AC Josephson effect between two superfluid time crystals

Quantum time crystals are systems characterized by spontaneously emerging periodic order in the time domain. While originally a phase of broken time translation symmetry was a mere speculation, a wide range of time crystals has been reported. However, the dynamics and interactions between such systems have not been investigated experimentally. Here we study two adjacent quantum time crystals realized by two magnon condensates in superfluid ³He-B. We observe an exchange of magnons between the time crystals leading to opposite-phase oscillations in their populations - a signature of the AC Josephson effect - while the defining periodic motion remains phase coherent throughout the experiment. Our results demonstrate that time crystals obey the general dynamics of quantum mechanics and offer a basis to further investigate the fundamental properties of these phases, opening pathways for possible applications in developing fields, such as quantum information processing.