Philip Walther

Despite the range of security guarantees offered by the laws of quantum theory, quantum-cryptographic schemes all face the same practical challenge: participants must ensure that the messages they receive originate from the right person. This authentication step is typically achieved by integrating classical methods, assumed hard for quantum computers, into quantum systems. Here, we propose an alternative solution to replace a central link of the authentication chain with a quantum commitment scheme that is information-theoretically secure against attackers holding a perfect quantum memory of thousands of qubits. In this way, all other parts of the authentication rely only on the fundamental and widely accepted conjecture that P/=NP. Our protocol can be implemented with all standard sources such as down-conversion and highly-attenuated laser states. We however employ a single-photon source emitting directly in the telecom C-band to show how its performance may be boosted by several orders of magnitude, even at moderate channel loss. Over a deployed optical fiber link, we achieve 128-bit security by detecting 10^9 photons, allowing for 66 secure re-uses of the public-private key pair.