  We describe several different methods for generating the entangled ancilla
states that are required for linear optics quantum computing. We show that
post-selection can be used in combination with linear optical elements to
generate the entangled ancilla, but with an exponentially-small efficiency.
Alternatively, the ancilla can be efficiently generated using solid-state
devices consisting of quantum wells coupled with tunnel junctions. Finally, we
consider the possibility of using encoded ancilla in order to reduce the
effects of decoherence and measurement errors.
