Simulation of a two-qubit quantum gate based on classical transverse spatial waveguide mode encoding

We demonstrate the simulation of a classical transverse mode-encoded controlled-NOT (CNOT) gate. Note that this integrated CNOT logic gate is composed of two 1/2 spatial mode beamsplitters and three 1/3 spatial mode beamsplitters, following the scheme proposed by Ralph. We will show how the CNOT gate can be analogously implemented by photonic integrated circuits in the classical regime, focusing on exploring the violation of Bell’s inequality by mode-entangled states. Our work may provide a promising approach to implementing small-scale quantum simulation and has the potential to enable practical and robust optical quantum information processing.