  The quantum digital signature protocol offers a replacement for most aspects
of public-key digital signatures ubiquitous in today's digital world. A major
advantage of a quantum-digital-signatures protocol is that it can have
information-theoretic security, whereas public-key cryptography cannot. Here we
demonstrate and characterize hardware to implement entanglement-based quantum
digital signatures over our campus network. Over 25 hours, we collect
measurements on our campus network, where we measure sufficiently low quantum
bit error rates (<5% in most cases) which in principle enable quantum digital
signatures at over 50 km as shown through rigorous simulation accompanied by a
noise model developed specifically for our implementation. These results show
quantum digital signatures can be successfully employed over deployed fiber.
Moreover, our reported method provides great flexibility in the number of
users, but with reduced entanglement rate per user. Finally, while the current
implementation of our entanglement-based approach has a low signature rate,
feasible upgrades would significantly increase the signature rate.
