  The "Ping-Pong" (PP) protocol is a two-way quantum key protocol based on
entanglement. In this protocol, Bob prepares one maximally entangled pair of
qubits, and sends one qubit to Alice. Then, Alice performs some necessary
operations on this qubit and sends it back to Bob. Although this protocol was
proposed in 2002, its security in the noisy and lossy channel has not been
proven. In this report, we add a simple and experimentally feasible
modification to the original PP protocol, and prove the security of this
modified PP protocol against collective attacks when the noisy and lossy
channel is taken into account. Simulation results show that our protocol is
practical.
