  Scalable quantum computation with linear optics was considered to be
impossible due to the lack of efficient two-qubit logic gates, despite its ease
of implementation of one-qubit gates. Two-qubit gates necessarily need a
nonlinear interaction between the two photons, and the efficiency of this
nonlinear interaction is typically very tiny in bulk materials. However, we
recently have shown that this barrier can be circumvented with effective
nonlinearities produced by projective measurements, and with this work
linear-optical quantum computing becomes a new possibility of scalable quantum
computation. We review several issues concerning its principles and
requirements.
