arXiv subjects

J. G. Muga

Publications and source records attributed to J. G. Muga.

At least 127 records · Page 7Linked to original sources

arXiv2008

Seeking better times: atomic clocks in the generalized Tonks-Girardeau regime

This article is based on the talk with the same title at the Blaubeuren meeting. First we discuss briefly the importance of time and time keeping, explaining the basic functioning of clocks in general and atomic clocks in particular, which rely on Ramsey interferometry. The usefulness of cold atoms is discussed as well as their limits in Bose-Einstein condensates. An alternative that we study is a different cold-atom regime: the Tonks-Girardeau (TG) gas of tightly confined and strongly interacting bosons. The TG gas is reviewed and then generalized for two-level atoms. Finally, we explore the combination of Ramsey interferometry and TG gases.

quant-ph↗

arXiv2008

Atom laser dynamics in a tight-waveguide

We study the transient dynamics that arise during the formation of an atom laser beam in a tight waveguide. During the time evolution the density profile develops a series of wiggles which are related to the diffraction in time phenomenon. The apodization of matter waves, which relies on the use of smooth aperture functions, allows to suppress such oscillations in a time interval, after which there is a revival of the diffraction in time. The revival time scale is directly related to the inverse of the harmonic trap frequency for the atom reservoir.

cond-mat.other↗

arXiv2007

Quantum matter wave dynamics with moving mirrors

When a stationary reflecting wall acting as a perfect mirror for an atomic beam with well defined incident velocity is suddenly removed, the density profile develops during the time evolution an oscillatory pattern known as diffraction in time. The interference fringes are suppressed or their visibility is diminished by several effects such as averaging over a distribution of incident velocities, apodization of the aperture function, atom-atom interactions, imperfect reflection or environmental noise. However, when the mirror moves with finite velocity along the direction of propagation of the beam, the visibility of the fringes is enhanced. For mirror velocities below beam velocity, as used for slowing down the beam, the matter wave splits into three regions separated by space-time points with classical analogues. For mirror velocities above beam velocity a visibility enhancement occurs without a classical counterpart. When the velocity of the beam approaches that of the mirror the density oscillations rise by a factor 1.8 over the stationary value.

quant-ph↗

arXiv2007

Long time deviation from exponential decay: non-integral power laws

Quantal systems are predicted to show a change-over from exponential decay to power law decay at very long times. Although most theoretical studies predict integer power-law exponents, recent measurements by Rothe et al. of decay luminescence of organic molecules in solution {Phys. Rev. Lett. 96 (2006) 163601} found non-integer exponents in most cases. We propose a physical mechanism, within the realm of scattering from potentials with long tails, which produces a continuous range of power law exponents. In the tractable case of the repulsive inverse square potential, we demonstrate a simple relation between the strength of the long range tail and the power law exponent. This system is amenable to experimental scrutiny.

quant-ph↗

arXiv2007

Motional frequency shifts of trapped ions in the Lamb-Dicke regime

First order Doppler effects are usually ignored in laser driven trapped ions when the recoil frequency is much smaller than the trapping frequency (Lamb-Dicke regime). This means that the central, carrier excitation band is supposed to be unaffected by vibronic transitions in which the vibrational number changes. While this is strictly true in the Lamb-Dicke limit (infinitely tight confinement), the vibronic transitions do play a role in the Lamb-Dicke regime. In this paper we quantify the asymptotic behaviour of their effect with respect to the Lamb-Dicke parameter. In particular, we give analytical expressions for the frequency shift, ``pulling'' or ``pushing'', produced in the carrier absorption band by the vibronic transitions both for Rabi and Ramsey schemes. This shift is shown to be independent of the initial vibrational state.

quant-ph↗

arXiv2007

Ramsey interferometry with a two-level Tonks-Girardeau gas

We propose a generalization of the Tonks-Girardeau model that describes a coherent gas of cold two-level Bosons which interact with two external fields in a Ramsey interferometer. They also interact among themselves by contact collisions with interchange of momentum and internal state. We study the corresponding Ramsey fringes and the quantum projection noise which, essentially unaffected by the interactions, remains that for ideal Bosons. The dual system of this gas, an ideal gas of two-level Fermions coupled by the interaction with the separated fields, produces the same fringes and noise fluctuations. The cases of time-separated and spatially-separated fields are studied. For spatially separated fields the fringes may be broadened slightly by increasing the number of particles, but only for large particle numbers far from present experiments with Tonks-Girardeau gases. The uncertainty in the determination of the atomic transition frequency diminishes, essentially with the inverse root of the particle number.

cond-mat.other↗

arXiv2007

Stability of spinor Fermi gases in tight waveguides

The two and three-body correlation functions of the ground state of an optically trapped ultracold spin-1/2 Fermi gas (SFG) in a tight waveguide (1D regime) are calculated in the plane of even and odd-wave coupling constants, assuming a 1D attractive zero-range odd-wave interaction induced by a 3D p-wave Feshbach resonance, as well as the usual repulsive zero-range even-wave interaction stemming from 3D s-wave scattering. The calculations are based on the exact mapping from the SFG to a ``Lieb-Liniger-Heisenberg'' model with delta-function repulsions depending on isotropic Heisenberg spin-spin interactions, and indicate that the SFG should be stable against three-body recombination in a large region of the coupling constant plane encompassing parts of both the ferromagnetic and antiferromagnetic phases. However, the limiting case of the fermionic Tonks-Girardeau gas (FTG), a spin-aligned 1D Fermi gas with infinitely attractive p-wave interactions, is unstable in this sense. Effects due to the dipolar interaction and a Zeeman term due to a resonance-generating magnetic field do not lead to shrinkage of the region of stability of the SFG.

cond-mat.other↗

arXiv2007

Three-dimensional effects in "atom diodes": atom-optical devices for one-way motion

The ``atom diode'' is a laser device that lets the ground state atom pass in one direction but not in the opposite direction. We examine three-dimensional effects of that device for arbitrary atomic incidence angles on flat laser sheets and set breakdown limiting angles and velocities. It is found that a correct diodic behavior independent of the incident angle can be obtained with blue detuned lasers.

quant-ph↗

arXiv2007

Time modulation of atom sources

Sudden turn-on of a matter-wave source leads to characteristic oscillations of the probability density which are the hallmark feature of diffraction in time. The apodization of matter waves relies on the use of smooth aperture functions which suppress such oscillations. The analytical dynamics arising with different aperture functions are discussed systematically for switching-on processes, and for single and many-pulse formation procedures. The possibility and time scale of a revival of the diffraction-in-time pattern is also analysed. Similar modulations in time of the pulsed output coupling in atom-lasers are responsible for the dynamical evolution and characteristics of the beam profile. For multiple pulses, different phase schemes and regimes are described and compared. Strongly overlapping pulses lead to a saturated, constant beam profile in time and space, up to the revival phenomenon.

cond-mat.other↗

arXiv2006

Suppression of Zeno effect for distant detectors

We describe the influence of continuous measurement in a decaying system and the role of the distance from the detector to the initial location of the system. The detector is modeled first by a step absorbing potential. For a close and strong detector, the decay rate of the system is reduced; weaker detectors do not modify the exponential decay rate but suppress the long-time deviations above a coupling threshold. Nevertheless, these perturbing effects of measurement disappear by increasing the distance between the initial state and the detector, as well as by improving the efficiency of the detector.

quant-ph↗

arXiv2006

Scattering of two-level atoms by delta lasers: Exactly solvable models in atom optics

We study the scattering of two-level atoms at narrow laser fields, modeled by a $δ$-shape intensity profile. The unique properties of these potentials allow us to give simple analytic solutions for one or two field zones. Several applications are studied: a single $δ$-laser may serve as a detector model for atom detection and arrival-time measurements, either by means of fluorescence or variations in occupation probabilities. We show that, in principle, this ideal detector can measure the particle density, the quantum mechanical flux, arrival time distributions or local kinetic energy densities. Moreover, two spatially separated $δ$-lasers are used to investigate quantized-motion effects on Ramsey interferometry.

quant-ph↗

arXiv2006

Exact propagators for atom-laser interactions

A class of exact propagators describing the interaction of an $N$-level atom with a set of on-resonance $δ$-lasers is obtained by means of the Laplace transform method. State-selective mirrors are described in the limit of strong lasers. The ladder, V and $Λ$ configurations for a three-level atom are discussed. For the two level case, the transient effects arising as result of the interaction between both a semi-infinite beam and a wavepacket with the on-resonance laser are examined.

quant-ph↗

arXiv2006

Vibronic "Rabi resonances" in harmonic and hard-wall ion-traps for arbitrary laser intensity and detuning

We investigate laser-driven vibronic transitions of a single two-level atomic ion in harmonic and hard wall traps. In the Lamb-Dicke regime, for tuned or detuned lasers with respect to the internal frequency of the ion, and weak or strong laser intensities, the vibronic transitions occur at well isolated "Rabi Resonances", where the detuning-adapted Rabi frequency coincides with the level spacing of the vibrational modes. These vibronic resonances are characterized as avoided crossings of the dressed levels (eigenvalues of the full Hamiltonian). Their peculiarities due to symmetry constraints and trapping potential are also examined.

quant-ph↗

arXiv2006

Ramsey interferometry with oppositely detuned fields

We report a narrowing of the interference pattern obtained in an atomic Ramsey interferometer if the two separated fields have different frequency and their phase difference is controlled. The width of the Ramsey fringes depends inversely on the free flight time of ground state atoms before entering the first field region in addition to the time between the fields. The effect is stable also for atomic wavepackets with initial position and momentum distributions and for realistic mode functions.

quant-ph↗

arXiv2006

Preparation of atomic velocities by bound-state to resonance conversion

A procedure is proposed to control the average and width of the velocity distribution of ultra-cold atoms. The atoms are set initially in a bound state of an optical trap formed by an inner red detuned laser and an outer blue detuned laser. The bound state is later converted into a resonance by a suitable change of the laser intensities. An optimal time dependence of the switching process, between the sudden and adiabatic limits, adjusts the final translational energies to the Lorentzian shape of the resonance state.

quant-ph↗

arXiv2006

Decay by tunneling of Bosonic and Fermionic Tonks-Girardeau Gases

We study the tunneling dynamics of bosonic and fermionic Tonks-Girardeau gases from a hard wall trap, in which one of the walls is substituted by a delta potential. Using the Fermi-Bose map, the decay of the probability to remain in the trap is studied as a function of both the number of particles and the intensity of the end-cap delta laser. The fermionic gas is shown to be a good candidate to study deviations of the non-exponential decay of the single-particle type, whereas for the bosonic case a novel regime of non-exponential decay appears due to the contributions of different resonances of the trap.

quant-ph↗

arXiv2006

Transversal laser excitation of atoms in a waveguide

We investigate the effect of a laser shining perpendicularly to a waveguide channeling two-level atoms. For weak transversal coupling the excitation of transverse atomic levels occurs at avoided crossings associated with "Rabi resonances" in which the Rabi frequency coincides with the transition frequency between the transverse levels.

quant-ph↗

arXiv2006

Matter-wave diffraction in time with a linear potential

Diffraction in time of matter waves incident on a shutter which is removed at time $t=0$ is studied in the presence of a linear potential. The solution is also discussed in phase space in terms of the Wigner function. An alternative configuration relevant to current experiments where particles are released from a hard wall trap is also analyzed for single-particle states and for a Tonks-Girardeau gas.

quant-ph↗