Quantum Horn's lemma, finite heat baths, and the third law of thermodynamics

TL;DR

This paper explores quantum state transitions with small heat baths, proving a quantum Horn's lemma for degenerate systems and illustrating the third law of thermodynamics for non-degenerate systems, highlighting quantum advantages.

Contribution

It establishes a quantum version of Horn's lemma for degenerate Hamiltonians and demonstrates the necessity of unbounded heat baths for certain transitions in non-degenerate systems.

Findings

Quantum Horn's lemma holds for fully degenerate Hamiltonians.

Some state transitions require unbounded heat baths in non-degenerate systems.

Quantum operations outperform classical ones with finite heat baths.

Abstract

Interactions of quantum systems with their environment play a crucial role in resource-theoretic approaches to thermodynamics in the microscopic regime. Here, we analyze the possible state transitions in the presence of "small" heat baths of bounded dimension and energy. We show that for operations on quantum systems with fully degenerate Hamiltonian (noisy operations), all possible state transitions can be realized exactly with a bath that is of the same size as the system or smaller, which proves a quantum version of Horn's lemma as conjectured by Bengtsson and Zyczkowski. On the other hand, if the system's Hamiltonian is not fully degenerate (thermal operations), we show that some possible transitions can only be performed with a heat bath that is unbounded in size and energy, which is an instance of the third law of thermodynamics. In both cases, we prove that quantum operations yield an advantage over classical ones for any given finite heat bath, by allowing a larger and more physically realistic set of state transitions.