Qubit dynamics under alternating controls

[摘要] In this thesis, we discuss two problems of quantum dynamics in the presence of alternating controls. Alternating controls arise in many protocols designed to extend the duration over which a qubit is a useful computational resource. This is accomplished by control sequences that either retard decoherence, or that accomplish a quantum operation in as short a time as possible. The first problem tackles the use of a composite-pulse control sequence known as ;;rotary-echo;; for quantum magnetometry purposes. The sequence consists in the continuous drive of a qubit, with field phases that alternate at specific intervals. We implement such a magnetometry protocol using an electronic qubit in diamond, and experimentally confirm the flexibility yielded by the tuning of sequence parameters that achieves a good compromise between decoherence resilience and sensitivity. The second problem theoretically investigates the time-optimal evolution of a qubit in the case of a restricted control set composed of alternating rotations around two non-parallel axes on the Bloch sphere. Using accessible algebraic methods, we show that experimental parameters, such as the angle between the two rotation axes, restrict the necessary structure of time-optimal sequences. We propose to implement such an evolution through alternate driving as an advantageous alternative to the slow, noisy direct addressing of a nuclear qubit anisotropically hyperfine-coupled to an electronic spin in diamond.

[发布机构] Massachusetts Institute of Technology