利用最优动力学解耦保持固态系统中电子自旋相干性

Preserving Electron Spin Coherence in Solids By Optimal Dynamical Decoupling

将量子力学和计算机科学结合并实现量子计算是人类的一大梦想,而实现这一梦想的关键挑战之一就是如何解决量子体系的退相干问题。如何在一个真实物理体系中保持量子相干,减小环境带来的退相干影响是量子信息和量子操控研究的基本前提和首要任务。近年来,一种被称为动力学解耦的有效对抗退相干效应的策略被提了出来,它是通过一串精心设计的微波脉冲序列来反复翻转电子自旋,从而有效地消去电子自旋与环境中核自旋之间的耦合,保护电子自旋的量子相干性。动力学解耦另外一个突出优势就是它可以很自然地与其它要实现的功能集成起来,如实现高精度的量子逻辑门。由于自旋反转难免会有错误发生,为达到最优效果,最近理论物理学家提出了所需脉冲数目最小化的最优动力学解耦序列和方案。然而在真实固体体系中,最优动力学解耦的可行性和有效性尚未得到实验验证。本研究通过选择合适的固体量子体系,利用精巧的脉冲控制(最优动力学解耦序列),使体系中环境对电子量子比特的不利影响被降到最小,从而大大减少量子体系中量子信息的流失,证明了这一技术的有效性,并成功厘清各种退相干机制在此类固体体系中的影响。

To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing,it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment.Dynamical decoupling,which uses stroboscopic spin flips to give an average coupling to the environment that is effectively zero,is a particularly promising strategy for combating decoherence because it can be naturally integrated with other desired functionalities,such as quantum gates.Errors are inevitably introduced in each spin flip,so it is desirable to minimize the number of control pulses used to realize dynamical decoupling having a given level of precision.Such optimal dynamical decoupling sequences have recently been explored.The experimental realization of optimal dynamical decoupling in solid-state systems,however,remains elusive.At present,Du and his colleagues use pulsed electron paramagnetic resonance to demonstrate experimentally optimal dynamical decoupling for preserving electron spin coherence in irradiated malonic acid crystals at temperatures from 50K to room temperature.Using a seven-pulse optimal dynamical decoupling sequence,we prolonged the spin coherence time to about 30s;it would otherwise be about 0.04s without control or 6.2s under one-pulse control.By comparing experiments with microscopic theories,we have identified the relevant electron spin decoherence mechanisms in the solid.