Hyperfine spectroscopy toward microwave quantum memory using rare-earth materials

A solution for practical quantum information processing is to develop quantum memories that store microwave photons. Today, most quantum memories work on optical modes while many quantum information systems operate in the microwave regime and are limited by relatively short coherence times. Rare-earth atoms in solids are a promising platform for both optical quantum memory and microwave to optical quantum transduction due to their extremely long coherence times, high densities of emitters, and more. In particular, certain isotopes with GHz-scale hyperfine splittings (including ¹⁶⁷Er³⁺, ¹⁴⁵Nd³⁺, and ¹⁷¹Yb³⁺) in yttrium-oxide crystalline hosts are well-suited for microwave quantum memory due to their minimal inhomogeneous broadening and optical addressability for spectroscopic investigations. For most microwave-regime quantum memory protocols, minimizing the inhomogeneous broadening of the spin transition is vital. We will present spectroscopic investigations of the hyperfine states of rare-earth ensembles at cryogenic temperatures to determine the dependence of the inhomogeneous broadening on temperature, magnetic field, doping concentration, and host material. We will discuss the implications on future microwave quantum memories with rare-earth doped crystals.