Generalised state space geometry in Hermitian and non-Hermitian quantum systems

TL;DR

This paper generalizes the geometric framework of quantum states by modifying the Hermitian tensor structure, enabling dual connections and classifying tensors in non-Hermitian quantum systems, with applications to quantum optimization.

Contribution

It introduces a family of dual connections in quantum geometry, classifies tensor types in non-Hermitian systems, and explores implications for quantum natural gradient methods.

Findings

Constructed dual connections related to the Fubini-Study tensor.

Classified four tensor types in non-Hermitian quantum dynamics.

Applied the formalism to quantum natural gradient descent.

Abstract

One of the key features of information geometry in the classical setting is the existence of a metric structure and a family of connections on the space of probability distributions. The uniqueness of the Fisher--Rao metric and the duality of these connections is at the heart of classical information geometry. However, these features do not carry over straightforwardly to quantum systems, where a Hermitian inner product structure on the Hilbert space induces a metric on the complex projective space of pure states -- the Fubini-Study tensor, which is preserved under the unitary evolution. In this work, we explore how modifying the Hermitian tensor structure on the projective space may affect the geometry of pure quantum states, and whether such generalisations can be used to define dual connections with a direct correspondence to classical probability distribution functions, modified by the presence of a non-trivial phase. We show that it is indeed possible to construct a family of connections that are dual to each other in a generalised sense with respect to the real-valued sector of the Fubini--Study tensor. Using this biorthogonal formalism, we systematically classify the four types of tensors that can arise when the dynamics of a quantum system are governed by a non-Hermitian Hamiltonian, identifying both the complex-valued metric and the Berry curvature. Finally, we elucidate the role of the metric in a quantum natural gradient descent optimisation problem, generalised to the non-Hermitian case for a suitable choice of cost function.