Its main purpose is to derive the laws of classical dissipation from the framework of quantum mechanics.
The typical approach to describe dissipation is to split the total system in two parts: the quantum system where dissipation occurs, and a so-called environment or bath into which the energy of the former will flow.
The way both systems are coupled depends on the details of the microscopic model, and hence, the description of the bath.
To include an irreversible flow of energy (i.e., to avoid Poincaré recurrences in which the energy eventually flows back to the system), requires that the bath contain an infinite number of degrees of freedom.
Notice that by virtue of the principle of universality, it is expected that the particular description of the bath will not affect the essential features of the dissipative process, as far as the model contains the minimal ingredients to provide the effect.
The simplest way to model the bath was proposed by Feynman and Vernon in a seminal paper from 1963.
[1] In this description the bath is a sum of an infinite number of harmonic oscillators, that in quantum mechanics represents a set of free bosonic particles.
In 1981, Amir Caldeira and Anthony J. Leggett proposed a simple model to study in detail the way dissipation arises from a quantum point of view.
[2] It describes a quantum particle in one dimension coupled to a bath.
The third term describes the bath as an infinite sum of harmonic oscillators with masses
The last term is a counter-term which must be included to ensure that dissipation is homogeneous in all space.
[3] To provide a good description of the dissipation mechanism, a relevant quantity is the bath spectral function, defined as follows: The bath spectral function provides a constraint in the choice of the coefficients
,[clarification needed] the corresponding classical kind of dissipation can be shown to be Ohmic.
As mentioned, the main idea in the field of quantum dissipation is to explain the way classical dissipation can be described from a quantum mechanics point of view.
To get the classical limit of the Caldeira–Leggett model, the bath must be integrated out (or traced out), which can be understood as taking the average over all the possible realizations of the bath and studying the effective dynamics of the quantum system.
To proceed with those technical steps mathematically, the path integral description of quantum mechanics is usually employed.
For so-called Markovian baths, which do not keep memory of the interaction with the system, and for Ohmic dissipation, the equations of motion simplify to the classical equations of motion of a particle with friction: Hence, one can see how Caldeira–Leggett model fulfills the goal of getting classical dissipation from the quantum mechanics framework.
The dissipative two-level system is a particular realization of the Caldeira–Leggett model that deserves special attention due to its interest in the field of quantum computation.
The aim of the model is to study the effects of dissipation in the dynamics of a particle that can hop between two different positions rather than a continuous degree of freedom.
This reduced Hilbert space allows the problem to be described in terms of 1/2-spin operators.
Notice that in this model the counter-term is no longer needed, as the coupling to
In the context of quantum computation, it represents a qubit coupled to an environment, which can produce decoherence.
In the study of amorphous solids, it provides the basis of the standard theory to describe their thermodynamic properties.
The dissipative two-level system represents also a paradigm in the study of quantum phase transitions.
For a critical value of the coupling to the bath it shows a phase transition from a regime in which the particle is delocalized among the two positions to another in which it is localized in only one of them.
The transition is of Kosterlitz–Thouless kind, as can be seen by deriving the renormalization group flow equations for the hopping term.
A different approach to describe energy dissipation is to consider time dependent Hamiltonians.
[4] However, the quantum mechanical state of the system stays pure, thus such an approach can not describe dephasing unless a subsystem is chosen and the reduced density matrix of this open quantum system is analyzed.
There a light pulse (described by a time dependent semi-classical Hamiltonian) can change the energy in the system by stimulated absorption or emission.