  Certain one-dimensional Fermi systems have an energy gap in the bulk spectrum
while boundary states are described by one Majorana operator per boundary
point. A finite system of length $L$ possesses two ground states with an energy
difference proportional to $\exp(-L/l_0)$ and different fermionic parities.
Such systems can be used as qubits since they are intrinsically immune to
decoherence. The property of a system to have boundary Majorana fermions is
expressed as a condition on the bulk electron spectrum. The condition is
satisfied in the presence of an arbitrary small energy gap induced by proximity
of a 3-dimensional p-wave superconductor, provided that the normal spectrum has
an odd number of Fermi points in each half of the Brillouin zone (each spin
component counts separately).
