Observing nonlinear spin squeezing and entanglement of non-Gaussian states with interconnected superconducting qubits*

Multipartite entanglement, witnessed by spin squeezing and Fisher information, is significant for both quantum information and quantum metrology. Using 20 interconnected superconducting qubits, we measure the nonlinear squeezing parameter, as a generalization of the Ramsey squeezing parameter with nonlinear observables, and identify the metrological enhancement of non-Gaussian states. By extracting the Fisher information, we observe a maximum metrological gain of 10.07 dB using 20 qubits. Merging different concepts of entanglement witnesses, our experiments characterize different classes of complicated many-body entangled states using superconducting qubits. Benefiting from high-fidelity full controls and addressable single-shot readouts, the superconducting processor with interconnected qubits is promising for improving the performance of quantum metrology and realizing other complex spin-squeezing algorithms.