Past exam of the laser science and quantum information processing course of the University of Oxford / 2019 / 5 / e / 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.