**Abstract:**
Quantum walks are quantum analogons of random walks, the main
difference of the definition of the two notion is that quantum walks
can be described by a deterministic (quantum) evolution, thus the
randomness emerges only at the time of observation. For coined quantum
walks this behaviour originates from the extension of the walker's
phase space by the so-called coin space, a usually low dimensional
quantum system, and a unitary operator called coin operator. By this
extension, the stochastic part of quantum walks -- choice of direction
before each step, is eliminated. In the standard definition, at all
sites the same coin operator is applied. Moving away from this
restriction, a random, non-uniform distribution of coin operators give
rise to disorder. In our recent research, we studied the effect of
disorder on a quantum walk on a line. The coin operator belonging to
SU(2) in this case can be characterised by 3 parameters. Depending on
which parameters are varied, and the time scale of these variations
one obtains: Anderson-type exponential localization, decoherence due
to a depolarising channel, near-uniform distribution with linear
spreading. These behaviours were also observed experimentally, using a
fiber optical feedback loop with a delay line [1]. The non-uniform coin
operators have been implemented by a controlled fast switching electro
optical modulator.

[1] A. Schreiber, K. N. Cassemiro, V. Potoček, A. Gábris, I. Jex,
Ch. Silberhorn, arXiv:1101.2638