强调制脉冲方法制备NMR子空间有效纯态

使用强调制脉冲方法在核磁共振系统中实验制备出量子计算所需的有效纯态.在数值搜索方法的辅助下优化参数获得的强调制脉冲辐照能准确地执行三量子位系统中的核选幺正演化操作.与低功率的核选脉冲相比,强调制脉冲方案不仅减少实验脉冲序列中的脉冲数目,而且降低了脉冲（时间）宽度.还在实验上同时制备出了一对两量子位子空间有效纯态,由NMR频谱重构出的自旋量子系统状态与理论预期吻合.整个实验方案中所用到的强调制脉冲宽度都明显比执行同一幺正操作的低功率（核选）脉冲的宽度短,因此减少了脉冲操作时间段内系统弛豫以及环境噪声下的量子系统退相干对量子计算的影响.量子计算向更多的量子位扩展必然需要数目更多的脉冲操作,这时强调制脉冲的优势就会明显体现.

Preparation of subspace effective-pure state for NMR QIP by means of strongly modulating pulses

An alternative method for preparing an effective-pure state in a subsystem of a three-spin system for quantum information processing(QIP) in liquid-state nuclear magnetic resonance (NMR) was demonstrated experimentally. With the aid of numerical search methods, strongly modulating pulses (SMP) scheme was obtained and can perform accurate, arbitrary, selective gates on a three-qubit system. Compared with low-power (nuclear-selective) pulse scheme, the number of shaped pulses and the width of each pulse were both reduced. A pair of two-qubit subspace effective-pure states within a three-spin system was acquired simultaneously. The tomography for the spin system agreed with theoretical predictions, which shows that the experiments were successfully implemented. The short pulse-width of SMP substantially reduced the effects of relaxation within the period of pulse. This is very useful for NMR QIP involving larger numbers of qubits.