Cold Atoms Physics

We report the realisation and preliminary study of a frequency standard using a fountain of laser cooled caesium atoms. Our apparatus uses a magneto-optical trap as a source of cold atoms and optical pumping to prepare the atoms in the correct state before they enter the microwave cavity.

We consider classical dynamical properties of a particle in a constant gravitational force and making specular reflections with circular, elliptic or oval boundaries. The model and collision map are described and a detailed study of the energy regimes is made. The linear stability of fixed points is studied, yielding exact analytical expressions for parameter values at which a perioddoubling bifurcation occurs. The dynamics is apparently ergodic at certain energies in all three models, in contrast to the regularity of the circular and elliptic billiard dynamics in the field-free case. This finding is confirmed using a sensitive test involving Lyapunov weighted dynamics. In the last part of the paper a time dependence is introduced in the billiard boundary, where it is shown that for the circular billiard the average velocity saturates for zero gravitational force but in the presence of gravitational it increases with a very slow growth rate, which may be explained using Arnold diffusion. For the oval billiard, where chaos is present in the static case, the particle has an unlimited velocity growth with an exponent of approximately 1/6.

The atom -electromagnetic field interaction is studied in the Dicke model, wherein a single field mode is interacting with a collection of two level atoms at thermal equilibrium. It is found that in the superradiant phase of the system, wherein the Bose-Einstein condensation of photons takes place, the notion of photon as an elementary electromagnetic excitation ceases to exist. The phase and intensity excitations of the condensate are found to be the true excitations of electromagnetic field. It is found that in this phase, the atom interacts with these excitations in a distinct coherent transition process, apart from the known stimulated emission/absorption and spontaneous emission processes. In the coherent transition it is found that while the atomic state changes in course of the transition process, the state of electromagnetic field remains unaffected. It is found that the transition probability of such coherent transition process is macroscopically large compared to other stimulated emission/absorption and spontaneous emission processes.

We show that the gauge field induced due to non-uniform hopping, in gapped graphene, can give rise to a non-BCS type of superconductivity. Unlike the conventional mechanisms, this superconductivity phenomena does not require any pairing. We estimate the critical temperature for superconducting-to-normal transition via Berezinskii-Kosterlitz-Thouless mechanism. Possibility of observing the same in ultra cold atomic gases is also pointed out.

We present progress toward the observation of antiferromagnetic (AFM) ordering of fermionic atoms in an optical lattice using Bragg scattering of light. We first laser cool ^6Li atoms using the 2S1/2->2P3/2 transition and then further cool using the 2S1/2->3P3/2 transition to T ˜ 65 muK, leading to enhanced loading into a far detuned optical dipole trap. After forced evaporative cooling, an incoherent spin mixture of the two lowest magnetic sublevels of the ground state is adiabatically loaded into a 3D optical lattice. By adjusting the s-wave scattering length and the depth of the lattice, we tune the interaction and hopping terms of the Hubbard Hamiltonian. Bragg scattering of light from the lattice planes can be used to detect sample ordering such as in the Mott insulator state. At low temperatures and weak interactions, a phase transition to AFM ordering of the two spin states is predicted to occur. The increased symmetry of the AFM state allows for Bragg scattering of lig...

Context. H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds, respectively. However, until recently, we have not had spectrally resolved C + surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium, in particular, the H 2 gas without CO, referred to as CO-dark H 2 , in a large sample of interstellar clouds. Aims. We use a sparse Galactic plane survey of the 1.9 THz (158 μm) [C ii] spectral line from the Herschel open time key programme, Galactic Observations of Terahertz C+ (GOT C+), to characterize the H 2 gas without CO in a statistically significant sample of interstellar clouds. Methods. We identify individual clouds in the inner Galaxy by fitting the [C ii] and CO isotopologue spectra along each line of sight. We then combine these spectra with those of H i and use them along with excitation models and cloud models of C + to determine the column densities and fractional mass of CO-dark H 2 clouds. Results. We identify1804 narrow velocity [C ii] components corresponding to interstellar clouds in different categories and evolutionary states. About 840 are diffuse molecular clouds with no CO, ∼510 are transition clouds containing [C ii] and 12 CO, but no 13 CO, and the remainder are dense molecular clouds containing 13 CO emission. The CO-dark H 2 clouds are concentrated between Galactic radii of ∼3.5 to 7.5 kpc and the column density of the CO-dark H 2 layer varies significantly from cloud to cloud with a global average of 9 × 10 20 cm -2 . These clouds contain a significant fraction by mass of CO-dark H 2 , that varies from ∼75% for diffuse molecular clouds to ∼20% for dense molecular clouds. Conclusions. We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO, but is detected in [C ii]. The fraction of CO-dark H 2 is greatest in the diffuse clouds and decreases with increasing total column density, and is lowest in the massive clouds. The column densities and mass fraction of CO-dark H 2 are less than predicted by models of diffuse molecular clouds using solar metallicity, which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective. There is an overall trend towards a higher fraction of CO-dark H 2 in clouds with increasing Galactic radius, consistent with lower metallicity there.

We give an overview of the work done with the LNE-SYRTE fountain ensemble during the last five years. After a description of the clock ensemble, comprising three fountains FO1, FO2 and FOM, and its newest developments, we review recent studies of several systematic frequency shifts. This includes the distributed cavity phase shift which we evaluate for the FO1 and FOM fountains, applying the techniques of our recent work on FO2. We also report calculations of the microwave lensing frequency shift for the three fountains, review the status of the blackbody radiation shift, and summarize recent experimental work to control microwave leakage and spurious phase perturbations. We give current accuracy budgets. We also describe several applications in time and frequency metrology: fountain comparisons, calibrations of the international atomic time, secondary representation of the SI second based on the 87 Rb hyperfine frequency, absolute measurements of optical frequencies, tests of the T2L2 satellite laser link, and review fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally, we give a summary of the tests of the PHARAO cold atom space clock performed using the FOM transportable fountain.

Ground state properties of Bose-Einstein condensate of 20000 Na atoms confined in isotropic and highly anisotropic magnetic traps in the presence of three-body interaction, in addition to the two-body and hard-core interactions, have been theoretically studied by solving modified highly non-linear Gross-Pitaevskii-Ginzburg (GPG) equation. There is increasingly significant change in the chemical potential, total and differential energies per particle as the aspect ratio, λ, is increased from 0.1 to 1.0, with respect to the corresponding values when only two-body and hard-core interactions are considered. The condensate order parameter changes drastically and becomes quasi 1-dimensional, as λ is varied from 1 to 0.1.

We present here our study of the adiabatic quantum dynamics of a random Ising chain across its quantum critical point. The model investigated is an Ising chain in a transverse field with disorder present both in the exchange coupling and in the transverse field. The transverse field term is proportional to a function Γ(t) which, as in the Kibble-Zurek mechanism, is linearly reduced to zero in time with a rate τ -1 , Γ(t) = -t/τ , starting at t = -∞ from the quantum disordered phase (Γ = ∞) and ending at t = 0 in the classical ferromagnetic phase (Γ = 0). We first analyze the distribution of the gaps, occurring at the critical point Γc = 1, which are relevant for breaking the adiabaticity of the dynamics. We then present extensive numerical simulations for the residual energy Eres and density of defects ρ k at the end of the annealing, as a function of the annealing inverse rate τ . Both the average Eres(τ ) and ρ k (τ ) are found to behave logarithmically for large τ , but with different exponents, [Eres(τ )/L]av ∼ 1/ ln ζ (τ ) with ζ ≈ 3.4, and [ρ k (τ )]av ∼ 1/ ln 2 (τ ). We propose a mechanism for 1/ ln 2 τ -behavior of [ρ k ]av based on the Landau-Zener tunneling theory and on a Fisher's type real-space renormalization group analysis of the relevant gaps. The model proposed shows therefore a paradigmatic example of how an adiabatic quantum computation can become very slow when disorder is at play, even in absence of any source of frustration.

Atomic ensembles have many potential applications in quantum information science. Owing to collective enhancement, working with ensembles at high densities increases the overall efficiency of quantum operations, but at the same time also increases the collision rate and leads to faster decoherence. Here we report on experiments with optically trapped 87 Rb atoms demonstrating a 20-fold increase of the coherence time when a dynamical decoupling sequence with more than 200 pi-pulses is applied. We perform quantum process tomography and demonstrate that using the decoupling scheme a dense ensemble with an optical depth of 230 can be used as an atomic memory with coherence times exceeding 3 sec.

Collisional narrowing with optically trapped atoms YOAV SAGI, IDO ALMOG, NIR DAVIDSON, Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot, Israel -Cold atoms trapped in a conservative potential can be used for quantum information processing, where their high density can be employed to achieve high efficiency in quantum operations. The high atomic density, on the other hand, alters the atomic coherence dynamics and leads to a surprising prolongation of the coherence times, a phenomenon we call collisional narrowing in analog to the well known motional narrowing effect in NMR. Here we present our theoretical and experimental study of collisional narrowing in optically trapped atoms. We show that elastic collisions reduce the spread of the atomic phase distribution and modify its time evolution from ballistic to diffusive. We perform Ramsey experiments on cold 87 Rb atoms trapped in a far-off-resonance laser, and observe a decrease of the dephasing rate for an increasing collision rate. We also show that the new emerging dephasing timescale depends only on the atomic phase space density of the ensemble.

We employ a continuous dynamical decoupling scheme to suppress the decoherence induced by elastic collisions of cold atoms. Using a continuous echo pulse we achieve a thirty-fold increase in the coherence time of 87 Rb atoms trapped in a dipole trap. Coherence times of more than 100ms are demonstrated for an ensemble with an optical depth of 120.

We show that synthetic spin-orbit coupling for ultracold atoms in optical Raman potentials can be exploited to build versatile quantum simulators of correlated Chern insulators connected to strongly coupled four-Fermi field theories similar to the Gross-Neveu model in (2 + 1) dimensions. Exploiting this multidisciplinary perspective, we identify a large-N quantum anomalous Hall (QAH) effect in absence of any external magnetic field, and use it to delimit regions in parameter space where these correlated topological phases appear, the boundaries of which are controlled by strongly coupled fixed points of these four-Fermi relativistic field theories. We further show how, for strong interactions, the QAH effect gives way to magnetic phases described by a two-dimensional quantum compass model in a transverse field. We present a detailed description of the phase diagram using the large-N effective potential, and variational techniques such as projected entangled pairs.

We study the two mechanisms of the interplay of long-and shortrange interactions in different geometries of ultracold fermionic atomic or molecular gases. We show that in the range of validity of the one-dimensional (1D) approximation, both mechanisms yield similar superconductivity. We show that electromagnetically induced isotropic dipole-dipole interactions in a spinpolarized non-degenerate fermionic gas can cause an extremely exothermic phase transition, analogous to the isothermal collapse in gravitationally interacting star clusters. This collapse may result in fragmentation of the gas into a hot 'halo' and a highly degenerate 'core'. Possible realization is envisaged in microwave-illuminated fermionic molecular gases at microkelvin temperatures.

While quantum accelerometers sense with extremely low drift and low bias, their practical sensing capabilities face at least two limitations compared with classical accelerometers: a lower sample rate due to cold atom interrogation time; and a reduced dynamic range due to signal phase wrapping. In this paper, we propose a maximum likelihood probabilistic data fusion method, under which the actual phase of the quantum accelerometer can be unwrapped by fusing it with the output of a classical accelerometer on the platform. Consequently, the recovered measurement from the quantum accelerometer is used to estimate bias and drift of the classical accelerometer which is then removed from the system output. We demonstrate the enhanced error performance achieved by the proposed fusion method using a simulated 1D accelerometer precision test scenario. We conclude with a discussion on fusion error and potential solutions.

In the formal development of optical response theory in terms of susceptibilities, proper representation of the optical frequency dependence necessitates modeling both the discrete linewidth and the finite signal enhancement associated with the onset of resonance. Such dispersion behavior is generally accommodated by damping factors, featured in both resonant and non-resonant susceptibility terms. For the resonant terms, the sign of such damping corrections is unequivocal; however the correct choice of sign for non-resonant terms has become a matter of debate, heightened by the discovery that entirely opposite conventions are applied in mainstream literature on Raman scattering and nonlinear optics. Where the two conventions are applied to electro-optical processes in fluids there are significant and potentially verifiable differences between the associated results . Through a full thorough quantum electrodynamical treatment the universal correctness of one convention can be ascertained and flaws in the counter-convention identified. Resolution of the central issue requires consideration of a number of fundamental questions concerning the nature of dissipation in quantum mechanical systems. It is concluded that optical susceptibilities formulated with correct signing of the damping corrections must fulfill several fundamental tests: satisfaction of a new sum rule; invariance of the associated quantum amplitudes under time -reversal symmetry, and a resilience to canonical transformation.

We launch Cs atoms using a moving three-dimensional ͑3D͒ optical lattice. Atoms are initially spin polarized and cooled to the ground state of the optical potential using 3D Raman sideband cooling and then accelerated in the lattice to velocities of up to 3 m/s. Subsequent adiabatic lowering of the potential releases the atoms from the lattice. Three-dimensional kinetic temperatures as low as 200 nK were achieved. The observed temperature of the launch is independent of the acceleration and final velocity of the atoms. In an alternative approach, we first accelerate the atoms to velocities of up to 5 m/s using moving molasses and then cool them with the lattice in the comoving frame of the atoms to temperatures as low as 150 nK.

We propose to control spin-mixing dynamics in a gas of spinor atoms, via the combination of two off-resonant Raman transition pathways, enabled by a common cavity mode and a bichromatic pump laser. The mixing rate, which is proportional to the synthesized spin-exchange interaction strength, and the effective atomic quadratic Zeeman shift (QZS), can both be tuned by changing the pump laser parameters. Quench and driving dynamics of the atomic collective spin are shown to be controllable on a faster time scale than in existing experiments based on inherent spin-exchange collision interactions. The results we present open a promising avenue for exploring spin-mixing physics of atomic ensembles accessible in current experiments.

We present a simple scheme for implementing a one-dimensional (1D) magnetic-flux lattice of ultracold fermionic spin-1/2 atoms. The resulting tight-binding model supports gapped and gapless topological phases, and chiral currents for Meissner and vortex phases. Its single-particle spectra exhibit topological flat bands at small flux, and the flatness sensitively depends on hopping strength. An effective p-wave interaction arises in a s-wave paired superfluid. Treating atomic internal states as forming a synthetic dimension and balancing the interplay of magnetic flux and Zeeman field, our model describes a tunable topological Fermi superfluid, which paves the way towards experimental explorations of non-Abelian topological matter in 1D atomic quantum gases.

We propose a scheme to dynamically generate optical flux lattices with nontrivial band topology using amplitude-modulated Raman lasers and radio-frequency (rf) magnetic fields. By tuning the strength of Raman and rf fields, three distinct phases are realized at unit filling for a unit cell. Respectively, these three phases correspond to normal insulator, topological Chern insulator, and semimetal. Nearly nondispersive bands are found to appear in the topological phase, which promises opportunities for investigating strongly correlated quantum states within a simple cold-atom setup. The validity of our proposal is confirmed by comparing the Floquet quasienergies from the evolution operator with the spectrum of the effective Hamiltonian.

We propose an efficient mechanism for the evaporative cooling of trapped fermions directly into quantum degeneracy. Our idea is based on an electric field induced elastic interaction between trapped atoms in spin symmetric states. We discuss some novel general features of fermionic evaporative cooling and present numerical studies demonstrating the feasibility for the cooling of alkali metal fermionic species 6 Li, 40 K, and 82,84,86 Rb. We also discuss the sympathetic cooling of fermionic hyperfine spin mixtures, including the effects of anisotropic interactions.

We develop a general scheme for detecting spin correlations inside a two-component lattice gas of bosonic atoms, stimulated by the recent theoretical and experimental advances on analogous systems for a single component quantum gas. Within a linearized theory for the transmission spectra of the cavity mode field, different magnetic phases of a two-component (spin 1/2) lattice bosons become clearly distinguishable. In the Mott-insulating (MI) state with unit filling for the two-component lattice bosons, three different phases: antiferromagnetic, ferromagnetic, and the XY phases are found to be associated with drastically different cavity photon numbers. Our suggested study can be straightforwardly implemented with current cold atom experiments.

Quantum memories are essential elements in long-distance quantum networks and quantum computation. Significant advances have been achieved in demonstrating relative long-lived single-channel memory at single-photon level in cold atomic media. However, the qubit memory corresponding to store two-channel spin-wave excitations (SWEs) still faces challenges, including the limitations resulting from Larmor procession, fluctuating ambient magnetic field, and manipulation/measurement of the relative

It is shown that by an appropriate modification of the trapping potential one may create collective excitation in cold atom Bose-Einstein condensate. The proposed method is complementary to earlier suggestions. It seems to be feasible experimentally -it requires only a proper change in time of the potential in atomic traps, as realized in laboratories already.

The quantum phase transition from the superfluid to the Mott insulator phase is predicted by the Bose-Hubbard model and realized for optical lattice ultracold atoms. We extend the model to include excited atoms and their coupling to cavity photons. In applying a mean field theory we calculate the phase diagram, where the Mott insulator reappears for deeper optical lattices [1].

We give a general overview of the high-frequency regime in periodically driven systems and identify three distinct classes of driving protocols in which the infinite-frequency Floquet Hamiltonian is not equal to the time-averaged Hamiltonian. These classes cover systems, such as the Kapitza pendulum, the Harper-Hofstadter model of neutral atoms in a magnetic field, the Haldane Floquet Chern insulator and others. In all setups considered, we discuss both the infinite-frequency limit and the leading finite-frequency corrections to the Floquet Hamiltonian. We provide a short overview of Floquet theory focusing on the gauge structure associated with the choice of stroboscopic frame and the differences between stroboscopic and non-stroboscopic dynamics. In the latter case one has to work with dressed operators representing observables and a dressed density matrix. We also comment on the application of Floquet Theory to systems described by static Hamiltonians with well-separated energy scales and, in particular, discuss parallels between the inverse-frequency expansion and the Schrieffer-Wolff transformation extending the latter to driven systems.

This colloquium gives an overview of recent theoretical and experimental progress in the area of nonequilibrium dynamics of isolated quantum systems. We particularly focus on quantum quenches: the temporal evolution following a sudden or slow change of the coupling constants of the system Hamiltonian. We discuss several aspects of the slow dynamics in driven systems and emphasize the universality of such dynamics in gapless systems with specific focus on dynamics near continuous quantum phase transitions. We also review recent progress on understanding thermalization in closed systems through the eigenstate thermalization hypothesis and discuss relaxation in integrable systems. Finally we overview key experiments probing quantum dynamics in cold atom systems and put them in the context of our current theoretical understanding. I. Introduction 1 II. Nearly adiabatic dynamics in quantum systems 3 A. Universality in a nutshell 3 B. Universal dynamics near quantum critical points 3 C. Slow dynamics in gapped and gapless systems. 7 D. Effects of finite temperature. 8 E. Open problems 9 III. Effects of Integrability and its breaking: ergodicity and thermalization. 9 A. Quantum and classical ergodicity. 10 B. Nonergodic behavior in integrable systems: the generalized Gibbs ensemble 11 C. Breaking integrability: eigenstate thermalization. 14 D. Outlook and open problems: quantum KAM threshold as a many-body delocalization transition ? 15 IV. Experimental progress in quantum dynamics in cold atoms and other systems 16 V. Outlook 20 VI. acknowledgements 20 References 20

When an isolated system is brought in contact with a heat bath its final energy is random and follows the Gibbs distribution -a cornerstone of statistical physics. The system's energy can also be changed by performing non-adiabatic work using a cyclic process. Almost nothing is known about the resulting energy distribution in this setup, which is especially relevant to recent experimental progress in cold atoms, ions traps, superconducting qubits and other systems. Here we show that when the non-adiabatic process comprises of many repeated cyclic processes the resulting energy distribution is universal and different from the Gibbs ensemble. We predict the existence of two qualitatively different regimes with a continuous second order like transition between them. We illustrate our approach performing explicit calculations for both interacting and non-interacting systems.

We propose a Doppler tracking system for gravitational wave detection via Double Optical Clocks in Space (DOCS). In this configuration two spacecrafts (each containing an optical clock) are launched to space for Doppler shift observations. Compared to the similar attempt of gravitational wave detection in the Cassini mission, the radio signal of DOCS that contains the relative frequency changes avoids completely noise effects due for instance to troposphere, ionosphere, ground-based antenna and transponder. Given the high stabilities of the two optical clocks (Allan deviation ∼ 4.1 × 10 -17 @ 1000 s), an overall estimated sensitivity of 5 × 10 -19 could be achieved with an observation time of 2 years, and would allow to detect gravitational waves in the frequency range from ∼ 10 -4 Hz to ∼ 10 -2 Hz.

We report on the development of a sapphire cryogenic microwave resonator oscillator with long-term fractional frequency stability of 2 × 10 -17 √τ for integration times τ >10 3 s and a negative drift of about 2.2 × 10 -15 /day. The short-term frequency instability of the oscillator is highly reproducible and also state-of-the-art: 5.6 ×10 -16 for an integration time of τ ≈ 20 s.

We theoretically study the superradiant gain and the direction of this coherent radiant for an array of Bose-Einstein condensates in an optical lattice. We find that the density grating is formed to amplify the scattering light within the phase match condition. The scattering spectroscopy in the momentum space can provide a method for measuring the overlap of wavefunction between the neighboring sites, which is related to their inner-site and inter-site coherence.

Superradiance scattering from a Bose-Einstein condensate is studied with a two-frequency pumping beam. We demonstrate the possibility of fully tuning the backward mode population as a function of the locked initial relative phase between the two frequency components of the pumping beam. This result comes from an imprinting of this initial relative phase on two matter wave gratings, formed by the forward mode or backward mode condensate plus the condensate at rest, so that cooperative scattering is affected. A numerical simulation using a semiclassical model agrees with our observations.

A microwave atomic clock scheme based on Rb and Cs atoms trapped in optical lattice with magic wavelength for clock transition is proposed. The ac Stark shift of clock transition due to trapping laser can be canceled at some specific laser wavelengths. Comparing with in fountain clock, the cavity related shifts, the collision shift, and the Doppler effect are eliminated or suppressed dramatically in atomic clock when the magic optical lattice is exploited. By carefully analyzing various sources of clock uncertainty, we conclude that a microwave atomic clock with an accuracy of better than 2 × 10 -17 is feasible, which is of the same accuracy as the expected best optical atomic clock.

Ever since the invention of the cesium beam atomic clock in 1955, quantum frequency standards have seen considerable development over the decades, as a representative of quantum precision measurement. The progress in frequency measurements achieved in the past allows one to perform measurements of other physical and technical quantities with unprecedented precision, whenever they could be traced back to frequency measurements. Using atomic transitions as frequency references, quantum frequency standards are far less susceptible to external perturbations, and the quantum identity principle of microscopic particles allows easy replication of quantum frequency standards with the same frequency at any time and place. With laser cooling and trapping, cold atomic ensembles eliminate Doppler shift broadening and have become the go-to quantum reference when precision is required. The advancement of laser cooling and cold atom physics, in addition to novel physical matter states such as Bose-Einstein Condensation, gives rise to new experimental techniques in quantum precision measurement, especially quantum frequency standards, such as cesium fountain clocks dictating the SI second, as well as optical lattice clocks and single-ion optical clocks pushing the Frontier of Quantum Metrology. Other areas of quantum metrology, such as gravitometers and magnetometers, also benefit greatly from cold atoms. Challenges still remain, as researchers strive to push forward the limit in quantum precision measurement and search for novel physical phenomena in cold atomic systems. In this regard, we organized the Research Topic "Quantum Precision Measurement and Cold Atom Physics" in Frontiers in Physics. As a tribute to his 90th birthday on September 20, we dedicate this Research Topic to the venerable Prof. Yiqiu Wang, a pioneering figure in China's development of quantum precision measurement and cold atom physics. Chen et al. chronicled the scientific career

We study quantum dynamics of Grover's adiabatic search algorithm with the equivalent two-level system. Its adiabatic and non-adiabatic evolutions are visualized as trajectories of Bloch vectors on a Bloch sphere. We find the change in the non-adiabatic transition probability from exponential decay for short running time to inverse-square decay for long running time. The size dependence of the critical running time is expressed in terms of Lambert W function. The transitionless driving Hamiltonian is obtained to make a quantum state follow the adiabatic path. We demonstrate that a constant Hamiltonian, approximate to the exact time-dependent driving Hamiltonian, can alter the non-adiabatic transition probability from the inverse square decay to the inverse fourth power decay with running time. This may open up a new way of reducing errors in adiabatic quantum computation.

In atomic systems, the spatially nonuniform distribution of a coupling field leads to the focusing of a probe beam producing a recipe for electromagnetically induced focusing (EIF). A diffraction-like pattern for the output probe beam can arise, and the probe radiation experiences focusing and defocusing across an electromagnetically induced transparency window. This phenomenon has critical implications for experiments on atomic coherence effects. Here, we study the EIF in a four-level closed-loop atomic system and show that full width at halfmaximum (FWHM) of focused output probe intensity in the focal plane can be controlled by both intensity profile and relative phase of applied fields. We also demonstrate that the FWHM of the focused output probe intensity in focal length is much smaller than that of the input probe intensity, and in addition, through a special set of parameters, the minimum value of the FWHM can be obtained. Moreover, the FWHM can be made small by increasing Rabi frequency of the Gaussian signal field. Furthermore, the Gaussian probe intensity profile switches to a doughnut-like one just by changing the relative phase of applied fields. Finally, we apply a Laguerre-Gaussian signal field and find that the characteristics of the output probe field depend on the intensity profile of the signal field. Our results can be used to design a lens-like device with a controllable focal length and focusing strength that would pave the way toward all-optical switching devices.

We consider the problem of estimating quantum observables on a collection of qubits, given as a linear combination of Pauli operators, with shallow quantum circuits consisting of singlequbit rotations. We introduce estimators based on randomised measurements, which use decision diagrams to sample from probability distributions on measurement bases. This approach generalises previously known uniform and locally-biased randomised estimators. The decision diagrams are constructed given target quantum operators and can be optimised considering different strategies. We show numerically that the estimators introduced here can produce more precise estimates on some quantum chemistry Hamiltonians, compared to previously known randomised protocols and Pauli grouping methods.

La quantum biology (QB) è un campo di ricerca emergente che cerca di affronta- re fenomeni quantistici non triviali all’interno dei contesti biologici dotandosi di dati sperimentali di esplorazioni teoriche e tecniche numeriche. I sistemi biologici sono per definizione sistemi aperti, caldi,umidi e rumorosi, e queste condizioni sono per loro imprenscindibili; si pensa sia un sistema soggetto ad una veloce decoerenza che sopprime ogni dinamica quantistica controllata. La QB, tramite i principi di noise assisted transport e di antenna fononica sostiene che la presenza di un adeguato livello di rumore ambientale aumenti l’efficienza di un network di trasporto,inoltre se all’interno dello spettro ambientale vi sono specifici modi vibrazionali persistenti si hanno effetti di risonanza che rigenerano la coerenza quantistica. L’interazione ambiente-sistema è di tipo non Markoviano,non perturbativo e di forte non equi- librio, ed il rumore non è trattato come tradizionale rumore bianco. ...

Photoionization of a cold atomic sample offers intriguing possibilities to observe collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap with laser pulses ionizing through 1-photon and 2-photon absorption processes has been performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample have been examined as function of the ionizing laser parameters. Ionization cross-sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

We introduce a new technique to probe the properties of an interacting cold atomic gas that can be viewed as a dynamical compressibility measurement. We apply this technique to the study of the superfluid to Mott insulator quantum phase transition in one and three dimensions for a bosonic gas trapped in an optical lattice. Excitations of the system are detected by time-of-flight measurements. The experimental data for the one-dimensional case are in good agreement with the results of a time-dependent density matrix renormalization group calculation.

The Chern-Hopf insulator is an unconventional three-dimensional topological insulator with a bulk gap and gapless boundary states without protection from global discrete symmetries. This study investigates its fate in the presence of disorder. We find it stable up to moderate disorder by analyzing the surface states and the zero energy bulk density of states using large-scale numerical simulation and the self-consistent Born approximation. The disordered Chern-Hopf insulator shows reentrant behavior: the disorder initially enhances the topological phase before driving it across an insulator-diffusive metal transition. We examine the associated critical exponents via finite-size scaling of the bulk density of states, participation entropy, and two-terminal conductance. We estimate the correlation length exponent ν ≃ 1.0(1), consistent with the clean two-dimensional Chern universality and distinct from the integer quantum Hall exponent.

The cold-atom on a two-dimensional square optical lattice is studied within the hard-core boson Hubbard model with an alternating potential. In terms of the quantum Monte Carlo method, it is shown explicitly that a supersolid phase emerges due to the presence of the alternating potential. For the weak alternating potential, the supersolid state appears for the whole range of hard-core boson densities except the half-filling case, where the system is a Mott insulator. However, for the strong alternating potential, besides the supersolid and Mott insulating states, a charge density wave phase appears.

Quantum computing requires a system of qubits with two distinguishable quantum states. Conditions for successful quantum computer implementation are system scalability, the ability to initialize qubit states, long decoherence times, execution of universal quantum gates, and read out of these states. Neutral atom quantum computers use light patterns to trap neutral atoms and meet all of the above criteria, save scalability, though much progress has recently been made. Our experiment aims to fill and characterize novel atomic dipole traps in the diffraction pattern of a pinhole [1]. The traps will be projected via a lens into the chamber of a magneto-optical trap containing laser cooled atoms [2]. A two-dimensional array of pinholes would create a two-dimensional qubit array that would be suitable for quantum computing [3]. We will present an experiment schematic and progress toward experimentally verifying our simulations of these traps. [1] G. D. Gillen, et al., Phys. Rev. A 73, 013409 (2006), [2] K. Gillen-Christandl, et al., Phys. Rev. A 82, 063420 (2010), [3] K. Gillen-Christandl, et al., Phys. Rev. A 83, 023408 (2011).

Photoionization of a cold atomic sample offers intriguing possibilities to observe collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap with laser pulses ionizing through 1-photon and 2-photon absorption processes has been performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample have been examined as function of the ionizing laser parameters. Ionization cross-sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

Shear viscosity is a measure of the amount of dissipation in a simple fluid. In kinetic theory shear viscosity is related to the rate of momentum transport by quasiparticles, and the uncertainty relation suggests that the ratio of shear viscosity η to entropy density s in units of h/k B is bounded by a constant. Here, h is Planck's constant and k B is Boltzmann's constant. A specific bound has been proposed on the basis of string theory where, for a large class of theories, one can show that η/s ≥ h/(4πk B ). We will refer to a fluid that saturates the string theory bound as a perfect fluid. In this review we summarize theoretical and experimental information on the properties of the three main classes of quantum fluids that are known to have values of η/s that are smaller than h/k B . These fluids are strongly coupled Bose fluids, in particular liquid helium, strongly correlated ultracold Fermi gases, and the quark gluon plasma. We discuss the main theoretical approaches to transport properties of these fluids: kinetic theory, numerical simulations based on linear response theory, and holographic dualities. We also summarize the experimental situation, in particular with regard to the observation of hydrodynamic behavior in ultracold Fermi gases and the quark gluon plasma.

Cold atomic gases placed in optical lattices enable studies of simple condensed matter theory models with parameters that may be tuned relatively easily. When the optical potential is randomized (e.g. using laser speckle to create a random intensity distribution) one may be able to observe Anderson localization of matter waves for non-interacting bosons, the so-called Bose glass in the presence of interactions, as well as the Fermi glass or quantum spin glass for mixtures of fermions and bosons.

Hollow-core photonic-crystal waveguides filled with cold atoms can support giant optical nonlinearities through nondispersive propagation of light tightly confined in the transverse direction. Here we explore electromagnetically induced transparency is such structures, considering a pair of counter-propagating weak quantum fields in the medium of coherently driven atoms in the ladder configuration. Strong dipole-dipole interactions between optically excited, polarized Rydberg states of the atoms translate into a large dispersive interaction between the two fields. This can be used to attain a spatially-homogeneous conditional phase shift of π for two single-photon pulses, realizing a deterministic photonic phase gate, or to implement a quantum nondemolition measurement of the photon number in the signal pulse by a coherent probe, thereby achieving a heralded source of single or few photon pulses.

Very high frequency oscillations of intense light fields interact with micron-size dielectric objects to exert dc optical forces that allow polarizable particles to levitate, to be trapped and to be bound. Such optical forces are also suitable to arrange cold at oms in optical lattices. Various assemblages of optical traps, including periodic arrays, can be constructed either with independent lasers, or with a single laser beam split into different parts later recombined by interference, as well as through the use of diffractive elements. These optical-well arrays serve as templates for writing and erasing dynamic two-dimensional and three-dimensional "optical crystals", composed of mono-dispersed polystyrene spheres in water. Subsequently, the crystals become diffractive structures themselves. The association of micro-fluidics and optical trapping allows for the formation of optical traps into micro-channels. This leads to perform microchemistry experiments, such as fluorescence detection, on individual bodies attached to trapped particles. Self-trapping due to the optical binding force relates to the interaction between different dielectric objects located in an electromagnetic field; each one reacts not only to the field of the incident beam, but also to the induced fields radiated coherently by all other particles. Optical binding strongly influences the equilibrium state and the behavior of optical crystals. It must have the potential for creating collective effects.

We evaluate the Bose-Einstein condensate density and the superfluid fraction of bosons in a periodic external potential using path-integral Monte Carlo methods. The periodic lattice consists of a cubic cell containing a potential well that is replicated along one dimension using periodic boundary conditions. The aim is to describe bosons in a one-dimensional optical lattice or helium confined in a periodic porous medium. The one-body density matrix is evaluated and diagonalized numerically to obtain the single boson natural orbitals and the occupation of these orbitals. The condensate fraction is obtained as the fraction of bosons in the orbital that has the highest occupation. The superfluid density is obtained from the winding number. From the condensate orbital and superfluid fraction, we investigate ͑1͒ the impact of the periodic external potential on the spatial distribution of the condensate, and ͑2͒ the correlation of localizing the condensate into separated parts and the loss of superflow along the lattice. For high-density systems, as the well depth increases, the condensate becomes depleted in the wells and confined to the plateaus between successive wells, as in pores between necks in a porous medium. For low-density systems, as the well depth increases the BEC is localized at the center of the wells ͑tight binding͒ and depleted between the wells. In both cases, the localization of the condensate suppresses superflow leading to a superfluid-insulator crossover. The impact of the external potential on the temperature dependence of the superfluidity is also investigated. The external potential suppresses the superfluid fraction at all temperatures, with a superfluid fraction significantly less than one at low temperature. The addition of an external potential does not, however, significantly reduce the transition temperature.