Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An artificial atom locked to natural atoms   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.

17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
21
The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
14
The categories of this paper are: math-ph
18
The categories of this paper are: econ
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio