DOCSLIB.ORG

Physics 551, Problem Set 2 Due: Wednesday, September 26 at 10Am Grader: M

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Physics 551, Problem Set 2 Due: Wednesday, September 26 at 10Am Grader: M Physics 551, Problem Set 2 due: Wednesday, September 26 at 10am grader: M. York Please give your completed problem sets to me at the beginning of class or place them in the \Physics 551" box in the physics department mailroom (Rutherford 103b) before the due date. You are encouraged to discuss these problems with your colleagues, but you must write up your own solutions; the solutions you hand in should reflect your own work and understanding. Late problem sets will be penalized 10% per day late, unless an extension has been obtained from me or a TA before the due date. Late problem sets will not be accepted after solutions are handed out. Reading: Chapters 1 & 2 of Sakurai. Chapters 1 { 4 of Dirac's \Principles of Quantum Mechanics." 1. Chapter 1, problem 29a. 1 2 2. Consider a free particle of mass m moving in one dimension, where H = 2m p . Let x(t) and p(t) be the position and momentum operators in Heisenberg picture. (a) Using the Heisenberg equation of motion show that p(t) = p(0) is independent of time. (b) Show that x(t) = x(0) + (p(0)=m)t. Conclude that the expectation value of the position hx(t)i takes the exact same form as in the classical mechanics of a free particle. (c) Compute [x(t); x(0)]. (d) Show that h(∆x(t))2ih(∆x(0))2i ≥ ct2 where c is a constant you should compute. Thus even though the average position of the particle obeys the classical equation of motion, the uncertainty in the position grows without bound at late times. 1 2 3. Consider a particle of mass m moving in one dimension with Hamiltonian H = 2m p +V (x) where the potential V (x) is non-zero. Let x(t) and p(t) be the position and momentum operators in Heisenberg picture. 1 (a) Write down the Heisenberg equations of motion and show that they imply that the expectation value hx(t)i obeys the classical equation of motion d2hxi m = −hV 0(x)i dt2 This is known as Ehrenfest's Theorem. n (b) Consider the case V = V0x . Recall that classical dynamical systems obey the \virial theorem" which states that the average value of the kinetic energy is proportional to the average value of the potential energy: 1 n h p2i = hV i 2m 2 where hi denotes the long time average of a classical variable. Show that this is not quite true in quantum mechanics, and instead quantum expectation values obey 1 n 1 dhxpi h p2i = hV i + 2m 2 2 dt d(xp) (Hint: Compute dt .) 4. Consider two operators A and B whose commutator is a constant, say [A; B] = c. (a) Show that eABe−A = B + c (b) Compute eABne−A.(Hint: 1 = e−AeA). (c) Show that eAeBe−Ae−B = ec. This is a simple version of the Campbell-Baker- Hausdorf forumla. 5. Consider a one dimensional particle with Schrodinger picture observables x and p. (a) Show that the operators U = eip and V = eix are unitary. Let jxi and jpi denote position and momentum eigenstates. Show that U njxi is an eigenstate of x for any n and compute it's eigenvalue. Show that V njpi is an eigenstate of p and compute it's eigenvalue. We often say that the operator U is a translation in position space, and V is a translation in momentum space. (b) Show that the operator W = ei¯h is unitary. What happens to a physical state when we act on it with the operator W n? (c) Show that U nV mU −nV −m = W p for some value of p which you should compute. 2 (d) Consider an operator of the form U nV mW p. Explain why it is that any product of the U's, V 's and W 's can be written in this form, with the U's on the left, the V 's in the middle and the W 's on the right. Show that if we multiply and two operators of this form we obtain another operator of this form. That is to say, show that (U n1 V m1 W p1 )(U n2 V m2 W p2 ) = U n3 V m3 W p3 where n3; m3; p3 are numbers you should compute in terms of n1; n2; m1; m2; p1 and n m p p2. This means that operators of the form U V W form what is known as a group. The formula you have just derived is called the group multiplication law. (e) The group you have discovered is called the Heisenberg group. In fact, this group has another very simple form. Consider the set of 3 × 3 matrices of the form 0 1 a c 1 B C @ 0 1 b A 0 0 1 A matrix of this form is called unitriangular. The Heisenberg group can be thought of as the set of 3 × 3 unitriangular matrices. To see this, show that 0 1 0 1 0 1 1 a1 c1 1 a2 c2 1 a3 c3 B C B C B C @ 0 1 b1 A @ 0 1 b2 A = @ 0 1 b3 A 0 0 1 0 0 1 0 0 1 where a3; b3; c3 are numbers you should compute in terms of a1; a2; b1; b2; c1 and c2. Show that this multiplication law is the same as the multiplication law you derived in part (d), provided a, b and c are identified with n, m and p in some way (which you should determine). 3.
Load more

Recommended publications
  • Ehrenfest Theorem
    Ehrenfest theorem Consider two cases, A=r and A=p: d 1 1 p2 1 < r > = < [r,H] > = < [r, + V(r)] > = < [r,p2 ] > dt ih ih 2m 2imh 2 [r,p ] = [r.p]p + p[r,p] = 2ihp d 1 d < p > ∴ < r > = < 2ihp > ⇒ < r > = dt 2imh dt m d 1 1 p2 1 < p > = < [p,H] > = < [p, + V(r)] > = < [p,V(r)] > dt ih ih 2m ih [p,V(r)] ψ = − ih∇V(r)ψ − ()− ihV(r)∇ ψ = −ihψ∇V(r) ⇒ [p,V(r)] = −ih∇V(r) d 1 d ∴ < p > = < −ih∇V(r) > ⇒ < p > = < −∇V(r) > dt ih dt d d Compare this with classical result: m vr = pv and pv = − ∇V dt dt We see the correspondence between expectation of an operator and classical variable. Poisson brackets and commutators Poisson brackets in classical mechanics: ⎛ ∂A ∂B ∂A ∂B ⎞ {A, B} = ⎜ − ⎟ ∑ ⎜ ⎟ j ⎝ ∂q j ∂p j ∂p j ∂q j ⎠ dA ⎛ ∂A ∂A ⎞ ∂A ⎛ ∂A ∂H ∂A ∂H ⎞ ∂A = ⎜ q + p ⎟ + = ⎜ − ⎟ + ∑ ⎜ & j & j ⎟ ∑ ⎜ ⎟ dt j ⎝ ∂q j ∂p j ⎠ ∂t j ⎝ ∂q j ∂q j ∂p j ∂p j ⎠ ∂t dA ∂A ∴ = {A,H}+ Classical equation of motion dt ∂ t Compare this with quantum mechanical result: d 1 ∂A < A > = < [A,H] > + dt ih ∂ t We see the correspondence between classical mechanics and quantum mechanics: 1 [A,H] → {A, H} classical ih Quantum mechanics and classical mechanics 1. Quantum mechanics is more general and cover classical mechanics as a limiting case. 2. Quantum mechanics can be approximated as classical mechanics when the action is much larger than h.
    [Show full text]
  • Lecture 8 Symmetries, Conserved Quantities, and the Labeling of States Angular Momentum
    Lecture 8 Symmetries, conserved quantities, and the labeling of states Angular Momentum Today’s Program: 1. Symmetries and conserved quantities – labeling of states 2. Ehrenfest Theorem – the greatest theorem of all times (in Prof. Anikeeva’s opinion) 3. Angular momentum in QM ˆ2 ˆ 4. Finding the eigenfunctions of L and Lz - the spherical harmonics. Questions you will by able to answer by the end of today’s lecture 1. How are constants of motion used to label states? 2. How to use Ehrenfest theorem to determine whether the physical quantity is a constant of motion (i.e. does not change in time)? 3. What is the connection between symmetries and constant of motion? 4. What are the properties of “conservative systems”? 5. What is the dispersion relation for a free particle, why are E and k used as labels? 6. How to derive the orbital angular momentum observable? 7. How to check if a given vector observable is in fact an angular momentum? References: 1. Introductory Quantum and Statistical Mechanics, Hagelstein, Senturia, Orlando. 2. Principles of Quantum Mechanics, Shankar. 3. http://mathworld.wolfram.com/SphericalHarmonic.html 1 Symmetries, conserved quantities and constants of motion – how do we identify and label states (good quantum numbers) The connection between symmetries and conserved quantities: In the previous section we showed that the Hamiltonian function plays a major role in our understanding of quantum mechanics using it we could find both the eigenfunctions of the Hamiltonian and the time evolution of the system. What do we mean by when we say an object is symmetric?�What we mean is that if we take the object perform a particular operation on it and then compare the result to the initial situation they are indistinguishable.�When one speaks of a symmetry it is critical to state symmetric with respect to which operation.
    [Show full text]
  • Schrödinger and Heisenberg Representations
    p. 33 SCHRÖDINGER AND HEISENBERG REPRESENTATIONS The mathematical formulation of the dynamics of a quantum system is not unique. So far we have described the dynamics by propagating the wavefunction, which encodes probability densities. This is known as the Schrödinger representation of quantum mechanics. Ultimately, since we can’t measure a wavefunction, we are interested in observables (probability amplitudes associated with Hermetian operators). Looking at a time-evolving expectation value suggests an alternate interpretation of the quantum observable: Atˆ () = ψ ()t Aˆ ψ ()t = ψ ()0 U † AˆUψ ()0 = ( ψ ()0 UA† ) (U ψ ()0 ) (4.1) = ψ ()0 (UA † U) ψ ()0 The last two expressions here suggest alternate transformation that can describe the dynamics. These have different physical interpretations: 1) Transform the eigenvectors: ψ (t) →U ψ . Leave operators unchanged. 2) Transform the operators: Atˆ ( ) →U† AˆU. Leave eigenvectors unchanged. (1) Schrödinger Picture: Everything we have done so far. Operators are stationary. Eigenvectors evolve under Ut( ,t0 ) . (2) Heisenberg Picture: Use unitary property of U to transform operators so they evolve in time. The wavefunction is stationary. This is a physically appealing picture, because particles move – there is a time-dependence to position and momentum. Let’s look at time-evolution in these two pictures: Schrödinger Picture We have talked about the time-development of ψ , which is governed by ∂ i ψ = H ψ (4.2) ∂t p. 34 in differential form, or alternatively ψ (t) = U( t, t0 ) ψ (t0 ) in an
    [Show full text]
  • Ph125: Quantum Mechanics
    Section 10 Classical Limit Page 511 Lecture 28: Classical Limit Date Given: 2008/12/05 Date Revised: 2008/12/05 Page 512 Ehrenfest’s Theorem Ehrenfest’s Theorem How do expectation values evolve in time? We expect that, as quantum effects becomes small, the fractional uncertainty in a physical observable Ω, given by q h(∆Ω)2i/hΩi, for a state |ψ i, becomes small and we need only consider the expectation value hΩi, not the full state |ψ i. So, the evolution of expectation values should approach classical equations of motion as ~ → 0. To check this, we must first calculate how expectation values time-evolve, which is easy to do: d „ d « „ d « „ dΩ « hΩi = hψ | Ω |ψ i + hψ |Ω |ψ i + hψ | |ψ i dt dt dt dt i „ dΩ « = − [− (hψ |H)Ω |ψ i + hψ |Ω(H|ψ i)] + hψ | |ψ i ~ dt i „ dΩ « = − hψ |[Ω, H]|ψ i + hψ | |ψ i ~ dt i fi dΩ fl = − h[Ω, H]i + (10.1) ~ dt Section 10.1 Classical Limit: Ehrenfest’s Theorem Page 513 Ehrenfest’s Theorem (cont.) If the observable Ω has no explicit time-dependence, this reduces to d i hΩi = − h[Ω, H]i (10.2) dt ~ Equations 10.1 and 10.2 are known as the Ehrenfest Theorem. Section 10.1 Classical Limit: Ehrenfest’s Theorem Page 514 Ehrenfest’s Theorem (cont.) Applications of the Ehrenfest Theorem To make use of it, let’s consider some examples. For Ω = P, we have d i hPi = − h[P, H]i dt ~ We know P commutes with P2/2 m, so the only interesting term will be [P, V (X )].
    [Show full text]
  • Ehrenfest's Theorem and Nonclassical States of Light
    GENERAL ARTICLE Ehrenfest’s Theorem and Nonclassical States of Light 1. Ehrenfest’s Theorem in Quantum Mechanics Lijo T George, C Sudheesh, S Lakshmibala and V Balakrishnan When it was ¯rst enunciated, Ehrenfest's The- orem provided a necessary and important link between classical mechanics and quantum me- chanics. Today, the content of the theorem is 1 2 understood to be a natural and immediate con- sequence of the equation of motion for opera- tors when quantum mechanics is formulated in 3 4 the Heisenberg picture. Nevertheless, the theo- rem leads to useful approximations when systems 1 Lijo T George got his with Hamiltonians of higher than quadratic or- Masters degree in physics from IIT Madras, and is der in the dynamical variables are considered. presently a doctoral student In this two-part article, we use it to provide a at RRI, Bangalore. His convenient illustration of the di®erences between research interests include so-called classical and nonclassical states of radi- quantum mechanics and ation. cosmology. 2C Sudheesh is with the 1. Paul Ehrenfest and the Quantum{Classical Department of Physics, Connection (IIST), Thiruvanan- thapuram. His research The Austrian-Dutch physicist Paul Ehrenfest (1880{1933) interests are nonlinear stands out among those who worked in the grey area dynamics, quantum optics of semiclassical physics straddling quantum mechanics and quantum information. (QM) and classical mechanics (CM). His work provided 3S Lakshmibala is with the a tangible correspondence between the quantum and Department of Physics, IIT Madras. Her research classical worlds, through the Ehrenfest theorem. This interests are dynamical theorem brought comfort to the practitioners of physics systems, quantum dynamics at a time in history when glimpses of the quantum world and quantum optics.
    [Show full text]
  • Classical from Quantum Arxiv:1803.04700V2 [Quant-Ph] 20 Apr 2018
    Classical from Quantum Timothy J. Hollowood Department of Physics, Swansea University, Swansea, SA2 8PP, United Kingdom E-mail: [email protected] Abstract: We consider the quantum-to-classical transition for macroscopic systems coupled to their environments. By applying Born's rule, we are led to a particular set of quantum trajectories, or an unravelling, that describes the state of the system from the frame of reference of the subsystem. The unravelling involves a branch dependent Schmidt decomposition of the total state vector. The state in the subsystem frame, the conditioned state, is described by a Poisson process that involves a non-linear deterministic effective Schr¨odingerequation interspersed with quantum jumps into orthogonal states. We then consider a system whose classical analogue is a generic chaotic system. Although the state spreads out exponentially over phase space, the state in the frame of the subsystem localizes onto a narrow wave packet that follows the classical trajectory due to Ehrenfest's Theorem. Quantum jumps occur with a rate that is the order of the effective Lyapunov exponent of the classical chaotic system and imply that the wave packet undergoes random kicks described by the classical Langevin equation of Brownian motion. The implication of the analysis is that this theory can explain in detail how classical mechanics arises from quantum mechanics by using only unitary evolution and Born's rule applied to a subsystem. arXiv:1803.04700v2 [quant-ph] 20 Apr 2018 1 Introduction Quantum mechanics is a remarkable fusion of a linear, deterministic theory where evolution involves unitary rotations of a vector in a Hilbert space, with stochastic evolution, in the form of Born's rule.
    [Show full text]
  • Equations of Motion for a Relativistic Wave Packet
    PRAMANA c Indian Academy of Sciences Vol. 78, No. 5 — journal of May 2012 physics pp. 679–685 Equations of motion for a relativistic wave packet LKOCIS1,2 1Julius Kruttschnitt Mineral Research Centre, The University of Queensland, Isles Road, Indooroopilly, Queensland 4068, Australia 2Present address: Peranga Court Unit 4, 43 Fifth Avenue, Sandgate, Queensland 4017, Australia E-mail: [email protected] MS received 15 April 2011; revised 23 November 2011; accepted 23 December 2011 Abstract. The time derivative of the position of a relativistic wave packet is evaluated. It is found that it is equal to the mean value of the momentum of the wave packet divided by the mass of the particle. The equation derived represents a relativistic version of the second Ehrenfest theorem. Keywords. Klein–Gordon equation; classical limit; Ehrenfest theorems. PACS Nos 03.65.−w; 03.65.Pm 1. Introduction In this article we consider a small relativistic wave packet that moves in external static fields and evaluate the time derivative of its position. The classical limit of the Klein–Gordon equation can be achieved by assuming that the Planck’s constant converges to zero [1] or by assuming that the coordinate and momentum dispersions of a relativistic wave packet are small [2]. At this point it is ne- cessary to give a brief review of what was achieved in ref. [2]. In ref. [2] substitution i ψ = R θ , exp (1.1) where R and θ are real functions, was applied in the Klein–Gordon formula for the particle probability density, namely i ∂ψ ∂ψ∗ 2 eφ ρ = ψ∗ − ψ − ψ∗ψ.
    [Show full text]
  • Validity of Ehrenfest's Theorem for Generalized Fields of Dyons
    Turkish Journal of Physics Turk J Phys (2016) 40: 244 { 255 http://journals.tubitak.gov.tr/physics/ ⃝c TUB¨ ITAK_ Research Article doi:10.3906/fiz-1512-11 Validity of Ehrenfest's theorem for generalized fields of dyons Gaurav KARNATAK∗, Praveen Singh BISHT, Om Prakash Singh NEGI Department of Physics, Kumaun University, S. S. J. Campus, Almora, Uttarakhand, India Received: 28.12.2015 • Accepted/Published Online: 18.04.2016 • Final Version: 01.12.2016 Abstract: The validity of Ehrenfest's theorem with its classical correspondence has been justified for the manifestly covariant equations of dyons. We have also developed accordingly the Lagrangian formulation for electromagnetic fields in a minimum coupled source giving rise to conserved current of dyons. Applying the Gupta subsidiary condition we have extended the validity of the Ehrenfest's theorem for U (1) × U (1) Abelian gauge theory of dyons. It is shown that the expectation value of the quantum equation of motion reproduces the classical equation of motion, which is the generalized form of Ehrenfest's theorem in quantum field theory. Key words: Dyons, Ehrenfest's theorem, Lagrangian, quantum field theory, electromagnetic fields 1. Introduction In classical mechanics the dynamical state of each particle is defined by its position and momentum, at a given instant. In quantum mechanics the dynamical state of the system is represented by its wave function [1]. The expectation values of displacement and momentum obey time evolution equations, which are analogous to those of classical mechanics. This result is called Ehrenfest's theorem [2]. The value of the quantum equation of motion taken with the states in the physical subspace reproduces the classical equations of motion.
    [Show full text]
  • Technische Universität München Department of Mathematics
    Technische Universität München Department of Mathematics Evolution of Angular Momentum Expectation in Quantum Mechanics Bachelor’s Thesis by Thomas Wiatowski supervisor : Prof. Dr. Gero Friesecke academic advisor : Dipl.-Math. Mario Koppen submission date : August 9, 2010 Ich erkläre hiermit, daß ich die Bachelorarbeit selbständig und nur mit den angegebe- nen Hilfsmitteln angefertigt habe. ............................................................. (München, der 09. August 2010), (Unterschrift) ii Acknowledgements I would like to thank all people who have helped and inspired me during my bachelor study. Especially I want to thank my academic advisor, Dipl.-Math. Mario Koppen, for his guidance during my bachelor thesis in the sixth semester at the Technische Universität München. He was always accessible and willing to help me with the research on my thesis. As a result, research and writing became smooth and rewarding for me. I was delighted to interact with Prof. Dr. Gero Friesecke by attending his class “Funk- tionalanalysis”, which gave me the mathematical background for this thesis and to have him as my supervisor. I am indebted to many of my colleagues who supported me, especially Friedrich Prade. His perpetual energy and enthusiasm in physics has motivated and helped me throughout my thesis. My deepest gratitude goes to my parents, Anna and Norbert, for their enduring love and support throughout my life, supporting me and my sister and sparing no effort to provide the best possible environment for us to grow up and attend school. Thank you. iii Abstract A mathematically rigorous derivation of the evolution of angular momentum expec- tation value is given, under assumptions which include Hamiltonians with potential wells.
    [Show full text]
  • On the Emergent Aspects of Quantum Mechanics in Relation to the Thermodynamics of Irreversible Processes
    1 ON THE EMERGENT ASPECTS OF QUANTUM MECHANICS IN RELATION TO THE THERMODYNAMICS OF IRREVERSIBLE PROCESSES AND EMERGENT GRAVITY 2 3 Candidato a doctor: Acosta Iglesias, Dagoberto. T´ıtulo: Aspectos emergentes de la mecanica´ cuantica:´ relacion´ con la termodinamica´ de los procesos irreversibles y con la gravedad emergente. Director: Fernandez´ De Cordoba´ Castella,´ Pedro Jose.´ Director: Ferrando Cogollos, Albert. Director: Isidro San Juan, Jose´ Mar´ıa. Entidad Academica´ Responsable: Departamento de Matematica´ Aplicada. Programa: Programa de Doctorado en Matematicas.´ 4 Abstract This PhD thesis elaborates on a proposal made by the Dutch theoretical physicist G. ’t Hooft (1999 Nobel prize in physics), to the effect that quantum mechanics is the emergent theory of some underlying, deterministic theory. According to this proposal, information–loss effects in the underlying deterministic theory lead to the arrangement of states of the latter into equivalence classes, that one identifies as quantum states of the emergent quantum mechanics. In brief, quantisation is dissipation, according to ’t Hooft. In our thesis we present two mechanisms whereby quantum mechanics is explicitly seen to emerge, thus explicitly realising ’t Hooft’s proposal. The first mechanism makes use of Verlinde’s approach to classical mechanics and general relativity via holographic screens. This technique, first presented in 2010 in order to understand the emergent nature of spacetime and gravity, is applied in our thesis to the case of quantum mechanics. The second mechanism presented to support ’t Hooft’s statement is based on a dictionary, also developed by the authors, between semiclassical quantum mechanics, on the one hand, and the classical theory of irreversible thermodynamics, on the other.
    [Show full text]
  • Arxiv:1609.07380V1 [Quant-Ph] 23 Sep 2016 Where Nntl Ag Oeta Nry Uha H Ia Et Ucinpo Function Delta Confi [1]
    Direct manifestation of Ehrenfest’s theorem in the infinite square well model Chyi-Lung Lin∗ Department of Physics, Soochow University, Taipei,Taiwan, R.O.C. Ehrenfest’s theorem in the infinite square well is up to now only manifested indirectly. The manifestation of this theorem is first done in the finite square well, and then consider the infinite square well as the limit of the finite well. For a direct manifestation, we need a more precise formula to describe the degree of infiniteness of the divergent potential energy. We show that the potential energy term term, V (x)Ψn(x), which is the product of the potential energy and the energy eigenfunction, is a well defined function which can be expressed in terms of Dirac delta functions. This means that the infinity in this model is not that vague but has obtained a specification. This results that expectation values can be calculated precisely and Ehrenfest’s thereom can be confirmed directly. PACS numbers: 03.65.-w, Quantum mechanics; 03.65.Ge, solution of wave equations: bound states Keywords: Infinite square well; Ehrenfest theorem; Time independent Schr¨odinger equation; Step function; Dirac delta function; Boundary condition; I. INTRODUCTION The infinite square well is a model using infinitely high potential barrier to confine particles inside a well. This is a basic model in quantum mechanics, and is a standard model for confining particles. We also have other models using infinitely large potential energy, such as the Dirac delta function potential. Belloni and Robinett have given a review on these models [1]. Curiously, for infinite square well, we still do not have a direct manifestation of Ehrenfest’s theorem in this funda- mental model.
    [Show full text]
  • Exact Time Evolution and Master Equations for the Damped Harmonic Oscillator Abstract
    CORE Metadata, citation and similar papers at core.ac.uk Provided by CERN DocumentExact Server Time Evolution and Master Equations for the Damped Harmonic Oscillator Robert Karrlein and Hermann Grabert Fakult¨at f¨ur Physik der Albert–Ludwigs–Universit¨at, Hermann-Herder-Str. 3, D-79104 Freiburg, Germany (August 26, 1996) Abstract Using the exact path integral solution for the damped harmonic oscillator it is shown that in general there does not exist an exact dissipative Liouville operator describing the dynamics of the oscillator for arbitrary initial bath preparations. Exact non-stationary Liouville operators can be found only for particular preparations. Three physically meaningful examples are exam- ined. An exact new master equation is derived for thermal initial conditions. Second, the Liouville operator governing the time-evolution of equilibrium correlations is obtained. Third, factorizing initial conditions are studied. Ad- ditionally, one can show that there are approximate Liouville operators inde- pendent of the initial preparation describing the long time dynamics under appropriate conditions. The general form of these approximate master equa- tions is derived and the coefficients are determined for special cases of the bath spectral density including the Ohmic, Drude and weak coupling cases. The connection with earlier work is discussed. Typeset using REVTEX 1 I. INTRODUCTION Recently the problem of the reduced dynamics of a quantum system in contact with a reservoir has gained renewed interest [1–3]. While the foundations of quantum dissipative processes were layed already in the sixties [4], this early work was mainly concerned with weakly damped systems and has relied on the Born and Markov approximations.
    [Show full text]