  A large payload quantum steganography protocol based on cavity quantum
electrodynamics (QED) is presented in the paper, which effectively uses the
evolution law of atom in cavity QED. The protocol builds up hidden channel to
transmit secret messages using entanglement swapping between one GHZ state and
one Bell state in cavity QED together with the Hadamard operation. The quantum
steganography protocol is insensitive to cavity decay and thermal field. The
capacity, imperceptibility and security against eavesdropping are analyzed in
detail in the protocol. It turns out that the protocol not only has good
imperceptibility but also possesses good security against eavesdropping. In
addition, its capacity of hidden channel achieves five bits, larger than most
of those previous quantum steganography protocols.
