  Nanoscale systems offer key capabilities for quantum technologies that
include single qubit control and readout, multiple qubit gate operation,
extremely sensitive and localized sensing and imaging, as well as the ability
to build hybrid quantum systems. To fully exploit these functionalities,
multiple degrees of freedom are highly desirable: in this respect, nanoscale
systems that coherently couple to light and possess spins, allow for storage of
photonic qubits or light-matter entanglement together with processing
capabilities. In addition, all-optical control of spins can be possible for
faster gate operations and higher spatial selectivity compared to direct RF
excitation. Such systems are therefore of high interest for quantum
communications and processing. However, an outstanding challenge is to preserve
properties, and especially optical and spin coherence lifetimes, at the
nanoscale. Indeed, interactions with surfaces related perturbations strongly
increase as sizes decrease, although the smallest objects present the highest
flexibility for integration with other systems. Here, we demonstrate optically
controlled nuclear spins with long coherence lifetimes (T2) in rare earth doped
nanoparticles. We observed spins echoes and measured T2 of 2.9 +/- 0.3 ms at 5
K and under a magnetic field of 9 mT, a value comparable to those obtained in
bulk single crystals. Moreover, we achieve, for the first time, spin T2
extension using all-optical spin dynamical decoupling and observe high fidelity
between excitation and echo phases. Rare-earth doped nanoparticles are thus the
only reported nano-materials in which optically controlled spins with
millisecond coherence lifetimes have been observed. These results open the way
to providing quantum light-atom-spin interfaces with long storage time within
hybrid architectures.
