Efficient Optical Quantum State Engineering

We discuss a novel method of efficiently producing multiphoton states using repeated spontaneous parametric down-conversion. Specifically, by attempting down-conversion several times, we can pseudo-deterministically add photons to a mode, producing various several-photon states, e. g., Fock states and N00N states (number-path entangled states of the form |N-A, 0(B)> + |0(A), N-B >). This scheme is exponentially more efficient than previous proposals; we discuss expected performance and experimental limitations.