  A quantum processor to import, process, and export optical quantum states is
a common core technology enabling various photonic quantum information
processing. However, there has been no photonic processor which is
simultaneously universal, scalable, and programmable. Here, we report on an
original loop-based single-mode versatile photonic quantum processor which is
designed to be universal, scalable, and programmable. Our processor can perform
arbitrarily many steps of programmable quantum operations on a given
single-mode optical quantum state by time-domain processing in a dynamically
controlled loop-based optical circuit. We use this processor to demonstrate
programmable single-mode Gaussian gates and multi-step squeezing gates. In
addition, we prove that the processor can perform universal quantum operations
by injecting appropriate ancillary states and also be straightforwardly
extended to a multi-mode processor. These results show that our processor is
programmable, scalable, and potentially universal, leading to be suitable for
general-purpose applications.
