M.Sc Thesis


M.Sc StudentChen Avinadav
SubjectTime-Optimal Universal Control of Two-Level Systems
under Strong Driving
DepartmentDepartment of Physics
Supervisor Full Professors Gershoni David
Full Thesis textFull thesis text - English Version


Abstract

In this thesis we study the problem of implementing time-optimal quantum gates on two-level systems actings as quantum bits (qubits), in the regime of strong driving, where the driving field has a fixed orientation and bounded amplitude. The method of resonant harmonic excitation at this regime fails due to the breakdown of the rotating wave approximation, and new driving techniques must be considered. Using tools of optimal control theory we prove that time-optimal solutions to this problem consist only of bang pulses, where the qubit is driven by the maximum available control field, and drift periods, where the control field is zero, and explicitly find these sequences by numerical optimization.


We then propose a novel idea for creating a well-controlled strongly-driven qubit system, using radiation-dressed of electron spins in nitrogen-vacancy centers in diamond. We realize this system on a self-built fluorescence microscopy apparatus, and demonstrate a qubit which is driven four times faster than its precession frequency. Using this unique system we demonstrate our designed optimal pulse sequences, where the qubit undergoes complete pi-rotations in less than half of its precession period, and characterize the designed gates using quantum process tomography. Finally we implement a dual-axis control sequence with more than a hundred operations with an inter-pulse delay of only two precession cycles, representing a regime of high-density pulse sequences which is impossible to reach with traditional weak driving.