Threshold quantum cryptograph based on Grover's algorithm

We propose a threshold quantum protocol based on Grover's operator and permutation operator on one two-qubit signal. The protocol is secure because the dishonest parties can only extract 2 bits from 3 bits information of operation on one two-qubit signal while they have to introduce error probability 3/8. The protocol includes a detection scheme to resist Trojan horse attack. With probability 1/2, the detection scheme can detect a multi-qubit signal that is used to replace a single-qubit signal, while it makes every legitimate qubit invariant.