  A two-layer quantum protocol for secure transmission of data using qubits is
presented. The protocol is an improvement over the BB84 QKD protocol. BB84, in
conjunction with the one-time pad algorithm, has been shown to be
unconditionally secure. However it suffers from two drawbacks: (1) Its security
relies on the assumption that Alice's qubit source is perfect in the sense that
it does not inadvertently emit multiple copies of the same qubit. A multi-qubit
emission attack can be launched if this assumption is violated. (2) BB84 cannot
transfer predetermined keys; the keys it can distribute are generated in the
process. Our protocol does not have these drawbacks.
  As in BB84, our protocol requires an authenticated public channel so as to
detect an intruder's interaction with the quantum channel, but unlike in
symmetric-key cryptography, the confidentiality of transmitted data does not
rely on a shared secret key.
