arXiv subjects

M. Fleischhauer

Publications and source records attributed to M. Fleischhauer.

At least 37 records · Page 2Linked to original sources

arXiv2008

Confining stationary light: Dirac dynamics and Klein tunneling

We discuss the properties of 1D stationary pulses of light in atomic ensemble with electromagnetically induced transparency in the limit of tight spatial confinement. When the size of the wavepacket becomes comparable or smaller than the absorption length of the medium, it must be described by a two-component vector which obeys the one-dimensional two-component Dirac equation with an effective mass $m^*$ and effective speed of light $c^*$. Then a fundamental lower limit to the spatial width in an external potential arises from Klein tunneling and is given by the effective Compton length $λ_C = \hbar/(m^* c^*)$. Since $c^*$ and $m^*$ can be externally controlled and can be made small it is possible to observe effects of the relativistic dispersion for rather low energies or correspondingly on macroscopic length scales.

quant-ph↗

arXiv2008

Dark-State Polaritons for multi-component and stationary light fields

We present a general scheme to determine the loss-free adiabatic eigensolutions (dark-state polaritons) of the interaction of multiple probe laser beams with a coherently driven atomic ensemble under conditions of electromagnetically induced transparency. To this end we generalize the Morris-Shore transformation to linearized Heisenberg-Langevin equations describing the coupled light-matter system in the weak excitation limit. For the simple lambda-type coupling scheme the generalized Morris-Shore transformation reproduces the dark-state polariton solutions of slow light. Here we treat a closed-loop dual-V scheme wherein two counter-propagating control fields generate a quasi stationary pattern of two counter-propagating probe fields -- so-called stationary light. We show that contrary to previous predictions,there exists a single unique dark-state polariton; it obeys a simple propagation equation.

quant-ph↗

arXiv2008

Ultracold bosons in disordered superlattices: Mott-insulators induced by tunneling

We analyse the phase diagram of ultra-cold bosons in a one-dimensional superlattice potential with disorder using the time evolving block decimation algorithm for infinite sized systems (iTEBD). For degenerate potential energies within the unit cell of the superlattice loophole-shaped insulating phases with non-integer filling emerge with a particle-hole gap proportional to the boson hopping. Adding a small amount of disorder destroys this gap. For not too large disorder the loophole Mott regions detach from the axis of vanishing hopping giving rise to insulating islands. Thus the system shows a transition from a compressible Bose-glass to a Mott-insulating phase with increasing hopping amplitude. We present a straight forward effective model for the dynamics within a unit cell which provides a simple explanation for the emergence of Mott-insulating islands. In particular it gives rather accurate predictions for the inner critical point of the Bose-glass to Mott-insulator transition.

cond-mat.other↗

arXiv2008

Many-body protected entanglement generation in interacting spin systems

We discuss a method to achieve decoherence resistent entanglement generation in two level spin systems governed by gapped and multi-degenerate Hamiltonians. In such systems, while the large number of degrees of freedom in the ground state levels allows to create various quantum superpositions, the energy gap prevents decoherence. We apply the protected evolution to achieve decoherence resistent generation of many particle GHZ states and show it can significantly increase the sensitivity in frequency spectroscopy. We discuss how to engineer the desired many-body protected manifold in two specific physical systems, trapped ions and neutral atoms in optical lattices, and present simple expressions for the fidelity of GHZ generation under non-ideal conditions.

cond-mat.other↗

arXiv2007

Continuous variable versus EIT-based quantum memories

We discuss a general model of a quantum memory for a single light mode in a collective mode of atomic oscillators. The model includes interaction Hamiltonians that are of second order in the canonical position and momentum operators of the light- and atomic oscillator modes. We also consider the possibility of measurement and feedback. We identify an interaction Hamiltonian that leads to an ideal mapping by pure unitary evolution and compare several schemes which realize this mapping using a common continuous-variable description. In particular we discuss schemes based on the off-resonant Faraday effect supplemented by measurement and feedback and proper preparation of the atoms in a squeezed state and schemes based on off-resonant Raman coupling as well as electromagnetically induced transparency (EIT).

quant-ph↗

arXiv2007

Formation of solitons in atomic Bose-Einstein condensates by dark-state adiabatic passage

We propose a new method of creating solitons in elongated Bose-Einstein Condensates (BECs) by sweeping three laser beams through the BEC. If one of the beams is in the first order (TEM10) Hermite-Gaussian mode, its amplitude has a transversal phase slip which can be transferred to the atoms creating a soliton. Using this method it is possible to circumvent the restriction set by the diffraction limit inherent to conventional methods such as phase imprinting. The method allows one to create multicomponent (vector) solitons of the dark-bright form as well as the dark-dark combination. In addition it is possible to create in a controllable way two or more dark solitons with very small velocity and close to each other for studying their collisional properties.

cond-mat.soft↗

arXiv2007

The one-dimensional Bose-Fermi-Hubbard model in the heavy-fermion limit

We study the phase diagram of the zero-temperature, one-dimensional Bose-Fermi-Hubbard model for fixed fermion density in the limit of small fermionic hopping. This model can be regarded as an instance of a disordered Bose-Hubbard model with dichotomic values of the stochastic variables. Phase boundaries between compressible, incompressible (Mott-insulating) and partially compressible phases are derived analytically within a generalized strong-coupling expansion and numerically using density matrix renormalization group (DMRG) methods. We show that first-order correlations in the partially compressible phases decay exponentially, indicating a glass-type behaviour. Fluctuations within the respective incompressible phases are determined using perturbation theory and are compared to DMRG results.

cond-mat.other↗

arXiv2007

Entanglement of Collectively Interacting Harmonic Chains

We study the ground-state entanglement of one-dimensional harmonic chains that are coupled to each other by a collective interaction as realized e.g. in an anisotropic ion crystal. Due to the collective type of coupling, where each chain interacts with every other one in the same way,the total system shows critical behavior in the direction orthogonal to the chains while the isolated harmonic chains can be gapped and non-critical. We derive lower and most importantly upper bounds for the entanglement,quantified by the von Neumann entropy, between a compact block of oscillators and its environment. For sufficiently large size of the subsystems the bounds coincide and show that the area law for entanglement is violated by a logarithmic correction.

quant-ph↗

arXiv2007

Noise-resistant entanglement of strongly interacting spin systems

We propose and analyze a scheme that makes use of interactions between spins to protect certain correlated many-body states from decoherence. The method exploits the finite energy gap of properly designed Hamiltonians to generate a manifold insensitive to local noise fluctuations. We apply the scheme to achieve decoherence-resistant generation of many particle GHZ states and show that it can improve the sensitivity in precision spectroscopy with trapped ions. Finally we also show that cold atoms in optical lattices interacting via short range interactions can be utilized to engineer the required long range interactions for a robust generation of entangled states.

cond-mat.other↗

arXiv2006

Quantum sensitivity limit of a Sagnac hybrid interferometer based on slow-light propagation in ultra-cold gases

The light--matter-wave Sagnac interferometer based on ultra-slow light proposed recently in (Phys. Rev. Lett. 92, 253201 (2004)) is analyzed in detail. In particular the effect of confining potentials is examined and it is shown that the ultra-slow light attains a rotational phase shift equivalent to that of a matter wave, if and only if the coherence transfer from light to atoms associated with slow light is associated with a momentum transfer and if an ultra-cold gas in a ring trap is used. The quantum sensitivity limit of the Sagnac interferometer is determined and the minimum detectable rotation rate calculated. It is shown that the slow-light interferometer allows for a significantly higher signal-to-noise ratio as possible in current matter-wave gyroscopes.

quant-ph↗

arXiv2006

Many-body effects on adiabatic passage through Feshbach resonances

We theoretically study the dynamics of an adiabatic sweep through a Feshbach resonance, thereby converting a degenerate quantum gas of fermionic atoms into a degenerate quantum gas of bosonic dimers. Our analysis relies on a zero temperature mean-field theory which accurately accounts for initial molecular quantum fluctuations, triggering the association process. The structure of the resulting semiclassical phase space is investigated, highlighting the dynamical instability of the system towards association, for sufficiently small detuning from resonance. It is shown that this instability significantly modifies the finite-rate efficiency of the sweep, transforming the single-pair exponential Landau-Zener behavior of the remnant fraction of atoms Gamma on sweep rate alpha, into a power-law dependence as the number of atoms increases. The obtained nonadiabaticity is determined from the interplay of characteristic time scales for the motion of adiabatic eigenstates and for fast periodic motion around them. Critical slowing-down of these precessions near the instability leads to the power-law dependence. A linear power law $Gamma\propto alpha$ is obtained when the initial molecular fraction is smaller than the 1/N quantum fluctuations, and a cubic-root power law $Gamma\propto alpha^{1/3}$ is attained when it is larger. Our mean-field analysis is confirmed by exact calculations, using Fock-space expansions. Finally, we fit experimental low temperature Feshbach sweep data with a power-law dependence. While the agreement with the experimental data is well within experimental error bars, similar accuracy can be obtained with an exponential fit, making additional data highly desirable.

cond-mat.other↗

arXiv2006

Coherent Control of Stationary Light Pulses

We present a detailed analysis of the recently demonstrated technique to generate quasi-stationary pulses of light [M. Bajcsy {\it et al.}, Nature (London) \textbf{426}, 638 (2003)] based on electromagnetically induced transparency. We show that the use of counter-propagating control fields to retrieve a light pulse, previously stored in a collective atomic Raman excitation, leads to quasi-stationary light field that undergoes a slow diffusive spread. The underlying physics of this process is identified as pulse matching of probe and control fields. We then show that spatially modulated control-field amplitudes allow us to coherently manipulate and compress the spatial shape of the stationary light pulse. These techniques can provide valuable tools for quantum nonlinear optics and quantum information processing.

quant-ph↗

arXiv2006

Filled Landau levels in neutral quantum gases

We consider the signatures of the Integer Quantum Hall Effect in a degenerate gas of electrically neutral atomic fermions. An effective magnetic field is achieved by applying two incident light beams with a high orbital angular momentum. We show how states corresponding to completely filled Landau levels are obtained and discuss various possibilities to measure the incompressible nature of the trapped two-dimensional gas

cond-mat.mes-hall↗

arXiv2005

Light induced effective magnetic fields for ultra-cold atoms in planar geometries

We propose a scheme to create an effective magnetic field for ultra-cold atoms in a planar geometry. The set-up allows the experimental study of classical and quantum Hall effects in close analogy to solid-state systems including the possibility of finite currents. The present scheme is an extention of the proposal in Phys. Rev. Lett. 93, 033602 (2004) where the effective magnetic field is now induced for three-level Lambda-type atoms by two counterpropagating laser beams with shifted spatial profiles. Under conditions of electromagentically induced transparency the atom-light interaction has a space dependent dark state, and the adiabatic center of mass motion of atoms in this state experiences effective vector and scalar potentials. The associated magnetic field is oriented perpendicular to the propagation direction of the laser beams. The field strength achievable is one flux quantum over an area given by the transverse beam separation and the laser wavelength. For a sufficiently dilute gas the field is strong enough to reach the lowest Landau level regime.

quant-ph↗

arXiv2005

Entanglement and criticality in translational invariant harmonic lattice systems with finite-range interactions

We discuss the relation between entanglement and criticality in translationally invariant harmonic lattice systems with non-randon, finite-range interactions. We show that the criticality of the system as well as validity or break-down of the entanglement area law are solely determined by the analytic properties of the spectral function of the oscillator system, which can easily be computed. In particular for finite-range couplings we find a one-to-one correspondence between an area-law scaling of the bi-partite entanglement and a finite correlation length. This relation is strict in the one-dimensional case and there is strog evidence for the multi-dimensional case. We also discuss generalizations to couplings with infinite range. Finally, to illustrate our results, a specific 1D example with nearest and next-nearest neighbor coupling is analyzed.

quant-ph↗

arXiv2005

Correlation in superradiance: A closed-form approach to cooperative effects

We have developed a novel method to describe superradiance and related cooperative and collective effects in a closed form. Using the method we derive a two-atom master equation in which any complexity of atomic levels, semiclassical coupling fields and quantum fluctuations in the fields can be included, at least in principle. As an example, we consider the dynamics of an initially inverted two-level system and show how even such in a simple system phenomena such as the initial radiation burst or broadening due to dipole-dipole interactions occur, but it is also possible to estimate the population of the subradiant state during the radiative decay. Finally, we find that correlation only, not entanglement is responsible for superradiance.

quant-ph↗

arXiv2005

Stochastic Simulation of a finite-temperature one-dimensional Bose-Gas: from Bogoliubov to Tonks-Girardeau regime

We present an ab initio stochastic method for calculating thermal properties of a trapped, 1D Bose-gas covering the whole range from weak to strong interactions. Discretization of the problem results in a Bose-Hubbard-like Hamiltonian, whose imaginary time evolution is made computationally accessible by stochastic factorization of the kinetic energy. To achieve convergence for low enough temperatures such that quantum fluctuations are essential, the stochastic factorization is generalized to blocks, and ideas from density-matrix renormalization are employed. We compare our numerical results for density and first-order correlations with analytic predictions.

cond-mat.stat-mech↗

arXiv2005

Nonlinear adiabatic passage from fermion atoms to boson molecules

We study the dynamics of an adiabatic sweep through a Feshbach resonance in a quantum gas of fermionic atoms. Analysis of the dynamical equations, supported by mean-field and many-body numerical results, shows that the dependence of the remaining atomic fraction $Γ$ on the sweep rate $α$ varies from exponential Landau-Zener behavior for a single pair of particles to a power-law dependence for large particle number $N$. The power-law is linear, $Γ\propto α$, when the initial molecular fraction is smaller than the 1/N quantum fluctuations, and $Γ\propto α^{1/3}$ when it is larger. Experimental data agree better with a linear dependence than with an exponential Landau-Zener fit, indicating that many-body effects are significant in the atom-molecule conversion process.

cond-mat.other↗