Generation of entangled ancilla states for use in linear optics quantum computing

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.