Quantum State Generation with Nonlinear Integrated Photonics: Conventional and Unconventional Approaches

Abstract

In this thesis we study the generation of non-classical states of light using nonlinear integrated photonics. We focus first on third-order parametric down-conversion (TOPDC), a process in which pump photons can be down-converted into photon triplets, and which could serve as a source of non-Gaussian states and tripartite discrete-variable entanglement. We investigate the possibility of implementing TOPDC in conventional integrated structures. By comparing the estimated triplet generation rates in a waveguide and a microring resonator, we establish that non-resonant structures appear to be a more promising platform for a near-term demonstration of TOPDC, due to the high generation bandwidth enhancing the rate of triplet generation. We estimate generation rates of ~10 triplets per second for a 1-cm-long waveguide driven by a 100 mW CW pump, a rate that is comparable to or higher than those quoted for other proposals. We then estimate the non-Gaussianity that might be expected in a supermode of the field generated by our sample structures. The predicted non-Gaussianity is negligible in both structures, for different reasons: The generation rate in the ring is too low, while the field generated by the waveguide is highly multimode. We conclude that although a simple SiN waveguide may be a suitable source for a first demonstration of optical TOPDC, the design will need to be improved significantly for observations of non-Gaussianity or tripartite entanglement in the generated light. Indeed, state generation protocols based on conventional spontaneous four-wave mixing sources and postselection appear to be more promising in the near term. We adopt this approach to design integrated sources of postselected multipartite states in a frequency bin encoding scheme. We introduce sources of W states and GHZ states, with kHz generation rates expected for picojoule pump pulses. This platform appears to be promising for the generation of more arbitrary states, including high-dimensional ones consisting of entangled qudits. We review a graph-based strategy that may be useful for designing other devices, and for understanding the limitations of this platform.