Time/frequency high-dimensional entanglement via engineered parametric down conversion*

Photonic quantum technologies rely on the deterministic preparation of qubits encoded in single-photons degrees of freedom (DoF). While polarisation, orbital angular momentum and path have been routinely used since the early days of quantum information, the last few years have seen an increasing interest in the frequency-time encoding, due to the possibility of generating high-dimensional states combined with the compatibility of frequency modes with standard optical components.
Here we present a new scheme for generating frequency-entangled photon pairs in parametric down-conversion (PDC) processes. By means of our novel non-linearity engineering technique, we deterministically choose the orientation of each ferroelectric domain of a poled crystal to tailor its phase-matching function as a multi-peaked function. The photons produced in such crystals emerge in a coherent superposition of an arbitrary number of orthogonal spectral Schmidt modes, allowing us to explore high-dimensional Hilbert spaces.
Our technique can easily be implemented as requires no additional components respect to a standard PDC setup, it’s highly compatible with other DoF encodings, and can be adapted to integrated waveguide sources.