  We investigate the definition of security for encryption scheme in quantum
context. We systematically define the indistinguishability and semantic
security for quantum public-key and private-key encryption schemes, and for
computational security, physical security and information-theoretic security.
Based on our definition, we present a necessary and sufficient condition that
leads to information-theoretic indistinguishability for quantum encryption
scheme. The equivalence between the indistinguishability and semantic security
of quantum encryption scheme is also proved.
