Anyon

Template:Statistics (stat. mech.) In mathematics and physics, an anyon is a type of particle that occurs only in two-dimensional systems. It is a generalization of the fermion and boson concept.

In physics

This mathematical concept becomes useful in the physics of two-dimensional systems such as sheets of graphene or the quantum Hall effect.

In space of three or more dimensions, particles are restricted to being fermions or bosons, according to their statistical behaviour. Fermions respect the so-called Fermi-Dirac statistics while bosons respect the Bose-Einstein statistics. In the language of quantum physics this is formulated as the behavior of multiparticle states under the exchange of particles. This is in particular for a two-particle state (in Dirac notation):

$\left|\psi _{1}\psi _{2}\right\rangle =\pm \left|\psi _{2}\psi _{1}\right\rangle$

(where the first entry in $\left|\dots \right\rangle$ is the state of particle 1 and the second entry is the state of particle 2. So for example the left hand side is read as "Particle 1 is in state $\psi _{1}$ and particle 2 in state $\psi _{2}$"). Here the "+" corresponds to both particles being bosons and the "−" to both particles being fermions (composite states of fermions and bosons are not possible).

In two-dimensional systems, however, quasiparticles can be observed which obey statistics ranging continuously between Fermi-Dirac and Bose-Einstein statistics, as was first shown by Jon Magne Leinaas and Jan Myrheim of the University of Oslo in 1977. In our above example of two particles this looks as follows:

$\left|\psi _{1}\psi _{2}\right\rangle =e^{i\,\theta }\left|\psi _{2}\psi _{1}\right\rangle$

With "i" being the imaginary unit from the calculus of complex numbers and $\theta$ a real number. Recall that $|e^{i\theta }|=1$ and $e^{2i\pi }=1$ as well as $e^{i\pi }=-1$. So in the case $\theta =\pi$ we recover the Fermi-Dirac statistics (minus sign) and in the case $\theta =2\pi$ the Bose-Einstein statistics (plus sign). In between we have something different. Frank Wilczek coined the term "anyon" to describe such particles, since they can have any phase when particles are interchanged.

Topological basis

In more than two dimensions, the spin-statistics connection states that any multiparticle state has to obey either Bose-Einstein or Fermi-Dirac statistics. This is related to the first homotopy group of SO(n,1) (and also Poincaré(n,1)) with $n>2$, which is $\mathrm {Z} _{2}$ (the cyclic group consisting of two elements). Therefore only two possibilities remain. (The details are more involved than that, but this is the crucial point.)

The situation changes in two dimensions. Here the first homotopy group of SO(2,1) (and also Poincaré(2,1)) is Z (infinite cyclic). This means that Spin(2,1) is not the universal cover: it is not simply connected. In detail, there are projective representations of the special orthogonal group SO(2,1) which do not arise from linear representations of SO(2,1), or of its double cover, the spin group Spin(2,1). These representations are called anyons.

This concept also applies to nonrelativistic systems. The relevant part here is that the spatial rotation group is SO(2), which has an infinite first homotopy group.

This fact is also related to the braid groups well known in knot theory. The relation can be understood when one considers the fact that in two dimensions the group of permutations of two particles is no longer the symmetric group $S_{2}$ (2-dimensional) but rather the braid group $B_{2}$ (infinite dimensional).

A very different approach to the stability-decoherence problem in quantum computing is to create a topological quantum computer with anyons, quasi-particles used as threads and relying on braid theory to form stable logic gates.

See also

• plekton
• fractional quantum Hall effect

External links

• Interview with Frank Wilczek on anyons and superconductivity