= Solution
As a single trapped-ion chain grows, it has more collective vibrational modes with increasingly crowded frequencies. A drive intended to address one <motional sideband> can excite nearby modes, leaving unwanted spin-motion entanglement and crosstalk. Resolving the modes requires longer or more carefully shaped gates, while heating and <frequency> drift become more consequential. Thus the shared motional bus that is simple for a few <ions> becomes difficult to isolate and return cleanly to its initial state for large registers. This is a control and scaling limitation, rather than a failure of <qubits> or gates to exist.
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