Ultrafast quantum optics is a young research field. Its focus lies at the study of quantum phenomena at extreme timescales of a few hundreds of femtoseconds. In this thesis, we investigate the intricate time-frequency (TF) structure of ultrafast quantumstates of light. This structure is of particular interest, because it is the natural basis of energytime entanglement, a resource for high-dimensional quantum information applications. We study the process of parametric down-conversion (PDC) and introduce a novel measurefor energy-time entanglement which is applicable to many current PDC sources. Moreover, we experimentally investigate the correlation time of the photon pair, a measure of the simultaneityof the photons, and find that is independent of the spectral-temporal properties of the PDC pump. The main work is dedicated to two novel devices for high-dimensional TF quantum networks, the quantum pulse gate and quantum pulse shaper. Both are based on dispersion engineered frequency conversion in nonlinear waveguides and facilitate a mode-selective operation on TF modes of ultrafast quantum states. We develop a theoretical framework, similar to the existing PDC description, to describe the TF structure of our devices and identify ideal operation parameters. These are used to realise aquantum pulse gate in the laboratory and verify its mode-selective operation.