Publications

Identifying a bath-induced Bose liquid in interacting spin-boson models
Cai, Zi, Schollwöck, Ulrich and Pollet, Lode
Phys. Rev. Lett. , Volume 113,, page: 260403
2014

Abstract: We study the ground state phase diagram of a one-dimensional hard-core bosonic model with nearest-neighbor interactions (XXZ model) where every site is coupled Ohmically to an independent but identical reservoir, hereby generalizing spin-boson models to interacting spin-boson systems. We show that a bath-induced Bose metal phase can occur in the ground state phase diagram away from half filling. This phase is compressible, gapless, and conducting but not superfluid. At haf-filling, only a Luttinger liquid and a charge density wave are found. The phase transition between them is of Kosterlitz-Thouless type where the Luttinger parameter takes a non-universal value.The applied quantum Monte Carlo method can be used for all open bosonic and unfrustrated spin systems, regardless of their dimension, filling factor and spectrum of the dissipation as long as the quantum system couples to the bath via the density operators.

Highly coherent electron beam from a laser-triggered tungsten needle tip
Ehberger, Dominik, Hammer, Jakob, Eisele, Max, Kr, Michael, Noe, Jonathan, H, Alexander and Hommelhoff, Peter
2014

Abstract: We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80\pm 0.05)\,nm in laser-triggered and (0.55\pm 0.02)\,nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.

Frustrated Quantum Magnetism with Laser-Dressed Rydberg Atoms
W. Glaetzle, Alexander, Dalmonte, Marcello, Nath, Rejish, Gross, Christian, Bloch, Immanuel and Zoller, Peter
Phys. Rev. Lett. , Volume 114(173002)
2014

Abstract: We show how a broad class of lattice spin-1/2 models with angular- and distance-dependent couplings can be realized with cold alkali atoms stored in optical or magnetic trap arrays. The effective spin-1/2 is represented by a pair of atomic ground states, and spin-spin interactions are obtained by admixing van der Waals interactions between fine-structure split Rydberg states with laser light. The strengths of the diagonal spin interactions as well as the "flip-flop", and "flip-flip" and "flop-flop" interactions can be tuned by exploiting quantum interference, thus realizing different spin symmetries. The resulting energy scales of interactions compare well with typical temperatures and decoherence time-scales, making the exploration of exotic forms of quantum magnetism, including emergent gauge theories and compass models, accessible within state-of-the-art experiments.

Rotational state detection of electrically trapped polyatomic molecules
Gl, Rosa, Prehn, Alexander, Rempe, Gerhard and Zeppenfeld, Martin
2014

Abstract: Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multi photon ionization (REMPI) or laser-induced fluorescense (LIF) do not work. As this is the case for most polyatomic molecule species, we here investigate an alternative based on state selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge as the frequency differences between various transitions is easily substantially less than the Stark broadening in an electric trap. However, making use of a unique trap design providing homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M-substate.

The Ground State Energy of Heavy Atoms: the Leading Correction
Handrek, Michael and Siedentop, Heinz
2014

Abstract: For heavy atoms (large atomic number Z) described by no-pair operators in the Furry picture we find the ground state's leading energy correction. We compare the result with (semi-)empirical values and Schwinger's prediction showing more than qualitative agreement.

Multipartite entanglement dectection with minimal effort
Knips, Lukas, Schwemmer, Christian, Klein, Nico, Wie?niak, Marcin and Weinfurter, Harald
2014

Abstract: Certifying entanglement in a multipartite state is a demanding task. A state of N qubits is parametrized by 4^N-1 real numbers, so, at first glimpse, one may expect that the measurement complexity of generic entanglement detection is also exponential with N. However, here we show how to design indicators for genuine multipartite quantum entanglement which require only two correlation measurements for prominent quantum states. We introduce a constructive method to derive such criteria and apply them in experiments for four-qubit Greenberger-Horne-Zeilinger states, cluster states and Dicke states.

Diagrammatic Monte Carlo study of mass-imbalanced Fermi-polaron system
Kroiss, Peter and Pollet, Lode
2014

Abstract: We apply the diagrammatic Monte Carlo approach to three-dimensional Fermi-polaron systems with mass-imbalance, where an impurity interacts resonantly with a noninteracting Fermi sea whose atoms have a different mass. This method allows to go beyond frequently used variational techniques by stochastically summing all relevant impurity Feynman diagrams up to a maximum expansion order limited by the sign problem. Polaron energy and quasiparticle residue can be accurately determined over a broad range of impurity masses. Furthermore, the spectral function of an imbalanced polaron demonstrates the stability of the quasiparticle and allows to locate in addition also the repulsive polaron as an excited state. The quantitative exactness of two-particle-hole wave-functions is investigated, resulting in a relative lowering of polaronic energies in the mass-imbalance phase diagram. Tan's contact coefficient for the mass-balanced polaron system is found in good agreement with variational methods. Mass-imbalanced systems can be studied experimentally by ultracold atom mixtures like ^6Li-40K.

Spin and charge dynamics of a quasi-one-dimensional antiferromagnetic metal
Raczkowski, Marcin, F. Assaad, Fakher and Pollet, Lode
Phys. Rev. B , Volume 91,, page: 045137
2014

Abstract: We use quantum Monte Carlo simulations to study a finite-temperature dimensional-crossover-driven evolution of spin and charge dynamics in weakly coupled Hubbard chains with a half-filled band. The low-temperature behavior of the charge gap indicates a crossover between two distinct energy scales: a high-energy one-dimensional (1D) Mott gap due to the umklapp process and a low-energy gap which stems from long-range antiferromagnetic (AF) fluctuations. Away from the 1D regime and at temperature scales above the charge gap, the emergence of a zero-frequency Drude-like feature in the interchain optical conductivity \sigma\perp(ω) implies the onset of a higher-dimensional metal. In this metallic phase, enhanced quasiparticle scattering off finite-range AF fluctuations results in incoherent single-particle dynamics. The coupling between spin and charge fluctuations is also seen in the spin dynamical structure factor S(\pmb q,ω) displaying damped spin excitations (paramagnons) close to the AF wave-vector \pmb q=(π,π) and particle-hole continua near 1D momentum transfers spanning quasiparticles at the Fermi surface. We relate our results to the charge deconfinement in quasi-1D organic Bechgaard-Fabre salts.

A Quantum Gate between a Flying Optical Photon and a Single Trapped Atom
Reiserer, Andreas, Kalb, Norbert, Rempe, Gerhard and Ritter, Stephan
Nature , Volume 508,, page: 237-240
2014

Abstract: The steady increase in control over individual quantum systems has backed the dream of a quantum technology that provides functionalities beyond any classical device. Two particularly promising applications have been explored during the past decade: First, photon-based quantum communication, which guarantees unbreakable encryption but still has to be scaled to high rates over large distances. Second, quantum computation, which will fundamentally enhance computability if it can be scaled to a large number of quantum bits. It was realized early on that a hybrid system of light and matter qubits could solve the scalability problem of both fields - that of communication via quantum repeaters, that of computation via an optical interconnect between smaller quantum processors. To this end, the development of a robust two-qubit gate that allows to link distant computational nodes is "a pressing challenge". Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The presented gate mechanism is deterministic, robust and expected to be applicable to almost any matter qubit. It is based on reflecting the photonic qubit from a cavity that provides strong light-matter coupling. To demonstrate its versatility, we use the quantum gate to create atom-photon, atom-photon-photon, and photon-photon entangled states from separable input states. We expect our experiment to break ground for various applications, including the generation of atomic and photonic cluster states, Schrödinger-cat states, deterministic photonic Bell-state measurements, and quantum communication using a redundant quantum parity code.

Cavity-based quantum networks with single atoms and optical photons
Reiserer, Andreas and Rempe, Gerhard
2014

Abstract: The implementation of a large-scale quantum network is a key challenge for quantum science. Such network consists of stationary quantum nodes that can store and process quantum information locally. The nodes are connected by quantum channels for flying information carriers, i.e. photons. These channels serve both to directly exchange quantum information between nodes as well as to distribute entanglement over the whole network. In order to scale such network to many particles and long distances, an efficient interface between the nodes and the channels is required. This article describes the cavity-based approach to this goal, with an emphasis on experimental systems in which single atoms are trapped in and coupled to optical resonators. Besides being conceptually appealing, this approach is promising for quantum networks on larger scales, as it gives access to long qubit coherence times and high light-matter coupling efficiencies. Thus, it allows one to generate entangled photons on the push of a button, to reversibly map the quantum state of a photon onto an atom, to transfer and teleport quantum states between remote atoms, to entangle distant atoms, to detect optical photons nondestructively, to perform entangling quantum gates between an atom and one or several photons, and even provides a route towards efficient heralded quantum memories for future repeaters. The presented general protocols and the identification of key parameters are applicable to other experimental systems.

Experimental multipartite entanglement without multipartite correlations
Schwemmer, Christian, Knips, Lukas, Cong Tran, Minh, de Rosier, Anna, Laskowski, Wieslaw, Paterek, Tomasz and Weinfurter, Harald
2014

Abstract: Non-classical correlations between measurement results make entanglement the essence of quantum physics and the main resource for quantum information applications. Surprisingly, there are n-particle states which do not exhibit n-partite correlations at all but still are genuinely n-partite entangled. We introduce a general construction principle for such states, implement them in a multiphoton experiment and analyze their properties in detail. Remarkably, even without n-partite correlations, these states do violate Bell inequalities showing that there is no local realistic model describing their properties.

Experimental Comparison of Efficient Tomography Schemes for a Six-Qubit State
Schwemmer, Christian, Toth, Geza, Niggebaum, Alexander, Moroder, Tobias, Gross, David, G, Otfried and Weinfurter, Harald
Phys. Rev. Lett. , Volume 113,, page: 040503
2014

Abstract: Quantum state tomography suffers from the measurement effort increasing exponentially with the number of qubits. Here, we demonstrate permutationally invariant tomography for which, contrary to conventional tomography, all resources scale polynomially with the number of qubits both in terms of the measurement effort as well as the computational power needed to process and store the recorded data. We demonstrate the benefits of combining permutationally invariant tomography with compressed sensing by studying the influence of the pump power on the noise present in a six-qubit symmetric Dicke state, a case where full tomography is possible only for very high pump powers.

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