Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Room-temperature biphoton source with a spectral brightness near the
  ultimate limit   The biphotons generated from a hot atomic vapor via the process of
spontaneous four-wave mixing (SFWM) have the following merits: stable and
tunable frequencies as well as linewidth. Such merits are very useful in the
applications of long-distance quantum communication. However, the hot-atom SFWM
biphoton sources previously had far lower values of generation rate per
linewidth, i.e., spectral brightness, as compared with the sources of biphotons
generated by the spontaneous parametric down conversion (SPDC) process. Here,
we report a hot-atom SFWM source of 960-kHz biphotons with a generation rate of
3.7$\times$ $10^5$ pairs/s. The high generation rate, together with the narrow
linewidth, results in a spectral brightness of 3.8$\times$ $10^5$ pairs/s/MHz,
which is 17 times of the previous best result with atomic vapors and also
better than all known results with all kinds of media. The all-copropagating
scheme together with a large optical depth (OD) of the atomic vapor is the key
improvement, enabling the achieved spectral brightness to be about one quarter
of the ultimate limit. Furthermore, this biphoton source had a
signal-to-background ratio (SBR) of 2.7, which violated the Cauchy-Schwartz
inequality for classical light by about two-fold. Although an increasing
spectral brightness usually leads to a decreasing SBR, our systematic study
indicates that both of the present spectral brightness and SBR can be enhanced
by further increasing the OD. This work demonstrates a significant advancement
and provides useful knowledge in the quantum technology using photons.

28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
29
The categories of this paper are: funct-an
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
23
The categories of this paper are: solv-int
6
The categories of this paper are: astro-ph
0
The categories of this paper are: hep-ph
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
18
The categories of this paper are: econ
2
The categories of this paper are: math
27
The categories of this paper are: comp-gas
12
The categories of this paper are: nucl-th