Integrated quantum interferometry with three-dimensional geometry

Summary form only given. The main features of quantum mechanics reside into interference deriving from the superposition of different quantum objects. The Hong-Ou-Mandel effect is a purely quantum phenomenon deriving from the interference of two photons. It is a consequence of the bosonic nature of light and can be observed when two identical photons impinge simultaneously on a two-port balanced beam splitter and, as a result, both are forced to exit from the same port. While current quantum optical technology enables two-photon interference both in bulk and integrated systems, simultaneous interference of more than two particles, leading to richer quantum phenomena, is still a challenging task.The emergence of integrated quantum photonics have opened exciting perspectives for the realization of scalable, miniaturized and intrinsically stable optical setups. In particular, the ultrafast laser-writing technique has proved to be a powerful tool for demonstrating new quantum integrated-optics devices, able to perform quantum logic operations as well as two-photon quantum walks. Here we report the experimental observation of threephoton interference in an integrated three-port directional coupler realized by ultrafast laser-writing. By exploiting the capability of this technique to produce three-dimensional structures, we realized and tested in the quantum regime a three-photon beam splitter, namely a tritter, which allowed us to observe bosonic coalescence of three photons [1]. Such a system can be exploited as a building block to implement a generalized class of multi-arm interferometers. We introduce the concept of 3D multi-photon interferometry [2], and we discuss their application in the context of quantum interferometry and quantum phase estimation. We propose novel geometries for integrated multi-arm interferometers based on three-port (tritter) and four-port (quarter) devices, and we discuss possible measurement protocols, based on the injection of multi-photon- Fock states in this kind of multi-port devices, demonstrating relevant metrological advantages in phase-estimation tasks. Furthermore, we provide a first insight on the application of this technology in the still unexplored field of multiparameter estimation.Several other contexts may benefit from the adoption of 3-dimensional integrated interferometers realized by femtosecond laser-writing. For instance, they can be adopted for the realization of “proof-of-principle” quantum simulators [3], and, combined in a modular structure, they can be used to realize full-scale quantum simulators for large size quantum systems. The adoption of multiphoton-multimode platforms may indeed disclose the “hardto-simulate” scenario by adopting linear optics to implement a computational power beyond the one of a classical computer.