  Macroscopic quantum phenomena such as lasers, Bose-Einstein condensates,
superfluids, and superconductors are of great importance in foundations and
applications of quantum mechanics. In particular, quantum superposition of a
large number of spins in solids is highly desirable for both quantum
information processing and ultrasensitive magnetometry. Spin ensembles in
solids, however, have rather short collective coherence time (typically less
than microseconds). Here we demonstrate that under realistic conditions it is
possible to maintain macroscopic quantum superposition of a large spin ensemble
(such as about ~10^{14} nitrogen-vacancy center electron spins in diamond) with
an extremely long coherence time ~10^8 sec under readily accessible conditions.
The scheme, following the mechanism of superradiant lasers, is based on
superradiant masing due to coherent coupling between collective spin
excitations (magnons) and microwave cavity photons. The coherence time of the
macroscopic quantum superposition is the sum of the magnon life time and the
cavity lifetime, further elongated by the total number of coherent magnons and
photons, which have macroscopic values when masing occurs. The macroscopic
quantum coherence of spin ensembles can be exploited for magnetometry with
sensitivity ~10 fT/(Hz)^{1/2}. The long-living collective states of spin
ensembles in solids will provide a new platform for studying macroscopic
quantum phenomena and are useful for quantum technologies.
