Large quantum systems are the fundament of modern quantum technologies with use cases in quantum sensing, communication, simulation, and computing. We are particularly interested in photonic quantum systems because they support applications in all these fields. Typically, large photonic quantum systems are realized in the spatial domain, where a network of beam splitters and phase shifters is used to realize a unitary operation on an input state, followed by detection in the output modes of the network. Naturally, this approach faces challenges in terms of scalability because ever more elements are required to build larger networks.

Here, we discuss an alternative approach to realizing large photonic quantum systems: all optical frequency bin networks. In this system, frequency bins take on the role of spatial channels. The network is realized with a so-called quantum pulse gate. This device converts a user-chosen superposition of frequency bins into an output channel at a different central wavelength. The exact superposition–and thus the underlying unitary of the network–is chosen by shaping the classical pump pulses of the QPG; arbitrary networks can be realized without changing the experiment. We present a proof-of-concept experiment and highlights steps towards all-optical quantum networks for applications in quantum simulation and computation.