Publications

Decoherence of an entangled state of a strongly-correlated double quantum dot structure through tunneling processes
A. Büsser, C., de Vega, I. and Heidrich-Meisner, F.
2014

Abstract: We consider two quantum dots described by the Anderson-impurity model with one electron per dot. The goal of our work is to study the decay of a maximally entangled state between the two electrons localized in the dots. We prepare the system in a perfect singlet and then tunnel-couple one of the dots to leads, which induces the non-equilibrium dynamics. We identify two cases: if the leads are subject to a sufficiently large voltage and thus a finite current, then direct tunneling processes cause decoherence and the entanglement as well as spin correlations decay exponentially fast. At zero voltage or small voltages and beyond the mixed-valence regime, virtual tunneling processes dominate and lead to a slower loss of coherence. We analyze this problem by studying the real-time dynamics of the spin correlations and the concurrence using two techniques, namely the time-dependent density matrix renormalization group method and a master-equation method. The results from these two approaches are in excellent agreement in the direct-tunneling regime for the case in which the dot is weakly tunnel-coupled to the leads. We present a quantitative analysis of the decay rates of the spin correlations and the concurrence as a function of tunneling rate, interaction strength, and voltage.

Mean-field phase diagram of the Bose-Fermi Hubbard model
Bukov, Marin and Pollet, Lode
Phys. Rev. B , Volume 89, page: 094502
2014

Abstract: We analyze the ground-state properties of mixtures consisting of scalar bosons and spin-12 fermions using a mean-field treatment of the local boson-fermion interaction on a simple cubic lattice. In the deep superfluid limit of the boson sector and the BCS regime of the fermion sector, we derive BCS-type equations to determine the phase diagram of the system. We find a competition between a charge density wave and a superconducting phase. In the opposite limit, we study the Mott-insulator-to-superfluid transition of the boson sector in the presence of a staggered density-induced alternating potential provided by the fermions, and determine the mean-field transition line. In the two-superfluids phase of the mixture, we restrict to nearest-neighbor-induced interactions between the fermions and consider the extended Hubbard model. We perform a mean-field analysis of the critical temperature for the formation of boson-assisted s-, extended s−-, d-, and p-wave pairs at fermionic half-filling. We compare our results with a recent dynamical mean-field study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012)].

Universal Conductivity in a Two-Dimensional Superfluid-to-Insulator Quantum Critical System
Chen, Kun, Liu, Longxiang, Deng, Youjin, Pollet, Lode and Prokof'ev, Nikolay
Phys. Rev. Lett. , Volume 112, page: 030402
2014

Abstract: We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional J-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, σ(∞)=0.359(4)σQ with σQ the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottoms-up AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D 83, 066017 (2011)]. For the first time, the shape of the σ(iωn)−σ(∞) function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particlelike nature of charge transport. We find that the holographic gauge-gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.

Dipoles in Graphene Have Infinitely Many Bound States
Cuenin, Jean-Claude and Siedentop, Heinz
2014

Abstract: We show that in graphene charge distributions with non-vanishing dipole moment have infinitely many bound states. The corresponding eigenvalues accumulate at the edges of the gap faster than any power.

An Aharonov-Bohm interferometer for determining Bloch band topology
Duca, Lucia, Li, Tracy, Reitter, Martin, Bloch, Immanuel, Schleier-Smith, Monika and Schneider, Ulrich
2014

Abstract: The geometric structure of an energy band in a solid is fundamental for a wide range of many-body phenomena in condensed matter and is uniquely characterized by the distribution of Berry curvature over the Brillouin zone. In analogy to an Aharonov-Bohm interferometer that measures the magnetic flux penetrating a given area in real space, we realize an atomic interferometer to measure Berry flux in momentum space. We demonstrate the interferometer for a graphene-type hexagonal lattice, where it has allowed us to directly detect the singular π Berry flux localized at each Dirac point. We show that the interferometer enables one to determine the distribution of Berry curvature with high momentum resolution. Our work forms the basis for a general framework to fully characterize topological band structures and can also facilitate holonomic quantum computing through controlled exploitation of the geometry of Hilbert space.

Polynomial cubic differentials and convex polygons in the projective plane
Dumas, David and Wolf, Michael
2014

Abstract: We construct and study a natural homeomorphism between the moduli space of polynomial cubic differentials of degree d on the complex plane and the space of projective equivalence classes of oriented convex polygons with d+3 vertices. This map arises from the construction of a complete hyperbolic affine sphere with prescribed Pick differential, and can be seen as an analogue of the Labourie-Loftin parameterization of convex RP^2 structures on a compact surface by the bundle of holomorphic cubic differentials over Teichmuller space.

Multiparticle localization for disordered systems on continuous space via the fractional moment method
Fauser, Michael and Warzel, Simone
2014

Abstract: We investigate spectral and dynamical localization of a quantum system of n particles on \mathbbR^d which are subject to a random potential and interact through a pair potential which may have infinite range. We establish two conditions which ensure spectral and dynamical localization near the bottom of the spectrum of the n -particle system: i)localization is established in the regime of weak interactions supposing one-particle localization, and ii)localization is also established under a Lifshitz-tail type condition on the sparsity of the spectrum. In case of polynomially decaying interactions, we provide an upper bound on the number of particles up to which these conditions apply.

Quantum channels with polytopic images and image additivity
Fukuda, Motohisa, Nechita, Ion and M. Wolf, Michael
2014

Abstract: We study quantum channels with respect to their image, i.e., the image of the set of density operators under the action of the channel. We first characterize the set of quantum channels having polytopic images and show that additivity of the minimal output entropy can be violated in this class. We then provide a complete characterization of quantum channels T that are universally image additive in the sense that for any quantum channel S, the image of T \otimes S is the convex hull of the tensor product of the images of T and S. These channels turn out to form a strict subset of entanglement breaking channels with polytopic images and a strict superset of classical-quantum channels.

The exponent in the orthogonality catastrophe for Fermi gases
Gebert, Martin, Küttler, Heinrich, Müller, Peter and Otte, Peter
2014

Abstract: We quantify the asymptotic vanishing of the ground-state overlap of two non-interacting Fermi gases in d-dimensional Euclidean space in the thermodynamic limit. Given two one-particle Schrödinger operators in finite-volume which differ by a compactly supported bounded potential, we prove a power-law upper bound on the ground-state overlap of the corresponding non-interacting N-particle systems. We interpret the decay exponent γ in terms of scattering theory and find γ = π-2{\lVert\arcsin\lvert T_E/2\rvert}\rVert_{\mathrmHS}^2, where T_E is the transition matrix at the Fermi energy E. This exponent reduces to the one predicted by Anderson [Phys. Rev. 164, 352--359 (1967)] for the exact asymptotics in the special case of a point-like perturbation. We therefore expect the upper bound to coincide with the exact asymptotics of the overlap.

Ergodicity and dynamical localization for Delone-Anderson operators
Germinet, Francois, Müller, Peter and Rojas-Molina, Constanza
2014

Abstract: We study the ergodic properties of Delone-Anderson operators, using the framework of randomly coloured Delone sets and Delone dynamical systems. In particular, we show the existence of the integrated density of states and, under some assumptions on the geometric complexity of the underlying Delone sets, we obtain information on the almost-sure spectrum of the family of random operators. We then exploit these results to study the Lifshitz-tail behaviour of the integrated density of states of a Delone-Anderson operator at the bottom of the spectrum. This is used as an input for the multi scale analysis to prove dynamical localization. We also estimate the size of the spectral region where dynamical localization occurs.

Construction of spin models displaying quantum criticality from quantum field theory
Glasser, Ivan, Ignacio Cirac, J., Sierra, German and E. B. Nielsen, Anne
Nuclear Physics B , Volume 886,, page: 63-74
2014

Abstract: We provide a method for constructing finite temperature states of one-dimensional spin chains displaying quantum criticality. These models are constructed using correlators of products of quantum fields and have an analytical purification. Their properties can be investigated by Monte-Carlo simulations, which enable us to study the low-temperature phase diagram and to show that it displays a region of quantum criticality. The mixed states obtained are shown to be close to the thermal state of a simple nearest neighbour Hamiltonian.

The effect of spin-orbit interactions on the 0.7-anomaly in quantum point contacts
Goulko, Olga, Bauer, Florian, Heyder, Jan and von Delft, Jan
2014

Abstract: We study how the conductance of a quantum point contact is affected by spin-orbit interactions, for systems at zero temperature both with and without electron-electron interactions. In the presence of spin-orbit coupling, tuning the strength and direction of an external magnetic field can change the dispersion relation and hence the local density of states in the point contact region. This modifies the effect of electron-electron interactions, implying striking changes in the shape of the 0.7-anomaly and introducing additional distinctive features in the first conductance step.

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