Publications

Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas
Piazza, Francesco and Strack, Philipp
Phys. Rev. Lett. , Volume 112,, page: 143003
2013

Abstract: The quantum dynamics of the electromagnetic light mode of an optical cavity filled with a coherently driven Fermi gas of ultracold atoms strongly depends on geometry of the Fermi surface. Superradiant light generation and self-organization of the atoms can be achieved at low pumping threshold due to resonant atom-photon Umklapp processes, where the fermions are scattered from one side of the Fermi surface to the other by exchanging photon momenta. The cavity spectrum exhibits sidebands, that, despite strong atom-light coupling and cavity decay, retain narrow linewidth, due to absorptionless transparency windows outside the atomic particle-hole continuum and the suppression of inhomogeneous broadening and thermal fluctuations in the collisionless Fermi gas.

Field-theoretical study of the Bose polaron
Patrick Rath, Steffen and Schmidt, Richard
Phys. Rev. A , Volume 88,, page: 053632
2013

Abstract: We study the properties of the Bose polaron, an impurity strongly interacting with a Bose-Einstein condensate, using a field-theoretic approach and make predictions for the spectral function and various quasiparticle properties that can be tested in experiment. We find that most of the spectral weight is contained in a coherent attractive and a metastable repulsive polaron branch. We show that the qualitative behavior of the Bose polaron is well described by a non-selfconsistent T-matrix approximation by comparing analytical results to numerical data obtained from a fully selfconsistent T-matrix approach. The latter takes into account an infinite number of bosons excited from the condensate.

Confinement and Interaction of Single Indirect Excitons in a Voltage-Controlled Trap Formed Inside Double InGaAs Quantum Wells
Schinner, G. J., Repp, J., Schubert, E., Rai, A. K., Reuter, D., Wieck, A. D., Govorov, A. O., Holleitner, A. W. and Kotthaus, J. P.
Phys. Rev. Lett. , Volume 110, page: 127403
2013

Abstract: Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.

Bright, long-lived and coherent excitons in carbon nanotube quantum dots
Hofmann, Matthias S., Gluckert, Jan T., Noe, Jonathan, Bourjau, Christian, Dehmel, Raphael and Högele, Alexander
Nat Nano , Volume 8(7), page: 502-505
2013

Abstract: Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances (1), promise long electron spin coherence times in a nuclear-spin free lattice (2, 3) for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions (4, 5) for optoelectronic applications (6). Excitons in semiconducting single-walled carbon nanotubes (7, 8) could facilitate the upconversion of spin (9), mechanical (10) or hybrid spin–mechanical (11) degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites (12), environmental dephasing (13) and emission intermittence (14). Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes (15, 16, 17) and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.

Classical simulations of Abelian-group normalizer circuits with intermediate measurements
Bermejo-Vega, Juan and Van den Nest, Maarten
2012

Abstract: Quantum normalizer circuits were recently introduced as generalizations of Clifford circuits [arXiv:1201.4867]: a normalizer circuit over a finite Abelian group G is composed of the quantum Fourier transform (QFT) over G, together with gates which compute quadratic functions and automorphisms. In [arXiv:1201.4867] it was shown that every normalizer circuit can be simulated efficiently classically. This result provides a nontrivial example of a family of quantum circuits that cannot yield exponential speed-ups in spite of usage of the QFT, the latter being a central quantum algorithmic primitive. Here we extend the aforementioned result in several ways. Most importantly, we show that normalizer circuits supplemented with intermediate measurements can also be simulated efficiently classically, even when the computation proceeds adaptively. This yields a generalization of the Gottesman-Knill theorem (valid for n-qubit Clifford operations [quant-ph/9705052, quant-ph/9807006] to quantum circuits described by arbitrary finite Abelian groups. Moreover, our simulations are twofold: we present efficient classical algorithms to sample the measurement probability distribution of any adaptive-normalizer computation, as well as to compute the amplitudes of the state vector in every step of it. Finally we develop a generalization of the stabilizer formalism [quant-ph/9705052, quant-ph/9807006] relative to arbitrary finite Abelian groups: for example we characterize how to update stabilizers under generalized Pauli measurements and provide a normal form of the amplitudes of generalized stabilizer states using quadratic functions and subgroup cosets.

On the Maximal Excess Charge of the Chandrasekhar-Coulomb Hamiltonian in Two Dimensions
Handrek, Michael and Siedentop, Heinz
2012

Abstract: We show that for the straightforward quantized relativistic Coulomb Hamiltonian of a two-dimensional atom -- or the corresponding magnetic quantum dot -- the maximal number of electrons does not exceed twice the nuclear charge. It result is then generalized to the presence of external magnetic fields and atomic Hamiltonians. This is based on the positivity of |\bx| T(\bp) + T(\bp) |\bx| which -- in two dimensions -- is false for the non-relativistic case T(\bp) = \bp^2, but is proven in this paper for T(\bp) = |\bp|, i.e., the ultra-relativistic kinetic energy.

Scaling of the thermal spectral function for quantum critical bosons in one dimension
Barthel, Thomas, Schollw, Ulrich and Sachdev, Subir
2012

Abstract: We present an improved scheme for the precise evaluation of finite-temperature response functions of strongly correlated systems in the framework of the time-dependent density matrix renormalization group. The maximum times that we can reach at finite temperatures T are typically increased by a factor of two, when compared against the earlier approaches. This novel scheme, complemented with linear prediction, allows us now to evaluate dynamic correlators for interacting bosons in one dimension. We demonstrate that the considered spectral function in the quantum critical regime with dynamic critical exponent z=2 is captured by the universal scaling form S(k,omega)=(1/T)*Phi(k/sqrt(T),omega/T) and calculate the scaling function precisely.

Nuclear spin physics in quantum dots: an optical investigation
Urbaszek, Bernhard, Marie, Xavier, Amand, Thierry, Krebs, Olivier, Voisin, Paul, Maletinsky, Patrick, Högele, Alexander and Imamoglu, Atac
Reviews of Modern Physics , Volume 85,, page: 79
2012

Abstract: The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.

Real analyticity away from the nucleus of pseudorelativistic Hartree-Fock orbitals
Dall'Acqua, Anna, Fournais, Søren, Østergaard Sørensen, Thomas and Stockmeyer, Edgardo
Analysis & PDE , Volume 5, page: ,no.3,657--691
2011

Abstract: We prove that the Hartree--Fock orbitals of pseudorelativistic atoms, that is, atoms where the kinetic energy of the electrons is given by the pseudorelativistic operator sqrt-Delta+1-1, are real analytic away from the origin. As a consequence, the quantum mechanical ground state of such atoms is never a Hartree-Fock state. Our proof is inspired by the classical proof of analyticity by nested balls of Morrey and Nirenberg. However, the technique has to be adapted to take care of the non-local pseudodifferential operator, the singularity of the potential at the origin, and the non-linear terms in the equation.

Quasi-locality and efficient simulation of Markovian quantum dynamics
Barthel, Thomas and Kliesch, Martin
Phys. Rev. Lett. , Volume 108,, page: 230504
2011

Abstract: We consider open many-body systems governed by a time-dependent quantum master equation with short-range interactions. With a generalized Lieb-Robinson bound, we show that the evolution in this very generic framework is quasi-local, i.e., the evolution of observables can be approximated by implementing the dynamics only in a vicinity of the observables' support. The precision increases exponentially with the diameter of the considered subsystem. Hence, the time-evolution can be simulated on classical computers with a cost that is independent of the system size. Providing error bounds for Trotter decompositions, we conclude that the simulation on a quantum computer is additionally efficient in time. For experiments and simulations, our result can be used to rigorously bound finite-size effects.

Spectral functions in one-dimensional quantum systems at T>0
Barthel, Thomas, Schollwöck, Ulrich and R. White, Steven
Phys. Rev. B , Volume 79,, page: 245101
2009

Abstract: We present for the first time time-dependent density-matrix renormalization-group simulations (t-DMRG) at finite temperatures. It is demonstrated how a combination of finite-temperature t-DMRG and time-series prediction allows for an easy and very accurate calculation of spectral functions in one-dimensional quantum systems, irrespective of their statistics, for arbitrary temperatures. This is illustrated with spin structure factors of XX and XXX spin-1/2 chains. For the XX model we can compare against an exact solution and for the XXX model (Heisenberg antiferromagnet) against a Bethe Ansatz solution and quantum Monte Carlo data.

Many-Body Physics with Ultracold Gases
Bloch, I., Dalibard, J. and Zwerger, R.
Rev. Mod. Phys. , Volume 80,, page: 885
2007

Abstract: This article reviews recent experimental and theoretical progress on many-body phenomena in dilute, ultracold gases. Its focus are effects beyond standard weak-coupling descriptions, like the Mott-Hubbard-transition in optical lattices, strongly interacting gases in one and two dimensions or lowest Landau level physics in quasi two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near Feshbach resonances in the BCS-BEC crossover.

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