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The synchronization of coherent states of light has long been an important subject of basic research and technology. Recently, a new concept for analog computers has emerged where this synchronization process can be exploited to solve computationally hard problems - potentially faster and more energy-efficient than what can be achieved with conventional computer technology today. The unit cell of such systems consists of two coherent centers that are coupled to one another in a controlled manner. Here, we experimentally characterize and analyze the synchronization process of two photon Bose-Einstein condensates, which are coupled to one another, either dispersively or dissipatively. We show that both types of coupling are robust against a detuning of the condensate frequencies and show similar time constants in establishing mutual coherence. Significant differences between these couplings arise in the behaviour of the condensate populations under imbalanced optical pumping. The combination of these two types of coupling extends the class of physical models that can be investigated using analog simulations.

Efficient and reliable measurement of photonic indistinguishability is crucial for any claim of a quantum advantage in photonics. Using an integrated photonic processor, we introduce and demonstrate a semi-device-independent witness of photonic indistinguishability.

We propose an integrated network of ring resonators on a silicon-nitride chip for use as an all-optical single-spatial-mode physical unclonable key, enabling secret-free optical authentication with standard communication channels such as telecom fibres.

We report the realization of the largest reconfigurable quantum photonic processor enabling arbitrary unitary transformations on its 20 input & output modes with an average fibre-to-fibre loss of 2.9 dB/channel. High-fidelity operation and high-visibility quantum interference is demonstrated.

We study the programming fidelity of a 12-mode quantum photonic processor, the largest universal quantum photonic processor to date. The processor is a fully reconfigurable linear interferometer using silicon nitride waveguide technology.

Open quantum systems can be systematically controlled by making changes to their environment. A prerequisite for a deliberate control of an open quantum system is a sufficient understanding of its underlying physical mechanisms. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback realized by a potential landscape that effectively acts as a Mach-Zehnder interferometer. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that the condensation process becomes an interplay between minimizing the energy of the condensate, minimizing the particle losses and maximizing the constructive feedback from the environment. [1] M. Vretenar et al., arXiv:2105.10708 [cond-mat.quant-gas]