Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Coupling of an erbium spin ensemble to a superconducting resonator   A quantum coherent interface between optical and microwave photons can be
used as a basic building block within a future quantum information network. The
interface is envisioned as an ensemble of rare-earth ions coupled to a
superconducting resonator, allowing for coherent transfer between optical and
microwave photons. Towards this end, we have realized a hybrid device coupling
a Er$^{3+}$ doped Y$_2$SiO$_5$ crystal in a superconducting coplanar waveguide
cavity. We observe a collective spin coupling of 4 MHz and a spin linewdith of
down to 75 MHz.

17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
30
The categories of this paper are: q-alg
18
The categories of this paper are: econ
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an