  We consider a system consisting of single electrons moving along a 1D wire in
the presence of two magnetic impurities. Such system shows strong analogies
with a Fabry - Perot interferometer in which the impurities play the role of
two mirrors with a quantum degree of freedom: the spin. We have analysed the
electron transmittivity of the wire in the presence of entanglement between the
impurity spins. The main result of our analysis is that, for suitable values of
the electron momentum, there are two maximally entangled state of the impurity
spins the first of which makes the wire transparent whatever the electron spin
state while the other strongly inhibits the electron transmittivity. Such
predicted striking effect is experimentally observable with present day
technology.
