Tailored quantum light for photonic quantum technologies

Abstract

Photonic quantum technologies rely on the deterministic preparation of qubits encoded in quantum states of light: advances in this field are therefore contingent with the development of reliable photon sources. In this Thesis, I address this challenge presenting a novel and versatile approach to single-photon generation based on nonlinearity engineering in parametric down-conversion. By tailoring the effective nonlinearity of a crystal, this scheme enables access to the spectral degree of freedom of photonic qubits with unprecedented precision, and translates into a number of different applications based on the manipulation of the biphoton spectral/temporal properties. A thorough theoretical and numerical description of such approach is provided and paired with experimental benchmarks conducted in three main experiments. The first experiment tackles the single-photon spectral purity problem in down-conversion sources: pure photons are in fact required for achieving perfect two-photon interference, a keystone of most quantum protocols. The second experiment demonstrates the feasibility of nonlinearity engineering to produce tailored entanglement encoded in the spectrum of biphoton states. Finally, the third experiment certifies the compatibility of this technique with different degrees of freedom, demonstrating hyperentanglement of spatially and spectrally structured quantum light. In conclusion, this Thesis stands as a cookbook for designing simple yet flexible and highly-efficient single-photon sources based on tailored parametric down-conversion processes.