Quantum state fusion in photons

Summary form only given. 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.Quantum state fusion is here defined as the physical process by which the internal quantum state of two particles (e.g., photons) is transferred into the four-dimensional internal quantum state of a single particle (photon) (Fig. 1). In terms of information content, the quantum fusion has no effect, since the same two qubits are present at the input and at the output. In other words, state fusion is not a quantum logical gate for information processing. However, physically there is an important transformation, as the two qubits are moved from two separate particles into a single one. This is analogous to what occurs in quantum teleportation, in which the information content is unchanged, but there is a physical transformation in the information localization. We describe the experimental implementation of the quantum state fusion process from two polarizations qubit encoded in two separate photons input state to a hybrid polarization-path single photon state, implemented in an intrinsecally stable interferometric setup with linear optics and an ancillary photon. Furthermore, we discuss a scheme to reverse the quantum state fusion process into its quant- m state fission counterpart, where the information is transferred from two degrees of freedom of a single photon to two separate particles.