Phase and amplitude controlled heralding of N00N states.

Entangled photons, an essential resource in quantum technology, are mostly generated in spontaneous processes, making it impossible to know if the quantum state is available for use; giving only a posteriori knowledge of the quantum state via destructive photon detection processes. There are schemes for heralding the generation of entangled photons but the heralding schemes developed to date only inform the generation of a predetermined quantum state with no capability of state control. Here, we report the phase and (probability-) amplitude controlled heralding, i.e., complete quantum state heralding, of multiphoton entangled states or N00N states. Since the phase and amplitude controls are inseparably integrated into the heralding mechanism, our scheme enables generation of N00N states with arbitrary phases and amplitudes. Such a flexible heralding scheme is expected to play important roles in various photonic quantum information applications.