50 research outputs found

    Periodically-driven cold atoms: the role of the phase

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    Numerous theoretical and experimental studies have investigated the dynamics of cold atoms subjected to time periodic fields. Novel effects dependent on the amplitude and frequency of the driving field, such as Coherent Destruction of Tunneling have been identified and observed. However, in the last year or so, three distinct types of experiments have demonstrated for the first time, interesting behaviour associated with the driving phase: i.e. for systems experiencing a driving field of general form V(x)sin(ωt+ϕ)V(x)\sin (\omega t + \phi), different types of large scale oscillations and directed motion were observed. We investigate and explain the phenomenon of Super-Bloch Oscillations (SBOs) in relation to the other experiments and address the role of initial phase in general. We analyse and compare the role of ϕ\phi in systems with homogeneous forces (V(x)=constV'(x)= const), such as cold atoms in shaken or amplitude-modulated optical lattices, as well as non-homogeneous forces (V(x)constV'(x)\neq const), such as the sloshing of atoms in driven traps, and clarify the physical origin of the different ϕ\phi-dependent effects.Comment: 10 pages, 1 figur

    Atomic motion in tilted optical lattices

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    This paper presents a formalism describing the dynamics of a quantum particle in a one-dimensional, time-dependent, tilted lattice. The formalism uses the Wannier-Stark states, which are localized in each site of the lattice, and provides a simple framework allowing fully-analytical developments. Analytic solutions describing the particle motion are explicit derived, and the resulting dynamics is studied.Comment: 6 pages, 2 figs, submitted to EPJD, Springer Verlag styl

    Wavepacket reconstruction via local dynamics in a parabolic lattice

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    We study the dynamics of a wavepacket in a potential formed by the sum of a periodic lattice and of a parabolic potential. The dynamics of the wavepacket is essentially a superposition of ``local Bloch oscillations'', whose frequency is proportional to the local slope of the parabolic potential. We show that the amplitude and the phase of the Fourier transform of a signal characterizing this dynamics contains information about the amplitude and the phase of the wavepacket at a given lattice site. Hence, {\em complete} reconstruction of the the wavepacket in the real space can be performed from the study of the dynamics of the system.Comment: 4 pages, 3 figures, RevTex

    Damped Bloch oscillations of cold atoms in optical lattices

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    The paper studies Bloch oscillations of cold neutral atoms in the optical lattice. The effect of spontaneous emission on the dynamics of the system is analyzed both analytically and numerically. The spontaneous emission is shown to cause (i) the decay of Bloch oscillations with the decrement given by the rate of spontaneous emission and (ii) the diffusive spreading of the atoms with a diffusion coefficient depending on {\em both} the rate of spontaneous emission and the Bloch frequency.Comment: 10 pages, 8 figure

    Dynamics of one-dimensional tight-binding models with arbitrary time-dependent external homogeneous fields

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    The exact propagators of two one-dimensional systems with time-dependent external fields are presented by following the path-integral method. It is shown that the Bloch acceleration theorem can be generalized to the impulse-momentum theorem in quantum version. We demonstrate that an evolved Gaussian wave packet always keeps its shape in an arbitrary time-dependent homogeneous driven field. Moreover, that stopping and accelerating of a wave packet can be achieved by the pulsed field in a diabatic way.Comment: 8 pages, 6 figure

    Optical lattice quantum simulator for QED in strong external fields: spontaneous pair creation and the Sauter-Schwinger effect

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    Spontaneous creation of electron-positron pairs out of the vacuum due to a strong electric field is a spectacular manifestation of the relativistic energy-momentum relation for the Dirac fermions. This fundamental prediction of Quantum Electrodynamics (QED) has not yet been confirmed experimentally as the generation of a sufficiently strong electric field extending over a large enough space-time volume still presents a challenge. Surprisingly, distant areas of physics may help us to circumvent this difficulty. In condensed matter and solid state physics (areas commonly considered as low energy physics), one usually deals with quasi-particles instead of real electrons and positrons. Since their mass gap can often be freely tuned, it is much easier to create these light quasi-particles by an analogue of the Sauter-Schwinger effect. This motivates our proposal of a quantum simulator in which excitations of ultra-cold atoms moving in a bichromatic optical lattice represent particles and antiparticles (holes) satisfying a discretized version of the Dirac equation together with fermionic anti-commutation relations. Using the language of second quantization, we are able to construct an analogue of the spontaneous pair creation which can be realized in an (almost) table-top experiment.Comment: 21 pages, 10 figure

    The nonlinear Schroedinger equation for the delta-comb potential: quasi-classical chaos and bifurcations of periodic stationary solutions

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    The nonlinear Schroedinger equation is studied for a periodic sequence of delta-potentials (a delta-comb) or narrow Gaussian potentials. For the delta-comb the time-independent nonlinear Schroedinger equation can be solved analytically in terms of Jacobi elliptic functions and thus provides useful insight into the features of nonlinear stationary states of periodic potentials. Phenomena well-known from classical chaos are found, such as a bifurcation of periodic stationary states and a transition to spatial chaos. The relation of new features of nonlinear Bloch bands, such as looped and period doubled bands, are analyzed in detail. An analytic expression for the critical nonlinearity for the emergence of looped bands is derived. The results for the delta-comb are generalized to a more realistic potential consisting of a periodic sequence of narrow Gaussian peaks and the dynamical stability of periodic solutions in a Gaussian comb is discussed.Comment: Enhanced and revised version, to appear in J. Nonlin. Math. Phy

    Mammalian cell sensitivity to hyperthermia in various cell lines: a new universal and predictive description

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    Introduction The Cumulative Equivalent Minute at 43 °C (CEM43) thermal dose model has been empirically derived more than 30 years ago and still serves as a benchmark for hyperthermia protocols despite the advent of regulatory network models. However, CEM43 suffers from several limitations regarding its inability to predict the effect of complex time varying profiles (thermotolerance, step-down heating), to predict synergistic effects with drug treatments or to explain the specificity of a cell line in thermal resistance. Objective Define a new generic predictive tool for thermal injury based on regulatory network models. Identify the biological parameters that account for the thermal resistance. Materials Comparative study of cell survival upon hyperthermia collected from literature (17 sets in 11 publications that cover 14 different cell lines from 8 different tissues). Results A dynamical model describes accurately cell survival according to the amplitude and duration of exposure but also molecular chaperone expression level. In the case of square shape hyperthermia, approximated analytical expression of the cell survival is derived from the dynamical model and compared to CEM43 description. The molecular chaperone expression level defines the thermal resistance of a given cell line and can be estimated from a single experimental result through an easy-to-use graphical tool. Conclusion The tools offered here can be useful for designing treatments combining hyperthermia and chemotherapy targeting molecular chaperones, but also for designing personalized hyperthermic treatment by prior biochemical screening of molecular chaperones. These tools could advantageously replace the description of CEM43

    Dynamical thermal dose models and dose time-profile effects

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    International audienceIntroduction:Models of dose-effect relationships seek systematic and predictive descriptions of how cell survival depends on the level and duration of the stressor. The CEM43 thermal dose model has been empirically derived more than thirty years ago and still serves as a benchmark for hyperthermia protocols despitethe advent of regulatory network models.Objective:In this paper, we propose and realize a simple experimental test to assess whether mech-anistic models can prove more reliable indicators for some protocols. We define two time-asymmetric hyperthermia profiles, faster rise than decay or slower rise than decay, for which the CEM43 model predicts the same survival while a regulatory network model predicts significant differences.Materials:Experimental data (both control 37°C and hyperthermia assays) were collected from duplicate HeLa cell cultures. Cells were imaged before and 24, 48 and 72 h after the hyperthermia assay doubl -stained with fluorescein-5-isothiocyanate (FITC)-labeled annexin V and propidium iodide fordetecting cell death.Results:Survival experiments of HeLa cells show that a fast temperature rise followed by a slow decay can be twice more lethal than the opposite, consistently with the prediction of the network model.Conclusions:Using a model reduction approach, we obtained a simple nonlinear dynamic equation that identifies the limited repair capacity as the main factor underlying the dose-asymmetry effect and that could be useful for refining thermal doses for dynamic protocols