  The well-known Robertson-Schroedinger uncertainty relations miss an
irreducible lower bound. This is widely attributed to the lower bound's
state-dependence. Therefore, Abbott \emph{et al.} introduced a general approach
to derive tight state-independent uncertainty relations for qubit measurements
[Mathematics 4, 8 (2016)]. The relations are expressed in two measures of
uncertainty, which are standard deviation and entropy, both functions of the
expectation value. Here, we present a neutron optical test of the tight
state-independent preparation uncertainty relations for non-commuting Pauli
spin observables with mixed spin states. The final results, obtained in a
polarimetric experiment, reproduce the theoretical predictions evidently for
arbitrary initial states of variable degree of polarization.
