Ring-down data for a 7-mg stepped fused-silica pendulum with a nanohertz mechanical linewidth

This dataset contains four ring-down measurement records for thefundamental pendulum mode of a 7-mg mirror suspended by a steppedfused-silica fiber. The four records were acquired on 27 June 2025, 7 July 2025,16 October 2025, and 6 November 2025. They were used for the jointenergy-decay fit reported in the associated manuscript,“Nanohertz Pendulum toward Macroscopic Entanglement under StructuralDamping.”