Publications (40)36.96 Total impact
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ABSTRACT: We investigate the possibility of manybody localization in translation invariant Hamiltonian systems. A key feature of manybody localized disordered systems is recovered, namely the fact that resonant spots are rare and farbetween. However, we point out that resonant spots are mobile, unlike in models with quenched disorder, and that they could therefore in principle delocalize the system. The motion of these resonant spots is reminiscent of motion in kinetically constrained models, the frustration effect being caused by a mismatch of energies rather than by geometric constraints. We provide examples where, in first order in the hopping, the resonant spots are effectively trapped (analogous to a jammed phase) and therefore localization is not ruled out, but we also have examples where, despite the rareness of resonant spots, the resonant spots are not trapped and hence this obvious scenario for manybody localization is not realized. We comment on the analysis of higher orders, but we did not reach a final conclusion yet.05/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the possibility of manybody localization in translation invariant Hamiltonian systems. A key feature of manybody localized disordered systems is recovered, namely the fact that resonant spots are rare and farbetween. However, we point out that resonant spots are mobile, unlike in models with quenched disorder, and that they could therefore in principle delocalize the system. The motion of these resonant spots is reminiscent of motion in kinetically constrained models, the frustration effect being caused by a mismatch of energies rather than by geometric constraints. We provide examples where, in first order in the hopping, the resonant spots are effectively trapped (analogous to a jammed phase) and therefore localization is not ruled out, but we also have examples where, despite the rareness of resonant spots, the resonant spots are not trapped and hence this obvious scenario for manybody localization is not realized. We comment on the analysis of higher orders, but we did not reach a final conclusion yet.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We consider a linear Boltzmann equation that arises in a model for quantum friction. It describes a particle that is slowed down by the emission of bosons. We study the stochastic process generated by this Boltzmann equation and we show convergence of its spatial trajectory to a multiple of Brownian motion with exponential scaling. The asymptotic position of the particle is finite in mean, even though its absolute value is typically infinite. This is contrasted to an approximation that neglects the influence of fluctuations, where the mean asymptotic position is infinite.Journal of Physics A Mathematical and Theoretical 03/2014; 47(27). · 1.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the projection on classical spins starting from quantum equilibria. We show Gibbsianness or quasilocality of the resulting classical spin system for a class of gapped quantum systems at low temperatures including quantum ground states. A consequence of Gibbsianness is the validity of a large deviation principle in the quantum system which is known and here recovered in regimes of high temperature or for thermal states in one dimension. On the other hand we give an example of a quantum ground state with strong nonlocality in the classical restriction, giving rise to what we call measurement induced entanglement, and still satisfying a large deviation principle.12/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We consider a quantum lattice system with infinitedimensional onsite Hilbert space, very similar to the BoseHubbard model. We investigate manybody localization in this model, induced by thermal fluctuations rather than disorder in the Hamiltonian. We provide evidence that the GreenKubo conductivity $\kappa(\beta)$, defined as the timeintegrated current autocorrelation function, decays faster than any polynomial in the inverse temperature $\beta$ as $\beta \to 0$. More precisely, we define approximations $\kappa_{\tau}(\beta)$ to $\kappa(\beta)$ by integrating the currentcurrent autocorrelation function up to a large but finite time $\tau$ and we rigorously show that $\be^{n}\kappa_{\be^{m}}(\beta)$ vanishes as $\be \to 0$, for any $n,m \in \bbN$ such that $mn$ is sufficiently large.08/2013;  [Show abstract] [Hide abstract]
ABSTRACT: This paper is a companion to 'Quantum Diffusion with Drift and the Einstein Relation I' (jointly submitted to arXiv). Its purpose is to describe and prove a certain number of technical results used in 'Quantum Diffusion with Drift and the Einstein Relation I', but not proven there. Both papers concern longtime properties (diffusion, drift) of the motion of a driven quantum particle coupled to an array of thermal reservoirs. The main technical results derived in the present paper are $(1)$ an asymptotic perturbation theory applicable for small driving, and, $(2)$ the construction of timedependent correlation functions of particle observables.06/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We study the dynamics of a quantum particle hopping on a simple cubic lattice and driven by a constant external force. It is coupled to an array of identical, independent thermal reservoirs consisting of free, massless Bose fields, one at each site of the lattice. When the particle visits a site x of the lattice it can emit or absorb field quanta of the reservoir at x. Under the assumption that the coupling between the particle and the reservoirs and the driving force are sufficiently small, we establish the following results: The ergodic average over time of the state of the particle approaches a nonequilibrium steady state (NESS) describing a nonzero mean drift of the particle. Its motion around the mean drift is diffusive, and the diffusion constant and the drift velocity are related to one another by the Einstein relation.06/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We study two popular onedimensional chains of classical anharmonic oscillators: the rotor chain and a version of the discrete nonlinear Schr\"odinger chain. We assume that the interaction between neighboring oscillators, controlled by the parameter $\epsilon >0$, is small. We rigorously establish that the thermal conductivity of the chains has a nonperturbative origin, with respect to the coupling constant $\epsilon$, and we provide strong evidence that it decays faster than any power law in $\epsilon$ as $\epsilon \rightarrow 0$. The weak coupling regime also translates into a high temperature regime, suggesting that the conductivity vanishes faster than any power of the inverse temperature.05/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We consider generalized versions of the massless spinboson model. Building on the recent work in 'Approach to ground state and timeindependent photon bound for massless spinboson models' (arXiv:1109.5582, Annales H. Poincare, 2012) and 'Propagation bounds and soft photon bounds for the massless spinboson model' (submitted to arXiv jointly with the present paper), we prove asymptotic completeness.01/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We consider generalized versions of the massless spinboson model. We prove detailed bounds on the number of bosons in certain spatial regions (propagation bounds) and on the number of bosons with low momentum (soft photon bounds). This work is an extension of our earlier work in 'Approach to ground state and timeindependent photon bound for massless spinboson models' (arXiv:1109.5582, Ann. H. Poincare, 2012). Together with the results in arXiv:1109.5582, the bounds of the present paper suffice to prove asymptotic completeness, as we describe in a joint submission to arXiv: 'Asymptotic completeness for the massless spinboson model'.01/2013; 
Article: Derivation of some translationinvariant Lindblad equations for a quantum Brownian particle
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ABSTRACT: We study the dynamics of a Brownian quantum particle hopping on an infinite lattice with a spin degree of freedom. This particle is coupled to free boson gases via a translationinvariant Hamiltonian which is linear in the creation and annihilation operators of the bosons. We derive the time evolution of the reduced density matrix of the particle in the van Hove limit in which we also rescale the hopping rate. This corresponds to a situation in which both the systembath interactions and the hopping between neighboring sites are small and they are effective on the same time scale. The reduced evolution is given by a translationinvariant Lindblad master equation which is derived explicitly.Journal of Statistical Physics 08/2012; 150(2). · 1.40 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: It is widely believed that an atom interacting with the electromagnetic field (with total initial energy wellbelow the ionization threshold) relaxes to its ground state while its excess energy is emitted as radiation. Hence, for large times, the state of the atom+field system should consist of the atom in its ground state, and a few free photons that travel off to spatial infinity. Mathematically, this picture is captured by the notion of asymptotic completeness. Despite some recent progress on the spectral theory of such systems, a proof of relaxation to the ground state and asymptotic completeness was/is still missing, except in some special cases (massive photons, small perturbations of harmonic potentials). In this paper, we partially fill this gap by proving relaxation to an invariant state in the case where the atom is modelled by a finitelevel system. If the coupling to the field is sufficiently infraredregular so that the coupled system admits a ground state, then this invariant state necessarily corresponds to the ground state. Assuming slightly more infrared regularity, we show that the number of emitted photons remains bounded in time. We hope that these results bring a proof of asymptotic completeness within reach.Annales Henri Poincare 09/2011; · 1.53 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We prove diffusion for a quantum particle coupled to a field of bosons (phonons or photons). The importance of this result lies in the fact that our model is fully Hamiltonian and randomness enters only via the initial (thermal) state of the bosons. This model is closely related to the one considered in [De Roeck, Fr\"ohlich 2011], but various restrictive assumptions of the latter have been eliminated. In particular, depending on the dispersion relation of the bosons, the present result holds in dimension $d \geq 3$.Communications in Mathematical Physics 07/2011; · 1.97 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recently, several authors studied small quantum systems weakly coupled to free boson fields at positive density. All the approaches we are aware of employ complex deformations of Liouvillians or Mourre theory (the infinitesimal version of the former). We present an approach based on polymer expansions of statistical mechanics. Despite the fact that our approach is elementary, our results are slightly sharper than those contained in the literature up to now. Essentially, we show that, whenever the small quantum system is known to admit a Markov approximation (Pauli master equation \emph{aka} Lindblad equation) in the weak coupling limit, and the Markov approximation is exponentially mixing, then the weakly coupled system approaches a unique invariant state that is perturbatively close to its Markov approximation. Comment: 23 pages, v1>v2: inconsequential error in Section 3 correctedCommunications in Mathematical Physics 05/2010; · 1.97 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that, in a model where a nonrelativistic particle is coupled to a quantized relativistic scalar Bose field, the embedded mass shell of the particle dissolves in the continuum when the interaction is turned on, provided the coupling constant is sufficiently small. More precisely, under the assumption that the fiber eigenvectors corresponding to the putative mass shell are differentiable as functions of the total momentum of the system, we show that a mass shell could exist only at a strictly positive distance from the unperturbed embedded mass shell near the boundary of the energy–momentum spectrum.Annales Henri Poincare 01/2010; 11(8):15451589. · 1.53 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study weakly disordered quantum wires whose width is large compared to the Fermi wavelength. It is conjectured that such wires diplay universal metallic behaviour as long as their length is shorter than the localization length (which increases with the width). The random matrix theory that accounts for this behaviour  the DMPK theory rests on assumptions that are in general not satisfied by realistic microscopic models. Starting from the Anderson model on a strip, we show that a twofold scaling limit nevertheless allows to recover rigorously the fundaments of DMPK theory, thus opening a way to settle some conjectures on universal metallic behaviour.Journal of Statistical Physics 12/2009; 139(4). · 1.40 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider a heavy quantum particle with an internal degree of freedom moving on the $d$dimensional lattice $\bbZ^d$ (e.g., a heavy atom with finitely many excited states). The particle is coupled to a thermal medium (bath) consisting of free relativistic bosons through an interaction of strength $\la$ linear in creation and annihilation operators. The mass of the quantum particle is assumed to be of order $\la^{2}$, and we assume that the internal degree of freedom is coupled "effectively" to the thermal medium. We prove that the motion of the quantum particle is diffusive in $d\geq 4$ and for $\la$ small enough.Communications in Mathematical Physics 06/2009; · 1.97 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider a finite quantum system coupled to quasifree thermal reservoirs at different temperatures. We construct the statistics of energy transport between the reservoirs and we show that the corresponding large deviation generating function exists and it is analytic on a compact set. This result is valid for small coupling and exponentially decaying reservoir correlation functions. Our technique consists of a diagrammatic expansion that uses the Markovian limit of the system as a reference. As a corollary, we derive the GallavottiCohen fluctuation relation for the entropy production.Reviews in Mathematical Physics 01/2009; 21:549585. · 1.09 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study a general class of translation invariant quantum Markov evolutions for a particle on $\bbZ^d$. The evolution consists of free flow, interrupted by scattering events. We assume spatial locality of the scattering events and exponentially fast relaxation of the momentum distribution. It is shown that the particle position diffuses in the long time limit. This generalizes standard results about central limit theorems for classical (nonquantum) Markov processes.Journal of Mathematical Physics 12/2008; · 1.30 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We continue the study of the free energy of quantum lattice spin systems where to the local Hamiltonian $H$ an arbitrary mean field term is added, a polynomial function of the arithmetic mean of some local observables $X$ and $Y$ that do not necessarily commute. By slightly extending a recent paper by Hiai, Mosonyi, Ohno and Petz [9], we prove in general that the free energy is given by a variational principle over the range of the operators $X$ and $Y$. As in [9], the result is a noncommutative extension of the LaplaceVaradhan asymptotic formula. Comment: v1>v2, 15 pages, the conditions of the theorems have been relaxed and their proof has been simplified. The whole paper was therefore restyled. A new author is addedReviews in Mathematical Physics 08/2008; · 1.09 Impact Factor
Publication Stats
246  Citations  
36.96  Total Impact Points  
Top Journals
Institutions

2008–2012

Heidelberg University
 Institute of Theoretical Physics
Heidelburg, BadenWürttemberg, Germany 
ETH Zurich
 Institute for Theoretical Physics
Zürich, ZH, Switzerland


2009

University of Helsinki
 Department of Mathematics and Statistics
Helsinki, Southern Finland Province, Finland
