A thulium-doped crystal is experimentally shown to be an excellent candidate for broadband quantum storage in a solid-state medium. For the first time, nuclear spin coherence is optically excited, detected and characterized in such a crystal. The lifetime of the spin coherence -the potential storage entity -is measured by means of Raman echo to be about 300 µs over a wide range of ground state splittings. This flexibility, attractive for broadband operation, and well fitted to existing quantum sources, results from the simple hyperfine structure, contrasting with Pr-and Eu-doped crystals.So far, the mapping of a quantum state of light onto an atomic ensemble has been implemented in atomic vapors [1] and cold atom clouds [2]. Because of their long optical coherence lifetimes at low temperature, rare-earth ion-doped crystals (REIC) have been extensively investigated for optical data storage [3] and data processing [4,5]. There has recently been renewed interest in these materials, stimulated by proposals to explore their adequacy to quantum memories [6,7,8,9,10]. To some extent, REIC at low impurity concentration are similar to atomic vapors with the advantage of no atomic diffusion.Most optical quantum memory protocols rely on the transfer of a quantum state of light into a long-lived atomic spin coherence that is free from decoherence via spontaneous emission. This can be achieved in a Λ-type three-level system where two hyperfine or spin sublevels are optically connected to a common upper level. The presence of a Λ-system ensures efficient coupling between light and matter together with long storage times. A Λ-system also represents a basic device where, in a simple way, the transition to be excited by the quantum field can be triggered for storage or restitution by an external control field. In the prospect of quantum storage, electromagnetically induced transparency (EIT) protocols involving Λ-type have been studied in praseodymium-doped materials [6,7].It is noteworthy that the essence of Λ-system operation, namely the optical excitation of nuclear spin coherence, has been practised in REIC for almost 30 years [11,12,13,14,15,16], but always in praseodymium-or europium-doped crystals. However, quantum storage demonstration in Pr-or Eudoped compounds is limited by the smallness of their hyperfine structure, not really matching the bandwidth of existing quantum sources. We recently demonstrated the existence of a Λ-system with adjustable ground state splitting in thuliumdoped YAG [17]. Widely adjustable splitting might help to match the bandwidth of existing quantum sources. In this Letter, for the first time to the best of our knowledge, we optically excite, characterize and detect the nuclear spin coherence in the electronic ground state of a Tm-doped crystal.In crystals doped with Pr 3+ or Eu 3+ , Λ-systems are built on the hyperfine structure of the ground level with a sublevel splitting up to a few tens of MHz. This spacing represents the memory bandwidth. Indeed, the two transitions of the Λ cannot be po...