Multi-photon path-entangled states, known as NOON states, are a class of quantum states featuring N indistinguishable photons in an equal superposition of being simultaneously all in either one of two possible paths. These states are "Schrödinger cats" as they consist of a superposition between two highly distinct states. Such states are potentially useful in various quantum metrology schemes, such as quantum lithography and Heisenberg limited interferometry. We generate high-NOON states (up to 5 photons) by interference of quantum down-converted light with a classical coherent state, in an approach that is inherently scalable [1]. We verify the high fidelity of the generated states by performing quantum state tomography. The scheme's limitations and the feasibility of measuring states with even larger N in the future are discussed.

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[1]  I. Afek, O. Ambar, Y. Silberberg, Science 328, 879. (2010).