Sustained macroscopic coherence in a superradiant diamond maser under ambient conditions

Superradiant laser or maser provides a route to sustain macroscopic coherence of many photons and a large ensemble of emitters. Its realization, however, is mostly in gaseous atomic systems, where the number of emitters is relatively small. Solids can accommodate a large number of emitters, but the complex environments can rapidly break the macroscopic coherence and therefore stringent conditions like extremely low temperature are usually required. Here we realize a superradiant diamond maser under ambient conditions, in which optical pumping sustains long-lived coherence among about 10^14 nitrogen-vacancy centre spins with coherence time (2.1 ms) much longer than the spin dephasing time (5.7 us) and the cavity photon lifetime (0.66 us). Continuous-wave emission reaches an output power of -52.3 dBm, corresponding to an intracavity occupation of approximately 10^9 microwave photons.By tuning the pump and cavity conditions, we observe continuous-wave masing, periodic amplitude modulation and sequences of superradiant bursts, which are attributed, correspondingly, to macroscopic spin coherence synchronized at a fixed frequency, a synchronized limit cycle with a frequency comb, and unsynchronized superradiant transients. This work establishes a new platform for exploring macroscopic quantum coherence, with potential applications in quantum metrology and bright quantum light sources.