  Physics at the Planck scale could be revealed by looking for tiny violations
of fundamental symmetries in low energy experiments. In 2008, a sensitive test
of the isotropy of the Universe using has been performed with stored ultracold
neutrons (UCN), this is the first clock-comparison experiment performed with
free neutrons. During several days we monitored the Larmor frequency of neutron
spins in a weak magnetic field using the Ramsey resonance technique. An
non-zero cosmic axial field, violating rotational symmetry, would induce a
daily variation of the precession frequency. Our null result constitutes one of
the most stringent tests of Lorentz invariance to date.
