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We investigate numerically and experimentally the near-field response of disordered arrays comprising asymmetrically split ring resonators that exhibit strong cooperative response. Our simulations treat the unit cell split ring resonators as discrete pointlike oscillators with associated electric and magnetic point dipole radiation, while the strong cooperative radiative coupling between the different split rings is fully included at all orders. The methods allow to calculate local field and Purcell factor enhancement arising from the collective electric and magnetic excitations. We find substantially increased standard deviation of the Purcell-enhancement with disorder, making it increasingly likely to find collective excitation eigenmodes with very high Purcell factors that are also stronger for magnetic than electric excitations. We show that disorder can dramatically modify the cooperative response of the metamaterial even in the presence of strong dissipation losses as is the case for plasmonic systems. Our analysis in terms of collective eigenmodes paves a way for controlled engineering of electromagnetic device functionalities based on strongly interacting metamaterial arrays., 10 pages, 8 figures

Systems of closely-spaced resonators can be strongly coupled by interactions mediated by scattered electromagnetic fields. In large systems the resulting response has been shown to be more sensitive to these collective interactions than to the detailed structure of individual resonators. Attempts to describe such systems have resulted in point-dipole approximations to resonators that are computationally efficient for large resonator ensembles. Here we provide a detailed study for the validity of point dipole approximations in small systems of strongly coupled plasmonic nanorods, including the cases of both super-radiant and subradiant excitations, where the characteristics of the excitation depends on the spatial separation between the nanorods. We show that over an appreciable range of rod lengths centered on 210 nm, when the relative separation kl in terms of the resonance wave number of light k satisfies $$kl\gtrsim \pi /2$$ k l ≳ π / 2 , the point electric dipole model becomes accurate. However, when the resonators are closer, the finite-size and geometry of the resonators modifies the excitation modes, in particular the cooperative mode line shifts of the point dipole approximation begin to rapidly diverge at small separations. We also construct simplified effective models by describing a pair of nanorods as a single effective metamolecule.

We provide detailed comparisons between experimental findings and numerical simulations of large coopera- tively interacting, spatially disordered metamaterial arrays, consisting of asymmetrically split rings. Simulation methods fully incorporate strong field-mediated inter-meta-atom interactions between discrete resonators and statistical properties of disorder, while approximating the resonators’ internal structure. Despite the large system size, we find a qualitative agreement between the simulations and experiments and characterize the microscopic origins of the observed disorder response. Our microscopic description of macroscopic electrodynamics reveals how the response of disordered arrays with strong field-mediated interactions is inherently linked to their cooperative response to electromagnetic waves where the multiple scattering induces strong correlations between the excitations of individual resonators. Whereas for a regular array the response can be overwhelmingly dominated by a spatially extended collective eigenmode with subradiant characteristics, a gradual increase of the positional disorder rapidly leads to a spatial localization of both the electric and magnetic dipolar excitation profile of this eigenmode. We show how the effects of disorder and cooperative interactions are mapped onto the transmission resonance in the far field spectrum and measure the “cooperative Lamb shift” of the resonance that is shifting toward the red as the disorder increases. The interplay between the disorder and interactions generally is most dramatic in the microwave arrays, but we find that in suitable regimes the strong disorder effects can be achieved also for plasmonic optical systems.

We study the collective optical response of an atomic ensemble confined within a single-mode optical cavity by stochastic electrodynamics simulations that include the effects of atomic position correlations, internal level structure, and spatial variations in cavity coupling strength and atom density. In the limit of low light intensity the simulations exactly reproduce the full quantum field-theoretical description for cold stationary atoms and at higher light intensities we introduce semiclassical approximations to atomic saturation that we compare with the exact solution in the case of two atoms. We find that collective subradiant modes of the atoms, with very narrow linewidths, can be coupled to the cavity field by spatial variation of the atomic transition frequency and resolved at low intensities, and show that they can be specifically driven by tailored transverse pumping beams. We show that the cavity optical response, in particular both the subradiant mode profile and the resonance shift of the cavity mode, can be used as a diagnostic tool for the position correlations of the atoms and hence the atomic quantum many-body phase. The quantum effects are found to be most prominent close to the narrow subradiant mode resonances at high light intensities. Although an optical cavity can generally strongly enhance quantum fluctuations via light confinement, we show that the semiclassical approximation to the stochastic electrodynamics model provides at least a qualitative agreement with the exact optical response outside the subradiant mode resonances even in the presence of significant saturation of the atoms.

We develop a theoretical formalism for collectively responding point scatterers where the radiating electromagnetic fields from each emitter are considered in the electric dipole, magnetic dipole, and electric quadrupole approximation. The contributions of the electric quadrupole moment to electromagnetically-mediated interactions between the scatterers are derived in detail for a system where each scatterer represents a linear $RLC$ circuit resonator, representing common metamaterial resonators in radiofrequency, microwave, and optical regimes. The resulting theory includes a closed set of equations for an ensemble of discrete resonators that are radiatively coupled to each other by propagating electromagnetic fields, incorporating potentially strong interactions and recurrent scattering processes. The effective model is illustrated and tested for examples of pairs of interacting point electric dipoles, where each pair can be qualitatively replaced by a model point emitter with different multipole radiation moments.

Subradiant excitations, originally predicted by Dicke, have posed a long-standing challenge in physics owing to their weak radiative coupling to environment. Here we engineer massive coherently driven classical subradiance in planar metamaterial arrays as a spatially extended eigenmode comprising over 1000 metamolecules. By comparing the near- and far-field response in large-scale numerical simulations with those in experimental observations we identify strong evidence for classically correlated multimetamolecule subradiant states that dominate the total excitation energy. We show that similar spatially extended many-body subradiance can also exist in plasmonic metamaterial arrays at optical frequencies. MOE (Min. of Education, S’pore) Published version

We show how strong light-mediated resonant dipole-dipole interactions between atoms can be utilized in a control and storage of light. The method is based on a high-fidelity preparation of a collective atomic excitation in a single correlated subradiant eigenmode in a lattice. We demonstrate how a simple phenomenological model captures the qualitative features of the dynamics and sharp transmission resonances that may find applications in sensing.

International audience; We study the emergence of a collective optical response of a cold and dense 87 Rb atomic cloud to a near-resonant low-intensity light when the atom number is gradually increased. Experimental observations are compared with microscopic stochastic simulations of recurrent scattering processes between the atoms that incorporate the atomic multilevel structure and the optical measurement setup. We analyze the optical response of an inhomogeneously broadened gas and find that the experimental observations of the resonance line shifts and the total collected scattered light intensity in cold atom clouds substantially deviate from those of thermal atomic ensembles, indicating strong light-induced resonant dipole-dipole interactions between the atoms. At high densities, the simulations also predict a significantly slower decay of light-induced excitations in cold than in thermal atom clouds. The role of dipole-dipole interactions is discussed in terms of resonant coupling examples and the collective radiative excitation eigenmodes of the system.

We derive equations for the strongly coupled system of light and dense atomic ensembles. The formalism includes an arbitrary internal-level structure for the atoms and is not restricted to weak excitation of atoms by light. In the low-light-intensity limit for atoms with a single electronic ground state, the full quantum field-theoretical representation of the model can be solved exactly by means of classical stochastic electrodynamics simulations for stationary atoms that represent cold atomic ensembles. Simulations for the optical response of atoms in a quantum degenerate regime require one to synthesize a stochastic ensemble of atomic positions that generates the corresponding quantum statistical position correlations between the atoms. In the case of multiple ground levels or at light intensities where saturation becomes important, the classical simulations require approximations that neglect quantum fluctuations between the levels. We show how the model is extended to incorporate corrections due to quantum fluctuations that result from virtual scattering processes. In the low-light-intensity limit, we illustrate the simulations in a system of atoms in a Mott-insulator state in a two-dimensional optical lattice, where recurrent scattering of light induces strong interatomic correlations. These correlations result in collective many-atom subradiant and superradiant states and a strong dependence of the response on the spatial confinement within the lattice sites

We show that the resonance shifts in the fluorescence of a cold gas of rubidium atoms substantially differ from those of thermal atomic ensembles that obey the standard continuous medium electrodynamics. The analysis is based on large-scale microscopic numerical simulations and experimental measurements of the resonance shifts in a steady-state response in light propagation., To appear in Phys. Rev. Lett

Atomic transitions afford a convenient way of storing quantum bits. However, there are few ground-state transitions suitable for use with light at telecommunication wavelengths. Now, researchers show that ensembles of cold rubidium atoms not only make good quantum memories, but can also noiselessly convert the emitted photons into and out of the telecoms band.

Two independent experiments that demonstrate memories for single quantum excitations with storage times on the order of a millisecond—two orders of magnitude longer than reported so far—should help to bring practical long-distance quantum-communication networks a step closer. Quantum memories for the storage and retrieval of quantum information are extremely sensitive to environmental influences, which limits their storage times. The ground states of atoms and ions are potential candidates for quantum memories, but although coherence times of the order of a few seconds for atoms1,2 and hundreds of seconds for ions3,4,5 have been demonstrated, long-lived storage and retrieval of single quantum excitations remains an outstanding challenge. Here, we report a quantum memory using the magnetically insensitive clock transition in atomic rubidium confined in a one-dimensional optical lattice. We observe quantum memory lifetimes exceeding 6 ms, more than two orders of magnitude longer than previously reported6. This advance is an important step towards the realization of long-distance quantum networks and the controlled production of complex entangled states of matter and light.

International audience; We measure the coherent scattering of light by a cloud of laser-cooled atoms with a size comparable to the wavelength of light. By interfering a laser beam tuned near an atomic resonance with the field scattered by the atoms, we observe a resonance with a redshift, a broadening, and a saturation of the extinction for increasing atom numbers. We attribute these features to enhanced light-induced dipole-dipole interactions in a cold, dense atomic ensemble that result in a failure of standard predictions such as the " cooperative Lamb shift ". The description of the atomic cloud by a mean-field model based on the Lorentz-Lorenz formula that ignores scattering events where light is scattered recurrently by the same atom and by a microscopic discrete dipole model that incorporates these effects lead to progressively closer agreement with the observations, despite remaining differences. The understanding of light propagation in dense media relies traditionally on a continuous description of the sample characterized by macroscopic quantities such as susceptibility or refractive index [1,2]. Their derivation from a microscopic theory is in general challenging owing to the interactions between the light-induced dipoles that can be large when the light is tuned near an atomic resonance. In dilute media, their role can be analyzed using the perturbative approach of Friedberg, Hartmann, and Manassah (FHM) [3], which predicts in particular a " cooperative Lamb shift " measured recently in inhomo-geneously broadened media [4,5] and cold dilute atomic gases [6]. For an atom slab, the FHM approach was shown to correspond to the low-density limit of the local-field model introduced by Lorentz [7], which replaces the action of all the atoms of the medium on a particular one by an average effective field [1,2], thus ignoring correlations between the light-induced dipoles. This mean-field approach leads to the Lorentz-Lorenz formula, which allows calculating the index of refraction of many dense media with an excellent accuracy [1,8]. However, it was pointed out [7,9] that in the absence of inhomo-geneous broadening, such as in cold atomic ensembles, the mean-field response may not be valid due to recurrent scattering where the field radiated by one atom can be scattered back by another atom [10,11]. Recurrent scattering should become important when the incident light (wavelength λ ¼ 2π=k) is tuned near an atomic resonance, and the atomic density approaches k 3. This calls for an experiment operating in this regime, where a comparison between the standard mean-field theories of light scattering and a microscopic approach, including recurrent scattering, can be performed. Here, we perform this comparison. To do so, we need to access a quantity relevant to both the macroscopic and the microscopic approaches. The coherent electric field hE sc i scattered by the cloud fulfills this condition: it is obtained by averaging the scattered field E sc over many realizations of the spatial random distribution of atoms, and its evolution is governed by the macroscopic Maxwell's equations in the cloud considered as an homogeneous medium described by a susceptibility. In the case of cold atomic gases, the near-resonance coherent optical response has been explored experimentally using mostly dilute, optically thick ensembles [12–19]. Recently, we studied the light scattered by a microscopic dense cloud of cold atoms at 90° of a near-resonant excitation laser [20]. In that situation, we were sensitive to the incoherent component hjE sc − hE sc ij 2 i of the scattered light. We could therefore not compare our results with mean-field predictions for continuous media, which are only relevant for the coherent part. In this work, we study the coherent scattering by our microscopic cloud. The cloud contains up to a few hundreds laser-cooled rubidium-87 atoms and has a size smaller than the wavelength of the optical D 2 transition. We illuminate the sample with a tightly focused laser with a waist larger than the cloud size. We access the coherent scattering by measuring the extinction resulting from the interference of the laser field with the field scattered by the cloud. We observe a saturation of the extinction, a broadening of the line, and a small redshift when we vary the number of atoms from 10 to 180. We show that the measured shift and width do not agree with the FHM perturbative approach. The description of the atomic cloud by a mean-field model based on the Lorentz-Lorenz

We show how the elusive toroidal dipole moment appears as a radiative excitation eigenmode in a metamolecule resonator that is formed by pairs of plasmonic nanorods. We analyze one such nanorod configuration - a toroidal metamolecule. We find that the radiative interactions in the toroidal metamolecule can be qualitatively represented by a theoretical model based on an electric point dipole arrangement. Both a finite-size rod model and the point dipole approximation demonstrate how the toroidal dipole moment is subradiant and difficult to excite by incident light. By means of breaking the geometric symmetry of the metamolecule, the toroidal mode can be excited by linearly polarized light and appears as a Fano resonance dip in the forward scattered light. We provide simple optimization protocols for maximizing the toroidal dipole mode excitation. This opens up possibilities for simplified control and driving of metamaterial arrays consisting of toroidal dipole unit-cell resonators., Comment: to appear in PRB

We study the emergence of collective scattering in the presence of dipole-dipole interactions when we illuminate a cold cloud of rubidium atoms with a near-resonant and weak intensity laser. The size of the atomic sample is comparable to the wavelength of light. When we gradually increase the atom number from 1 to 450, we observe a broadening of the line, a small red shift and, consistently with these, a strong suppression of the scattered light with respect to the noninteracting atom case. Numerical simulations, which include the internal atomic level structure, agree with the data., 5 pages, 5 figures

We propose a cooperative asymmetry-induced transparency, CAIT, formed by collective excitations in metamaterial arrays of discrete resonators. CAIT can display a sharp transmission resonance even when the constituent resonators individually exhibit broad resonances. We further show how dynamically reconfiguring the metamaterial allows one to actively control the transparency. While reminiscent of electromagnetically induced transparency, which can be described by independent emitters, CAIT relies on a cooperative response resulting from strong radiative couplings between the resonators., Comment: 10 pages, 4 figures

Summary form only given. Multiple scattering of the electromagnetic (EM) field from an ensemble of resonators generates interactions which can lead to a cooperative response. In atomic gases, the cooperative response is often washed out due to fluctuations in atomic positions. When these fluctuations are restricted, as in an optical lattice in a Mott-insulator state with precisely one atom per lattice site, a cooperative response can be observed. The ability to fabricate metamaterials whose constituent circuit elements, meta-atoms at fixed positions, interact in prescribed ways with EM fields also permits the construction of systems in which cooperative phenomena emerge. In this work, we show that the cooperative response of an ensemble of discrete resonators can result in a transmission resonance whose quality factor increases with the size of the system. This behaviour arises from the formation of a collective eigenmode of excitation that possesses a suppressed, or subradiant, emission rate. To describe the metamaterial, we developed a model in which one dynamic variable represents a meta-atom interacting with the EM fields. Collective modes emerge from interactions between meta-atoms mediated by the EM field, each mode with its own resonance frequency and decay rate.

We theoretically analyze the experimental observations of a spectral line collapse in a metamaterial array of asymmetric split ring resonators [Fedotov et al., Phys. Rev. Lett. 104, 223901 (2010)]. We show that the ensemble of closely-spaced resonators exhibits cooperative response, explaining the observed system-size dependent narrowing of the transmission resonance linewidth. We further show that this cooperative narrowing depends sensitively on the lattice spacing and that significantly stronger narrowing could be achieved in media with suppressed ohmic losses., 19 pages, 6 figures, to appear in New Journal of Physics

We show that a cooperative atom response in an optical lattice to resonant incident light can be employed for precise control and manipulation of light on a subwavelength scale. Specific collective excitation modes of the system that result from strong light-mediated dipole-dipole interactions can be addressed by tailoring the spatial phase-profile of the incident light. We demonstrate how the collective response can be used to produce optical excitations at well-isolated sites on the lattice., 8 pages, 1 figure

We examine the effect of inhomogeneous broadening on the collective response of a planar metamaterial consisting of asymmetric split ring resonators. We show that such a response leads to a transmission resonance that can persist when the broadening of individual meta-atom resonance frequencies is roughly one half the frequency characterizing the split ring asymmetry. We also find that larger degrees of inhomogeneous broadening can drastically alter the cooperative response, destroying this resonance. The reduced effect of cooperative response due to inhomogeneous broadening may find applications in producing metamaterial samples that more closely mimic homogeneous magneto-dielectric medium with well-defined susceptibility and permittivity., Comment: 10 pages, 6 figures

We develop a general formalism to describe the propagation of a near-resonant electromagnetic field in a medium composed of magnetodielectric resonators. As the size and the spatial separation of nanofabricated resonators in a metamaterial array is frequently less than the wavelength, we describe them as discrete scatterers, supporting a single mode of current oscillation represented by a single dynamic variable. We derive a Lagrangian and Hamiltonian formalism for the coupled electromagnetic fields and oscillating currents in the length gauge, obtained by the Power-Zienau-Woolley transformation. The response of each resonator to electromagnetic field is then described by polarization and magnetization densities that, to the lowest order in a multipole expansion, generate electric and magnetic dipole excitations. We derive a closed set of equations for the coherently scattered field and normal mode amplitudes of current oscillations of each resonator both within the rotating wave approximation, in which case the radiative decay rate is much smaller than the resonance frequency, and without such an assumption. The set of equations includes the radiative couplings between a discrete set of resonators mediated by the electromagnetic field, fully incorporating recurrent scattering processes to all orders. By considering an example of a two-dimensional split ring resonator metamaterial array, we show that the system responds cooperatively to near-resonant field, exhibiting collective eigenmodes, resonance frequencies, and radiative linewidths that result from strong radiative interactions between closely-spaced resonators., Comment: 34 pages, 6 figures

Long-distance quantum information networks require information storage, retrieval and transmission at telecommunication wavelengths. A quantum memory and its interface to the telecom window based on cold rubidium in an optical lattice is described. With this system it has been demonstrated (i) interconversion of 795 and 1367 nm light with efficiency in excess of 50% and (ii) measurement of non-classical correlations between telecom light and atomic spin-wave excitations.

We sugest a new paradigm for achieving prescribed localization of optical energy with nanoscale accuracy. Well isolated energy hot-spots as small as lambda/10 can be created and positioned at the chosen meta-molecules on the metamaterial landscape.

Continuous measurements on correlated quantum systems, in addition to providing information on the state vector of the system in question, induce evolution in the unmeasured degrees of freedom conditioned on the measurement outcome. However, experimentally accessing these nontrivial regimes requires high-efficiency measurements over time scales much longer than the temporal resolution of the measurement apparatus. We report the observation of such a continuous conditioned evolution in the state of a light-collective atomic excitation system undergoing photoelectric measurement.

Precise control and manipulation of optical fields on a nanoscale is one of the most important and challenging problems in "nanophotonics". Since optical wavelength is on a much larger microscale, it is impossible to employ conventional focusing for that purpose. We show the strong optically-induced interactions between discrete meta-molecules in a metamaterial system and coherent monochromatic continuous light beam with a spatially-tailored phase profile can be used to prepare a sub-wavelength scale energy localization. Well isolated energy hot-spots as small as $\lambda/10$ can be created and positioned at will on the metamaterial landscape offering new opportunities for data storage and imaging applications., Comment: 7 pages, 4 figures

Long-distance quantum information networks require information storage, retrieval and transmission at telecommunications wavelengths. We demonstrate conversion between 795 and 1367 nm light with efficiency 52% and measure non-classical correlations between telecom light and memory.

We demonstrate the existence of three-dimensional localized structures, known as cavity light bullets, that circulate in a driven cavity filled with a self-focusing dispersive medium. Though unstable, we show through a linear stability analysis how a multiphoton absorption can stabilize these structures. Bullets may be written and erased from the cavity at controlled positions, allowing them to act as elements in three-dimensional pixel arrays. We illustrate the process of writing and erasing bullets in the cavity through dynamic numerical simulations.

We propose and implement a scheme to produce long-lived entanglement between a signal field and a magnetically insensitive collective excitation in an atomic cloud cooled in a one-dimensional optical lattice. After a programmable storage time, we convert the spin-wave excitation into an idler field, and demonstrate violation of Bell's inequality for storage times in excess of 3 ms.

A key ingredient for a practical quantum repeater is a long memory coherence time. We describe a quantum memory using the magnetically-insensitive clock transition in atomic rubidium confined in a 1D optical lattice. We observe quantum lifetimes exceeding 6 milliseconds. We also demonstrate a dozen independent quantum memory elements within a single cold sample, and describe matter-light entanglement generation involving arbitrary pairs of these elements.

The authors propose and demonstrate an atomic qubit based on a cold rubidium isotopic mixture, entangled with a frequency-encoded optical qubit. We discuss the use of such matter qubits in memory-insensitive multiplexed quantum repeaters.

For long distances direct quantum communication via optical fiber is not viable, due to fiber losses. We will outline our program on the use of atomic ensembles for long distance quantum networks.

We propose and demonstrate an atomic qubit based on a cold $^{85}$Rb-$^{87}$Rb isotopic mixture, entangled with a frequency-encoded optical qubit. The interface of an atomic qubit with a single spatial light mode, and the ability to independently address the two atomic qubit states, should provide the basic element of an interferometrically robust quantum network., 5 pages, 3 figures

We observe quantum, Hong-Ou-Mandel, interference of fields produced by two remote atomic memories. High-visibility interference is obtained by utilizing the finite atomic memory time in four-photon delayed coincidence measurements. Interference of fields from remote atomic memories is a crucial element in protocols for scalable generation of multi-node remote qubit entanglement., 4 pages, 3 figures

Long-distance quantum communication via distant pairs of entangled quantum bits (qubits) is the first step towards more secure message transmission and distributed quantum computing. To date, the most promising proposals require quantum repeaters to mitigate the exponential decrease in communication rate due to optical fiber losses. However, these are exquisitely sensitive to the lifetimes of their memory elements. We propose a multiplexing of quantum nodes that should enable the construction of quantum networks that are largely insensitive to the coherence times of the quantum memory elements., Comment: 5 pages, 4 figures. Accepted for publication in PRL

Quantum mechanics provides a mechanism for absolutely secure communication between remote parties. For distances greater than 100 km, direct quantum communication via optical fiber is not viable, owing to fiber losses, and intermediate storage of the quantum information along the transmission channel is necessary. This leads to the concept of the quantum repeater, proposed by Briegel et al. [Phys. Rev. Lett. 81, 169 (1998)]. Duan [Nature 414, 413 (2001)] have proposed to use atomic ensembles as the basic memory elements for the quantum repeater. We provide an overview of our program on the use of atomic ensembles as an interface for quantum information transfer and the prospects for long-distance quantum networks.

We have realized a conical matter wave lens. The repulsive potential of a focused laser beam was used to launch a Bose-Einstein condensate into a radially expanding wavepacket whose perfect ring shape was ensured by energy conservation. In spite of significant interactions between atoms, the spatial and velocity widths of the ring along its radial dimension remained extremely narrow, as also confirmed by numerical simulations. Our results open the possibility for cylindrical atom optics without the perturbing effect of mean-field interactions., 11 pages, 5 figures, Multimedia files (movies) available in our website: http://www.physics.gatech.edu/chandra/index.htm

A source of deterministic single photons is proposed and demonstrated by the application of a measurement-based feedback protocol to a heralded single photon source consisting of an ensemble of cold rubidium atoms. Our source is stationary and produces a photoelectric detection record with sub-Poissonian statistics., Comment: 5 pages, 3 figures

We investigate the dynamics of dark-state polaritons in an atomic ensemble with ground-state degeneracy. A signal light pulse may be stored and retrieved from the atomic sample by adiabatic variation of the amplitude of a control field. During the storage process, a magnetic field causes rotation of the atomic hyperfine coherences, leading to collapses and revivals of the dark-state polariton number. These collapses and revivals should be observable in measurements of the retrieved signal field, as a function of storage time and magnetic field orientation., 5 pages, 3 figures

By time-dependent variation of a control field, both coherent and single photon states of light are stored in, and retrieved from, a cold atomic gas. The efficiency of retrieval is studied as a function of the storage time in an applied magnetic field. A series of collapses and revivals is observed, in very good agreement with theoretical predictions. The observations are interpreted in terms of the time evolution of the collective excitation of atomic spin wave and light wave, known as the dark-state polariton., 5 pages, 4 figures

A quantum repeater at telecommunications wavelengths with long-lived atomic memory is proposed, and its critical elements are experimentally demonstrated using a cold atomic ensemble. Via atomic cascade emission, an entangled pair of 1530 nm and 780 nm photons is generated. The former is ideal for long-distance quantum communication, and the latter is naturally suited for mapping to a long-lived atomic memory. Together with our previous demonstration of photonic-to-atomic qubit conversion, both of the essential elements for the proposed telecommunications quantum repeater have now been realized., 5 pages, 4 figures

We describe some recent investigations of the use of cold atomic ensembles as an interface for quantum information processing.

We report generation, transmission, storage and retrieval of single photons using remote atomic ensembles as quantum memories, as well as collapses and revivals of these memories.

An elementary quantum network operation involves storing a qubit state in an atomic quantum memory node, and then retrieving and transporting the information through a single photon excitation to a remote quantum memory node for further storage or analysis. Implementations of quantum network operations are thus conditioned on the ability to realize such matter-to-light and/or light-to-matter quantum state mappings. Here, we report generation, transmission, storage and retrieval of single quanta using two remote atomic ensembles. A single photon is generated from a cold atomic ensemble at Site A via the protocol of Duan, Lukin, Cirac, and Zoller (DLCZ) [Nature v.414, 413 (2001)] and is directed to Site B through a 100 meter long optical fiber. The photon is converted into a single collective excitation via the dark-state polariton approach of Fleischhauer and Lukin [Phys. Rev. Lett. v.84, 5094 (2000)]. After a programmable storage time the atomic excitation is converted back into a single photon. This is demonstrated experimentally, for a storage time of 500 nanoseconds, by measurement of an anticorrelation parameter a. Storage times exceeding ten microseconds are observed by intensity cross-correlation measurements. The length of the storage period is two orders of magnitude longer than the time to achieve conversion between photonic and atomic quanta. The controlled transfer of single quanta between remote quantum memories constitutes an important step towards distributed quantum networks., Comment: 15 pages, 6 figures

We report observations of entanglement of two remote atomic qubits, achieved by generating an entangled state of an atomic qubit and a single photon at Site A, transmitting the photon to Site B in an adjacent laboratory through an optical fiber, and converting the photon into an atomic qubit. Entanglement of the two remote atomic qubits is inferred by performing, locally, quantum state transfer of each of the atomic qubits onto a photonic qubit and subsequent measurement of polarization correlations in violation of the Bell inequality |S, 5 pages, 3 figures

We describe a new experimental approach to probabilistic atom-photon (signal) entanglement. Two qubit states are encoded as orthogonal collective spin excitations of an unpolarized atomic ensemble. After a programmable delay, the atomic excitation is converted into a photon (idler). Polarization states of both the signal and the idler are recorded and are found to be in violation of the Bell inequality. Atomic coherence times exceeding several microseconds are achieved by switching off all the trapping fields - including the quadrupole magnetic field of the magneto-optical trap - and zeroing out the residual ambient magnetic field., Phys. Rev. Lett., in press (submitted April 29, 2005), 4 pages, 3 figures

We study the impact of the beam diameter on the spectrum of the instability in second harmonic generation, and we report direct evidence of a quantum origin of the instability of a spatial soliton. © 2005 IEEE.

We report the first observation of temporal modulation instability of spatial solitons produced in second harmonic generation. Using quantum theory we interpret the observed instability to result from quantum fluctuations of the vacuum field

We analyze the dynamical stability of common spatial, or breathe-together, modes for a system of two trapped interacting Bose-Einstein condensates, when the condensates are resonantly coupled by a strong driving field. We show analytically that the dynamical stability is governed by a set of dressed atom collision parameters, and that the driving field typically destabilizes breathe-together modes which are stable in the absence of driving and vice versa. By means of numerical solutions we illustrate the consequences of this phenomenon for a number of interacting condensates consisting of pairs of ground hyperfine states of both sodium and rubidium.

We consider a collection of atoms prepared in a Bose-Einstein condensate which interact via two-body elastic collisions. A resonant driving field couples two internal states which together constitute an effective spin 1/2 system for each atom, and from which a total spin for the gas can be defined. It is shown that in the limit of strong driving, the system dynamics reduces to a mechanism for spin squeezing similar to that of a recent proposal for undriven condensates. However, we find that the conditions for spatial stability in our driven system are complementary to those of the undriven system. Reasons for this difference, associated with the physics of dressed atom collisions, are discussed along with conditions for preparing and observing the spin squeezed state