Fast and Accurate GHZ Encoding Using All-to-all Interactions

The N-qubit Greenberger-Horne-Zeilinger (GHZ) state is an important resource for quantum technologies. We consider the task of GHZ encoding using all-to-all interactions, which prepares the GHZ state in a special case, and is furthermore useful for quantum error correction and enhancing the rate of qubit interactions. The naive protocol based on parallelizing CNOT gates takes O(1)-time of Hamiltonian evolution. In this work, we propose a fast protocol that achieves GHZ encoding with high accuracy. The evolution time O(log²N/N) almost saturates the theoretical limit Ω(log N/N). Moreover, the final state is close to the ideal encoded one with high fidelity > 1-10⁻³, up to large system sizes N 2000. The protocol only requires a few stages of time-independent Hamiltonian evolution; the key idea is to use the data qubit as control, and to use fast spin-squeezing dynamics generated by e.g. two-axis-twisting.