Context:
In a measurement on a BEC of non-entangled atoms, on average half of the atoms are found in the ground state (spin downwards), the other half in the excited state (spin upwards). "Deviations from this mean value that occur from measurement to measurement, lead to a quantum noise that is evenly distributed among the
spin components orthogonal to the mean spin," adds Pascal Böhi, another doctoral student.
In order to investigate the influence of the state-dependent potential on the quantum noise the scientists determined the noise for each spin component using yet another microwave pulse. As they could clearly demonstrate, for one
spin component the noise could be "squeezed" below the limit given by the Heisenberg uncertainty relation. From the observed noise reduction the scientists concluded that inside the BEC clusters of at least four atoms are entangled.
More:
http://www.sciencedaily.com/releases/2010/04/100401101810.htm