Programmable quantum interference in massively multichannel networks

Abstract

Multiphoton quantum correlations are crucial for quantum information processing and quantum communication protocols in linear optical networks. For large-scale implementation of quantum information processing, such as quantum simulators, boson sampling or programmable quantum logic gates, a programmable functionality of a large multichannel network is required. In this thesis, we describe and demonstrate programmable quantum interference between multiple single-photon states in massively multichannel networks. Using a theoretical analysis we first show that losses in optical networks, which are unavoidable in experiments, introduce new freedom, in that the functionality of the network, and thus also quantum interference in there, is programmable to an extent not possible with lossless networks. We introduce a method to program the functionality of general multichannel linear optical networks by phase modulation of incident wavefronts, which we apply to opaque scattering media as well as integrated optics. To demonstrate quantum interference in massively multichannel networks we require a source of multiple indistinguishable single-photon states. Therefore, we have constructed and characterized a versatile quantum light source based on spontaneous parametric down-conversion. For the first time, we demonstrate two-photon quantum interference in a massively multichannel linear optical network realized in an opaque scattering medium. Using adaptive phase-modulation of the incident photons, the scattering medium is transformed to behave as a fully programmable beam splitter that is freely tunable in functionality. Exploiting this freedom, we not only show the well-known Hong-Ou-Mandel bunching of photons, but also demonstrate that this bunching can be made to vanish, or be transformed into antibunching. Our results establish opaque scattering media as a platform for massively multichannel linear optical networks with programmable quantum correlations. Finally, the high number of available channels offered by opaque scattering media can directly be employed for authentication of secure communication. We present an authenticated protocol for quantum key distribution that removes the need of an initial shared secret for authentication. Moreover, we introduce and demonstrate the first protocol for authenticated and asymmetric quantum communication, based on the quantum-secure readout of a physical unclonable function.