  The crucial issue of quantum communication protocol is its security. In this
paper, we show that all the deterministic and direct two-way quantum
communication protocols, sometimes called ping-pong (PP) protocols, are
insecure when an eavesdropper uses the invisible photon to eavesdrop on the
communication. With our invisible photon eavesdropping (IPE) scheme, the
eavesdropper can obtain full information of the communication with zero risk of
being detected. We show that this IPE scheme can be implemented experimentally
with current technology. Finally, a possible improvement of PP communication
protocols security is proposed. PACS:03.67.Hk
