We propose and demonstrate a scheme in which ancillary photons for heralding a photon-number entangled state can control the state to be heralded. This scheme can herald photon-number entangled states having arbitrary number of photons.
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.
Entangled states of photons form the backbone of many quantum technologies. Due to the lack of effective photon-photon interactions, the generation of these states is typically probabilistic. In the prevailing but fundamentally limited generation technique, known as postselection, the target photons are measured destructively in the generation process. By contrast, in the alternative approach-heralded state generation-the successful creation of a desired state is verified by the detection of ancillary photons. Heralded state generation is superior to postselection in several critical ways: it enables free usage of the prepared states, allows for the success probability to be arbitrarily increased via multiplexing, and provides a scalable route to quantum information processing using photons. Here, we review theoretical proposals and experimental realisations of heralded entangled photonic state generation, as well as the impact of realistic experimental errors. We then discuss the wide-ranging applications of these states for quantum technologies, including resource states in linear optical quantum computing, entanglement swapping for repeater networks, fundamental physics, and quantum metrology.