Fundamental Limitation on Cooling under Classical Noise

TL;DR

This paper proves a fundamental limit on cooling quantum systems under classical noise, showing that cooling via dynamical control alone cannot reduce a system's population of excited states unless it starts in a pure state.

Contribution

The authors establish a rigorous theorem that constrains the maximum achievable cooling of open quantum systems under classical noise, assuming initial factorization.

Findings

Cooling cannot increase the population of any eigenstate under classical noise.

A system cannot be cooled to a pure state through dynamical control alone.

The theorem provides a fundamental bound on open system cooling processes.

Abstract

We prove a general theorem that the action of arbitrary classical noise or random unitary channels can not increase the maximum population of any eigenstate of an open quantum system, assuming initial system-environment factorization. Such factorization is the conventional starting point for descriptions of open system dynamics. In particular, our theorem implies that a system can not be ideally cooled down unless it is initially prepared as a pure state. The resultant inequality rigorously constrains the possibility of cooling the system solely through temporal manipulation, i.e., dynamical control over the system Hamiltonian without resorting to measurement based cooling methods.