Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct measurement of superluminal group velocity and of signal velocity
  in an optical fiber   We present an easy way of observing superluminal group velocities using a
birefringent optical fiber and other standard devices. In the theoretical
analysis, we show that the optical properties of the setup can be described
using the notion of "weak value". The experiment shows that the group velocity
can indeed exceed c in the fiber; and we report the first direct observation of
the so-called "signal velocity", the speed at which information propagates and
that cannot exceed c.

14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
25
The categories of this paper are: patt-sol
0
The categories of this paper are: hep-ph
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
15
The categories of this paper are: nucl-ex
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
5
The categories of this paper are: gr-qc
27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
6
The categories of this paper are: astro-ph