High-rate multiplexed silicon-integrated sources of entangled states of light for quantum communication

I present an elaborate which focuses on integrated devices with quantum communication applications. In particular, I concentrate on silicon integrated sources of entangled photon pairs, addressing all the main steps needed for their realization: design, fabrication and characterization. The first source I describe consists in a large ring resonator, designed to reach a high rate of entangled photon pairs by employing many frequency pairs. The resonator is followed by an integrated broadband demultiplexer, which is employed to separate the generated pairs into two multiplexed channels, one with the signal, the other with the idler comb. For what concerns this device, I was in charge of every realization step: I started with the modeling and simulation of the structures, I supervised the fabrication process thanks to the opportunity of spending 16 months at CEA-Leti in Grenoble (France), finally I performed linear and quantum characterization measurements. As a result, I demonstrate a generation rate of entangled photon pairs that exceeds 1 Gpair/s. This achievement is of great relevance for quantum communication applications, especially satellite-based ones, where the channel losses strongly limit the bit-rate. In addition, by performing a Bell measurement and a quantum state tomography, I prove high fidelity and purity for a frequency-bin entangled Bell state. These outcomes are crucial for the implementation of device-independent quantum key distribution. The two other devices I report on share some common concepts, such as the fact that they are photonic molecules that include more than one ring and that they are to some extent programmable. In the first case, two resonators that are linearly uncoupled but that can interact nonlinearly are used to selectively enhance dual-pump spontaneous four-wave mixing while suppressing parasitic single-pump spontaneous four-wave mixing. Furthermore, by employing a Mach-Zehnder interferometer in the coupling region instead of a simple directional coupler, I show how it is possible to extend the bandwidth of the device. As for the second case, tunable rings pumped with the same signal are used to generate on-demand frequency-bin maximally entangled states. Engineerability and selectivity of the entangled state are of great relevance not only to the implementation of secure quantum key distribution protocols, but also for what concerns quantum computing and information processing.