History
The first experiments searching for the electric dipole moment of the neutron used beams of thermal (and later cold) neutrons to conduct the measurement. It started with the experiment by Smith, Purcell and Ramsey in 1951 (and published in 1957) obtaining a limit of |dn| < 5×10−20e·cm. Beams of neutrons were used until 1977 for nEDM experiments. At this point, systematic effects related to the high velocities of the neutrons in the beam became insurmountable. The final limit obtained with a neutron beam amounts to |dn| < 3×10−24e·cm.
After that, experiments with ultracold neutrons took over. It started in 1980 with an experiment at the Leningrad Nuclear Physics Institute obtaining a limit of |dn| < 1.6×10−24e·cm. This experiment and especially the experiment starting in 1984 at the Institut Laue-Langevin pushed the limit down by another two orders of magnitude yielding the above quoted best upper limit in 2006.
During these 50 years of experiments, six orders of magnitude have been covered thereby putting stringent constraints on more theoretical models than probably any other experimental value.
Read more about this topic: Neutron Electric Dipole Moment
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