Spin Decoherence Measurements for Solid State Qubits

This project set out to measure and to understand the decoherence times of nuclear spins in semiconductors, to assess their potential as qubits in solid state quantum computer architectures. Our initial goal was to characterize P-31 nuclei in Silicon doped with Phosphorous SiP. While working towards this goal, we unexpectedly discovered an important discrepancy with the conventional theory of NMR. Specifically, well-known multiple Pi pulse spin echo experiments had the ability either to freeze out or to accelerate the signal decay expected due to the spin-spin dipolar coupling, reminiscent of the quantum Zeno effect. This result has now been seen in many nuclei e.g., Si-29, C-13, Y-89, H-1 in different samples including Silicon and buckyballs, and it is a robust phenomenon. This was probably always present, just not recognized, in magnetic resonance experiments. It appears to be a many-body effect arising from the tiny spin-spin interactions acting during strong, but finite, control pulses. Understanding this puzzle is essential, because it is likely to be relevant to most physical qubits not just spins driven by bang-bang control sequences, or other control pulse sequences.