  The ability of magnetic materials to modify superconducting systems is an
active research area for possible applications in thermoelectricity, quantum
sensing, and spintronics. We consider the fundamental properties of the
Josephson effect in a third class of magnetic materials beyond ferromagnets and
antiferromagnets: altermagnets. We show that despite having no net
magnetization, altermagnets induce $0$-$\pi$ oscillations. The decay length and
oscillation period of the Josephson coupling are qualitatively different from
ferromagnetic junctions and depend on the crystallographic orientation of the
altermagnet. The Josephson effect in altermagnets thus serves a dual purpose:
it acts as a signature that distinguishes altermagnetism from conventional
(anti)ferromagnetism and offers a way to tune the supercurrent via flow
direction anisotropy.
