Demonstration of Entanglement-Assisted Communication Surpassing the Ultimate Classical Channel Capacity*

The seminal work by Bennett, Shor, Smolin, and Thapliyal showed that pre-shared entanglement between communication parties can be harnessed to increase the rate of reliable classical communication over noisy and lossy channels, known as entanglement-assisted communication (EACOMM). Despite the advances of quantum technology in the last a few decades, EACOMM surpassing the ultimate classical channel capacity has never been experimentally demonstrated. We report the construction of an efficient entangled-photon source and a nontraditional quantum phase-conjugate receiver to realize EACOMM over a lossy and noisy bosonic channel. We show that EACOMM beats the classical capacity of the channel, quantified by the Holevo-Schumacher-Westmoreland formula, by up to 14.6% even though the initial entanglement is completely destroyed by loss and noise. As a practical performance benchmark, a classical communication protocol without entanglement assistance is implemented over the same bosonic channel, showing that EACOMM reduces the bit-error rate by up to 34%. Our experiment would open a promising avenue for entanglement-enhanced performance in deep-space communications, remote sensing, and absorption spectroscopy.