Publications (237)870.26 Total impact

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ABSTRACT: We discuss a protocol for growing states with topological order in interacting manybody 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. 
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ABSTRACT: 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 magneticdipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of EIT, with the controlled switching of the coupling between the magneticdipole 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 stateoftheart 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. 
Nature Physics 01/2015; DOI:10.1038/nphys3214 · 20.60 Impact Factor

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ABSTRACT: Controlling strongly interacting manybody systems enables the creation of tailored quantum matter, with properties surpassing single particle physics. Atomic ensembles which are optically driven to a Rydberg state provide many examples for this, such as atomatom entanglement [1,2], manybody Rabi oscillations [3], and strong photonphoton interaction [4]. In its most basic form, Rydberg quantum matter consists of an isolated ensemble of strongly interacting atoms spatially confined to the blockade volume  a socalled superatom. Here, we demonstrate the controlled creation and characterization of an isolated mesoscopic superatom by means of accurate density engineering and excitation to Rydberg pstates. Looking at continuous laserinduced ionization we observe a transition from strongly antibunched ion emission under blockade conditions to extremely bunched ion emission under offresonant excitation. Our experimental setup enables in vivo measurements of the superatom, yielding insight into the excitation statistics and dynamics. We anticipate straightforward applications in quantum optics and quantum information as well as future experiments on manybody physics with superatoms. 
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ABSTRACT: 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 onebyone. 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 twodimensional (2D) lattices subject to effective magnetic fields and strong onsite interactions. We present numerical simulations of small lattice systems and discuss also the influence of losses.Physical Review Letters 06/2014; 113(15). DOI:10.1103/PhysRevLett.113.155301 · 7.73 Impact Factor 
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ABSTRACT: We study the dynamics of dissipative spin lattices with powerlaw interactions, realized via fewlevel atoms driven by coherent lasercoupling and decoherence processes. Using MonteCarlo simulations, we determine the phase diagram in the steady state and analyze the dynamics of its generation. As opposed to meanfield predictions and nearestneighbour models there is no phase transition to longrange 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.Physical Review A 04/2014; 90(2). DOI:10.1103/PhysRevA.90.021603 · 2.99 Impact Factor 
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ABSTRACT: We propose a scheme to couple short single photon pulses to superconducting qubits. An optical photon is first absorbed into an inhomogeneously broadened rareearth doped crystal using controlled reversible inhomogeneous broadening. The optical excitation is then mapped into a spin state using a series of $\pi$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 $\pi$pulses. We predict total transfer efficiencies on the order of 90%.Physical Review Letters 02/2014; 113(6). DOI:10.1103/PhysRevLett.113.063603 · 7.73 Impact Factor 
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ABSTRACT: We analyze the eigenstates of a twodimensional 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 lowenergy regime, where lattice effects are small, all states are transporting topologically nontrivial states. For large energies above a certain critical value, the periodic lattice causes localization of all states through a mechanism similar to WannierStark localization. In the intermediate energy regime transporting, topologically non trivial states coexist with topologically trivial countertransporting chaotic states. The character of the eigenstates, in particular their transport properties are studied numerically and are explained using a semiclassical analysis.Physical Review A 12/2013; 89(3). DOI:10.1103/PhysRevA.89.033607 · 2.99 Impact Factor 
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ABSTRACT: The coupling of weak light fields to Rydberg states of atoms under conditions of electromagnetically induced transparency leads to the formation of Rydberg polaritons which are quasiparticles with tunable effective mass and nonlocal interactions. Confined to one spatial dimension their low energy physics is that of a movingframe Luttinger liquid which, due to the nonlocal character of the repulsive interaction, can form a Wigner crystal of individual photons. We calculate the Luttinger K parameter using densitymatrix renormalization group simulations and find that under typical slowlight 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 us to generate true crystalline order over a finite length. The dynamics of this process and asymptotic correlations are analyzed in terms of a timedependent Luttinger theory.Physical Review Letters 09/2013; 111(11):113001. DOI:10.1103/PhysRevLett.111.113001 · 7.73 Impact Factor 
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ABSTRACT: We analyze interacting ultracold bosonic atoms in a onedimensional superlattice potential with alternating tunneling rates t_{1} and t_{2} and inversion symmetry, which is the bosonic analogue of the SuSchriefferHeeger model. A Z_{2} 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 nontrivial, which results in manybody edge states at open boundaries. In contrast to the SuSchriefferHeeger model the bosonic counterpart lacks chiral symmetry and the edge states are no longer midgap. This leads to a generalization of the bulkedge 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. The shape and energy of the edge states as well as the conditions for their occupation are determined analytically in the strong coupling limit and in general by densitymatrix renormalization group simulations.Physical Review Letters 06/2013; 110(26):260405. DOI:10.1103/PhysRevLett.110.260405 · 7.73 Impact Factor 
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ABSTRACT: We study the effects of the fourwave 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, alloptical 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.Physical Review A 04/2013; 88(1). DOI:10.1103/PhysRevA.88.013823 · 2.99 Impact Factor 
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ABSTRACT: We study the emergence of manybody correlations in the stationary state of continuouslydriven, stronglyinteracting dissipative system. Specifically, we examine resonant optical excitations of Rydberg states of atoms interacting via longrange dipoledipole and van der Waals potentials employing exact numerical solutions of the density matrix equations and MonteCarlo 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 uniformlydriven, extended onedimensional system, the saturation of superatoms leads to quasicrystallization 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 "hardrod" interatomic potential.Physical Review A 12/2012; 87(5). DOI:10.1103/PhysRevA.87.053414 · 2.99 Impact Factor 
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ABSTRACT: We investigate the longrange coupling of individual atoms coupled to plasmon modes of metallic nanostructures. Placing a pair of emitters along a thin metallic wire, we observe a strong, wire mediated longrange interaction between the emitters. As a result, super and subradiance can occur over distances large compared to the resonant wavelength. The states with enhanced or suppressed decay rate are the symmetric or antisymmetric singleexcitation states. Coupling more atoms to a wire network with a nontrivial coupling topology leads to interesting entangled subradiant states of the system. A similar longrange superradiance effect can be observed when two emitters are coupled by a metamaterial slab (also known as a perfect lens) having a refractive index n=1. Besides the modification of decay rates, dipoledipole shifts enter due to the plasmonmediated interaction. Based on the superradiance effect, we propose setups for building a twoqubit quantum phase gate for quantum emitters coupled by a nanowire and a perfect lens, respectively, where the qubits are strongly interacting and individually addressable at the same time.Proceedings of SPIE  The International Society for Optical Engineering 10/2012; DOI:10.1117/12.929618 · 0.20 Impact Factor 
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ABSTRACT: We study resonant optical excitations of atoms in a onedimensional lattice to the Rydberg states interacting via the van der Waals potential which suppresses simultaneous excitation of neighboring atoms. Considering two and threelevel excitation schemes, we analyze the dynamics and stationary state of the continuouslydriven, dissipative manybody system employing timedependent densitymatrix renormalization group (tDMRG) simulations. We show that twolevel atoms can exhibit only nearest neighbor correlations, while threelevel atoms under darkstate resonant driving can develop finiterange crystalline order of Rydberg excitations. We present an approximate rate equation model whose analytic solution yields qualitative understanding of the numerical results.Physical Review A 08/2012; 87(2). DOI:10.1103/PhysRevA.87.023401 · 2.99 Impact Factor 
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ABSTRACT: Building on the recent experimental achievements obtained with scanning electron microscopy on ultracold atoms, we study onedimensional Bose gases in the crossover between the weakly (quasicondensate) and the strongly interacting (TonksGirardeau) regime. We measure the temporal twoparticle correlation function and compare it with calculations performed using the timeevolving block decimation algorithm. More pronounced antibunching is observed when entering the more strongly interacting regime. Even though this mimics the onset of a fermionic behavior, we highlight that the exact and simple duality between onedimensional bosons and fermions does not hold when such a dynamical response is probed. The onset of fermionization is also reflected in the density distribution, which we measure in situ to extract the relevant parameters and to identify the different regimes. Our results show agreement between experiment and theory and give insight into the dynamics of strongly correlated manybody systems.Physical Review A 08/2012; 86(2):21601. DOI:10.1103/PhysRevA.86.021601 · 2.99 Impact Factor 
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ABSTRACT: We study ultracold Rydbergdressed Bose gases subject to artificial gauge fields in the fractional quantum Hall (FQH) regime. The characteristics of the Rydberg interaction gives rise to interesting manybody ground states different from standard FQH physics in the lowest Landau level (LLL). The nonlocal 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 $\nu<1/12$, a correlated crystal of composite particles emerges already for $\nu \leq 1/6$ 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 $\nu\lesssim 1/4$ 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.Physical Review A 07/2012; 87(4). DOI:10.1103/PhysRevA.87.043628 · 2.99 Impact Factor 
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ABSTRACT: We discuss reservoirinduced phase transitions of lattice fermions in the nonequilibrium steady state of an open system with local reservoirs. These systems may become critical in the sense of a diverging correlation length on 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 reservoirinduced quantum phase transition between distinct phases with a nonvanishing 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 reservoirinduced 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).Physical Review A 07/2012; 86(1). DOI:10.1103/PhysRevA.86.013606 · 2.99 Impact Factor 
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ABSTRACT: We discuss reservoir driven phase transitions to critical states in onedimensional bosonic lattice systems subject to local dissipation. By coupling to local reservoirs fermionic and bosonic lattice systems can be driven to a steady state which shows criticality in the sense of a diverging correlation length. For free lattice bosons this criticality is generically associated with a dynamical instability of the system. To avoid this instability we introduce a nonlinearity by saturating the dissipative gain. We consider coupling of the lattice sites to common local reservoirs of different range and derive correlations as well as critical exponents of the induced quasiphase transition in a meanfield approximation. 
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ABSTRACT: We develop the theory of light propagation under the conditions of electromagnetically induced transparency in systems involving strongly interacting Rydberg states. Taking into account the quantum nature of light, we compute the propagation of an arbitrary input pulse in the limit of strong RydbergRydberg interactions. We also solve the case of a fewphoton pulse for arbitrary RydbergRydberg interaction strengths [PRL 107, 133602 (2011)]. We show that this system can be used for the generation of nonclassical states of light including single photons and trains of single photons with an avoided volume between them, for implementing photonphoton gates, as well as for studying manybody phenomena with strongly correlated photons. 
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ABSTRACT: We show that excitations in a gas of atoms driven to Rydberg states by nearresonant laser radiation in a twophoton coupling scheme experience a photon mediated transport. Thus even if the centerofmass motion of the atoms can be neglected, this results in a kinetic Hamiltonian for the Rydberg excitations. The corresponding mass is identical to that of the darkstate polaritons of the optical coupling scheme. The kinetic energy competes with the Rydberg dipoledipole interactions and can prevent the formation of quasicrystal structures. Using DMRG simulations we calculate the Luttinger parameter for a onedimensional gas of resonantly driven Rydberg atoms taking into account the photon mediated transport and derive conditions under which quasicrystallization can be observed.
Publication Stats
9k  Citations  
870.26  Total Impact Points  
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Institutions

2001–2015

Technische Universität Kaiserslautern
 Department of Physics
Kaiserlautern, RheinlandPfalz, Germany


1994–2007

LudwigMaximilianUniversity of Munich
 Department of Physics
München, Bavaria, Germany 
Houston Advanced Research Center
The Woodlands, Texas, United States


2006

Harvard University
 Department of Physics
Cambridge, MA, United States


1992–2001

Texas A&M University
 Department of Electrical and Computer Engineering
College Station, Texas, United States 
University of New Mexico
 Department of Physics & Astronomy
Albuquerque, New Mexico, United States


2000

University Hospital München
München, Bavaria, Germany


1999–2000

HarvardSmithsonian Center for Astrophysics
 Institute for Theoretical Atomic, Molecular and Optical Physics
Cambridge, Massachusetts, United States


1992–1994

Max Planck Institute of Quantum Optics
Arching, Bavaria, Germany
