  Recently, a new category of superfluids and superconductors has been
discovered in various systems. These could be linked to the idea of a
supersolid phase, featuring a macroscopic wavefunction with spatial modulation
resulting from simultaneous, spontaneous breaking of gauge and translational
symmetries. However, this relation has only been recognized in some cases and
there is the need for universal properties quantifying the differences between
supersolids and ordinary superfluids/superconductors or crystals. A key
property is the superfluid fraction, which measures the reduction in superfluid
stiffness due to spatial modulation, leading to the non-standard superfluid
dynamics of supersolids. Here we employ the Josephson effect, common in
superfluids and superconductors, to measure the superfluid fraction in a
supersolid. Even without a physical barrier, the Josephson effect arises
spontaneously in a supersolid due to spatial modulation. Individual lattice
cells act as self-induced Josephson junctions, allowing the direct
determination of the local superfluid fraction. We studied a cold-atom dipolar
supersolid, revealing a significant sub-unity superfluid fraction. Our results
open new research directions, enabling the exploration of novel phenomena like
partially quantized vortices and supercurrents, potentially unifying the
understanding of supersolid-like systems, and introducing a new type of
Josephson junction.
