  State-of-the-art quantum key distribution systems are based on the BB84
protocol and single photons generated by lasers. These implementations suffer
from range limitations and security loopholes, which require expensive
adaptation. The use of polarization entangled photon pairs substantially
alleviates the security threads while allowing for basically arbitrary
transmission distances when embedded in quantum repeater schemes. Semiconductor
quantum dots are capable of emitting highly entangled photon pairs with
ultra-low multi-pair emission probability even at maximum brightness. Here we
report on the first implementation of the BBM92 protocol using a quantum dot
source with an entanglement fidelity as high as 0.97(1). For a proof of
principle, the key generation is performed between two buildings, connected by
350 metre long fiber, resulting in an average key rate of 135 bits/s and a
qubit error rate of 0.019 over a time span of 13 hours, without resorting to
time- or frequency-filtering techniques. Our work demonstrates the viability of
quantum dots as light sources for entanglement-based quantum key distribution
and quantum networks. By embedding them in state-of-the-art photonic
structures, key generation rates in the Gbit/s range are at reach.
