In order to implement the measurement described in the thought experiment, you basically have to uncompute Charlie's state back to the start of the experiment (

In order to implement the measurement described in the thought experiment, you basically have to uncompute Charlie's state back to the start of the experiment (to disentangle him from the qubit you want to measure), do the measurement on the single qubit, then recompute Charlie forward. This means that when Charlie is making deductions about the current state of affairs, like "if my qubit is |0> then the other qubit is |+>, that thinking needs to be valid during the computation and during the recomputation. But the conclusions Charlie is making are not correct during the recomputation, e.g. it is no longer the case that if his qubit is |0> then the other qubit is |+> at that time, and as a result the whole argument (based on Charlie's reasoning being valid so that an outside agent can use his conclusions) falls apart. More details: https://algassert.com/post/1904 https://algassert.com/post/1904