  We investigate the computational power of passive and active linear optical
elements and photo-detectors. We show that single photon sources, passive
linear optics and photo-detectors are sufficient for implementing reliable
quantum algorithms. Feedback from the detectors to the optical elements is
required for this implementation. Without feedback, non-deterministic quantum
computation is possible. A single photon source sufficient for quantum
computation can be built with an active linear optical element (squeezer) and a
photo-detector. The overheads associated with using only linear optics appear
to be sufficiently low to make quantum computation based on our proposal a
viable alternative.
