  The preparation of completely non-polarized light is seemingly easy: an
everyday example is sunlight. The task is much more difficult if light has to
be in a pure quantum state, as required by most quantum-technology
applications. The pure quantum states of light obtained so far are either
polarized or, in rare cases, manifest hidden polarization: even if their
intensities are invariant to polarization transformations, higher-order moments
are not. We experimentally demonstrate the preparation of the macroscopic
singlet Bell state, which is pure, completely non-polarized, and has no
polarization noise. Simultaneous fluctuation suppression in three Stokes
observables below the shot-noise limit is demonstrated, opening perspectives
for noiseless polarization measurements. The state is shown to be invariant to
polarization transformations. This robust highly entangled isotropic state
promises to fuel important applications in photonic quantum technologies.
