Integrated photonic source of Gottesman-Kitaev-Preskill qubits.

Building a useful photonic quantum computer requires robust techniques to synthesize optical states that can encode qubits. Gottesman-Kitaev-Preskill (GKP) states offer one of the most attractive classes of such qubit encodings, as they enable the implementation of universal gate sets with straightforward, deterministic and room temperature-compatible Gaussian operations. Existing pioneering demonstrations generating optical GKP states and other complex non-Gaussian states have relied on free-space optical components, hindering the scaling eventually required for a utility-scale system. Here we use an ultra-low-loss integrated photonic chip fabricated on a customized multilayer silicon nitride 300-mm wafer platform, coupled over fibre with high-efficiency photon number resolving detectors, to generate GKP qubit states. These states show critical mode-level features necessary for fault tolerance, including at least four resolvable peaks in both p and q quadratures, and a clear lattice structure of negative Wigner function regions, in this case a 3 × 3 grid. We also show that our GKP states show sufficient structure to indicate that the devices used to make them could, after further reduction in optical losses, yield states for the fault-tolerant regime. This experiment validates a key pillar of bosonic architectures for photonic quantum computing, paving the way for arrays of GKP sources that will supply future fault-tolerant machines.