  An on-demand source of bright entangled photon pairs is desirable for quantum
key distribution (QKD) and quantum repeaters. The leading candidate to generate
entangled photon pairs is based on spontaneous parametric down-conversion
(SPDC) in a non-linear crystal. However, there exists a fundamental trade-off
between entanglement fidelity and efficiency in SPDC sources due to multiphoton
emission at high brightness, which limits the pair extraction efficiency to
0.1% when operating at near-unity fidelity. Quantum dots in photonic
nanostructures can in principle overcome this trade-off; however, the quantum
dots that have achieved entanglement fidelities on par with SPDC sources (99%)
have poor pair extraction efficiencies of 0.01%. Here, we demonstrate a 65-fold
increase in the pair extraction efficiency compared to quantum dots with
equivalent peak fidelity from an InAsP quantum dot in a photonic nanowire
waveguide. We measure a raw peak concurrence and fidelity of 95.3% $\pm$ 0.5%
and 97.5% $\pm$ 0.8%, respectively. Finally, we show that an oscillating
two-photon Bell state generated by a semiconductor quantum dot can be utilized
to establish a secure key for QKD, alleviating the need to remove the quantum
dot energy splitting of the intermediate exciton states in the
biexciton-exciton cascade.
