At the risk of missing something obvious, any distributed quantum circuit without a measurement step it is not possible for Kevin and Charlie to learn anything about the plaintext per the no cloning theorem.
Eavesdropping in the middle of the circuit should lead to measurable statistical anomalies due to projecting the state onto the measurement basis.
(I’ll add a caveat that I am talking about theoretical quantum circuits and ignoring any nuances that emerge from their physical implementations.)
Edit:
On posting, I think I realize my error. We need Kevin and Charlie to not have knowledge of the specific gates that they are implementing as well.
At the risk of missing something obvious, any distributed quantum circuit without a measurement step it is not possible for Kevin and Charlie to learn anything about the plaintext per the no cloning theorem.
Eavesdropping in the middle of the circuit should lead to measurable statistical anomalies due to projecting the state onto the measurement basis.
(I’ll add a caveat that I am talking about theoretical quantum circuits and ignoring any nuances that emerge from their physical implementations.)
Edit:
On posting, I think I realize my error.
We need Kevin and Charlie to not have knowledge of the specific gates that they are implementing as well.