  The maximum distance of quantum communication is limited due to the photon
loss and detector noise. Exploiting entanglement swapping, quantum relay could
offer ways to extend the achievable distance by increasing the signal to noise
ratio. In this letter we present an experimental simulation of long distance
quantum communication, in which the superiority of quantum relay is
demonstrated. Assisted by quantum relay, we greatly extend the distance limit
of unconditional secure quantum communication.
