Geometric phases of mixed quantum states: A comparative study of interferometric and Uhlmann phases

TL;DR

This paper compares two geometric phases of mixed quantum states, the interferometric and Uhlmann phases, highlighting their differences in finite-temperature transitions in two- and three-level systems.

Contribution

It provides a detailed comparison of the interferometric and Uhlmann phases, including exact solutions and conditions for their accumulation in quantum systems.

Findings

Interferometric phase shows finite-temperature transitions only in three-level systems.

Uhlmann phase exhibits finite-temperature transitions in both two- and three-level systems.

Comparison reveals the rich topological physics of mixed quantum states.

Abstract

Two geometric phases of mixed quantum states, known as the interferometric phase and Uhlmann phase, are generalizations of the Berry phase of pure states. After reviewing the two geometric phases and examining their parallel-transport conditions, we specify a class of cyclic processes that are compatible with both conditions and therefore accumulate both phases through their definitions, respectively. Those processes then facilitate a fair comparison between the two phases. We present exact solutions of two-level and three-level systems to contrast the two phases. While the interferometric phase exhibits finite-temperature transitions only in the three-level system but not the two-level system, the Uhlmann phase shows finite-temperature transitions in both cases. Thus, using the two geometric phases as finite-temperature topological indicators demonstrates the rich physics of topology of mixed states.