Standard quantum circuit descriptions represent processes by linear wiring and sequential composition, while concurrency is typically reduced to commutation checks and tensor factorisation. This manuscript introduces Quantum Higher-Dimensional Automata, a geometric model in which executions are directed paths on a cubical complex and quantum coherence is carried by structured projective unitary data on cells. The construction defines a quantum cubical set whose vertices represent configurations and whose higher cells represent coherent multi-parameter evolutions that witness compatibility of concurrent actions. Within this framework, compatibility is promoted from an algebraic condition on operators to a directed homotopy invariant condition on executions, and geometric phase is interpreted as curvature on execution surfaces when smooth lifts exist. The model yields a homotopy based compatibility criterion and supplies obstruction based notions of deadlock grounded in phase holonomy on cubical loops. The aim is an operational semantics in which concrete families of quantum controls induce computable invariants on an explicit concurrency geometry.
Standard quantum circuit descriptions represent processes by linear wiring and sequential composition, while concurrency is typically reduced to commutation checks and tensor factorisation. This manuscript introduces Quantum Higher-Dimensional Automata, a geometric model in which executions are directed paths on a cubical complex and quantum coherence is carried by structured projective unitary data on cells. The construction defines a quantum cubical set whose vertices represent configurations and whose higher cells represent coherent multi-parameter evolutions that witness compatibility of concurrent actions. Within this framework, compatibility is promoted from an algebraic condition on operators to a directed homotopy invariant condition on executions, and geometric phase is interpreted as curvature on execution surfaces when smooth lifts exist. The model yields a homotopy based compatibility criterion and supplies obstruction based notions of deadlock grounded in phase holonomy on cubical loops. The aim is an operational semantics in which concrete families of quantum controls induce computable invariants on an explicit concurrency geometry.
Quantum holonomies are investigated in different contexts. A geometric phase is proposed for decomposition dependent evolution, where each component of a given decomposition of a mixed state evolves independently. It is shown that this geometric phase only depends on the path traversed in the space of decompositions. A holonomy is associated to general paths of subspaces of a Hilbert space, both discrete and continuous. This opens up the possibility of constructing quantum holonomic gates in the open path setting. In the discrete case it is shown that it is possible to associate two distinct holonomies to a given path. Interferometric setups for measuring both holonomies are provided. It is further shown that there are cases when the holonomy is only partially defined. This has no counterpart in the Abelian setting. An operational interpretation of amplitudes of density operators is provided. This allows for a direct interferometric realization of Uhlmann's parallelity condition, and the possibility of measuring the Uhlmann holonomy for sequences of density operators. Off-diagonal geometric phases are generalized to the non-Abelian case. These off-diagonal holonomies are undefined for cyclic evolution, but must contain members of non-zero rank if all standard holonomies are undefined. Experimental setups for measuring the off-diagonal holonomies are proposed. The concept of nodal free geometric phases is introduced. These are constructed from gauge invariant quantities, but do not share the nodal point structure of geometric phases and off-diagonal geometric phases. An interferometric setup for measuring nodal free geometric phases is provided, and it is shown that these phases could be useful in geometric quantum computation. A holonomy associated to a sequence of quantum maps is introduced. It is shown that this holonomy is related to the Uhlmann holonomy. Explicit examples are provided to illustrate the general idea.
This preprint presents a unified holonomy-first, invariant-only simulator architecture for loop-based quantum operations, integrating two complementary components:(i) an operational framework for discovering, diagnosing, and compiling native holonomic operations from closed-loop executions, and(ii) a minimal semantic language that characterizes what such loop-based operations mean when only conjugacy-invariant data are accessible. The simulator treats holonomy words—compositions of calibrated loop primitives—as the fundamental operational objects, rather than Hamiltonians, generators, or gauge-fixed operators. All semantics are defined at the level of conjugacy classes, represented operationally by invariant fingerprints such as trace moments or overlap-derived scalars. This invariant-first stance enables robust workflows that avoid full operator reconstruction, tomography, or fixed-basis assumptions. On this substrate, the simulator introduces a geometric compilation pipeline that: Extracts interaction structure from short loop compositions using commutator and nested-commutator diagnostics; Organizes noncommutativity and curvature information into a gauge-invariant interaction spectrum; Promotes reproducible, scale-consistent loop words into reusable native operations. A central contribution of the unified framework is a theorem-level semantic obstruction: the default single-system semantic tier is generically incomplete in the presence of entanglement-sensitive loop effects. This is demonstrated by introducing entanglement holonomy, a word-level invariant accessible via joint return amplitudes of entangled probes yet invisible to any reconstruction based solely on single-system class functions or mode-preserving data. As a consequence, the simulator treats the effective representation on which holonomies act as provisional rather than fixed. When invariant closure and scaling tests fail at a given tier, the simulator is forced to lift representation (e.g., from single-particle to multi-particle or encoded subspaces) as an operational necessity rather than a modeling choice. An explicit minimal example of an irreducible two-particle holonomic gate is provided within a quadratic-mode backend. The simulator does not attempt to unify classical and quantum dynamics or to provide a theory of quantum gravity. Instead, it supplies a shared holonomy-based semantic layer in which classical geometric transport and genuinely quantum effects (including entanglement) can be jointly represented and operationally distinguished using the same invariant diagnostics. This unified manuscript synthesizes and extends two previously developed components—an operational framework (GQF, DOI: 10.5281/zenodo.18303288) and a semantic foundation with an obstruction result (MHQL, DOI: 10.5281/zenodo.18332083)—to expose a structural point not apparent when they are treated separately: invariant-first compilation and representation lifting are linked by a principled semantic incompleteness theorem. The result is a rare simulator architecture that is holonomy-driven, gauge-invariant by construction, and representation-adaptive when compelled by invariant evidence.
Quantum entanglement is a basic resource of quantum theory, but its usual definition assumes a fixed decomposition into subsystems. We develop an algebro--geometric framework to address quantum entanglement. For the Severi--Brauer scheme SB(A)→X of an Azumaya algebra and a factorization type d, we prove that a global locus of product states exists exactly when the associated torsor under PGLn reduces to the stabilizer Gd of the Segre variety. Thus entanglement in families is measured by a geometric obstruction to global subsystem structure.
Beyond this reduction criterion, we identify the cohomology that carries computable entangling obstructions. After determinant and incidence data L⊂R⊂K are chosen, the relevant classes lie in the mapping cone relative cohomology E∙(BL→BR). Their absolute images on X, modulo classes induced from BK, give determinant entangling obstructions beyond the Brauer class. We construct the moduli of subsystem structures as the quotient P/Gd and realize it as a locally closed locus in the relative Hilbert scheme, with a compactification by degenerations of product state loci. In the bipartite case, a chosen subsystem structure globalizes the Schmidt rank stratification to a flat filtration with base change compatible resolutions and fiberwise constant numerical invariants. Examples show that reducibility can depend on the underlying torsor, not only on the Brauer class, with an interpretation by entangling monodromy.
Standard quantum information is formulated over complex Hilbert spaces, where the pure state space of a single qubit is geometrically encoded by the first Hopf fibration S3→S2. Beyond this familiar setting, the normed division algebras C, H and O provide a hierarchy of increasingly rich algebraic and topological structures that has motivated several extensions of quantum theory and quantum computation. This review synthesizes the literature connecting quaternionic and octonionic frameworks, Clifford algebras, spinors, projective spaces and Hopf fibrations. We emphasize a central conceptual point that is often blurred in the literature: the second and third Hopf fibrations play two distinct roles, namely, (i) as kinematical descriptions of hypercomplex single-particle state spaces such as the quaternionic projective line HP1≃S4, and (ii) as entanglement-sensitive descriptions of multi-qubit complex systems, especially two- and three-qubit Hilbert spaces. On the algebraic side, Clifford and geometric algebras provide a natural language for rotations, spinors, and gate synthesis, while quaternionic Hilbert modules furnish a mathematically consistent extension of standard qubit kinematics and dynamics. By contrast, octonionic models face major obstructions due to non-associativity, which affects inner products, tensor products, spectral theory and circuit composition. We therefore distinguish carefully between robust results, partial constructions and speculative directions. The outcome is a unified geometric review of hypercomplex quantum information, together with a map of open problems at the interface of topology, noncommutative algebra, and quantum computation.
Quantum entanglement is a fundamental feature of quantum mechanics whose underlying physical interpretation remains an active topic of research. In this work, we develop a geometric framework in which physical states are defined on a higher-dimensional manifold, while observable states arise from a coherent projection onto spacetime. We show that this projection acts as a non-injective mapping on the underlying state space, generically transforming separable higher-dimensional states into non-factorizable states in the observable domain. Within this construction, quantum correlations emerge as a consequence of interference along a compact dimension. Under conditions of phase coherence, the standard quantum predictions are recovered. More generally, incomplete phase cancellation leads to a well-defined deviation from the usual correlation structure, introducing an additional angular dependence beyond the relative measurement settings. The framework is fully compatible with the standard quantum formalism at the effective level, while extending it through experimentally testable predictions. Moreover, the presence of such additional angular contributions provides a potential signature of projection-induced effects. These results suggest that quantum entanglement may be interpreted as an emergent phenomenon associated with geometric projection, in which effective nonlocal behavior arises from dimensional reduction.
Quantum computations operate in the quantum world. For their results to be useful in any way, there is an intrinsic necessity of cooperation and communication controlled by the classical world. As a consequence, full formal descriptions of algorithms making use of quantum principles must take into account both quantum and classical computing components and assemble them so that they communicate and cooperate. This paper aims at defining a high level language allowing the description of classical and quantum programming, and their cooperation. Since process algebras provide a framework to model cooperating computations and have well defined semantics, they have been chosen as a basis for this language. Starting with a classical process algebra, this paper explains how to transform it for including quantum computation. The result is a quantum process algebra with its operational semantics, which can be used to fully describe quantum algorithms in their classical context.