# Simulating quantum thermodynamics of a finite system and bath with variable temperature

## TL;DR

This paper models a finite quantum bath with variable temperature to simulate heat exchange and thermalization in small quantum systems, revealing finite-size effects and deviations from classical thermodynamic relations.

## Contribution

It introduces a finite, variable-temperature quantum bath with non-identical oscillators for simulating quantum thermodynamics, highlighting finite-size effects and deviations from classical behavior.

## Key findings

- Finite baths exhibit higher temperatures than infinite baths at the same energy.
- The temperature approaches the infinite bath limit as the number of oscillators increases.
- Significant deviations from classical energy-temperature relations are observed in small quantum systems.

## Abstract

We construct a finite bath with variable temperature for quantum thermodynamic simulations in which heat flows between a system $\mathcal{S}$ and the bath environment $\mathcal{E}$ in time evolution of an initial $\mathcal{SE}$ pure state. The bath consists of harmonic oscillators that are not necessarily identical. Baths of various numbers of oscillators are considered; a bath with five oscillators is used in the simulations. The bath has a temperature-like level distribution. This leads to definition of a system-environment microcanonical temperature $T_\mathcal{SE}(t)$ which varies with time. The quantum state evolves toward an equilibrium state which is thermal-like, but there is significant deviation from the ordinary energy-temperature relation that holds for an infinite quantum bath, e.g. an infinite system of identical oscillators. There are also deviations from the Einstein quantum heat capacity. The temperature of the finite bath is systematically greater for a given energy than the infinite bath temperature, and asymptotically approaches the latter as the number of oscillators increases. It is suggested that realizations of these finite-size effects may be attained in computational and experimental dynamics of small molecules.