Uhm... no, I think not: to get information out of the quantum computer as output for the ”previous user” it needs to go through an explicit phase of observation

Uhm... no, I think not: to get information out of the quantum computer as output for the ”previous user” it needs to go through an explicit phase of observation, busting all the state (?), and loading up new data for the subsequent user’s computation also constitutes ’observation’ since there’s knowledge of the inner state of the quantum system outside the strict confines of the quantum system. I really don’t think you could at any moment in time get information out of a side-channel, otherwise the thing wouldn’t compute. (Yup, of course they’re extremely simplistic in terms of inner architecture, but that’s quite beside the point, as I think we both agree.)