  Blind quantum computing protocols enable a client, who can generate or
measure single-qubit states, to delegate quantum computing to a remote quantum
server protecting the client's privacy (i.e., input, output, and program). With
current technologies, generations or measurements of single-qubit states are
not too much burden for the client. In other words, secure delegated quantum
computing is possible for "almost classical" clients. However, is it possible
for a "completely classical" client? Here we consider a one-round
perfectly-secure delegated quantum computing, and show that the protocol cannot
satisfy both the correctness (i.e., the correct result is obtained when the
server is honest) and the perfect blindness (i.e., the client's privacy is
completely protected) simultaneously unless BQP is in NP. Since BQP is not
believed to be in NP, the result suggests the impossibility of the one-round
perfectly-secure delegated quantum computing.
