arXiv subjects

Michael Fleischhauer

Publications and source records attributed to Michael Fleischhauer.

At least 91 records · Page 5Linked to original sources

arXiv2014

Interfacing Superconducting Qubits and Telecom Photons via a Rare-Earth Doped Crystal

We propose a scheme to couple short single photon pulses to superconducting qubits. An optical photon is first absorbed into an inhomogeneously broadened rare-earth doped crystal using controlled reversible inhomogeneous broadening. The optical excitation is then mapped into a spin state using a series of $π$-pulses and subsequently transferred to a superconducting qubit via a microwave cavity. To overcome the intrinsic and engineered inhomogeneous broadening of the optical and spin transitions in rare earth doped crystals, we make use of a special transfer protocol using staggered $π$-pulses. We predict total transfer efficiencies on the order of 90%.

quant-ph↗

arXiv2014

Antiferromagnetic long-range order in dissipative Rydberg lattices

We study the dynamics of dissipative spin lattices with power-law interactions, realized via few-level atoms driven by coherent laser-coupling and decoherence processes. Using Monte-Carlo simulations, we determine the phase diagram in the steady state and analyze the dynamics of its generation. As opposed to mean-field predictions and nearest-neighbour models there is no phase transition to long-range ordered phases for realistic interactions and resonant driving. However, for finite laser detunings, we demonstrate the emergence of crystalline order with a vanishing dissipative gap. Although the found steady states differ considerably from those of an equilibrium Ising magnet, the critical exponent of the revealed dissipative phase transition falls into the 2D Ising universality class. Two complementary schemes for an experimental implementation with cold Rydberg atoms are discussed.

cond-mat.quant-gas↗

arXiv2014

Quantum particle in a parabolic lattice in the presence of a gauge field

We analyze the eigenstates of a two-dimensional lattice with additional harmonic confinement in the presence of an artificial magnetic field. While the softness of the confinement makes a distinction between bulk and edge states difficult, the interplay of harmonic potential and lattice leads to a different classification of states in three energy regions: In the low-energy regime, where lattice effects are small, all states are transporting topologically non-trivial states. For large energies above a certain critical value, the periodic lattice causes localization of all states through a mechanism similar to Wannier-Stark localization. In the intermediate energy regime transporting, topologically non- trivial states coexist with topologically trivial counter-transporting chaotic states. The character of the eigenstates, in particular their transport properties are studied numerically and are explained using a semiclassical analysis.

cond-mat.quant-gas↗

arXiv2013

Wigner Crystallization of Single Photons in Cold Rydberg Ensemble

The coupling of weak light fields to Rydberg states of atoms under conditions of electromagnetically induced transparency (EIT) leads to the formation of Rydberg polaritons which are quasi-particles with tunable effective mass and long-range interactions. Confined to one spatial dimension their low energy physics is that of a moving-frame Luttinger liquid which due to the long-range character of the repulsive interaction can form a Wigner crystal. We calculate the Luttinger $K$ parameter using density-matrix renormalization group (DMRG) simulations and find that under typical slow-light conditions kinetic energy contributions are too strong for crystal formation. However, adiabatically increasing the polariton mass by turning a light pulse into stationary spin excitations allows to generate true crystalline order over a finite length. The dynamics of this process and asymptotic correlations are analyzed in terms of a time-dependent Luttinger theory.

quant-ph↗

arXiv2013

Fidelity of photon propagation in electromagnetically induced transparency in the presence of four-wave mixing

We study the effects of the four-wave mixing (4WM) in a quantum memory scheme based on electromagnetically induced transparency (EIT). We treat the problem of field propagation on the quantum mechanical level, which allows us to calculate the fidelity for propagation for a quantum light pulse such as a single photon. While 4WM can be beneficial for classical, all-optical information storage, the quantum noise associated with the signal amplification and idler generation is in general detrimental for a quantum memory. We identify a range of parameters where 4WM makes a single photon quantum memory impossible.

quant-ph↗

arXiv2013

Topological edge states in the one-dimensional super-lattice Bose-Hubbard model

We analyze interacting ultra-cold bosonic atoms in a one-dimensional (1D) super-lattice potential with alternating tunneling rates t_1 and t_2 and inversion symmetry, which is the bosonic analogue of the Su-Schrieffer-Heeger (SSH) model. A Z2 topological order parameter is introduced which is quantized for the Mott insulating (MI) phases. Depending on the ratio t_1/t_2 the n=1/2 MI phase is topologically non-trivial, which results in many-body edge states at open boundaries. In contrast to the SSH model the bosonic counterpart lacks chiral symmetry and the edge states are no longer mid-gap. This leads to a generalization of the bulk-edge correspondence, which we discuss in detail. The edge states can be observed in cold atom experiments by creating a step in the effective confining potential, e.g. by a second heavy atom species, which leads to an interface between two MI regions with filling n=1 and n=1/2. Shape and energy of the edge states as well as conditions for their occupation are determined analytically in the strong coupling limit and in general by density-matrix renormalization group (DMRG) simulations.

cond-mat.quant-gas↗

arXiv2013

Fractional quantum Hall physics with ultracold Rydberg gases in artificial gauge fields

We study ultracold Rydberg-dressed Bose gases subject to artificial gauge fields in the fractional quantum Hall (FQH) regime. The characteristics of the Rydberg interaction gives rise to interesting many-body ground states different from standard FQH physics in the lowest Landau level (LLL). The non-local but rapidly decreasing interaction potential favors crystalline ground states for very dilute systems. While a simple Wigner crystal becomes energetically favorable compared to the Laughlin liquid for filling fractions $ν<1/12$, a correlated crystal of composite particles emerges already for $ν\leq 1/6$ with a large energy gap to the simple Wigner crystal. The presence of a new length scale, the Rydberg blockade radius $a_B$, gives rise to a bubble crystal phase for $ν\lesssim 1/4$ when the average particle distance becomes less than $a_B$, which describes the region of saturated, almost constant interaction potential. For larger fillings indications for strongly correlated cluster liquids are found.

cond-mat.quant-gas↗

arXiv2012

Spatial correlations of Rydberg excitations in optically driven atomic ensembles

We study the emergence of many-body correlations in the stationary state of continuously-driven, strongly-interacting dissipative system. Specifically, we examine resonant optical excitations of Rydberg states of atoms interacting via long-range dipole-dipole and van der Waals potentials employing exact numerical solutions of the density matrix equations and Monte-Carlo simulations. Collection of atoms within a blockade distance form a "superatom" that can accommodate at most one Rydberg excitation. The superatom excitation probability saturates to 1/2 for coherently driven atoms, but is significantly higher for incoherent driving, approaching unity as the number of atoms increases. In the steady state of uniformly-driven, extended one-dimensional system, the saturation of superatoms leads to quasi-crystallization of Rydberg excitations whose correlations exhibit damped spatial oscillations. The behavior of the system under the van der Waals interaction potential can be approximated by an analytically soluble model based on a "hard-rod" interatomic potential.

physics.atom-ph↗

arXiv2012

Steady-state crystallization of Rydberg excitations in an optically driven lattice gas

We study resonant optical excitations of atoms in a one-dimensional lattice to the Rydberg states interacting via the van der Waals potential which suppresses simultaneous excitation of neighboring atoms. Considering two- and three-level excitation schemes, we analyze the dynamics and stationary state of the continuously-driven, dissipative many-body system employing time-dependent density-matrix renormalization group (t-DMRG) simulations. We show that two-level atoms can exhibit only nearest neighbor correlations, while three-level atoms under dark-state resonant driving can develop finite-range crystalline order of Rydberg excitations. We present an approximate rate equation model whose analytic solution yields qualitative understanding of the numerical results.

quant-ph↗

arXiv2012

Transport induced melting of crystals of Rydberg dressed atoms in a one dimensional lattice

We discuss the many-body physics of an ensemble of Rydberg dressed atoms with van der Waals dipole-dipole interactions in a one-dimensional lattice. Using a strong coupling expansion and numerical density-matrix renormalisation group simulations, we calculate the many-body phase diagram. A devil's staircase structure emerges with Mott-insulating phases at any rational filling fraction. Closed analytic expressions are given for the phase boundaries in second order of the tunnelling amplitude and shown to agree very well with the numerical results. The transition point where the incompressible phases melt due to the kinetic energy term depends strongly on the denominator of the filling fraction and varies over many orders of magnitude between different phases.

quant-ph↗

arXiv2012

Critical exponents of steady-state phase transitions in fermionic lattice models

We discuss reservoir induced phase transitions of lattice fermions in the non-equilibrium steady state (NESS) of an open system with local reservoirs. These systems may become critical in the sense of a diverging correlation length upon changing the reservoir coupling. We here show that the transition to a critical state is associated with a vanishing gap in the damping spectrum. It is shown that although in linear systems there can be a transition to a critical state there is no reservoir-induced quantum phase transition between distinct phases with non-vanishing damping gap. We derive the static and dynamical critical exponents corresponding to the transition to a critical state and show that their possible values, defining universality classes of reservoir-induced phase transitions are determined by the coupling range of the independent local reservoirs. If a reservoir couples to N neighboring lattice sites, the critical exponent can assume all fractions from 1 to 1/(N - 1).

quant-ph↗

arXiv2011

Dynamics and evaporation of defects in Mott-insulating clusters of boson pairs

Repulsively bound pairs of particles in a lattice governed by the Bose-Hubbard model can form stable incompressible clusters of dimers corresponding to finite-size n=2 Mott insulators. Here we study the dynamics of hole defects in such clusters corresponding to unpaired particles which can resonantly tunnel out of the cluster into the lattice vacuum. Due to bosonic statistics, the unpaired particles have different effective mass inside and outside the cluster, and "evaporation" of hole defects from the cluster boundaries is possible only when their quasi-momenta are within a certain transmission range. We show that quasi-thermalization of hole defects occurs in the presence of catalyzing particle defects which thereby purify the Mott insulating clusters. We study the dynamics of one-dimensional system using analytical techniques and numerically exact t-DMRG simulations. We derive an effective strong-interaction model that enables simulations of the system dynamics for much longer times. We also discuss a more general case of two bosonic species which reduces to the fermionic Hubbard model in the strong interaction limit.

quant-ph↗

arXiv2011

Wigner crystal vs. Friedel oscillations in the 1D Hubbard model

We analyze the fermion density of the one-dimensional Hubbard model using bosonization and numerical DMRG calculations. For finite systems we find a relatively sharp crossover even for moderate short range interactions into a region with $4k_F$ density waves as a function of density. The results show that the unstable fixed point of a spin-incoherent state can dominate the physical behavior in a large region of parameter space in finite systems. The crossover may be observable in ultra cold fermionic gases in optical lattices and in finite quantum wires.

cond-mat.str-el↗

arXiv2011

Electromagnetically induced transparency with Rydberg atoms

We present a theory of electromagnetically induced transparency in a cold ensemble of strongly interacting Rydberg atoms. Long-range interactions between the atoms constrain the medium to behave as a collection of superatoms, each comprising a blockade volume that can accommodate at most one Rydberg excitation. The propagation of a probe field is affected by its two-photon correlations within the blockade distance, which are strongly damped due to low saturation threshold of the superatoms. Our model is computationally very efficient and is in quantitative agreement with the results of recent experiment of Pritchard et al. [Phys. Rev. Lett. 105, 193603 (2010)].

quant-ph↗

arXiv2011

Dipole-dipole shift of quantum emitters coupled to surface plasmons of a nano-wire

Placing quantum emitters close to a metallic nano-wire, an effective interaction can be achieved over distances large compared to the resonance wavelength due to the strong coupling between emitters and the surface plasmon modes of the wire. This leads to modified collective decay rates, as well as to Lamb and dipole-dipole shifts. In this paper we present a general method for calculating these level shifts which we subsequently apply to the case of a pair of atoms coupled to the guided modes of a nano-wire.

quant-ph↗

arXiv2011

Multi-band and nonlinear hopping corrections to the 3D Bose-Fermi Hubbard model

Recent experiments revealed the importance of higher-band effects for the Mott insulator (MI) -- superfluid transition (SF) of ultracold bosonic atoms or mixtures of bosons and fermions in deep optical lattices [Best \emph{et al.}, PRL \textbf{102}, 030408 (2009); Will \emph{et al.}, Nature \textbf{465}, 197 (2010)]. In the present work, we derive an effective lowest-band Hamiltonian in 3D that generalizes the standard Bose-Fermi Hubbard model taking these effects as well as nonlinear corrections of the tunneling amplitudes mediated by interspecies interactions into account. It is shown that a correct description of the lattice states in terms of the bare-lattice Wannier functions rather than approximations such as harmonic oscillator states is essential. In contrast to self-consistent approaches based on effective Wannier functions our approach provides a quantitative explanation of the observed reduction of the superfluid phase for repulsive interspecies interactions.

cond-mat.quant-gas↗

arXiv2011

Photon-Photon Interactions via Rydberg Blockade

We develop the theory of light propagation under the conditions of electromagnetically induced transparency (EIT) in systems involving strongly interacting Rydberg states. Taking into account the quantum nature and the spatial propagation of light, we analyze interactions involving few-photon pulses. We demonstrate that this system can be used for the generation of nonclassical states of light including trains of single photons with an avoided volume between them, for implementing photon-photon quantum gates, as well as for studying many-body phenomena with strongly correlated photons.

quant-ph↗

arXiv2010

Dynamical simulation of integrable and non-integrable models in the Heisenberg picture

The numerical simulation of quantum many-body dynamics is typically limited by the linear growth of entanglement with time. Recently numerical studies have shown, however, that for 1D Bethe-integrable models the simulation of local operators in the Heisenberg picture can be efficient as the corresponding operator-space entanglement grows only logarithmically. Using the spin-1/2 XX chain as generic example of an integrabel model that can be mapped to free particles, we here provide a simple explanation for this. We show furthermore that the same reduction of complexity applies to operators that have a high-temperature auto correlation function which decays slower than exponential, i.e., with a power law. This is amongst others the case for models where the Blombergen-De Gennes conjecture of high-temperature diffusive dynamics holds. Thus efficient simulability may already be implied by a single conservation law (like that of total magnetization), as we will illustrate numerically for the spin-1 XXZ model.

quant-ph↗