  We report on deterministic generation of 18-qubit genuinely entangled
Greenberger-Horne-Zeilinger (GHZ) state and multi-component atomic
Schr\"{o}dinger cat states of up to 20 qubits on a quantum processor, which
features 20 superconducting qubits interconnected by a bus resonator. By
engineering a one-axis twisting Hamiltonian enabled by the resonator-mediated
interactions, the system of qubits initialized coherently evolves to an
over-squeezed, non-Gaussian regime, where atomic Schr\"{o}dinger cat states,
i.e., superpositions of atomic coherent states including GHZ state, appear at
specific time intervals in excellent agreement with theory. With high
controllability, we are able to take snapshots of the dynamics by plotting
quasidistribution $Q$-functions of the 20-qubit atomic cat states, and globally
characterize the 18-qubit GHZ state which yields a fidelity of $0.525\pm0.005$
confirming genuine eighteen-partite entanglement. Our results demonstrate the
largest entanglement controllably created so far in solid state architectures,
and the process of generating and detecting multipartite entanglement may
promise applications in practical quantum metrology, quantum information
processing and quantum computation.
