  The success of the emerging field of solid-state optical quantum information
processing (QIP) critically depends on the access to resonant optical
materials. Rare-earth ions (REIs) are suitable candidates for QIP protocols due
to their extraordinary photo-physical and magnetic quantum properties such as
long optical and spin coherence lifetimes ($T_2$). However, molecules
incorporating REIs, despite having advantageous properties such as atomically
exact quantum tunability, inherent scalability, and large portability, have not
yet been studied for QIP applications. As a first testimony of the usefulness
of REI molecules for optical QIP applications, we demonstrate in this study
that narrow spectral holes can be burned in the inhomogeneously broadened
$^5$D$_0\to^7$F$_0$ optical transition of a binuclear Eu(III) complex,
rendering a homogeneous linewidth ($\Gamma_h$) = 22 $\pm$ 1 MHz, which
translates as $T_2 = 14.5$ $\pm$ 0.7 ns at 1.4 K. Moreover, long-lived spectral
holes are observed, demonstrating efficient polarization of Eu(III) ground
state nuclear spins, a fundamental requirement for all-optical spin
initialization and addressing. These results elucidate the usefulness of
REI-based molecular complexes as versatile coherent light-spin interfaces for
applications in quantum communications and processing.
