OverviewThe fractional quantum Hall effect is a collective quantum phenomenon of two-dimensional electrons placed in a strong perpendicular magnetic field (B ∼ 1 − 10 Tesla) and at very low temperature (T ∼ 50 − 500 mK). It manifests itself in the measure of the transverse (σ xy ) and longitudinal (σ xx ) conductances as functions of the magnetic field. The Hall conductance σ xy shows the characteristic step-like behavior: at each step (plateau), it is quantized in rational multiples of the quantum unit of conductance e 2 /h: σ xy = ν e 2 h ; furthermore, at the plateaus centers the longitudinal resistance vanishes. These features are independent on the microscopic details of the samples; in particular, the quantization of σ xy is very accurate, with experimental errors of the order 10 −8 Ω. The rational number ν is the filling fraction of occupied one-particle states at the given value of magnetic field.The fractional Hall effect is characterized by a non-perturbative gap due to the Coulomb interaction among electrons in strong fields. The microscopic theory is impracticable, besides numerical analysis; thus, effective theories, effective field theories in particular, are employed to study these systems. The first step was made by Laughlin, who proposed the trial ground state wave function for fillings ν = , · · · describing the main physical properties of the Hall effect. One general feature is that the electrons form a fluid with gapped excitations in the bulk, the so-called incompressible fluid. In a finite sample, the incompressible fluid gives rise to edge excitations, that are gapless and responsible for the conduction properties. Therefore the low-energy effective field theory should describe these edge degrees of freedom.The Laughlin theory revealed that the excitations are "anyons", i.e. quasiparticles with fractional values of charge and exchange statistics. The latter is a possibility in bidimensional quantum systems, where the statistics of particles is described by the braid group instead of the permutation group of higher-dimensional systems. Each filling fraction corresponds to a different state of matter that is characterized by specific fractional values of charge and statistics. The braiding of quasiparticles can also occur for multiplets of degenerate excitations, by means of multi-dimensional unitary transformations; in this case, called non-Abelian fractional statistics, two different braidings do not commute. The corresponding non-Abelian quasiparticles are degenerate for fixed positions and given quantum numbers.Non-Abelian anyons are candidate for implementing topological quantum computations according to the proposal by Kitaev and others. Since Anyons are non-perturbative collective excitations, they are less affected by decoherence due to local disturbances.
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IVExperimental observation of non-Abelian statistics is a present challenge.The topological fluids, such as the quantum Hall fluid can be described by the ChernSimon effective field theory in the low-energy long-range limit. Th...