Minimising the Decoherence of Rare Earth Ion Solid State Spin Qubits

Abstract

This work has demonstrated that hyperfine decoherence times sufficiently long for QIP and quantum optics applications are achievable in rare earth ion centres. Prior to this work there were several QIP proposals using rare earth hyperfine states for long term coherent storage of optical interactions [1, 2, 3]. The very long T1 (∼weeks [4]) observed for rare-earth hyperfine transitions appears promising but hyperfine T2s were only a few ms, comparable to rareearth optical transitions and therefore the usefulness of such proposals was doubtful. This work demonstrated an increase in hyperfine T2 by a factor of ∼ 7 × 10 compared to the previously reported hyperfine T2 for Pr:Y2SiO5 through the application of static and dynamic magnetic field techniques. This increase in T2 makes previous QIP proposals useful and provides the first solid state optically active Λ system with very long hyperfine T2 for quantum optics applications. The first technique employed the conventional wisdom of applying a small static magnetic field to minimise the superhyperfine interaction [5, 6, 7], as studied in chapter 4. This resulted in hyperfine transition T2 an order of magnitude larger than the T2 of optical transitions, ranging fro 5 to 10 ms. The increase in T2 was not sufficient and consequently other approaches were required. Development of the critical point technique during this work was crucial to achieving further gains in T2. The critical point technique is the application of a static magnetic field such that the Zeeman shift of the hyperfine transition of interest has no first order component, thereby nulling decohering magnetic interactions to first order. This technique also represents a global minimum for back action of the Y spin bath due to a change in the Pr spin state, allowing the assumption that the Pr ion is surrounded by a thermal bath. The critical point technique resulted in a dramatic increase of the hyperfine transition T2 from ∼10 ms to 860 ms. Satisfied that the optimal static magnetic field configuration for increasing T2 had been achieved, dynamic magnetic field techniques, driving ei-