  In port-based teleportation, Alice teleports an unknown quantum state to one
of N ports at Bob's site. Alice applies a measurement and sends Bob the outcome
k. Bob only needs to select the kth port in order to obtain the state. We
present a theorem in the spirit of the no-cloning theorem, which says that it
is impossible to extract any information from an unknown quantum state if only
a single copy of it is provided and if the state remains unchanged. We use this
theorem and the no-signaling principle to prove an upper bound on the success
probability of port-based teleportation.
