  The notion of spontaneous symmetry breaking has been well established to
characterize classical and quantum phase transitions of matter, such as in
condensation, crystallization or quantum magnetism. Generalizations of this
paradigm to the time dimension can lead to a time crystal phase, which
spontaneously breaks the time translation symmetry of the system. Whereas the
existence of a continuous time crystal at equilibrium has been challenged by
no-go theorems, this difficulty can be circumvented by dissipation in an open
system. Here, we report the experimental observation of such dissipative time
crystalline order in a room-temperature atomic gas, where ground-state atoms
are continuously driven to Rydberg states. The emergent time crystal is
revealed by persistent oscillations of the photon transmission, and we show
that the observed limit cycles arise from the coexistence and competition
between distinct Rydberg components. The nondecaying autocorrelation of the
oscillation, together with the robustness against temporal noises, indicate the
establishment of true long-range temporal order and demonstrates the
realization of a continuous time crystal.
