  The security of conventional cryptography systems is threatened in the
forthcoming era of quantum computers. Quantum key distribution (QKD) features
fundamentally proven security and offers a promising option for quantum-proof
cryptography solution. Although prototype QKD systems over optical fiber have
been demonstrated over the years, the key generation rates remain several
orders-of-magnitude lower than current classical communication systems. In an
effort towards a commercially viable QKD system with improved key generation
rates, we developed a discrete-variable QKD system based on time-bin quantum
photonic states that is capable of generating provably-secure cryptographic
keys at megabit-per-second (Mbps) rates over metropolitan distances. We use
high-dimensional quantum states that transmit more than one secret bit per
received photon, alleviating detector saturation effects in the superconducting
nanowire single-photon detectors (SNSPDs) employed in our system that feature
very high detection efficiency (of over 70%) and low timing jitter (of less
than 40 ps). Our system is constructed using commercial off-the-shelf
components, and the adopted protocol can readily be extended to free-space
quantum channels. The security analysis adopted to distill the keys ensures
that the demonstrated protocol is robust against coherent attacks, finite-size
effects, and a broad class of experimental imperfections identified in our
system.
