  Storage lifetime and capacity are two important factors to characterize the
performance of a quantum memory. Here we report the stable trapping of above
200 ions in a cryogenic setup, and demonstrate the combination of the
multi-qubit capacity and long storage lifetime by measuring the coherence time
of randomly chosen ions to be on the order of hundreds of milliseconds. We
apply composite microwave pulses to manipulate qubit states globally for
efficient characterization of different storage units simultaneously, and we
compare the performance of the quantum memory with and without the sympathetic
cooling laser, thus unambiguously show the necessity of sympathetic cooling for
the long-time storage of multiple ionic qubits.
