  Quantum digital signature (QDS) is an approach to guarantee the
nonrepudiation, unforgeability and transferability of a signature with the
information-theoretical security. All previous experimental realizations of QDS
relied on an unrealistic assumption of secure channels and the longest distance
is only several kilometers. Here, we have experimentally demonstrated a
recently proposed QDS protocol without any secure channel. Exploiting the decoy
state modulation, we have successfully signed one bit message through up to 102
km optical fiber. Furthermore, we continuously run the system to sign the
longer message "USTC" with 32 bit at the distance of 51 km. Our results pave
the way towards the practical application of QDS.
