The "Ping-Pong" (PP) protocol is a two-way quantum key protocol based onentanglement. In this protocol, Bob prepares one maximally entangled pair ofqubits, and sends one qubit to Alice. Then, Alice performs some necessaryoperations on this qubit and sends it back to Bob. Although this protocol wasproposed in 2002, its security in the noisy and lossy channel has not beenproven. In this report, we add a simple and experimentally feasiblemodification to the original PP protocol, and prove the security of thismodified PP protocol against collective attacks when the noisy and lossychannel is taken into account. Simulation results show that our protocol ispractical.
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