Crystal growth and characterisation

The performance of any quantum information device is ultimately dependent on the material properties of the system used. Of particular importance are the coherence time, optical density, and optical inhomogeneous linewidth. The excellent optical properties of rare earth ions in solids have made them the most outstanding system for quantum memories, and are leading to an ever-developing list of other quantum information applications. However, we are starting to reach the performance limits of current materials.

A major theme of our research is pushing past these limits by developing new materials and protocols. We have focused on increasing the coherence times of rare earths for many years, which has led to three orders of magnitude improvement in just the past ten years. We are also addressing the linked properties of optical density and optical inhomogeneous linewidth, with the aim to see similar improvements.

Figure 1: When isotopically purified in chlorine, Eu³⁵Cl₃.6H₂O has an optical inhomogeneous linewidth of only 25 MHz on its ⁷F₀-⁵D₀ transition, the lowest linewidth seen in any crystal in the visible spectrum. This allows the resolution of the hyperfine structure of the two Eu isotopes, ¹⁵¹Eu and ¹⁵³Eu.

The optical density of a crystal is dependent on the rare earth concentration and the optical inhomogeneous linewidth, which is determined by the crystalline defect density. The doped rare earth crystals traditionally used for quantum information typically have broad linewidths, 100 MHz – 10 GHz, because the dopant itself is a defect that broadens the line.

To avoid this problem, we are concentrating on crystals fully concentrated (stoichiometric) in the rare earth ion, which have narrow linewidths combined with very high optical depths, around 1000 cm^-1. We are studying a series of mostly hydrated stoichiometric rare earth crystals to identify materials with ultra-narrow linewidths and optical properties suited to a variety of quantum information applications. Through a combination of high quality crystal growth and isotopic purification we have seen optical linewidths as low as 25 MHz, which is the lowest linewidth ever observed in the visible.

Selected publications