: Single Crystal X-Ray Diffraction Characterization, Hirshfeld Surface Analysis and DFT Calculations of N-Substituted Hexahydro-1,3,5-Triazines

In this manuscript, the synthesis and single crystal X-ray diffraction characterization of four N-substituted 1,3,5-triazinanes are reported along with a detailed analysis of the noncovalent interactions observed in the solid state architecture to these compounds, focusing on C–H···π and C–H···O H-bonding interactions. These noncovalent contacts have been characterized energetically by using DFT calculations and also by Hirshfeld surface analysis. In addition, the supramolecular assemblies have been characterized using the quantum theory of “atoms-in-molecules” (QTAIM) and molecular electrostatic potential (MEP) calculations. The XRD analysis revealed a never before observed feature of the crystalline structure of some molecules: symmetrically substituted 1,3,5-triazacyclohexanes possess two chemically identical sulfonamide nitrogen atoms in different sp2 and sp3-hybridizations.

Computational Evidence for Homonuclear GeIGeI Dative Bonds.

Dative bonds between identical atoms of the same formal oxidation state are formed as geometric and electronic constraints turn an otherwise lone pair into a bond. Calculations at the B3LYP/6-31+g(d,p) level supported by additional atoms in molecules (AIM) and electron localization function (ELF) analyses on selected germanium polycations are used to prove the concept. The electron density (ρ) is not equally shared between the two atoms of such a homonuclear dative bond (HDB), and the associated charge transfer results in formal charges contradicting chemical convention.

Collisions of solitary waves in condensates beyond mean-field theory

Bright solitary waves in a Bose-Einstein condensate contain thousands of identical atoms held together despite their only weakly attractive contact interactions. They nonetheless behave like a compound object, staying whole in collisions, with their collision properties strongly affected by inter-soliton quantum coherence. We show that separate solitary waves decohere due to phase diffusion, dependent on their effective ambient temperature, after which their initial mean-field relative phases are no longer well defined or relevant for collisions. In this situation, collisions occur predominantly repulsively and can no longer be described within mean field theory. When considering the time-scales involved in recent solitary wave experiments where non-equilibrium phenomena play an important role, these features could explain the predominantly repulsive collision dynamics observed in most condensate soliton train experiments.

Near-Resonant Light Scattering by a Subwavelength Ensemble of Identical Atoms.

We study theoretically the scattering of light by an ensemble of N resonant atoms in a subwavelength volume. We consider the low intensity regime so that each atom responds linearly to the field. While N noninteracting atoms would scatter N^{2} more than a single atom, we find that N interacting atoms scatter less than a single atom near resonance. In addition, the scattered power presents strong fluctuations, either from one realization to another or when varying the excitation frequency. We analyze this counterintuitive behavior in terms of collective modes resulting from the light-induced dipole-dipole interactions. We find that for small samples and sufficiently large atom number, their properties are governed only by their volume.

Interaction of a para-Bose state with two two-level atoms: control of dissipation by a local classical field

We consider a parity-deformed Jaynes–Cummings model (JCM) consisting of two identical two-level atoms interacting with a single-mode para-Bose field in a cavity. Compared to the standard JCM, this model introduces the action of a specific local classical field as an external control which can be simulated through an intensity-dependent two-atom JCM with a particular intensity-dependent function. The conservation of the total excitation number and overall parity provides a complete number parity basis for the Hilbert space of the system. In this basis, the matrix representation of the system’s Hamiltonian becomes block diagonal. The corresponding eigenvalues and eigenstates of the system are then analytically calculated by diagonalizing the block matrices. In continuum, we investigate the dynamical evolution of some physical properties such as entanglement, atomic inversion, photon statistics and atomic squeezing in the parity representation, with emphasis on the control role of the local external classical fields. In addition, in the presence of the cavity decay, we study the effect of cavity dissipation on the quantum dynamics of the system. In the ideal case (without the cavity dissipation), we show that the initial states of the system with even–odd parity manifest qualitatively distinct control role of the local external classical fields on the dynamical behavior of the physical properties. In the dissipation regime, we show that the control of local classical fields has the general effect to improve the stabilization of the quantum properties of the system generated during the time evolution.

Radiation-reaction-induced transitions of two maximally entangled atoms in noninertial motion

We apply the DDC formalism [proposed by Dalibard, Dupont-Roc and Cohen-Tannoudji] to study the average rate of change of energy of two identical two-level atoms interacting with the vacuum massless scalar field in synchronized motion along stationary trajectories. By separating the contributions of vacuum fluctuations and atomic radiation reaction, we first show that for the two-atom system initially prepared in the factorizable eigenstates $|g_Ag_B\rangle$ and $|e_Ae_B\rangle$, where $g$ and $e$ represent the ground state and the excited state of a single atom respectively, both vacuum fluctuations and atomic radiation reaction contribute to the average rate of change of energy of the two-atom system, and the contribution of vacuum fluctuations is independent of the interatomic separation while that of atomic radiation reaction is dependent on it. This is contrary to the existing results in the literature where vacuum fluctuations are interatomic-separation dependent. However, if the two-atom system is initially prepared in the unfactorizable symmetric/antisymmetric entangled state, the average rate of change of energy of the two-atom system is never perturbed by the vacuum fluctuations, but is totally a result of the atomic radiation reaction. We then consider two special cases of motion of the two-atom system which is initially prepared in the symmetric/antisymmetric entangled state, i.e., synchronized inertial motion and synchronized uniform acceleration. In contrast to the average rate of change of energy of a single uniformly accelerated atom, the average rate of change of energy of the uniformly accelerated two-atom system is nonthermal-like. The effects of noninertial motion on the transitions of states of the two correlated atoms are also discussed.

Concurrence of two identical atoms in a rectangular waveguide: linear approximation with single excitation

We study two two-level systems (TLSs) interacting with a reservoir of guided modes confined in a rectangular waveguide. For the energy separation of the identical TLSs far away from the cutoff frequencies of transverse modes, the delay-differential equations are obtained with single excitation initial in the TLSs. The effects of the inter-TLS distance on the time evolution of the concurrence of the TLSs are examined.

Matrix elements of the dipole-dipole interaction between two two-level atoms distanced arbitrarily from each other

Purpose. As a standard model for describing the processes of a resonant transmission of quantum information on arbitrary distances is the system of two identical two-level atoms, one of which is under radiation of the field of real photons. Such a system can serve as a basis for the construction of an element basis of quantum computers. The purpose of this paper is to study the different modes of dynamics of a system of two identical two-level atoms when they interacts with the field of real photons. Methods. In this paper, we propose a general approach to the description of the processes for the transfer of quantum information from one atom-qubit to another on the arbitrary interatomic distances, which includes two types of new physical effects: the attenuation of quantum states and the retardation of the dipole-dipole interaction. Results. The optical properties of a system of two identical two-level atoms in collective (symmetric Ψ s and antisymmetric Ψ a ) Bell states at arbitrary interatomic distances are investigated. The closed analytical expressions for the shifts and widths of the considered collective states are considered, taking into account the retarded dipole-dipole interaction of atoms. In calculation of the radial matrix elements of the dipole-dipole interaction, the wave functions of the model Fues potential are used. Conclusions. A detailed study of the mechanisms of resonant transmission of the excitation energy at arbitrary distances between the two-element atoms has an important practical significance for the physical realization of the logical operator CNOT.

Study That Led to the Discovery of Rhombohedral Close Packing and Hexagonal Close Packing as Two New and Independent Lattices

An ab-initio calculation of angles has been made between various directions on close packed basal plane with the tilted axis obtained by placing identical atoms on voids. Results obtained from simple calculations provide a lot of important information related to Rhombohedral Close Packed and Hexagonal Close Packed structures, effectively leading to their discovery as independent and genuine space lattices from close packed category of materials. En-route, we also propose a separate equation to calculate the angles between two given directions in the above mentioned close packed crystal systems. Modes of polytype formation in close packing of identical atoms is also briefly discussed.

Quantum metrology enhanced by coherence-induced driving in a cavity-QED setup

We propose a quantum metrology scheme in a cavity QED setup to achieve the Heisenberg limit. In our scheme, a series of identical two-level atoms randomly pass through and interact with a dissipative single-mode cavity. Different from the entanglement based Heisenberg limit metrology scheme, we do not need to prepare the atomic entangled states before they enter into the cavity. We show that the initial atomic coherence will induce an effective driving to the cavity field, whose steady state is an incoherent superposition of orthogonal states, with the superposition probabilities being dependent on the atom-cavity coupling strength. By measuring the average photon number of the cavity in the steady state, we demonstrate that the root-mean-square of the fluctuation of the atom-cavity coupling strength is proportional to $1/N_c^2$ ($N_c$ is the effective atom number interacting with the photon in the cavity during its lifetime). It implies that we have achieved the Heisenberg limit in our quantum metrology process. We also discuss the experimental feasibility of our theoretical proposal. Our findings may find potential applications in quantum metrology technology.

Purely long-range polar molecules composed of identical lanthanide atoms

Doubly polar molecules, possessing an electric dipole moment and a magnetic dipole moment, can strongly couple to both an external electric field and a magnetic field, providing unique opportunities to exert full control of the system quantum state at ultracold temperatures. We propose a method for creating a purely long-range doubly polar homonuclear molecule from a pair of strongly magnetic lanthanide atoms, one atom being in its ground level and the other in a superposition of quasi-degenerate opposite-parity excited levels [Phys.~Rev.~Lett.~\textbf{121}, 063201 (2018)]. The electric dipole moment is induced by coupling the excited levels with an external electric field. We derive the general expression of the long-range, Stark, and Zeeman interaction energies in the properly symmetrized and fully-coupled basis describing the diatomic complex. Taking the example of holmium, our calculations predict shallow long-range wells in the potential energy curves that may support vibrational levels accessible by direct photoassociation from pairs of ground-level atoms.

Experimental Study of Effective Mass and Spin-Orbital Energy of the Al2O3/NiO Nanoheterostructure Material.

The effective mass of the materials relies on exciton dynamics which is greatly dominated by energy gap of the materials. We examined the effective mass for the Al2O3/NiO nanoheterostructure material through polarization in the dielectric study. The mean optical effective mass was calculated to be mopt* = 5.69645 × 10-20 mop/m0 in the UV-visible region (1 × 106 to 5.4 × 106 Hz). From the group and phase velocity values, we observed that Al2O3/NiO is an anisotropic material. The electron spin-orbit energy splitting associated to electron in the valence band was identified using the Gaussian-confining 3D potential under normalized angular momentum (n, l = 0, 1, 2, 3). The 1S (1S1/2) and 2S (2S1/2) orbital ionization energies were calculated to be -6.08 and -5.99 eV for AlO3/NiO. The orbital ionization energies were established from the Bohr radius aB = 5.29 × 10-11 m and donor Rydberg constant Ry = 1.097 × 107 m-1 of the identical hydrogen atom. Our study gives insights into the exciton dynamics and calculation of orbital energy for the nanoheterostructure materials.

The distribution of stationary correlations between qubits and the environment

We study the distribution of the stationary correlations in a system of two nonidentical as well as identical two-level atoms separated by an arbitrary distance and interacting with a squeezed vacuum. We investigate how the interatomic distance and the squeezing mean number of photons influence the way that one of the atoms is quantum correlated with the environment. Both for nonidentical and identical atoms, the approach of the two atoms can maximize the entanglement and quantum discord between the pair of atoms. For nonidentical atoms, the bigger the mean number of photons, the stronger the stationary correlations, i.e., entanglement of formation (EOF) and quantum discord (QD) between the qubit and the environment as the increase of distance. The increase of EOF and QD with the environment is always accompanied by a decrease of the correlations between the subsystems with regard to the interatomic distance no matter what the mean number of photons is. For identical atoms, the stationary correlations EOF and QD between each partition behave similarly with respect to the distance when the mean number of photons is very small.

The entanglement of atoms with two-photon transitions in the presence of ac Stark shift of energy levels

In this paper, we investigated the dynamics of atomic entanglement in a quantum system consisting of two identical two-level atoms (qubits) resonantly interacting with a mode of a thermal cavity field through degenerate two-photon transitions, in the presence of a Stark energy level shift. An analytical expression is obtained for the atom entanglement (negativity) parameter for separable and entangled initial states of atoms. The influence of the Stark shift on the degree of atom-atom entanglement is considered. It is established that the Stark shift leads to a significant increase in the degree of atom entanglement in the case of separable initial states of atoms and to stabilization of atomic entanglement in the case of entangled initial states of atoms.