In this paper a quantization procedure is described based on the requirement that the configuration of a quantum system performs a diffusion making stationary the mean classical action with respect to variations of the sample paths. In the path-wise calculus of variations contributions of higher order than in the usual Ito^'s stochastic calculus are analyzed in the estimate of infinitesimal increments of the configuration and of its variations.
In this paper a quantization procedure is described based on the requirement that the configuration of a quantum system performs a diffusion making stationary the mean classical action with respect to variations of the sample paths. In the path-wise calculus of variations contributions of higher order than in the usual Ito^'s stochastic calculus are analyzed in the estimate of infinitesimal increments of the configuration and of its variations.
We study the stochastic quantization of finite dimensional systems via path-wise calculus of variations with the mean discretized classical action in the general case of electromagnetic interactions. We show that there exists a unique choice of the mean discretized action corresponding to the minimal classical magnetic coupling and derive the general equations of motion by means of a path-wise stochastic calculus of variations. In the case of purely scalar interactions, the total mean energy of the system (which gives the usual quantum mechanical expectation of the Hamiltonian in the canonical limit) works as a Lyapunov functional and the system relaxes on the canonical solutions, represented by Nelson's diffusions, which act as an attracting set. We show that, in presence of a minimal magnetic coupling, the mean energy is no longer a Lyapunov functional. We construct for a simple example, a new Lyapunov functional, and we show that the system can reach the dynamical equilibrium also by absorbing energy from the external magnetic field.
Within the framework of stochastic calculus of variations for time-symmetric semimartingales X(t,ω), we consider two different stochastic versions of Maupertuis’ least action principle, in Lagrangian and Hamiltonian terms. The general results are applied to classical statistical mechanics, where they coincide with those of classical calculus of variations, and to Nelson’s stochastic mechanics, an approach to quantum mechanics where a time-symmetric semimartingale represents the position of a particle and the dynamics is expressed by a stochastic version of Hamilton’s principle of least action. Some historic examples of old quantum theory are discussed.
The stochastic quantization of dissipative systems is discussed. It is shown that in order to stochastically quantize a system with dissipation, one has to restrict the Fourier transform of the space-time variable to the positive half domain in the complex plane. This breaks the time-reversal invariance, which manifests in the formulation through the resulting noninvariant forms for the propagators. The relation of the stochastic approach with the Caldeira and Leggett path-integral method is also analyzed. † tzani@vana.physto.se; address after September 1994: Imperial College, Physics Dept., Prince Consort Road, London SW72BZThe quantization of a classical system may, in general, be hindered by several obstacles, such as nonrenormalizability, topological obstructions, anomalies etc. There exists, moreover, a whole class of systems for which there is no available method of quantization, namely non-hamiltonian systems. The quantization of systems with no lagrangian or hamiltonian formulation, apart from the wellknown examples of systems with dissipation, includes several interesting physical problems, such as the Bargmann-Wigner higher spin field equations [1] and field theories of fundamental anyons [2]. For the Bargmann-Wigner system of equations there is no action which directly reproduces this set of equations, although the quantization for higher spin systems has been done using more complicated and indirect ways [3]. The case of anyonic systems is known to be unsolved. The most common example of a non-hamiltonian system is, nevertheless, the case of a particle on the line with friction. Its equation of motion is m¨x + γ ˙x + dV dx = 0 (1) where overdot denotes time derivative and V (x) is the potential of the particle. If we assume an action of the form S = f(x, ˙x, t) dt (2) for the above equation, the variation of the action gives δS δx = dt ( ∂f ∂ ∂f ∂ ∂f
The stochastic variational principle which applies to diffusions and extremises the mean value of the classical action is generalised to include non-Markovian diffusions. A whole family of different stochastic actions, labelled by a time-dependent parameter, is obtained and all of them are shown to lead to the Shrodinger equation. As a result, the well known degeneracy of the class of stochastic processes which correspond to a single quantum state is recovered. It is also related to an arbitrariness in the choice of the noise which can occur in two places: in the definition of the noise, through the time-dependent parameter, and in the auto-correlations of the noise which, although constant during the variational method, remain undetermined. The whole scheme is given a covariant form.
The basic integrator processes of quantum stochastic calculus, namely, creation, conservation, and annihilation, are introduced in the Hilbert space of square integrable Brownian functionals. Stochastic integrals with respect to these processes and a quantum Itô’s formula are described. As an application two examples of quantum stochastic differential equations are discussed. A continuous time version of Stinespring’s theorem on completely positive maps in C *-algebras is exploited to formulate the notion of a quantum Markov process and indicate how classical Markov processes are woven into the fabric of the Schodinger-Heisenberg dynamics of quantum theory.