Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical frequency transfer via a 660 km underground fiber link using a
  remote Brillouin amplifier   In long-haul optical continuous-wave frequency transfer via fiber, remote
bidirectional Er$^+$-doped fiber amplifiers are commonly used to mitigate
signal attenuation. We demonstrate for the first time the ultrastable transfer
of an optical frequency using a remote fiber Brillouin amplifier, placed in a
server room along the link. Using it as the only means of remote amplification,
on a 660 km loop of installed underground fiber we bridge distances of 250 km
and 160 km between amplifications. Over several days of uninterrupted
measurement we find an instability of the frequency transfer (Allan deviation
of $\Lambda$-weighted data with 1 s gate time) of around $1\times10^{-19}$ and
less for averaging times longer than 3000 s. The modified Allan deviation
reaches $3\times10^{-19}$ at an averaging time of 100 s, corresponding to the
current noise floor at this averaging time. For averaging times longer than
1000 s the modified Allan deviation is in the $10^{-20}$ range. A conservative
value of the overall accuracy is $1\times10^{-19}$.

12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
34
The categories of this paper are: chem-ph
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
10
The categories of this paper are: q-bio
5
The categories of this paper are: gr-qc
6
The categories of this paper are: astro-ph
1
The categories of this paper are: cs
29
The categories of this paper are: funct-an
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
8
The categories of this paper are: hep-ex