  Quantum digital signatures apply quantum mechanics to the problem of
guaranteeing message integrity and non-repudiation with information-theoretical
security, which are complementary to the confidentiality realized by quantum
key distribution. Previous experimental demonstrations have been limited to
transmission distances of less than 5-km of optical fiber in a laboratory
setting. Here we report the first demonstration of quantum digital signatures
over installed optical fiber as well as the longest transmission link reported
to date. This demonstration used a 90-km long differential phase shift quantum
key distribution system to achieve approximately one signed bit per second - an
increase in the signature generation rate of several orders of magnitude over
previous optical fiber demonstrations.
