  Time crystals are periodic states exhibiting spontaneous symmetry breaking in
either time-independent or periodically forced quantum many-body systems.
Spontaneous modification of discrete time translation symmetry in a
periodically driven physical system can create a discrete time crystal (DTC).
DTCs constitute a state of matter with properties such as temporal rigid
long-range order and coherence which are inherently desirable for quantum
computing and quantum information processing. Despite their appeal,
experimental demonstrations of DTCs are scarce and hence many significant
aspects of their behavior remain unexplored. Here, we report the experimental
observation and theoretical investigation of photonic DTCs in a Kerr-nonlinear
optical microcavity. Empowered by the simultaneous self-injection locking of
two independent lasers with arbitrarily large frequency separation to two
cavity modes and a dissipative soliton, this room-temperature all-optical
platform enables observing novel states like DTCs carrying defects, and
realizing long-awaited phenomena such as DTC phase transitions and mutual
interactions. Combined with monolithic fabrication, it can result in chip-scale
DTCs, paving the way for liberating time crystals from sophisticated laboratory
setups and propelling them toward real-world applications.
