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Journal of Physics A Mathematical and General

Low-temperature properties in the stochastic quantisation of the Brownian oscillator

S. De Martino, Silvio De Siena, Patrizia Ruggiero, Marco Zannetti

Published Jul 7, 1989• 3 citations
Publisher

Abstract

The microscopic model of Ullersma (1966) for a harmonic oscillator in contact with a thermal reservoir is quantised in the framework of Nelson's stochastic mechanics. Eliminating the degrees of freedom of the thermal reservoir the stochastic process of the quantum Brownian oscillator is obtained as the sum of the thermal contribution and the quantum (zero-temperature) contribution. The properties of the quantum fluctuations are studied in detail and the equation of motion is derived, obtaining the first example of a non-Markovian Nelson process.

DOI
10.1088/0305-4470/22/13/043

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The microscopic model of Ullersma (1966) for a harmonic oscillator in contact with a thermal reservoir is quantised in the framework of Nelson's stochastic mechanics. Eliminating the degrees of freedom of the thermal reservoir the stochastic process of the quantum Brownian oscillator is obtained as the sum of the thermal contribution and the quantum (zero-temperature) contribution. The properties of the quantum fluctuations are studied in detail and the equation of motion is derived, obtaining the first example of a non-Markovian Nelson process.
We generalize the classical theory of Brownian motion so as to reckon with non-Markovian effects on both Klein-Kramers and Smoluchowski equations. For a free particle and a harmonic oscillator, it is shown that such non-Markovian effects account for the differentiability of the Brownian trajectories as well as the breakdown of the energy equipartition of statistical mechanics at short times in some physical situations. This non-Markovian approach is also extended to look at anomalous diffusion. Next, we bring in the dynamical-quantization method for investigating open quantum systems, which does consist in quantizing the classical Brownian motion starting directly from our non-Markovian Klein-Kramers and Smoluchowski equations, without alluding to any model Hamiltonian. Accordingly, quantizing our non-Markovian Klein-Kramers in phase space gives rise to a non-Markovian quantum master equation in configuration space, whereas quantizing our non-Markovian Smoluchowski equation in configuration space leads to a non-Markovian quantum Smoluchowski equation in phase space. In addition, it is worth noticing that non-Markovian quantum Brownian motion takes place in presence of a generic environment (e.g. a non-thermal quantum fluid). As far as the special case of a heat bath comprising of quantum harmonic oscillators is concerned, a non-Markovian Caldeira-Leggett master equation and a thermal quantum Smoluchowski equation are derived and extended to bosonic and fermionic heat baths valid for all temperatures.
The theory of Brownian motion of a quantum oscillator is presented. The equation of motion for the density operator of the oscillator is obtained by a model Hamiltonian which characterizes the interaction between this oscillator and a reservoir. By applying iteration methods the equation of motion is solved and the time development of the density operator is obtained. The solution is then used to write the normally ordered generating functional for the field of the oscillator executing Brownian motion. This field appears to be the superposition of a thermal field in equilibrium with the reservoir, a chaotic field, and a field statistically similar to the one before the interaction. Next, it is shown that for large times the oscillator approaches equilibrium; moreover, it is shown that, if the reservoir is at zero temperature and initially the state of the oscillator is a coherent state, then it remains in a coherent state for later time. In order to obtain by differentiation two-time modal densities, a suitable two-time generating functional is introduced. This functional allows two-time averages to be computed from one-time averages before the interaction. Finally, some time-dependent statistical properties of the oscillator are studied.
Quantum Brownian motion is a frontier area of research in quantum optics (Louisell 1973; Sargent, Scully, and Lamb 1974; Meystre and Sargent 1991), condensed matter (Grabert, Schramm, and Ingold 1988; Weiss 1999), and chemical physics (Hänggi, Talkner, and Borkovec 1990; Zwanzig 1973) over the last several decades. The fundamental paradigm is the irregular movement of a particle in a thermal bath kept at finite temperature. The bath constitutes many, in principle, infinite degrees of freedom interacting with the particle. In general, the bath is considered to be a set of harmonic oscillators of varied frequency ranges and is therefore bosonic in character. The particle follows the reduced stochastic dynamics obtained after appropriate elimination of these reservoir degrees of freedom under suitable approximation schemes. The dynamical evolution is characterized by quantum dissipation and bosonic noise related to each other by fluctuation-dissipation (FD) relation, ensuring a detailed balance in the system. This chapter concerns a system-reservoir model where the reservoir is constituted by a set of two-level atoms or spin-1/2 systems.
The influence of a finite bath correlation time and the bath temperature on the onset of a non-Markovian behaviour in the relaxation of a harmonic oscillator linearly coupled to a coloured thermal bath is analysed. A fully quantum mechanical treatment of the time autocorrelation function of the oscillator-bath interaction is made. Special attention is paid to the temperature dependence. In the low-temperature regime a non-exponential time decay of the mean number of quanta is found even though the correlation time associated with the oscillator-bath interaction is zero.