  In 2001 all-optical quantum computing became feasible with the discovery that
scalable quantum computing is possible using only single photon sources, linear
optical elements, and single photon detectors. Although it was in principle
scalable, the massive resource overhead made the scheme practically daunting.
However, several simplifications were followed by proof-of-principle
demonstrations, and recent approaches based on cluster states or error encoding
have dramatically reduced this worrying resource overhead, making an
all-optical architecture a serious contender for the ultimate goal of a
large-scale quantum computer. Key challenges will be the realization of
high-efficiency sources of indistinguishable single photons, low-loss, scalable
optical circuits, high efficiency single photon detectors, and low-loss
interfacing of these components.
