Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Active optical frequency standards using cold atoms: perspectives and
  challenges   We consider various approaches to the creation of a high-stability active
optical frequency standard, where the atomic ensemble itself produces a highly
stable and accurate frequency signal. The short-time frequency stability of
such standards may overcome the stability of lasers stabilized to macroscopic
cavities which are used as local oscillators in the modern optical frequency
standard systems. The main idea is to create a "superradiant" laser operating
deep in the bad cavity regime, where the decay rate of the cavity field
significantly exceeds the decoherence rate of the lasing transition. Two main
approaches towards the realization of an active optical frequency standard have
been proposed already: the optical lattice laser, and the atomic beam laser. We
consider these and some alternative approaches, and discuss the parameters for
atomic ensembles necessary to attain the metrology relevant level of short-time
frequency stability, and various effects and main challenges critical for
practical implementations.

20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
30
The categories of this paper are: q-alg
2
The categories of this paper are: math
34
The categories of this paper are: chem-ph
16
The categories of this paper are: stat
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
24
The categories of this paper are: dg-ga
4
The categories of this paper are: cond-mat
33
The categories of this paper are: plasm-ph
26
The categories of this paper are: bayes-an