Realization of high-fidelity unitary operations on up to 64 frequency bins

Abstract: The ability to apply user-chosen large-scale unitary operations with high fidelity to a quantum state is key to realizing future photonic quantum technologies. Here, we realize the implementation of programmable unitary operations on up to 64 frequency-bin modes. To benchmark the performance of our system, we probe different quantum walk unitary operations, in particular Grover walks on four-dimensional hypercubes with similarities exceeding 95\% and quantum walks with 400 steps on circles and finite lines with similarities of 98\%. Our results open a new path towards implementing high-quality unitary operations, which can form the basis for applications in complex tasks, such as Gaussian boson sampling.