  Quantum time crystals are systems characterised by spontaneously emerging
periodic order in the time domain. A range of such phases has been reported.
The concept has even been discussed in popular literature, and deservedly so:
while the first speculation on a phase of broken time translation symmetry did
not use the name "time crystal", it was later adopted from 1980's popular
culture. For the physics community, however, the ultimate qualification of a
new concept is its ability to provide predictions and insight. Confirming that
time crystals manifest the basic dynamics of quantum mechanics is a necessary
step in that direction. We study two adjacent quantum time crystals
experimentally. The time crystals, realised by two magnon condensates in
superfluid $^3$He-B, exchange magnons leading to opposite-phase oscillations in
their populations -- AC Josephson effect -- while the defining periodic motion
remains phase coherent throughout the experiment.
