Single-photon sources that emit photons at the same energy play a key role in the emerging concepts of quantum information, such as entanglement swapping, quantum teleportation and quantum networks. They can be realized in a variety of systems, where
semiconductor quantum dots, or artificial atoms, are arguably among the most attractive. However, unlike natural atoms, no two artificial atoms are alike. This peculiarity is a serious hurdle for quantum information applications that require photonic quantum states with identical energies. Here we demonstrate a single artificial atom that generates photons with an absolute energy that is locked to an optical transition in a natural atom. Furthermore, we show that our system is robust and immune to drifts and fluctuations in the environment of the emitter. Our demonstration is crucial for realization of a large number of universally-indistinguishable solid-state systems at arbitrary remote locations, where frequency-locked artificial atoms might become fundamental ingredients.
We study the absorption and emission polarization of single semiconductor quantum dots in semiconductor nanowires. We show that the polarization of light absorbed or emitted by a nanowire quantum dot strongly depends on the orientation of the nanowir
e with respect to the directions along which light is incident or emitted. Light is preferentially linearly polarized when directed perpendicular to the nanowire elongation. In contrast, the degree of linear polarization is low for light directed along the nanowire. This result is vital for photonic applications based on intrinsic properties of quantum dots, such as generation of entangled photons. As an example, we demonstrate optical access to the spin states of a single nanowire quantum dot.
