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A. Kupiainen

Publications and source records attributed to A. Kupiainen.

At least 19 recordsLinked to original sources
arXiv2014

Minimal velocity estimates and soft mode bounds for the massless spin-boson model

We consider generalised versions of the spin-boson model at small coupling. We assume the spin (or atom) to sit at the origin $0 \in \mathbb{R}^d$ and the propagation speed $v_p$ of free bosons to be constant, i.e.\ independent of momentum. In particular, the bosons are massless. We prove detailed bounds on the mean number of bosons contained in the ball $\{|x| \leq v_p t \}$. In particular, we prove that, as $t \to \infty$, this number tends to an asymptotic value that can be naturally identified as the mean number of bosons bound to the atom in the ground state. Physically, this means that bosons that are not bound to the atom, are travelling outwards at a speed that is not lower than $v_p$, hence the term 'minimal velocity estimate'. Additionally, we prove bounds on the number of emitted bosons with low momentum (soft mode bounds). This paper is an extension of our earlier work in 'Approach to ground state and time-independent photon bound for massless spin-boson models' (arXiv:1109.5582, Ann. H. Poincare, 2012). Together with the results in that paper, the bounds of the present paper suffice to prove asymptotic completeness, as we describe in 'Asymptotic completeness for the massless spin-boson model'(arXiv:1301.2357).

math-ph↗
arXiv2013

Diffusion for a quantum particle coupled to phonons in $d\geq 3$

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, Fröhlich 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 $d \geq 3$.

math-ph↗
arXiv2013

Asymptotic completeness for the massless spin-boson model

We consider generalized versions of the massless spin-boson model. Building on the recent work in 'Approach to ground state and time-independent photon bound for massless spin-boson models' (arXiv:1109.5582, Annales H. Poincare, 2012) and 'Propagation bounds and soft photon bounds for the massless spin-boson model' (submitted to arXiv jointly with the present paper), we prove asymptotic completeness.

math-ph↗
arXiv2013

Approach to ground state and time-independent photon bound for massless spin-boson models

It is widely believed that an atom interacting with the electromagnetic field (with total initial energy well-below 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 finite-level system. If the coupling to the field is sufficiently infrared-regular 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.

math-ph↗
arXiv2010

'Return to equilibrium' for weakly coupled quantum systems: a simple polymer expansion

Recently, several authors studied small quantum systems weakly coupled to free boson or fermion fields at positive temperature. 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. 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.

math-ph↗
arXiv2009

Random Curves by Conformal Welding

We construct a conformally invariant random family of closed curves in the plane by welding of random homeomorphisms of the unit circle given in terms of the exponential of Gaussian Free Field. We conjecture that our curves are locally related to SLE$(κ)$ for $κ<4$.

math.CV↗
arXiv2009

Random Conformal Weldings

We construct a conformally invariant random family of closed curves in the plane by welding of random homeomorphisms of the unit circle. The homeomorphism is constructed using the exponential of $βX$ where $X$ is the restriction of the two dimensional free field on the circle and the parameter $β$ is in the "high temperature" regime $β<\sqrt 2$. The welding problem is solved by studying a non-uniformly elliptic Beltrami equation with a random complex dilatation. For the existence a method of Lehto is used. This requires sharp probabilistic estimates to control conformal moduli of annuli and they are proven by decomposing the free field as a sum of independent fixed scale fields and controlling the correlations of the complex dilation restricted to dyadic cells of various scales. For uniqueness we invoke a result by Jones and Smirnov on conformal removability of Hölder curves. We conjecture that our curves are locally related to SLE$(κ)$ for $κ<4$.

math.CV↗
arXiv2007

KAM Theorem and Renormalization Group

We give an elementary proof of the analytic KAM theorem by reducing it to a Picard iteration of a PDE with quadratic nonlinearity, the so called Polchinski renormalization group equation studied in quantum field theory.

math-ph↗
arXiv2003

Absolute continuity of projected SRB measures of coupled Arnold cat map lattices

We study a d-dimensional coupled map lattice consisting of hyperbolic toral automorphisms (Arnold cat maps) that are weakly coupled by an analytic coupling map. We construct the Sinai-Ruelle-Bowen measure for this system and study its marginals on the tori. We prove they are absolutely continuous with respect to the Lebesgue measure if and only if the coupling satisfies a nondegeneracy condition.

nlin.CD↗
arXiv2000

Exponential Mixing of the 2D Stochastic Navier-Stokes Dynamics

We consider the Navier-Stokes equation on a two dimensional torus with a random force which is white noise in time, and excites only a finite number of modes. The number of excited modes depends on the viscosity $ν$, and grows like $ν^{-3}$ when $ν$ goes to zero. We prove that this Markov process has a unique invariant measure and is exponentially mixing in time.

math-ph↗
arXiv1998

KAM Theorem and Quantum Field Theory

We give a new proof of the KAM theorem for analytic Hamiltonians. The proof is inspired by a quantum field theory formulation of the problem and is based on a renormalization group argument treating the small denominators inductively scale by scale. The crucial cancellations of resonances are shown to follow from the Ward identities expressing the translation invariance of the corresponding field theory.

chao-dyn↗
arXiv1995

Infinite dimensional SRB measures

We review the basic steps leading to the construction of a Sinai-Ruelle-Bowen (SRB) measure for an infinite lattice of weakly coupled expanding circle maps, and we show that this measure has exponential decay of space-time correlations. First, using the Perron-Frobenius operator, one connects the dynamical system of coupled maps on a $d$-dimensional lattice to an equilibrium statistical mechanical model on a lattice of dimension $d+1$. This lattice model is, for weakly coupled maps, in a high-temperature phase, and we use a general, but very elementary, method to prove exponential decay of correlations at high temperatures.

chao-dyn↗
arXiv1995

High Temperature Expansions and Dynamical Systems

We develop a resummed high-temperature expansion for lattice spin systems with long range interactions, in models where the free energy is not, in general, analytic. We establish uniqueness of the Gibbs state and exponential decay of the correlation functions. Then, we apply this expansion to the Perron-Frobenius operator of weakly coupled map lattices.

chao-dyn↗
arXiv1995

Global Large Time Self-similarity of a Thermal-Diffusive Combustion System with Critical Nonlinearity

We study the initial value problem of the thermal-diffusive combustion system: $u_{1,t} = u_{1,x,x} - u_1 u^2_2, u_{2,t} = d u_{2,xx} + u_1 u^2_2, x \in R^1$, for non-negative spatially decaying initial data of arbitrary size and for any positive constant $d$. We show that if the initial data decays to zero sufficiently fast at infinity, then the solution $(u_1,u_2)$ converges to a self-similar solution of the reduced system: $u_{1,t} = u_{1,xx} - u_1 u^2_2, u_{2,t} = d u_{2,xx}$, in the large time limit. In particular, $u_1$ decays to zero like ${\cal O}(t^{-\frac{1}{2}-δ})$, where $δ> 0$ is an anomalous exponent depending on the initial data, and $u_2$ decays to zero with normal rate ${\cal O}(t^{-\frac{1}{2}})$. The idea of the proof is to combine the a priori estimates for the decay of global solutions with the renormalization group (RG) method for establishing the self-similarity of the solutions in the large time limit.

chao-dyn↗
arXiv1994

Renormalizing Partial Differential Equations

In this review paper, we explain how to apply Renormalization Group ideas to the analysis of the long-time asymptotics of solutions of partial differential equations. We illustrate the method on several examples of nonlinear parabolic equations. We discuss many applications, including the stability of profiles and fronts in the Ginzburg-Landau equation, anomalous scaling laws in reaction-diffusion equations, and the shape of a solution near a blow-up point.

chao-dyn↗
arXiv1994

Coupled Analytic Maps

We consider a lattice of weakly coupled expanding circle maps. We construct, via a cluster expansion of the Perron-Frobenius operator, an invariant measure for these infinite dimensional dynamical systems which exhibits space-time-chaos.

chao-dyn↗
arXiv1993

Stable Non-Gaussian Diffusive Profiles

We prove two stability results for the scale invariant solutions of the nonlinear heat equation $\partial_t u=Δu - |u|^{p-1}u$ with $1<p<1+{2\over n}$, $n$ being the spatial dimension. The first result is that a small perturbation of a scale invariant solution vanishes as $t\rightarrow\infty$. The second result is global, with a positivity condition on the initial data.

chao-dyn↗