Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Balanced Detection in Femtosecond X-ray Absorption Spectroscopy to Reach
  the Ultimate Sensitivity Limit   X-ray absorption spectroscopy (XAS) is a powerful and well established
technique with sensitivity to elemental and chemical composition. Despite these
advantages, its implementation has not kept pace with the development of
ultrafast pulsed x-ray sources where XAS can capture femtosecond chemical
processes. X-ray Free Electron Lasers (XFELs) deliver femtosecond, narrow
bandwidth ($\frac{\Delta E}{E} < 0.5\%$) pulses containing $\sim 10^{10}$
photons. However, the energy contained in each pulse fluctuates thus
complicating pulse by pulse efforts to quantify the number of photons.
Improvements in counting the photons in each pulse have defined the state of
the art for XAS sensitivity. Here we demonstrate a final step in these
improvements through a balanced detection method that approaches the photon
counting shot noise limit. In doing so, we obtain high quality absorption
spectra from the insulator-metal transition in VO$_2$ and unlock a method to
explore dilute systems, subtle processes and nonlinear phenomena with ultrafast
x-rays. The method is especially beneficial for x-ray light sources where
integration and averaging are not viable options to improve sensitivity.

20
The categories of this paper are: acc-phys
31
The categories of this paper are: ao-sci
24
The categories of this paper are: dg-ga
2
The categories of this paper are: math
21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
6
The categories of this paper are: astro-ph
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom