arXiv subjects

V. S. Bagnato

Publications and source records attributed to V. S. Bagnato.

At least 73 records · Page 4Linked to original sources

arXiv2007

Ramsey fringes formation during excitation of topological modes in a Bose-Einstein condensate

The Ramsey fringes formation during the excitation of topological coherent modes of a Bose-Einstein condensate by an external modulating field is considered. The Ramsey fringes appear when a series of pulses of the excitation field is applied. In both Rabi and Ramsey interrogations, there is a shift of the population maximum transfer due to the strong non-linearity present in the system. It is found that the Ramsey pattern itself retains information about the accumulated relative phase between both ground and excited coherent modes.

cond-mat.stat-mech↗

arXiv2005

Mechanocaloric and Thermomechanical Effects in Bose-Einstein Condensed Systems

In this paper we extend previous hydrodynamic equations, governing the motion of Bose-Einstein-condensed fluids, to include temperature effects. This allows us to analyze some differences between a normal fluid and a Bose-Einstein-condensed one. We show that, in close analogy with superfluid He-4, a Bose-Einstein-condensed fluid exhibits the mechanocaloric and thermomechanical effects. In our approach we can explain both effects without using the hypothesis that the Bose-Einstein-condensed fluid has zero entropy. Such ideas could be investigated in existing experiments.

cond-mat.stat-mech↗

arXiv2005

Achievement of Quantum Degeneracy in a Na-QUIC trap in Brazil: an in situ observation

Using a system composed of a Quadrupole and Ioffe Configuration (QUIC) trap loaded from a slowed atomic beam, we have performed experiments to observe the Bose-Einstein Condensation of Na atoms. In order to obtain the atomic distribution in the trap, we use an in situ out of resonance absorption image through a probe beam, to determine temperature and density. The phase space density (D) is calculated using the density profile and the temperature. We have followed D as a function of the final evaporation frequency. The results show that at 1.65 MHz we crossed the value for D expected to correspond to the critical point to start de Bose-Condensation of the sample. Due to the low number of atoms remaining in the trap at the critical point, the interaction produces minor effects and therefore an ideal gas model explains well the observations. We analyze the obtained low number in terms of efficiency of evaporation. The utility of an in situ detection is illustrated by measuring the harmonic gas pressure of the trapped gas in the route to condensation.

cond-mat.other↗

arXiv2005

Topological Coherent Modes in Trapped Bose Gas

The report reviews the problem of topological coherent modes, which are nonlinear collective states of Bose-condensed atoms. Such modes can be generated by means of alternating external fields, whose frequencies are in resonance with the transition frequencies between the related modes. The Bose gas with generated topological coherent modes is a collective nonlinear analog of a resonant atom. Such systems exhibit a variety of nontrivial effects, e.g.: interference fringes, interference current, mode locking, dynamic transitions, critical phenomena, chaotic motion, harmonic generation, parametric conversion, atomic squeezing, and entanglement production.

cond-mat.mes-hall↗

arXiv2004

Creating a self-induced dark spontaneous-force optical trap for neutral atoms

This communication describes the observation of a new type of dark spontaneous-force optical trap (dark SPOT) obtained without the use of a mask blocking the central part of the repumper laser beam. We observe that loading a magneto-optical trap (MOT) from a continuous and intense flux of slowed atoms and by appropriately tuning the frequency of the repumper laser is possible to achieve basically the same effect of the dark SPOT, using a simpler apparatus. This work characterizes the new system through measurements of absorption and fluorescence imaging of the atomic cloud and presents a very simple model to explain the main features of our observations. We believe that this new approach may simplify the current experiments to produce quantum degenerated gases.

physics.atom-ph↗

arXiv2003

Coherent Resonance in Trapped Bose Condensates

Coherent resonance is the effect of resonant excitation of nonlinear coherent modes in trapped Bose condensates. This novel effect is shown to be feasible for Bose-condensed trapped gases. Conditions for realizing this effect are derived. A method of stabilizing Bose condensates with attractive interactions is advanced. The origin of dynamic critical phenomena is elucidated. Interference effects are studied. The existence of atomic squeezing and multiparticle coherent entanglement is demonstrated. Coherent resonance is a generalization of atomic resonance, involving internal states of an individual atom, to collective states of a multiparticle system.

cond-mat.soft↗

arXiv2003

Resonant Bose Condensate: Analog of Resonant Atom

The resonant formation of nonlinear coherent modes in trapped Bose-Einstein condensates is studied. These modes represent nonground-state Bose condensates. The methods of describing the spectrum of the nonlinear modes are discussed. The latter can be created by modulating the trapping potential with a frequency tuned close to the transition frequency between the two chosen modes. The requirement that the transition amplitudes be smaller than the transition frequency implies a constraint on the number of particles that can be transferred to an excited mode. The resonant Bose condensate serves as a collective analog of a single resonant atom. Such a condensate, displaying the coherent resonance, possesses several interesting features, among which are: mode locking, critical dynamics, interference patterns, interference current, atomic squeezing, and multiparticle entanglement.

cond-mat↗

arXiv2002

Nonlinear Coherent Modes of Trapped Bose-Einstein Condensates

Nonlinear coherent modes are the collective states of trapped Bose atoms, corresponding to different energy levels. These modes can be created starting from the ground state condensate that can be excited by means of a resonant alternating field. A thorough theory for the resonant excitation of the coherent modes is presented. The necessary and sufficient conditions for the feasibility of this process are found. Temporal behaviour of fractional populations and of relative phases exhibits dynamic critical phenomena on a critical line of the parametric manifold. The origin of these critical phenomena is elucidated by analyzing the structure of the phase space. An atomic cloud, containing the coherent modes, possesses several interesting features, such as interference patterns, interference current, spin squeezing, and massive entanglement. The developed theory suggests a generalization of resonant effects in optics to nonlinear systems of Bose-condensed atoms.

cond-mat↗

arXiv2002

Self-Similar Approximations for a Trapped Bose-Einstein Condensate

An approximate solution to the Gross-Pitaevskii equation for Bose-Einstein condensate in a spherical harmonic trap is suggested, which is valid in the whole interval of the coupling parameter, correctly interpolating between weak-coupling and strong-coupling limits. This solution is shown to be more accurate than the optimized Gaussian approximation as well as the Thomas-Fermi approximation. The derivation of the solution is based on the self-similar approximation theory. The possibility of obtaining interpolation formulas in the case of nonspherical traps is discussed.

cond-mat↗

arXiv2002

Spectrum of Coherent Modes for Trapped Bose Gas

The spectrum of coherent modes for dilute atomic Bose gas, confined in a cylindrical trap, is calculated by applying optimized perturbation theory and the technique of self-similar root approximants. The latter technique makes it possible to derive accurate analytical formulas. The obtained expressions are valid for arbitrary energy levels of all excited nonlinear coherent modes and for different traps, spherical, sigar-shape, and disk-shape.

cond-mat↗

arXiv2002

Critical Effects in Population Dynamics of Trapped Bose-Einstein Condensates

The population dynamics of a trapped Bose-Einstein condensate, subject to the action of an external field, is studied. This field produces a spatio-temporal modulation of the trapping potential with the frequency close to the transition frequency between the ground state and a higher energy level. For the evolution equations of fractional populations, a critical line is found. It is demonstrated that there exists a direct analogy between dynamical instability at this line and critical phenomena at a critical line of an averaged system. The related critical indices are calculated. The spatio-temporal evolution of atomic density is analyzed.

cond-mat↗

arXiv2001

Bose-Einstein Condensation of Trapped Atomic Gases

This article reviews recent investigations on the phenomenon of Bose-Einstein condensation of dilute gases. Since the experimental observation of quantum degeneracy in atomic gases, the research activity in the field of coherent matter-waves literally exploded. The present topical review aims to give an introduction into the thermodynamics of Bose-Einstein condensation, a general overview over experimental techniques and investigations, and a theoretical foundation for the description of bosonic many-body quantum systems.

cond-mat.soft↗

arXiv2001

Dynamic Critical Phenomena in Trapped Bose Gases

Nonlinear dynamics of a trapped Bose-Einstein condensate, subject to the action of a resonant external field, is studied. This field produces a spatio-temporal modulation of the trapping potential with the frequency close to the transition frequency between the ground state and a higher energy level. The evolution equations of fractional populations display a kind of critical phenomena at a critical line on the manifold of the system parameters. It is demonstrated that there exists a direct analogy between dynamical instability at this line and critical phenomena at a critical line of a related averaged system.

cond-mat↗

arXiv2001

Magnetic Field Tomography

Neutral atoms may be trapped via the interaction of their magnetic dipole moment with magnetic field gradients. One of the possible schemes is the cloverleaf trap. It is often desirable to have at hand a fast and precise technique for measuring the magnetic field distribution. We introduce a novel diagnostic tool for instantaneous imaging the equipotential lines of a magnetic field within a region of space (the vacuum recipient) that is not accessible to massive probes. Our technique is based on spatially resolved observation of the fluorescence emitted by a hot beam of sodium atoms crossing a thin slice of resonant laser light within the magnetic field region to be investigated. The inhomogeneous magnetic field spatially modulates the resonance condition between the Zeeman-shifted hyperfine sublevels and the laser light and therefore the amount of scattered photons. We demonstrate this technique by mapping the field of our cloverleaf trap in three dimensions under various conditions.

cond-mat.soft↗

arXiv2001

Comparative investigation of 39K and 40K trap loss rates: alternative loss channel at low light intensities

We report a comparative investigation of trap loss rates in a magneto-optical trap for two potassium isotopes, 39K and 40K, as a function of trap light intensity. The isotopes present a quite similar behavior for the loss rates at high intensities, and a sudden increase of the loss rates at low intensities is present in both cases. While for 39K such increase can be explained assuming that the major contribution to the losses comes from Hyperfine Changing Collisions, a different loss mechanism must be considered for 40K, which has an inverted ground state hyperfine structure. The experimental results of both isotopes are well reproduced by an alternative model based on radiative escape as the dominant loss mechanism.

physics.atom-ph↗

arXiv2001

Nonground State Condensates of Ultracold Trapped Atoms

The population dynamics of a trapped Bose-Einstein condensate, subject to the action of an oscillatory field, is studied. This field produces a modulation of the trapping potential with the frequency close to the transition frequency between the ground state and an excited energy level. Unusual critical effects are found exhibiting sharp qualitative changes in the population dynamics. An effective averaged system is constructed explicitly illustrating the occurrence of critical phenomena. The related critical indices are calculated.

cond-mat↗

arXiv2001

Capture Velocity for a Magneto-Optical Trap in a Broad Range of Light Intensity

In a recent paper, we have used the dark-spot Zeeman tuned slowing technique [Phys. Rev. A 62, 013404-1, (2000)] to measure the capture velocity as a function of laser intensity for a sodium magneto optical trap. Due to technical limitation we explored only the low light intensity regime, from 0 to 27 mW/cm^2. Now we complement that work measuring the capture velocity in a broader range of light intensities (from 0 to 400 mW/cm^2). New features, observed in this range, are important to understant the escape velocity behavior, which has been intensively used in the interpretation of cold collisions. In particular, we show in this brief report that the capture velocity has a maximum as function of the trap laser intensity, which would imply a minimum in the trap loss rates.

physics.atom-ph↗

arXiv2000

Excited Coherent Modes of Ultracold Trapped Atoms

A method of exciting coherent spatial modes of Bose-condensed trapped atoms is considered. The method is based on the resonance modulation of the trapping potential. The population dynamics of coherent modes is analysed. The method makes it possible to create mixtures of different spatial modes in arbitrary proportions, including the formation of pure excited coherent modes. Novel critical effects in the population dynamics are found.

cond-mat↗