Lifetime and Coherence of the Coupled Electron and Nuclear Spin Systems in Semiconductor Quantum Dots

This thesis presents experimental investigations into the underlying mechanisms limiting the spin state longevity of III-V InGaAs/GaAs self-assembled quantum dots (QDs), a key step towards realising QDs as a viable quantum computing resource. We investigate the coupled electron and nuclear spin system within the QD using a range of optical and nuclear magnetic resonance techniques to provide an in-depth understanding of the sources causing spin relaxation and decoherence.

Several InGaAs charge-tuneable structures were investigated with varying Fermi reservoir tunnel coupling. We present a comprehensive study of the spin lifetimes, T_1, for the electron and nuclear spin systems for a range of magnetic fields, charge states and tunnel couplings. We combine previously observed mechanisms affecting T_1 to estimate the fundamental limits of the spin lifetimes, in addition to demonstrating measurement of the longest observed InGaAs QD electron spin lifetime of ≈ 1 s.

Existing work suggested nuclear spin ensemble coherence time T_(2,N) is strongly affected by the presence of an electron, limiting the prospects of QD qubits. We reveal a T_(2,N) on the scale of milliseconds in a charged QD, in addition to developing a spectral diffusion model to explain nuclear spin decoherence in the presence of a fluctuating electron spin. We show that the nuclear spin bath can be used as an electron spin state sensor with a readout fidelity of F > 99.8%, improving on state of the art spin sensing techniques.

In addition to the studies on QDs, we present results from the testing of recently developed keyhole resonators to be used for fast coherent control of electron spins using magnetic resonance techniques. We demonstrate the ability of keyhole resonator to coherently control electron spins in diamond with high power microwave pulses and pulse lengths on the scale of tens of nanoseconds.