Multiple Round Quantum Dense Coding And Its Implementation Using Nuclear   Magnetic Resonance
  A multiple round quantum dense coding (MRQDC) scheme based on the quantum phase estimation algorithm is proposed. Using an $m+1$ qubit system, Bob can transmit $2^{m+1}$ messages to Alice, through manipulating only one qubit and exchanging it between Alice and Bob for $m$ rounds. The information capacity is enhanced to $m+1$ bits. We have implemented the scheme in a three- qubit nuclear magnetic resonance (NMR) quantum computer. The experimental results show a good agreement between theory and experiment. 
Jingfu Zhang, Jingyi Xie, Chuan Wang, Zhiwei Deng, Zhiheng Lu, and   Guilu Long