On-chip coherent conversion of photonic quantum entanglement between different degrees of freedom

Abstract

In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process. Harnessing multiple degrees of freedom of quantum states on chip could improve quantum information processing. Here, the authors demonstrate coherent conversion of quantum states between path, polarization and transverse waveguide-mode degrees of freedom in a quantum photonic integrated circuit.