Photonic circuits, engineered to couple optical modes according to a specific map, serve as processors for classical and quantum light. The number of components typically scales with that of processed modes, thus correlating system size, circuit complexity, and optical losses. We present a photonic-circuit technology implementing large-scale unitary maps in free space, coupling a single input to hundreds of output modes in a two-dimensional compact layout. The map corresponds to a quantum walk of structured photons, realized through light propagation in three liquid-crystal metasurfaces, having their optic axes artificially patterned. Theoretically, the walk length and the number of connected modes can be arbitrary while keeping losses constant. The patterns can be designed to replicate multiple unitary maps. We also discuss limited reconfigurability by adjusting the overall birefringence and the relative displacement of the optical elements. These results lay the basis for the design of low-loss nonintegrated photonic circuits, primarily for manipulating multiphoton states in quantum regimes.

Soliton molecules (SMs), bounded and self-assembled of particle-like dissipative solitons, exist with versatile mutual interactions and manifest substantial potential in soliton communication and optical data storage. However, controllable manipulation of the bounded molecular patterns remains challenging, as reaching a specific operation regime in lasers generally involves adjusting multiple control parameters in connection with a wide range of accessible pulse dynamics. An evolutionary algorithm is implemented for intelligent control of SMs in a 2 μm ultrafast fiber laser mode locked through nonlinear polarization rotation. Depending on the specifications of the merit function used for the optimization procedure, various SM operations are obtained, including spectra shape programming and controllable deterministic switching of doublet and triplet SMs operating in stationary or pulsation states with reconfigurable temporal separations, frequency locking of pulsation SMs, doublet and SM complexes with controllable pulsation ratio, etc. Digital encoding is further demonstrated in this platform by employing the self-assembled characteristics of SMs. Our work opens up an avenue for active SM control beyond conventional telecom bands and brings useful insights into nonlinear science and applications.