Assisted Quantum Secret Sharing

A restriction on quantum secret sharing (QSS) that comes from the no-cloning theorem is that any pair of authorized sets in an access structure should overlap. From the viewpoint of application, this places an unnatural constraint on secret sharing. We present a generalization, called assisted QSS (AQSS), where access structures without pairwise overlap of authorized sets is permissible, provided some shares are withheld by the share dealer. We show that no more than λ− 1 withheld shares are required, where λ is the minimum number of partially linked classes among the authorized sets for the QSS. This is useful in QSS schemes where the share dealer is honest by definition and is equivalent to a secret reconstructor. Our result means that such applications of QSS need not be thwarted by the no-cloning theorem.