  Quantum secret sharing schemes encrypting a quantum state into a multipartite
entangled state are treated. The lower bound on the dimension of each share
given by Gottesman [Phys. Rev. A \textbf{61}, 042311 (2000)] is revisited based
on a relation between the reversibility of quantum operations and the Holevo
information. We also propose a threshold ramp quantum secret sharing scheme and
evaluate its coding efficiency.
