Experimental unconditionally secure bit commitment

Quantum physics allows unconditionally secure communication between parties that trust each other. However, when they do not trust each other such as in the bit commitment, quantum physics is not enough to guarantee security. Only when relativistic causality constraints combined, the unconditional secure bit commitment becomes feasible. Here we experimentally implement a quantum bit commitment with relativistic constraints that offers unconditional security. The commitment is made through quantum measurements in two quantum key distribution systems in which the results are transmitted via free-space optical communication to two agents separated with more than $20$ km. Bits are successfully committed with less than $5.68\times10^{-2}$ cheating probability. This provides an experimental proof of unconditional secure bit commitment and demonstrates the feasibility of relativistic quantum communication.