  Coherence time is an essential parameter for quantum sensing, quantum
information, and quantum computation. In this work, we demonstrate electron
spin coherence times as long as 0.1 s for an ensemble of rubidium atoms trapped
in a solid parahydrogen matrix. We explore the underlying physics limiting the
coherence time. The properties of these matrix isolated atoms are very
promising for future applications, including quantum sensing of nuclear spins.
If combined with efficient single-atom readout, this would enable NMR and
magnetic resonance imaging of single molecules cotrapped with alkali-metal atom
quantum sensors within a parahydrogen matrix.
