Threshold quantum cryptography

We present the concept of threshold collaborative unitary-transformation or threshold quantum phy,which is a kind of quantum version of threshold cryptography. Threshold quantum cryptography. states that classical shared secrets are distributed to several parties and a subset of them, whose number is greater than a threshold, collaborates to compute a quantum cryptographic function, while keeping each share secretly inside each party. The shared secrets are reusable if no cheating is detected. As a concrete example of this concept, we show a distributed protocol (with threshold) of conjugate coding.