Photonic hyperentanglement in polarisation and frequency via joint spectrum shaping
Hyperentanglement offers enhanced capacity for quantum information processing and communication protocols, especially in combination with robust high-dimensional degrees of freedom such as frequency-bin encoding. Here, we present a single-pass, unfiltered, down-conversion source of hyperentangled photon pairs in polarisation and frequency-bin degrees of freedom with dynamically tunable state dimension and composition at telecom wavelengths. We achieve this by optimal tailoring of the photons' joint spectral amplitude via pump and nonlinearity shaping. Using polarisation-resolved time-of-flight spectrometry and Hong-Ou-Mandel interference, we characterise the hyperentangled states and demonstrate for the polarisation component fidelities exceeding 99% averaged over frequency bins and concurrences above 98%. The degree of spectral entanglement, quantified by the Hong-Ou-Mandel visibility, is measured as 90%, well in line with numerical simulations. This approach provides a scalable route toward high-dimensional quantum states for quantum communication and computing applications.