Quantum state fusion in photons

Quantum information technology is based on our ability to manipulate and transmit the internal quantum states of physical systems, such as photons, ions, atoms, superconducting circuits, etc [1,2]. In photonic systems, a strong research effort has been recently devoted to expanding the useful quantum space by two alternative approaches, which have proceeded in parallel. One of them relies on increasing the number of involved photons, while the other one relies on exploiting different degrees of freedom of the same photon, such as polarization, time-bin, wavelength, propagation paths, and orbital angular momentum. We introduce and prove experimentally a novel quantum-state manipulation processes, namely fusion, that can be used to bridge these two approaches, allowing for their full integration.