Coherence-controlled stationary entanglement between two atoms embedded in a bad cavity injected with squeezed vacuum

We investigate the entanglement between two atoms in an overdamped cavity injected with squeezed vacuum when these two atoms are initially prepared in coherent states. It is shown that the stationary entanglement exhibits a strong dependence on the initial state of the two atoms when the spontaneous emission rate of each atom is equal to the collective spontaneous emission rate, corresponding to the case where the two atoms are close together. It is found that the stationary entanglement of two atoms increases with decreasing effective atomic cooperativity parameter. The squeezed vacuum can enhance the entanglement of two atoms when the atoms are initially in coherent states. Valuably, this provides us with a feasible way to manipulate and control the entanglement, by changing the relative phases and the amplitudes of the polarized atoms and by varying the effective atomic cooperativity parameter of the system, even though the cavity is a bad one. When the spontaneous emission rate of each atom is not equal to the collective spontaneous emission rate, the steady-state entanglement of two atoms always maintains the same value, as the amplitudes of the polarized atoms varies. Moreover, the larger the degree of two-photon correlation, the stronger the steady-state entanglement between the atoms.     

Entanglement generation in trapped atoms

The two atoms in the ion trap are entangled by the interaction with an external excited atom. The evolution of the entanglement is analytically derived without the decoherence. Considering the spontaneous decay from the environment, the evolution of the entanglement is similar to the damping Rabi oscillation. The generation of entanglement is induced by the dipole-dipole type interaction of atoms. It is found that the entanglement of two trapped atoms is robust with the uniform interaction with the external atom. The collective spontaneous emission from the coupling between the atoms may enhance the entanglement.     

Frequency conversion and entanglement in V configuration atomic ensemble

The frequency conversion and entanglement is studied via polariton technique. We analyze the effect of detuning on the efficiency of frequency conversion (EFC). The results show that by adjusting the second mode frequency, the EFC still can be achieved perfectly when the incoming photon is off-resonant. The effect of the spontaneous emission of the atoms on EFC is included.     

Van der Waals interactions between atoms and dispersive surfaces at finite temperature

The long-range interactions between an atomic system in an arbitrary energy level and dispersive surfaces in thermal equilibrium at non-zero temperature are revisited within the framework of the quantum-mechanical linear response theory, using generalized susceptibilities for both atom and electromagnetic field. After defining the observables of interest, one presents a general analysis of the atomic level shift valid for any number and form of dielectric surfaces. It is shown that, at zero temperature, one recovers well-known results previously obtained in the linear response regime. The case of a plane dispersive surface is elaborated on in the non-retarded regime. Calculations are given in detail for a dielectric surface exhibiting a single polariton resonance. Theoretical predictions are presented within a physical viewpoint allowing one to discriminate between the various interaction processes: on one hand, the level shift induced by non-resonant quantum fluctuations, on the other hand, two potentially resonant atom-surface couplings. The first resonant process appears for excited-state atoms and originates in an atomic de-excitation channel resonantly coupled to the surface polariton mode. It exists also at zero temperature, and has been studied and observed previously. The second physical process, which exists at non-zero temperature only, corresponds to the reverse process in which a thermal quantum excitation of a surface polariton resonantly couples to an atomic absorption channel. This novel phenomenon is predicted as well for a ground state atom, and can turn the ordinary long-range van der Waals attraction of atoms into a surface repulsion at increasing temperatures. This opens the way to the control and engineering of the sign and amplitude of van der Waals forces via surface temperature adjustment.     

Position Dependent Spontaneous Emission Spectra of a Λ-Type Atomic System Embedded in a Defective Photonic Crystal

We investigate the position dependent spontaneous emission spectra of a Λ-type three-level atom with one transition coupled to the free vacuum reservoir and the other one coupled to a double-band photonic band gap reservoir with a defect mode in the band gap.It is shown that,for the atom at the defect location,we have a two-peak spectrum with a wide dark line due to the strong coupling between the atom and the defect mode.While,when the atom is far from the defect location(or in the absence of the defect mode),the spectrum has three peaks with two dark lines due to the coupling between the atom and the photonic band gap reservoir with the largest density of states near the band edges.On the other hand,we have a four-peak spectrum for the atom at the space in between.Moreover,the average spontaneous emission spectra of the atoms uniformly embedded in high dielectric or low dielectric regions are described.It is shown that the atoms embedded in high(low) dielectric regions far from the defect location,effectively couple to the modes of the lower(upper) photonic band.However,the atoms embedded in high dielectric or low dielectric regions at the defect location,are coupled mainly to the defect modes.While,the atoms uniformly embedded in high(low) dielectric regions with a normal distance from the defect location,are coupled to both of defect and lower(upper) photonic band modes.     

Bose-Einstein condensates in optical lattices: Spontaneous emission in the presence of photonic band gaps

An extended Bose-Einstein condensate (BEC) in an optical lattice provides a kind of periodic dielectric and causes band gaps to occur in the spectrum of light propagating through it. We examine the question whether these band gaps can modify the spontaneous emission rate of atoms excited from the BEC, and whether they can lead to a self-stabilization of the BEC against spontaneous emission. We find that self-stabilization is not possible for BECs with a density in the order of 10¹⁴ cm^-3. However, the corresponding non-Markovian behavior produces significant effects in the decay of excited atoms even for a homogeneous BEC interacting with a weak laser beam. These effects are caused by the occurrence of an avoided crossing in the photon (or rather polariton) spectrum. We also predict a new channel for spontaneous decay which arises from an interference between periodically excited atoms and periodic photon modes. This new channel should also occur in ordinary periodic dielectrics. Received 27 March 2000     

Collective effects in the self-interference of a single photon emitted by two atoms

Collective effects in the spontaneous emission pattern of two identical two-level atoms a fixed distance apart and sharing initially a single excitation are investigated. It is shown that the interference can take place even when it is known for certain which atom is excited initially. This interference is due solely to the atomic coherence established through multiple photon absorptions and reemissions and will disappear if it is ignored. The interference patterns with and without collective effects are compared for symmetric and antisymmetric initial states. The dark center from an antisymmetric state is shown both analytically and numerically.     

The emission properties of an atom inside a cavity when manipulating the atoms outside the cavity

Considering three two-level atoms initially in the GHZ state, then one atom of them is put into an initially empty cavity and made resonant interaction. It is shown that the emission properties of the atom inside the cavity can be affected only when both of the atoms outside the cavity have been manipulated. This conclusion can also be generalized to n two-level atoms.     

Modification of radiation pressure due to cooperative scattering of light

Cooperative spontaneous emission of a single photon from a cloud of N atoms modifies substantially the radiation pressure exerted by a far-detuned laser beam exciting the atoms. On one hand, the force induced by photon absorption depends on the collective decay rate of the excited atomic state. On the other hand, directional spontaneous emission counteracts the recoil induced by the absorption. We derive an analytical expression for the radiation pressure in steady-state. For a smooth extended atomic distribution we show that the radiation pressure depends on the atom number via cooperative scattering and that, for certain atom numbers, it can be suppressed or enhanced. Cooperative scattering of light by extended atomic clouds can become important in the presence of quasi-resonant light and could be addressed in many cold atoms experiments.     

Quantum theory of van der Waals interactions between excited atoms and birefringent dielectric surfaces

A theory of van der Waals (vdW) interaction between an atom (in ground or excited state) and a birefringent dielectric surface with an arbitrary orientation of the principal optic axis (C-axis) is presented. Our theoretical approach is based on quantum-mechanical linear response theory, using generalized susceptibilities for both atom and electromagnetic field. Resonant atom-surface coupling is predicted for excited-state atoms interacting with a dispersive dielectric surface, when an atom de-excitation channel gets into resonance with a surface polariton mode. In the non-retarded regime, this resonant coupling can lead to enhanced attractive or repulsive vdW surface forces, as well as to a dissipative coupling increasing the excited-state relaxation. We show that the strongly non-scalar character of the interaction with the birefringent surface produces a C-axis-dependent symmetry-breaking of the atomic wavefunction. Changes of the C-axis orientation may also lead to a frequency shift of the surface polariton mode, allowing for tuning on or off the resonant coupling, resulting in a special type of engineering of surface forces. This is analysed here in the case of cesium 6D _3/2 level interacting with a sapphire interface, where it is shown that an adequate choice of the sapphire C-axis orientation allows one to transform vdW surface attraction into repulsion, and to interpret recent experimental observations based on selective reflection methods [H. Failache etal., Phys. Rev. Lett. 83, 5467 (1999)]. Received 24 January 2001     

Bipolariton laser emission from a GaAs microcavity

Biexciton emission properties were studied in a single GaAs quantum well semiconductor planar microcavity by photoluminescence measurements at low temperatures. At high pump intensity a bipolariton emission appears close to the lower polariton mode. This new mode appears when we detune the cavity resonance out of the lower polariton branch, showing a laser-like behavior. Very small linewidths were measured, lying below 110 μeV and 150 μeV for polariton and bipolariton emission respectively. The input/output power (I/O) measurements show that the bipolariton emission has a weaker coupling efficiency compared to previous results for polariton emission. Varying the pump laser polarization, we were able to show the selection rules for the biexciton particle creation in the quantum well. Simultaneous photoluminescence and near-field measurements show that the polariton and bipolariton emission are spectrally and spatially separated.     

Spontaneous emission from a microwave-driven four-level atom in an anisotropic photonic crystal

The spontaneous emission from a microwave-driven four-level atom embedded in an anisotropic photonic crystal is studied. Due to the modified density of state(DOS) in the anisotropic photonic band gap(PBG) and the coherent control induced by the coupling fields, spontaneous emission can be significantly enhanced when the position of the spontaneous emission peak gets close to the band gap edge. As a result of the closed-loop interaction between the fields and the atom,the spontaneous emission depends on the dynamically induced Autler–Townes splitting and its position relative to the PBG.Interesting phenomena, such as spectral-line suppression, enhancement and narrowing, and fluorescence quenching, appear in the spontaneous emission spectra, which are modulated by amplitudes and phases of the coherently driven fields and the effect of PBG. This theoretical study can provide us with more efficient methods to manipulate the atomic spontaneous emission.     

Characteristics of Spontaneous Emission of Polarized Atoms in Metal-Dielectric Multiple Layer Structures

The spontaneous emission （SE） progress of polarized atoms in a stratified structure of air-dielectric（DO）-metal（M） dielectric（D1）-air can be controlled effectively by changing the thickness of the D1 layer and rotating the polarized direction of atoms. It is found that the normalized SE rate of atoms located inside the DO layer crucially depends on the atomic position and the thickness of the D1 layer. When the atom is located near the DO-M interface, the normalized atomic SE rate as a function of the atomic position is abruptly onset for the thin D1 layer. However, with the increasing thickness of the D1 layer, the corresponding curve profile exhibits plateau and stays nearly unchanged. The substantial change of the SE rate stems from the excitation of the surface plasmon polaritons in metal-dielectric interface, and the feature crucially depends on the thickness of D1 layer. If atoms are positioned near the DO-air interface, the substantial variation of the normalized SE rate appears when rotating the polarized direction of atoms. These findings manifest that the atomic SE processes can be flexibly controlled by altering the thickness of the dielectric layer D1 or rotating the orientation of the polarization of atoms.     

Temperature dependent polariton emission from strongly coupled organic semiconductor microcavities

We investigated the absorption and photoluminescence (PL) of J-aggregates of a cyanine dye both in a thin film format and when used as the active layer in a strongly-coupled microcavity. We show that as temperature is reduced, the absorption linewidth of the J-aggregates narrows and shifts to higher energy. When the J-aggregate is placed in a microcavity we find that the energy of the polariton modes also shifts to higher energies as temperature is reduced. We compare the intensity of PL emission from the upper and lower branches at resonance as a function of temperature, and find that it can be described by an activation energy of 25 meV. PL emission spectra at resonance also suggest that uncoupled excitons inside the microcavity populate the upper polariton branch states.     

Control of Group Velocity via Spontaneous Generated Coherence and Kerr Nonlinearity

A four-level N-type atomic medium is considered to study the effect of spontaneous generated coherence(SGC) and Kerr nonlinearity on light pulse propagation. A light pulse is propagating inside the medium where each atom follows four-level N-type atom-field configuration of rubidium(85Rb) atom. The atom-field interaction leads to electromagnetically induced transparency(EIT) process. The atom-field interaction is accompanied by normal dispersion and in the presence of SGC and Kerr nonlinearity the dispersion property of the proposed atomic medium is modified,which leads to enhancement of positive group index of the medium. The enhancement of positive group index then leads to slow group velocity inside the medium. A more slow group velocity is also investigated by incorporated the collective effect of SGC and Kerr nonlinearity. The control of group velocity inside a four-level N-type atomic medium via collective effect of SGC and Kerr nonlinearity is the major part of this work.     

Superradiance and Collective Gain in the Atom-Assisted Multimode Optomechanical System

We investigate that the muti-mode optomechanical system coupled with the two-level atoms. If the driving pump field is resonance with the anti-Stokes sideband, the system is at the superradiative state. For the driving filed in the Stokes sideband, the collective gain can be observed. We study a scheme that how the atomic medium affect these superradiance and collective gain. Our results show that the presence of the atom can enhance the superradiant behavior. In the mode splitting regime, the mode splits into thirds with the presence of the atoms with the anti-Stokes sideband. In addition, we also show that the use of atoms in this system could provide us a way to switch the system form superradiative state to collective gain.

     

Atoms in an optical cavity: Quantum electrodynamics in confined space

Cavity quantum electrodynamics studies the behaviour of atoms inside a low-loss cavity. The composite atom–cavity system constitutes a new ‘molecule’ with radiative properties which differ radically from those of the individual components. For example, spontaneous emission becomes reversible, the spectrum splits into several peaks and the intensity fluctuations of the light transmitted by this novel ‘molecule’ are below the classical shot noise limit. In terms of practical applications, the system renders possible, first, the detection of single atoms with a high sensitivity and, second, optical bistability for a few atoms and photons only.     

Control of Group Velocity via Spontaneous Generated Coherence and Kerr Nonlinearity

A four-level N-type atomic medium is considered to study the effect of spontaneous generated coherence (SGC) and Kerr nonlinearity on light pulse propagation. A light pulse is propagating inside the medium where each atom follows four-level N-type atom-field configuration of rubidium (⁸⁵Rb) atom. The atom-field interaction leads to electromagnetically induced transparency (EIT) process. The atom-field interaction is accompanied by normal dispersion and in the presence of SGC and Kerr nonlinearity the dispersion property of the proposed atomic medium is modified, which leads to enhancement of positive group index of the medium. The enhancement of positive group index then leads to slow group velocity inside the medium. A more slow group velocity is also investigated by incorporated the collective effect of SGC and Kerr nonlinearity. The control of group velocity inside a four-level N-type atomic medium via collective effect of SGC and Kerr nonlinearity is the major part of this work.     

Simultaneous jumps in two two-level atoms

We demonstrate the importance of collective behaviour in two identical two-level atoms. When the atoms become very close together a cascade three-level atom analysis can be adopted to show the increasing possibility of simultaneous jumps. Due to the dipole-dipole interaction the intermediate one-atom excited state is detuned by a large amount when the atoms are close together but leaving the upper two-atom excited state in two-photon resonance. The relative importance of multiple jumps compared with stepwise independent jumps is then changed dramatically.