In quantum mechanics separable states can be characterized as convex combinations of product states whereas non-separable states exhibit entanglement. Quantum entanglement has played a pivotal role in both theoretical investigations and practical applications within quantum information science. In this study, we explore the connection between product states and geometric structures, specifically manifolds and their associated geometric properties such as the first fundamental form (metric). We focus on the manifolds formed by the product states of M systems of N levels, examining the induced metric derived from the Euclidean metric. For elementary cases we will compute the Levi-Civita connection, and, where computationally tractable, the scalar curvature.
In quantum mechanics separable states can be characterized as convex combinations of product states whereas non-separable states exhibit entanglement. Quantum entanglement has played a pivotal role in both theoretical investigations and practical applications within quantum information science. In this study, we explore the connection between product states and geometric structures, specifically manifolds and their associated geometric properties such as the first fundamental form (metric). We focus on the manifolds formed by the product states of M systems of N levels, examining the induced metric derived from the Euclidean metric. For elementary cases we will compute the Levi-Civita connection, and, where computationally tractable, the scalar curvature.
Quantum entanglement has played a pivotal role in both theoretical investigations and practical applications within quantum information science. In this study, we explore the connection between entanglement and geometric structures, specifically manifolds and their associated geometric properties such as curvature. We focus on the manifolds formed by the product states of three qubits, examining the induced metric derived from the Euclidean metric, the Levi-Civita connection, and, where computationally tractable, the scalar curvature. Consequently, separable states can be characterized as convex combinations of points residing on these manifolds, whereas non-separable states exhibit entanglement.
We study the geometric properties of the manifold of states described as (uniform) matrix product states. Due to the parameter redundancy in the matrix product state representation, matrix product states have the mathematical structure of a (principal) fiber bundle. The total space or bundle space corresponds to the parameter space, i.e., the space of tensors associated to every physical site. The base manifold is embedded in Hilbert space and can be given the structure of a Kähler manifold by inducing the Hilbert space metric. Our main interest is in the states living in the tangent space to the base manifold, which have recently been shown to be interesting in relation to time dependence and elementary excitations. By lifting these tangent vectors to the (tangent space) of the bundle space using a well-chosen prescription (a principal bundle connection), we can define and efficiently compute an inverse metric, and introduce differential geometric concepts such as parallel transport (related to the Levi-Civita connection) and the Riemann curvature tensor.
This manuscript explores the Riemannian geometry of quantum state manifolds and its physical implications. We establish that quantum state manifolds, as submanifolds of Hilbert space, possess intrinsic Riemannian geometry characterized by a metric tensor derived from the Hilbert space inner product. This geometric framework reveals fundamental connections: the distance between nearby states scales with quantum fluctuations, curvature indicates state robustness, and geometric phase transitions occur in many-body systems. We review historical developments from early quantum geometry to modern quantum information geometry, and present methodologies for computing geometric quantities and their physical interpretations. Results include distance-fluctuation relations for coherent states, curvature calculations for SU(2) manifolds, and applications to quantum optimization and metrology via the quantum Fisher information metric. The discussion interprets geometric quantities in terms of uncertainty principles, state robustness, and quantum criticality, while outlining future research directions in quantum chaos-geometry correlations, non-associative geometry, and geometric quantum algorithm design. We conclude that Riemannian geometry provides a powerful framework for understanding and exploiting the structure of quantum state space, with applications across quantum information processing, metrology, and foundational physics.
Entanglement for pure bipartite states is most commonly quantified in a state-by-state manner to each pure state of a bipartite system a scalar quantity, such as the von Neumann entropy of a reduced density matrix. This provides a precise local characterization of how entangled a given state is. At the same time, this local description naturally invites a set of complementary, more global questions about the structure of the space of pure states: How abundant are the states with a given amount of entanglement within the full state space? Do the manifolds of constant entanglement exhibit distinct geometric regimes? These questions shift the focus from assigning an entanglement value to a single state to understanding the global organization and geometry of entanglement across the entire manifold of pure states.
In this work, we develop a geometric framework in which these questions become natural. We regard the projective Hilbert space of pure states, endowed with the Fubini-Study metric, as a Riemannian manifold and promote bipartite entanglement to a macroscopic functional on this manifold. Its level sets stratify the space of pure states into hypersurfaces of constant entanglement, and we define a geometric entanglement entropy as the log-volume of these hypersurfaces, weighted by the Fubini-Study gradient of entanglement. This quantity plays the role of a microcanonical entropy in entanglement space: it measures the degeneracy of a given entanglement value in the natural quantum geometry.
The framework is illustrated first in the simplest case of a single spin-1/2 and then for bipartite entanglement of spin systems, including a two-qubit example where explicit calculations can be carried out, along with a sketch of the extension to spin chains.
The geometrical description of a Hilbert space associated with a quantum system considers a Hermitian tensor to describe the scalar inner product of vectors which are now described by vector fields. The real part of this tensor represents a flat Riemannian metric tensor while the imaginary part represents a symplectic two-form. The immersion of classical manifolds in the complex projective space associated with the Hilbert space allows to pull-back tensor fields related to previous ones, via the immersion map. This makes available, on these selected manifolds of states, methods of usual Riemannian and symplectic geometry. Here, we consider these pulled-back tensor fields when the immersed submanifold contains separable states or entangled states. Geometrical tensors are shown to encode some properties of these states. These results are not unrelated with criteria already available in the literature. We explicitly deal with some of these relations.
The unification of quantum mechanics and geometric frameworks has historically relied on the formulation of state spaces as Riemannian or Kähler manifolds, wherein geometric properties such as the Fubini-Study metric and Berry curvature govern quantum evolution and fidelity. However, the physical consequences of these geometric structures have traditionally remained confined to abstract informational and phase-related phenomena, rarely translating into direct mechanical or dynamical forces within physical systems. This paper introduces the comprehensive theoretical framework of Quantum-State Curvature Pressure, designated as QSCP, which posits that the intrinsic curvature of quantum state spaces can act as a source of physical pressure. By defining a quantum-state metric tensor denoted as g_ij and introducing a scalar curvature of the quantum state space denoted as R_Q, we establish a thermodynamic and mechanical analogy where variations in state volume, denoted as V_state, induce a scalar pressure field. Specifically, the quantum-state curvature pressure is formulated as P_Q = -kappa * (partial R_Q / partial V_state). Consequently, alterations in quantum state paths and manifold deformations give rise to an emergent effective force defined as F_Q = -grad P_Q. This framework successfully bridges the chasm between abstract quantum geometry and tangible mechanical dynamics, offering profound implications for quantum control, quantum thermodynamics, and the fundamental interpretation of quantum-induced forces without relying on conventional environmental decoherence or external classical potentials.