Scalable creation of long-lived multipartite entanglement

Abstract: We demonstrate the deterministic generation of multipartite entanglement based on scalable methods. Four qubits are encoded in $^{40}$Ca$^+$, stored in a micro-structured segmented Paul trap. These qubits are sequentially entangled by laser-driven pairwise gate operations. Between these, the qubit register is dynamically reconfigured via ion shuttling operations, where ion crystals are separated and merged, and ions are moved in and out of a fixed laser interaction zone. A sequence consisting of three pairwise entangling gates yields a four-ion GHZ state $\vert\psi\rangle=\tfrac{1}{\sqrt{2}}\left(\vert 0000\rangle+\vert 1111\rangle\right)$, and full quantum state tomography reveals a Bell state fidelity of 94.4(3)\%. We analyze the decoherence of this state and employ dynamic decoupling on the spatially distributed constituents to maintain 69(5)\% coherence at a storage time of 1.1~seconds.