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Michael Fleischhauer

Publications and source records attributed to Michael Fleischhauer.

At least 73 records · Page 4Linked to original sources
arXiv2017

Prethermalization in the cooling dynamics of an impurity in a BEC

We discuss the cooling dynamics of heavy impurity atoms in a Bose-Einstein condensate (BEC) by emission of Cherenkov phonons from scattering with the condensate. In a weakly interacting, low-temperature condensate the superfluidity of the condensate results in a separation of time-scales of the thermalization dynamics. Pre-thermalized states are formed with distinct regions of impurity momenta determined by the mass ratio of impurity and BEC atoms. This can be employed to detect the mass renormalization of the impurity upon the formation of a polaron and paves the way to preparing non-equilibrium impurity-momentum distributions.

cond-mat.quant-gas↗
arXiv2017

Probing the topology of density matrices

The mixedness of a quantum state is usually seen as an adversary to topological quantization of observables. For example, exact quantization of the charge transported in a so-called Thouless adiabatic pump is lifted at any finite temperature in symmetry-protected topological insulators. Here, we show that certain directly observable many-body correlators preserve the integrity of topological invariants for mixed Gaussian quantum states in one dimension. Our approach relies on the expectation value of the many-body momentum-translation operator, and leads to a physical observable --- the "ensemble geometric phase" (EGP) --- which represents a bona fide geometric phase for mixed quantum states, in the thermodynamic limit. In cyclic protocols, the EGP provides a topologically quantized observable which detects encircled spectral singularities ("purity-gap" closing points) of density matrices. While we identify the many-body nature of the EGP as a key ingredient, we propose a conceptually simple, interferometric setup to directly measure the latter in experiments with mesoscopic ensembles of ultracold atoms.

cond-mat.quant-gas↗
arXiv2017

Finite-size corrections to quantized particle transport in topological charge pumps

We investigate the quantization of adiabatic charge transport in the insulating ground state of finite systems. Topological charge pumps are used in experiments as an indicator of topological order. In the thermodynamic limit the transport can be related to a topological Berry phase and is thus strictly quantized. This is no longer true for finite systems. We derive finite-size corrections to the transport both for non-interacting and interacting systems and relate them to analytic properties of the single- and many-body Berry curvature. We find that they depend on the details of experimental realizations of the pumps. While they can be non-negligible even in large systems, a proper choice of the pumping protocol can suppress these corrections.

cond-mat.quant-gas↗
arXiv2017

Many-body dynamics of holes in a driven, dissipative spin chain of Rydberg superatoms

Strong dipole-dipole interactions between atoms in high-lying Rydberg states can suppress multiple Rydberg excitations within a micron-sized trapping volume and yield sizable Rydberg level shifts at larger distances. Ensembles of atoms in optical microtraps then form Rydberg superatoms with collectively enhanced transition rates to the singly excited state. These superatoms can represent mesoscopic, strongly-interacting spins. We study a regular array of such effective spins driven by a laser field tuned to compensate the interaction-induced level shifts between neighboring superatoms. During the initial transient, a few excited superatoms seed a cascade of resonantly facilitated excitation of large clusters of superatoms. Due to spontaneous decay, the system then relaxes to the steady state having nearly universal Rydberg excitation density $ρ_{\mathrm{R}} = 2/3$. This state is characterized by highly-nontrivial equilibrium dynamics of quasi-particles -- excitation holes in the lattice of Rydberg excited superatoms. We derive an effective many-body model that accounts for hole mobility as well as continuous creation and annihilation of holes upon collisions with each other. We find that holes exhibit a nearly incompressible liquid phase with highly sub-Poissonian number statistics and finite-range density-density correlations.

physics.atom-ph↗
arXiv2017

Creation and detection of photonic molecules in Rydberg gases

We consider the propagation of photons in a gas of Rydberg atoms under conditions of electromagnetically induced transparency, where they form strongly interacting massive particles, termed Rydberg polaritons. Depending on the strength of the van der Waals-type interactions of the atoms either bunching or anti-bunching of photons can be observed when driving the atoms off-resonantly. The bunching is associated with the formation of bound states. We employ a Green's function approach and numerical wave-function simulations to analyze the conditions for the creation and the dynamics of these photonic molecules and their interplay with the scattering continuum which can also show photon bunching. Analytic solutions of the pair-propagation problem obtained from a pseudopotential approximation and verified numerically provide a detailed understanding of bound and scattering states. We find that the scattering contributions acquire asymptotically a robust relative phase which can be employed to separate bound-state and scattering contributions by a homodyne detection scheme.

quant-ph↗
arXiv2017

Bistability vs. Metastability in Driven Dissipative Rydberg Gases

We investigate the possibility of a bistable phase in an open many-body system. To this end we discuss the microscopic dynamics of a continuously off-resonantly driven Rydberg lattice gas in the regime of strong decoherence. Our experimental results reveal a prolongation of the temporal correlations with respect to the lifetime of a single Rydberg excitation and show strong evidence for the formation of finite-sized Rydberg excitation clusters in the steady state. We simulate our data using a simplified and a full many-body rate-equation model. The results are compatible with the formation of metastable states associated with a bimodal counting distribution as well as dynamic hysteresis. A scaling analysis reveals however, that the correlation times remain finite for all relevant system parameters. This suggest that the Rydberg aggregate is composed of many small clusters and all correlation lengths remain finite. This is a strong indication for the absence of a global bistable phase, previously suggested to exist in this system.

physics.atom-ph↗
arXiv2017

Dynamic defects in photonic Floquet topological insulators

Edge modes in topological insulators are known to be robust against defects. We investigate if this also holds true when the defect is not static, but varies in time. We study the influence of defects with time-dependent coupling on the robustness of the transport along the edge in a Floquet system of helically curved waveguides. Waveguide arrays are fabricated via direct laser writing in a negative tone photoresist. We find that single dynamic defects do not destroy the chiral edge current, even when the temporal modulation is strong. Quantitative numerical simulation of the intensity in the bulk and edge waveguides confirms our observation.

physics.optics↗
arXiv2016

Anomalous excitation facilitation in inhomogeneously broadened Rydberg gases

When atomic gases are laser driven to Rydberg states in an off resonant way, a single Rydberg atom may enhance the excitation rate of surrounding atoms. This leads to a facilitated excitation referred to as Rydberg anti-blockade. In the usual facilitation scenario, the detuning of the laser from resonance compensates the interaction shift. Here, we discuss a different excitation mechanism, which we call anomalous facilitation. This occurs on the "wrong side" of the resonance and originates from inhomogeneous broadening. The anomalous facilitation may be seen in experiments of attractively interacting atoms on the blue detuned side, where facilitation is not expected to appear.

physics.atom-ph↗
arXiv2016

Reservoir-induced Thouless pumping and symmetry protected topological order in open quantum chains

We introduce a classification scheme for symmetry protected topological phases applicable to stationary states of open systems based on a generalization of the many-body polarization. The polarization can be used to probe the topological properties of non-interacting and interacting closed and open systems as well and remains a meaningful quantity even in the presence of moderate particle-number fluctuations. As examples, we discuss two open-system versions of a topological Thouless pump in the steady state of one-dimensional lattices driven by Markovian reservoirs. In the analogous unitary system, the Rice-Mele model, symmetries enforce topological properties which lead to a non-trivial winding of the geometric Zak phase upon cyclic variations of model parameters. Associated with this is a winding of the many-body polarization, corresponding to a quantized transport in the bulk (Thouless pump). We here show that in the open system, where the Zak phase looses its meaning, the same symmetries enforce a winding of the generalized many- body polarization. This winding is shown to be robust against Hamiltonian perturbations as well as homogeneous dephasing and particle losses..

cond-mat.quant-gas↗
arXiv2016

A number-state filter for pulses of light

We present a detailed theoretical analysis of a Fock-state filter based on the photon-number dependent group delay in cavity induced transparency proposed in Phys. Rev. Lett. 105, 013601 (2010). We derive a general expression for the propagation velocity of different photon-number components of a light pulse interacting with an optically dense ensemble of three-level atoms coupled to a resonator mode under conditions of cavity induced transparency. These predictions are compared to numerical simulations of the propagation of few photon wave packets, and an estimation for experimental realization is made.

quant-ph↗
arXiv2016

On the adiabatic preparation of spatially-ordered Rydberg excitations of atoms in a one-dimensional optical lattice by laser frequency sweeps

We examine the adiabatic preparation of crystalline phases of Rydberg excitations in a one-dimensional lattice gas by frequency sweep of the excitation laser, as proposed by Pohl et al. [Phys. Rev. Lett. 104, 043002 (2010)] and recently realized experimentally by Schauß et al. [Science 347, 1455 (2015)]. We find that the preparation of crystals of a few Rydberg excitations in a unitary system of several tens of atoms requires exceedingly long times for the adiabatic following of the ground state of the system Hamiltonian. Using quantum stochastic (Monte-Carlo) wavefunction simulations, we show that realistic decay and dephasing processes affecting the atoms during the preparation lead to a final state of the system that has only a small overlap with the target crystalline state. Yet, the final number and highly sub-Poissonian statistics of Rydberg excitations and their spatial order are little affected by the relaxations.

quant-ph↗
arXiv2015

Interferometric Measurements of Many-body Topological Invariants using Mobile Impurities

Topological quantum phases cannot be characterized by Ginzburg-Landau type order parameters, and are instead described by non-local topological invariants. Experimental platforms capable of realizing such exotic states now include "synthetic" many-body systems such as ultracold atoms or photons. Unique tools available in these systems enable a new characterization of strongly correlated many-body states. Here we propose a general scheme for detecting topological order using interferometric measurements of elementary excitations. The key ingredient is the use of mobile impurities which bind to quasiparticles of a host many-body system. Specifically we show how fractional charges can be probed in the bulk of fractional quantum Hall systems. We demonstrate that combining Ramsey interference with Bloch oscillations can be used to measure Chern numbers of individual quasiparticles, which gives a direct probe of their fractional charges. We discuss possible extensions of our method to other topological many-body systems, such as spin liquids.

cond-mat.quant-gas↗
arXiv2015

Tunable Fröhlich Polarons of slow-light polaritons in a two-dimensional Bose-Einstein condensate

When an impurity interacts with a bath of phonons it forms a polaron. For increasing interaction strengths the mass of the polaron increases and it can become self-trapped. For impurity atoms inside an atomic Bose-Einstein condensate (BEC) the nature of this transition is subject of debate. While Feynman's variational approach predicts a sharp transition for light impurities, renormalization group studies always predict an extended intermediate-coupling region characterized by large phonon correlations. To investigate this intricate regime we suggest a versatile experimental setup that allows to tune both the mass of the impurity and its interactions with the BEC. The impurity is realized as a dark-state polariton (DSP) inside a quasi two-dimensional BEC. We show that its interactions with the Bogoliubov phonons lead to photonic polarons, described by the Bogoliubov-Fröhlich Hamiltonian, and make theoretical predictions using an extension of a recently introduced renormalization group approach to Fröhlich polarons.

cond-mat.quant-gas↗
arXiv2015

Many-body physics of Rydberg dark-state polaritons in the strongly interacting regime

Coupling light to Rydberg states of atoms under conditions of electromagnetically induced transparency (EIT) leads to the formation of strongly interacting quasi-particles, termed Rydberg polaritons. We derive a one-dimensional model describing the time evolution of these polaritons under paraxial propagation conditions, which we verify by numerical two-excitation simulations. We determine conditions allowing for a description by an effective Hamiltonian of a single-species polariton, and calculate ground-state correlations by use of the density matrix renormalization group (DMRG). Under typical stationary slow-light EIT conditions it is difficult to reach the strongly interacting regime where the interaction energy dominates the kinetic energy. We show that by employing time dependence of the control field the regime of strong interactions can be reached where the polaritons attain quasi crystalline order. We analyze the dynamics and resulting correlations for a translational invariant system in terms of a time-dependent Luttinger liquid theory and exact few-particle simulations and address the effects of nonadiabatic corrections and initial excitations.

quant-ph↗
arXiv2015

Growing quantum states with topological order

We discuss a protocol for growing states with topological order in interacting many-body systems using a sequence of flux quanta and particle insertion. We first consider a simple toy model, the superlattice Bose Hubbard model, to explain all required ingredients. Our protocol is then applied to fractional quantum Hall systems in both, continuum and lattice. We investigate in particular how the fidelity, with which a topologically ordered state can be grown, scales with increasing particle number N. For small systems exact diagonalization methods are used. To treat large systems with many particles, we introduce an effective model based on the composite fermion description of the fractional quantum Hall effect. This model also allows to take into account the effects of dispersive bands and edges in the system, which will be discussed in detail.

cond-mat.quant-gas↗
arXiv2015

Interfacing microwave qubits and optical photons via spin ensembles

A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of EIT, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for transferring coherently a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of NV centers and approaches 99% for cold atomic ensembles, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime so to efficiently combine superconducting and optical components in quantum networks.

quant-ph↗
arXiv2014

Topological growing of Laughlin states in synthetic gauge fields

We suggest a scheme for the preparation of highly correlated Laughlin (LN) states in the presence of synthetic gauge fields, realizing an analogue of the fractional quantum Hall effect in photonic or atomic systems of interacting bosons. It is based on the idea of growing such states by adding weakly interacting composite fermions (CF) along with magnetic flux quanta one-by-one. The topologically protected Thouless pump ("Laughlin's argument") is used to create two localized flux quanta and the resulting hole excitation is subsequently filled by a single boson, which, together with one of the flux quanta forms a CF. Using our protocol, filling 1/2 LN states can be grown with particle number N increasing linearly in time and strongly suppressed number fluctuations. To demonstrate the feasibility of our scheme, we consider two-dimensional (2D) lattices subject to effective magnetic fields and strong on-site interactions. We present numerical simulations of small lattice systems and discuss also the influence of losses.

cond-mat.quant-gas↗