报告题目：Quantum control for Quantum Information Processing with NMR system

报 告 人：Jonathan Jones, University of Oxford

报告时间：2018-03-28 14:00

报告地点：理科楼C109

报告摘要： Quantum information processing is the encoding of information in two-level quantum systems called qubits and the manipulation of this information through a series of unitary transformations which can be interpreted as logic gates. Building real quantum computers will require the ability to perform accurate unitary transformations on quantum systems in the presence of realistic errors. Such errors can be divided into two broad categories: random errors, arising from decoherence, which can be tackled by methods such as quantum error correction, and systematic errors, arising from imperfections in control fields. If systematic errors vary slowly in time, or if they vary over a spatial ensemble of qubits, it is necessary to design control sequences which are intrinsically tolerant of a range of error values.

One successful approach, adopted from Nuclear Magnetic Resonance (NMR) experiments, is the use of composite pulses. Here a single rotation about some axis in the xy plane is replaced by a sequence of rotations, such that the combined propagator implements the desired rotation in the absence of errors, while in the presence of small errors the errors in individual rotations do not accumulate but instead mostly cancel out. I will describe two methods to construct robust NOT gates, that is, π rotations about the x axis of the Bloch sphere, using only π rotations around axes in the xy plane. One group of solutions permits the correction of pulse strength errors to arbitrary accuracy using an analytic construction, while the second group seeks short and practical composite pulses which are still highly robust to both pulse strength and off-resonance errors in single-spin systems. Finally I will comment briefly on some recent practical developments in the use of gradient ascent pulse engineering to design control pulses in more complex NMR spin systems.