  Isolated Neutron Stars are some of the most exciting stellar objects known to
astronomers: they have the most extreme magnetic fields, with values up to
$10^{15}$ G, and, with the exception of stellar-mass black holes, they are the
most dense stars, with densities of $\approx 10^{14}$ g cm$^{-3}$. As such,
they are perfect laboratories to test theories of electromagnetism and nuclear
physics under conditions of magnetic field and density unattainable on Earth.
In particular, the interaction of radiation with strong magnetic fields is the
cause of the {\em vacuum birefringence}, an effect predicted by quantum
electrodynamics in 1936 but that lacked an observational evidence until now.
Here, we show how the study of the polarisation of the optical radiation from
the surface of an isolated neutron star yielded such an observational evidence,
opening exciting perspectives for similar studies at other wavelengths.
