We propose and fully analyze the simplest technique to date (to our knowledge) for generating light-based universal quantum computing resources, namely, 2D, 3D, and n -hypercubic cluster states in general. The technique uses two standard optical components: first, a single optical parametric oscillator pumped below threshold by a monochromatic field, which generates Einstein–Podolsky–Rosen entangled states, a.k.a. two-mode squeezed states, over the quantum optical frequency comb; second, phase modulation at frequencies that are multiples of the comb spacing (via RF or optical means). The compactness of this technique paves the way to implementing quantum computing on chip using quantum nanophotonics.

We investigate quantum properties of light in optical parametric oscillators (OPOs) based on four-wave mixing gain in media with third-order nonlinearities. In spite of other competing χ effects such as phase modulation, bipartite and tripartite entanglement is predicted above threshold. These findings are relevant for recent implementations of complementary metal-oxide-semiconductor (CMOS)-compatible on-chip OPOs.

We propose and experimentally demonstrate a quantum state tomography protocol that generalizes and improves upon the Wallentowitz-Vogel-Banaszek-Wódkiewicz point-by-point Wigner function reconstruction. The full density operator of an arbitrary quantum state is efficiently reconstructed in the Fock basis, using semidefinite programming, after interference with a small set of calibrated coherent states. This protocol is resource-and computationally efficient, is robust against noise, does not rely on approximate state displacements, and ensures the physicality of results.