  Quantum computing has been pursued with various hardware platforms, and an
optical system is one of the most reasonable choices for large-scale
computation. In the optical continuous-variable computation scheme, the
incorporation of Gaussian gates and a highly non-classical non-Gaussian state
enables universal quantum computation. Although basic technologies for Gaussian
gates and non-Gaussian state generations have long been developed, these
building blocks have not yet been integrated in a scalable fashion. Here, we
integrate them to develop a scalable and programmable optical quantum computing
platform that can sequentially perform an essential Gaussian gate, the
squeezing gate, on a non-Gaussian input state. The key enablers are a
loop-based optical circuit with dynamical and programmable controllability and
its time-synchronization with the probabilistic non-Gaussian state generation.
We verify the deterministic, programmable, and repeatable quantum gates on a
typical non-Gaussian state by implementing up to three-step gates. The gates
implemented are so high-quality that strong evidence of the states'
non-classicalities, negativities of the Wigner functions, are preserved even
after multistep gates. This platform is compatible with other non-Gaussian
states and realizes large-scale universal quantum computing by incorporating
other existing processing technologies.
