Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct measurement of the van der Waals interaction between two Rydberg
  atoms   We report on the direct measurement of the van der Waals interaction between
two isolated, single Rydberg atoms separated by a controlled distance of a few
micrometers. By working in a regime where the single-atom Rabi frequency of the
laser used for excitation to the Rydberg state is comparable to the interaction
energy, we observe a \emph{partial} Rydberg blockade, whereby the
time-dependent populations of the various two-atom states exhibit coherent
oscillations with several frequencies. A quantitative comparison of the data
with a simple model based on the optical Bloch equations allows us to extract
the van der Waals energy, and to observe its characteristic $C_6/R^6$
dependence. The magnitude of the measured $C_6$ coefficient agrees well with an
\emph{ab-initio} theoretical calculation, and we observe its dramatic increase
with the principal quantum number $n$ of the Rydberg state. Our results not
only allow to test an important physical law, but also demonstrate a degree of
experimental control which opens new perspectives in quantum information
processing and quantum simulation using long-range interactions between the
atoms.

25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
2
The categories of this paper are: math
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
18
The categories of this paper are: econ
19
The categories of this paper are: eess
24
The categories of this paper are: dg-ga