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.