  Bit commitment is a fundamental cryptographic task that guarantees a secure
commitment between two mutually mistrustful parties and is a building block for
many cryptographic primitives, including coin tossing, zero-knowledge proofs,
oblivious transfer and secure two-party computation. Unconditionally secure bit
commitment was thought to be impossible until recent theoretical protocols that
combine quantum mechanics and relativity were shown to elude previous
impossibility proofs. Here we implement such a bit commitment protocol. In the
experiment, the committer performs quantum measurements using two quantum key
distribution systems and the results are transmitted via free-space optical
communication to two agents separated with more than 20 km. The security of the
protocol relies on the properties of quantum information and relativity theory.
We show that, in each run of the experiment, a bit is successfully committed
with less than 5.68*10^-2 cheating probability. Our result demonstrates
unconditionally secure bit commitment and the experimental feasibility of
relativistic quantum communication.
