In quantum optics, a NOON state or N00N state is a quantum-mechanical many-body entangled state: | ψ NOON ⟩ = | N ⟩ a | 0 ⟩ b + e i N θ | 0 ⟩ a | N ⟩ b 2, {\displaystyle |\psi _{\text{NOON}}\rangle ={\frac {|N\rangle _{a}|0\rangle _{b}+e^{iN\theta }|{0}\rangle _{a}|{N}\rangle _{b}}{\sqrt {2}}},\,} which represents a superposition of N particles in mode a with zero particles in mode b, and vice versa. Wikipedia