Ultranarrow Optical Inhomogeneous Linewidth in a Stoichiometric Rare Earth Crystal.
We obtain a low optical inhomogeneous linewidth of 25 MHz in the stoichiometric rare earth crystal EuCl_{3}*6H_{2}O by isotopically purifying the crystal in {35}Cl. With this linewidth, an important limit for stoichiometric rare earth crystals is surpassed: the hyperfine structure of {153}Eu is spectrally resolved, allowing the whole population of {153}Eu^{3+ ions to be prepared in the same hyperfine state using hole burning techniques. This material also has a very high optical density, and can have long coherence times when deuterated. This combination of properties offers new prospects for quantum information applications. We consider two of these: quantum memories and quantum many body studies. We detail the improvements in the performance of current memory protocols possible in these high optical depth crystals, and describe how certain memory protocols, such as off resonant Raman memories, can be implemented for the first time in a solid state system. We explain how the strong excitation induced interactions observed in this material resemble those seen in Rydberg systems, and describe how these interactions can lead to quantum many body states that could be observed using standard optical spectroscopy techniques.
Physical review letters