The present thesis is concerned with the local quantum physics of relativistic particles and fields in three space-time dimensions, whose statistics is to be described by a representation of the braid group -- so-called plektons or, if the representation is Abelian, anyons. In particular the issue of the existence of free anyonic fields is addressed. In our context, these are operators affiliated with the `local´ field algebras and creating only single particle vectors from the vacuum. (Localizability here refers to regions extending to infinity in some spacelike direction.) Under a mild regularity condition for these fields, we can derive commutation relations which are not compatible with braid group statistics. Further, model-independent results concerning the PCT operator and the connection of spin and statistics are obtained. Assuming the observable algebra to satisfy the Bisognano-Wichmann theorem, a PCT theorem for plektons is derived. For anyons it is shown that the Bisognano-Wichmann property of the observable algebra lifts, in a `twisted´ version, to the field algebra. In particular, the PCT-operator coincides, up to a `twist operator´, with the modular conjugation of the field algebra associated to a standard wedge region. From this fact, the spin-statistics theorem for anyons is derived. The structure of the Hilbert space of scattering states of plektons and anyons is further clarified. It is known to be fixed by the relevant representations of the braid group and by the rules which determine the charges contained in the scattering states (the `fusion´ rules). We present Møller operators W+ and W- from the spaces of outgoing and incoming scattering states into a reference Hilbert space, which translate the physical ray representation of the Poincaré group on the spaces of scattering states into a canonical ray representation on the reference Hilbert space. A new result concerns the `incoming free PCT operator´, i.e. the product of the PCT operator with the S-matrix S = ( W+)* W- : It is exhibited how this model-independent operator acts in the reference Hilbert space. In the case of anyons, the fusion rules are trivial and the relevant representations of the braid group are classified by a finite set of complex numbers, namely the statistics parameters of the particles under consideration. Hence in the anyonic case the above results yield explicit formulae for the ray representation of the Poincaré group and for the incoming free PCT-operator. In view of the Bisognano-Wichmann property, we have thereby constructed the product of the Tomita operator associated to any given wedge region with the S-matrix, i.e. the `incoming free´ Tomita-operator of the anyonic field algebra.
The present thesis is concerned with the local quantum physics of relativistic particles and fields in three space-time dimensions, whose statistics is to be described by a representation of the braid group -- so-called plektons or, if the representation is Abelian, anyons. In particular the issue of the existence of free anyonic fields is addressed. In our context, these are operators affiliated with the local´ field algebras and creating only single particle vectors from the vacuum. (Localizability here refers to regions extending to infinity in some spacelike direction.) Under a mild regularity condition for these fields, we can derive commutation relations which are not compatible with braid group statistics. Further, model-independent results concerning the PCT operator and the connection of spin and statistics are obtained. Assuming the observable algebra to satisfy the Bisognano-Wichmann theorem, a PCT theorem for plektons is derived. For anyons it is shown that the Bisognano-Wichmann property of the observable algebra lifts, in a twisted´ version, to the field algebra. In particular, the PCT-operator coincides, up to a twist operator´, with the modular conjugation of the field algebra associated to a standard wedge region. From this fact, the spin-statistics theorem for anyons is derived. The structure of the Hilbert space of scattering states of plektons and anyons is further clarified. It is known to be fixed by the relevant representations of the braid group and by the rules which determine the charges contained in the scattering states (the fusion´ rules). We present Møller operators W+ and W- from the spaces of outgoing and incoming scattering states into a reference Hilbert space, which translate the physical ray representation of the Poincaré group on the spaces of scattering states into a canonical ray representation on the reference Hilbert space. A new result concerns the incoming free PCT operator´, i.e. the product of the PCT operator with the S-matrix S = ( W+)* W- : It is exhibited how this model-independent operator acts in the reference Hilbert space. In the case of anyons, the fusion rules are trivial and the relevant representations of the braid group are classified by a finite set of complex numbers, namely the statistics parameters of the particles under consideration. Hence in the anyonic case the above results yield explicit formulae for the ray representation of the Poincaré group and for the incoming free PCT-operator. In view of the Bisognano-Wichmann property, we have thereby constructed the product of the Tomita operator associated to any given wedge region with the S-matrix, i.e. the incoming free´ Tomita-operator of the anyonic field algebra.
Using the theory of fibre bundles, we provide several equivalent intrinsic descriptions for the Hilbert spaces of n ``free'' nonrelativistic and relativistic plektons in two space dimensions. These spaces carry a ray representation of the Galilei group and a unitary representation of the Poincaré group respectively. In the relativistic case we also discuss the situation where the braid group is replaced by the ribbon braid group.
We realize the physical N-anyon Hilbert spaces, introduced previously via unitary representations of the group of diffeomorphisms of the plane, as N-fold braided-symmetric tensor products of the 1-particle Hilbert space. This perspective provides a convenient Fock space construction for nonrelativistic anyon quantum fields along the more usual lines of boson and fermion fields, but in a braided category, and clarifies how discrete (lattice) anyon fields relate to anyon fields in the continuum. We also see how essential physical information is encoded. In particular, we show how the algebraic structure of the anyonic Fock space leads to a natural anyonic exclusion principle related to intermediate occupation number statistics, and obtain the partition function for an idealized gas of fixed anyonic vortices.
A central question in the theory of topological order asks whether the braid group representation BN carried by N non-Abelian anyons uniquely determines the modular data — the S and T matrices — of the underlying topological quantum field theory. We reconcile three lines of analysis into a single assessment. Structurally, we formalize the question as a reduction map Φ from modular tensor category data to projective braid group representations, whose kernel is the locus of potential counterexamples. Two rigorous finiteness results constrain the problem: braid representations from twisted quantum doubles of finite groups factor through finite groups, and the SO(N)2/O(N)2 braid representations are Gaussian with finite image. We then perform an explicit computation on the Ising anyon model — the canonical non-Abelian order of Majorana zero modes — reconstructing the topological spins θσ=eiπ/8, θψ=−1, the total quantum dimension D=2, and the full S matrix from the braid eigenvalues, with all arithmetic shown. We exhibit a Galois-conjugate deformation Ising(7) with identical fusion rules but distinct S matrix, and verify that its braid representation is inequivalent to the Ising representation on B3 by an explicit trace computation, while remaining indistinguishable at the level of fusion rules. We conclude that braid representations constrain but do not provably uniquely determine modular data in general: uniqueness can hold only under additional hypotheses, and the Majorana fusion-rule classification is not closed by braid data alone. The affirmative claim that braiding is a complete invariant is restricted to a narrower regime than sometimes asserted, and we document this divergence explicitly. ## 1. Introduction Non-Abelian anyons in two spatial dimensions are characterized by two layers of algebraic structure. The first is combinatorial: fusion rules Nabc specifying how anyon c Full text and updates: papers.qnfo.org/papers/braid-group-representations-and-modular-data-non-uniqueness-finite-images-and-th/
"Anyonic quantum statistics is an exotic phenomenon of identical particles in quantum mechanics. When particles are confined in two spatial dimensions, exchanges of identical particles can induce phase factors in the wave function of Abelian anyons or matrix valued transformations of the wave function of non-Abelian anyons. As a result, systems of anyons may have richer properties than those of bosons and fermions in three spatial dimensions. There is strong theoretical support for the existence of anyons in some engineered two-dimensional systems such as 2D electron gases, strongly correlated spin lattices and as edge modes of nanowires. In the future, anyons could be used in topological quantum computation to perform highly efficient information processing with very small error rates. The phases of matter described by dynamically interacting anyons have recently been studied in chains where anyons interact via Heisenberg-type exchange interactions. In this thesis, new kind of anyonic interactions are studied, induced purely by braiding during free evolution. Such interactions are of topological origin, and the information about the interactions is stored non-locally. A quantum walk model is used to study the effects of these braiding interactions on the dynamical behaviour of anyons. The anyonic quantum walk is a quasi-one-dimensional generalization of the discrete-time quantum walk which allows the simulation of anyonic dynamics analytically. The moving anyon and a chain of stationary anyons interact via braiding statistics, and the behaviour of the anyon is studied in three cases. Striking differences are found between particles with conventional boson or fermion statistics and non-Abelian anyonic statistics. The random walk is a dynamical model that describes the motion of a particle on a lattice. In physics, it is used to describe Brownian motion of fluids and gases. In such systems, energy transport is diffusive, and the order of the system approaches a highly mixed state without any information content. The quantum version of the random walk, the quantum walk, has nonintuitive properties. Generally, the information content in a unitarily evolving system is fixed, which leads to unexpected transport phenomena. An initially localized particle does not propagate diffusively, but escapes the starting point with ballistic speed. Most of our results for non-Abelian anyons use the Ising model anyon which is most likely to be measured in experiments. First, when each anyonic site is occupied by one Ising anyon, the propagation of the anyon becomes diffusive. More precisely, the variance of the spatial probability distribution of the particle depends linearly on the number of time steps. This is in stark contrast to bosons, fermions and Abelian anyons which propagate ballistically, and the variance depends linearly on the square of the number of time steps. The essential reason for this slowdown is that the non-Abelian anyons possess an extra degree of freedom called the fusion Hilbert space. This space can be viewed as an environment for the normal degrees of freedom of the particle, inducing decoherence in the quantum walk. This thesis opens the line for studies of this novel kind of decoherence mechanism in quantum walks. In the second case the study is extended to more general anyon models by losing some of the information about the history of the evolution. In this case the system is subject to decoherence, and the total system evolution is not unitary. The behaviour of the particle in this model is found to be diffusive for all the non-Abelian anyon models studied, while Abelian anyons behave ballistically also in this model. One peculiarity of quantum systems is their behaviour under disorder. Quantum mechanical particles moving in random local potentials are known to freeze and not move at all. Such a phenomenon is known as Anderson localization. Studies have shown that Anderson localization happens also in quantum walks with random spatial fluctuations in the coin parameters. In the third case, the transport properties of anyons are studied under topological randomness, allowing the occupations of anyons change between experiments. Bosons and fermions would propagate ballistically under such randomness, and Abelian anyons are shown to localize. The results show that non-Abelian anyons behave diffusively at short time scales, and it is argued that they do so in the long time limit as well. In all cases, non-Abelian anyons are shown to have very different dynamical properties than bosons, fermions and Abelian anyons. A possible simulation of the anyonic quantum walk in Fractional Quantum Hall systems is also discussed.
When leaving classical physics and entering the realm of quantum physics, there are many new concepts being introduced. One of the most fundamental ideas in quantum mechanics is that particles no longer have exact known positions, but instead expected values and prob- abilities. This leads to the phenomena of truly identical particles, since they no longer can be distinguished simply by their positions. An important property differentiating different kinds of particles is how a system behaves when two such identical particles are exchanged. Historically, this divided particles into bosons and fermions, corresponding to symmetry and antisymmetry under an exchange. However, in two dimensions a new type of particle appears. These particles are called anyons, and behave differently when particles are exchanged. Anyons can be further divided into abelian and non-abelian anyons, of which this thesis will focus on the latter. The ex- changes can then be represented by the fundamental group of the configuration space of the particles, and in two dimensions this fundamental group is the braid group. Using rotors from a Clifford algebra and studying excitations of Majorana fermions, this thesis will show a way to calculate the exchange matrices of non-abelian anyons, and their corresponding eigenvalues. Furthermore, suggestions on a generalization of this framework along with areas where it can be applied are given.
We investigate whether the braid group representation on N anyons uniquely determines the modular data (S and T matrices) of a non-Abelian topological order, and whether this framework classifies all fusion rules for Majorana zero modes (MZMs) in 2D topological superconductors. We show that the local N-anyon braid group representation constrains the modular data up to finite gauge ambiguity but does not uniquely fix the global S-matrix without specifying the underlying spin structure. For MZMs, the braid generators satisfy the Ising anyon R-matrix with the asymptotic Hecke relation σi2=1 (up to phase), which rigidly fixes the local F and R symbols of the Ising category. Consequently, the fusion rules are strictly of Ising type (σ×σ=1+ψ), precluding alternative non-Abelian fusion rules. The T-matrix is determined by topological spins, but the full S-matrix depends on the spin structure of the underlying manifold, reflecting the super-modular structure of the theory. ## 1. Introduction Topological quantum computation exploits the non-Abelian statistics of anyons—quasiparticle excitations in two-dimensional systems whose braiding operations are represented by non-commuting unitary matrices on a degenerate ground-state manifold [1]. A central question is whether the local braiding data of N anyons suffices to reconstruct the full modular data of the underlying topological order (TO), or whether additional global information is required. This question has direct physical consequences for 2D topological superconductors (TSCs) hosting Majorana zero modes (MZMs), where the anyonic excitations are vortices binding MZMs and the fermionic quasiparticle is a transparent (local) fermion [2,3]. The modular data of a TO consists of the S-matrix, which encodes the mutual braiding statistics of anyons, and the T-matrix, which encodes the topological spins. Together, these matrices form a projective representation of t