Bidirectional quantum key distribution protocol with practical faint laser pulses

We present a two-way protocol for the quantum key distribution with practical faint laser pulses. It is secure when the faint laser pulses contain no more than two photons. The key distribution task is completed in two transmissions. Bob first sends the laser pulses to Alice, and Alice encodes the key message through certain unitary operations and returns the laser pulses to Bob. Security is achieved by placing eavesdropping check procedures in both transmissions. This protocol is secure and is close to practical conditions. In addition, it does not require the exchange of measuring basis information between Alice and Bob, hence saving a lot of storage space.