  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.
