Neutral Atomic Systems Research Articles

Quantum state swap for two trapped Rydberg atoms

The quantum swap gate is one of the most useful gates for quantum computation. Two-qubit entanglement and a controlled-NOT quantum gate in a neutral Rydberg atom system have been achieved in recent experiments. It is therefore very interesting to propose a scheme here for swapping a quantum state between two trapped neutral atoms via the Rydberg blockade mechanism. The atoms interact with a sequence of laser pulses without individual addressing. The errors of the swap gate due to imprecision of pulse length, finite Rydberg interaction, and atomic spontaneous emission are discussed.

Transfer of entangled state, entanglement swapping and quantum information processing via the Rydberg blockade

We provide a scheme with which the transfer of the entangled state and the entanglement swapping can be realized in a system of neutral atoms via the Rydberg blockade. Our idea can be extended to teleport an unknown atomic state. According to the latest theoretical research of the Rydberg excitation and experimental reports of the Rydberg blockade effect in quantum information processing, we discuss the experimental feasibility of our scheme.

Atoms playing dress-up

The idea of using ultracold atoms to simulate the behaviour of electrons in new kinds of quantum systems — from topological insulators to exotic superfluids and superconductors — is a step closer to becoming a reality. See Letter p.83 Spin-orbit coupling describes the interaction between a quantum particle's spin and its momentum, and is important for many areas of physics from spintronics to the quantum spin Hall effect and topological insulators. However, in systems of ultracold neutral atoms, there is no coupling between the spin and the centre-of-mass motion of the atom. Lin et al. use lasers to engineer such spin-orbit coupling in a neutral atomic Bose–Einstein condensate, the first time this has been achieved for any bosonic system. This should lead to the realization of topological insulators in fermionic neutral atom systems.

Spin-orbit-coupled Bose-Einstein condensates.

Spin-orbit (SO) coupling--the interaction between a quantum particle's spin and its momentum--is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices. Quantum many-body systems of ultracold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom's intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose-Einstein condensate by dressing two atomic spin states with a pair of lasers. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling--equally applicable for bosons and fermions--sets the stage for the realization of topological insulators in fermionic neutral atom systems.

Strong correlations in quantum vortex nucleation of ultracold atomic gases

We review some recent developments in the theory of rotating atomic gases. These studies have thrown light on the process of nucleation of vortices in regimes where mean-field methods are inadequate. In our review, we shall describe and compare quantum vortex nucleation of a dilute ultracold bosonic gas trapped in three different configurations: a one-dimensional ring lattice, a one-dimensional ring superlattice and a two-dimensional asymmetric harmonic trap. In all of them, there is a critical rotation frequency, at which the particles in the ground state exhibit strong quantum correlations. However, the entanglement properties vary significantly from case to case. We explain these differences by characterizing the intermediate states that participate in the vortex nucleation process. Finally, we show that noise correlations are sensitive to these differences. These new studies have, therefore, shown how novel quantum states may be produced and probed in future experiments with rotating neutral atom systems.

Use of ab initio methods to classify four existing energy density functionals according to their possible variational validity

Density functional theory is, of course, fundamentally a variational method. Therefore, in this Letter we suggest a classification of available energy density functionals into two groups, which we term (i) heuristic (H) and (ii) possibly variationally valid (PV). Quantum-chemical ab initio methods are employed on some selected neutral and anionic atomic systems and the molecules H 2O and LiOB to make this separation into H and PV groups in the case of the exchange-correlation functionals LDA, PBE, PW91 and B3LYP. This study shows that while the PBE is a potential candidate for being variationally valid, the B3LYP, today's other most widely used density functional, shows its strongly heuristic character. The investigation of variational validity can be an important part of systematic functional development, which is today's biggest challenge in DFT.

Mixing of neutral atoms and lepton number oscillations

We discuss oscillations of two neutral atoms which proceed with the violation of lepton number. One of the neutral atoms is stable, the other one represents a quasistationary state subjected to electromagnetic deexcitation. The system of neutral atoms exhibits oscillations similar to those of the system of neutral kaons and neutron-antineutron oscillations in the nuclear medium. The underlying mechanism is a transition of two protons and two bound electrons to two neutrons p + p + eb− + eb− ↔ n + n. A signature of the oscillations might be an electromagnetic deexcitation of the involved unstable nucleus and atomic shell with the electron holes. A resonant enhancement of the neutrinoless double electron capture takes place when the atomic masses tend to be degenerate. Qualitative estimates show that in searches for lepton number violation oscillations of atoms might be a possible alternative to the conventional mechanism of the neutrinoless double β decay process with emission of two electrons.

SKYRMION EXCITATIONS OF SPIN HALL EFFECT IN ATOMS

By making use of ϕ-mapping topological current theory and the decomposition of gauge potential theory, we investigate the skyrmion excitations of spin Hall effect induced by optical fields in neutral atomic system. It is shown that the topological structures of these skyrmions are characterized by ϕ-mapping topological numbers: Hopf indices and Brouwer degrees. Based on the bifurcation theory of ϕ-mapping theory, it is found that the skyrmions can be generated or annihilated at the limit point and they encounter, split or merge at the bifurcation point.

Raman processes and effective gauge potentials

A new technique is described by which light-induced gauge potentials allow systems of ultra-cold neutral atoms to behave like charged particles in a magnetic field. Here, atoms move in a uniform laser field with a spatially varying Zeeman shift and experience an effective magnetic field. This technique is applicable for atoms with two or more internal ground states. Finally, an explicit model of the system using a single-mode 2D Gross-Pitaevskii equation yields the expected vortex lattice.

微小トロイド共振器を用いたキャビティQED

Cavity quantum electrodynamics (cQED) with systems of single neutral atoms or ions coupled to Fabry-

Photon-emission rate from atomic systems in the CSL model

We calculate the photon-emission rate from a general atomic system in the mass-proportional continuous spontaneous localization (CSL) model. For an isolated charged particle emitting kilovolt gamma rays, our results agree with those obtained by Fu. For a neutral atomic system, photon emission is strongly suppressed for photon wavelengths much larger than the atomic radius. However, for kilovolt gamma rays, Fu's result is modified by a structure factor that is of order unity, giving no rate suppression. Our calculation is readily generalized to the case of non-white noise, noise couplings that are not mass-proportional, and general (non-Gaussian) spatial correlation functions, and corresponding results are given. We briefly discuss the implications of our calculation for upper bounds on the CSL model parameters.

Controlled exchange interaction between pairs of neutral atoms in an optical lattice

Ultracold atoms trapped by light offer robust quantum coherence and controllability, providing an attractive system for quantum information processing and for the simulation of complex problems in condensed matter physics. Many quantum information processing schemes require the manipulation and deterministic entanglement of individual qubits; this would typically be accomplished using controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress towards this goal by demonstrating entanglement among an ensemble of atoms confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between atoms in isolated pairs. Here we use an optical lattice of double-well potentials to isolate and manipulate arrays of paired (87)Rb atoms, inducing controlled entangling interactions within each pair. Our experiment realizes proposals to use controlled exchange coupling in a system of neutral atoms. Although 87Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, the wavefunction exchange symmetry of these identical bosons leads to state-dependent dynamics. We observe repeated interchange of spin between atoms occupying different vibrational levels, with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits). Its 'half-implementation', the root SWAP gate, is entangling, and together with single-qubit rotations it forms a set of universal gates for quantum computation.

Optically Induced Spin-Hall Effect in Atoms

We propose an optical means to realize the spin-Hall effect (SHE) in a neutral atomic system by coupling the internal spin states of atoms to radiation. The interaction between the external optical fields and the atoms creates effective magnetic fields that act in opposite directions on "electrically" neutral atoms with opposite spin polarizations. This effect leads to a Landau level structure for each spin orientation in direct analogy with the familiar SHE in semiconductors. The conservation and topological properties of the spin current, and the creation of a pure spin current are discussed.

Diffusion Monte Carlo Study of Atomic Systems from Li to Ne

A systematic diffusion Monte Carlo (DMC) study of both neutral and charged atomic systems from Li to Ne in the ground state is performed to evaluate the correlation energy (Ec), the ionization energy (IE), and the electron affinity (EA) of these systems. The present study is based on the fixed-node approximation in which the nodal surfaces of the DMC wavefunction is assumed to be the same as those of the HartreeFock wavefunction. The present calculations reproduce 90 � 7% of the exact value of correlation energy for the cations, 91 � 3% for the neutral atoms, and 92 � 2% for the anions, respectively. The theoretical values of IE and EA in the present study are in good agreement with experimental values within an accuracy of 0.3 eV for IE and 0.1 eV for EA. The variation of Ec and IE with respect to the atomic number Z is interpreted. [doi:10.2320/matertrans.47.2612]

Dynamic electrostriction of a Bose condensate and of a system of neutral atoms

It is shown that the condensate of a Bose gas of neutral atoms has electrical properties that are reflected in the dynamics of the condensate and do not reduce to a trivial polarization of the atomic gases in an external electric field. The concept of an isotropic atomic quadrupole moment whose distribution creates a distribution of the macroscopic electric potential 〈φ〉 in a system of neutral atoms is used. The nonuniform distribution of atoms that arises in a sound wave, for example, will produce an electric field whose behavior is determined by the dynamics of 〈φ〉.

Effects of fermion-boson interaction in neutral atomic systems

We investigate the collective excitations of ${}^{3}\mathrm{He}{--}^{4}\mathrm{He}$ mixture films at zero temperature within the random phase approximation and linear response theory. In a low concentration regime of ${}^{3}\mathrm{He},$ a level repulsion between zero sound and third sound modes is derived, which opens the possibility to observe quantum mechanical coherence between a ${}^{3}\mathrm{He}$ particle-hole pair and the condensate of ${}^{4}\mathrm{He}.$ We also investigate the ${}^{3}\mathrm{He}{--}^{4}\mathrm{He}$ vertex corrections in the ladder approximation, and show that the third branch, the combined mode of fermionic particle-hole pair and the third sound quanta, provides a unique correction to the Landau f function. Some implications for a fermion-boson mixture of alkali atoms in a potential trap are discussed.

Physical explanation of the periodic table.

The Periodic Table of the elements, the most important generalization in chemistry, is often considered as a representative special case in the study of the relation between chemistry and physics. Its quantum interpretation was initiated, but not completed, by Niels Bohr. In this paper, post-Bohr conceptual developments are discussed from historical and epistemological points of view. The difference between high-precision numerical calculations for individual atoms and the theory of the periodic system as a whole is emphasized. Periodic laws met in Nature are not restricted to the chemical Periodic Table. A comparative study of these laws makes it possible to single out essential features that define the particular pattern of periodicity. It is shown that the periodic system of neutral ground state atoms now has a firm nonempirical quantum-theoretical basis. Alternative approaches, based on group theory and other mathematical schemes, are briefly discussed. It is argued that, while quantum theory is capable of fully accurate calculations for relatively simple atoms or molecular objects, the complexity of polyatomic molecules and chemical reactions guarantees the flourishing of chemistry as a separate scientific discipline.

Reconstruction of atomic effective potentials from isotropic scattering factors

We present a method for the approximate determination of one-electron effective potentials of many-electron systems from a finite number of values of the isotropic scattering factor. The method is based on the minimum cross-entropy technique. An application to some neutral ground-state atomic systems has been done within a Hartree-Fock framework.

Kinetic contribution to the correlation energy density: benchmark to Tc[ n] energy functionals

Since in density functional theory the classical form of the virial theorem does not hold due to the localization effect of exchange-correlation, it is important to investigate the kinetic contribution to correlation. Nearly exact t c([ n]; r) kinetic contributions to the correlation energy densities are constructed for the two-electron He and H 2 and for LiH and compared with various model t c mod([ n]; r) energy densities. For comparative purpose the “global” T c[ n] energies are also given for several atomic and molecular systems. The model functionals are derived using the Levy-Perdew relation. Stoll's correction is employed for those models, which are based on the electron gas model or capable of reproducing the uniform electron gas limit. This correction makes it possible to treat opposite- and parallel-spin correlation distinctly. The proper modelling of t c([ n]; r) functionals provides the opportunity of a separate treatment of the potential part W c[ n] and the T c[ n] part of the correlation. The virial ratio − T c/ E c is compared for several neutral and ionic systems and is found to be in the range 0.75–0.85 for neutral atomic systems, while for tightly bound atoms, like Be 2+ a value of 0.96 is computed.

Density functional calculations of positron annihilation lifetimes from positron bound states in atoms

Positron annihilation from positron bound states in atomic systems has been investigated by using density functional theory. The shifts of the positron lifetime have been analysed in terms of competing influences of correlation potentials. The results suggest that inclusion of electron - positron correlation is crucial in the assignment of positron orbitals to lifetime components in density functional investigations of bound states of the positron in negative and neutral atomic systems. The eigenvalues of positron bound states consistently correspond to the positron annihilation lifetimes in our calculations.