在天然硅中保持自旋量子位活着：比较最佳工作点和动态解耦
Keeping a spin qubit alive in natural silicon: Comparing optimal working points and dynamical decoupling
摘要：
有两种不同的技术证明有效地延长自旋量子位在其他自旋环境中的相干寿命。一种是动态解耦，即量子比特受到精心定时的控制脉冲序列的影响;另一种是将量子比特调谐到“最佳工作点”（OWP），这是磁场中减少退相干的最佳点。利用量子多体计算方法，研究了远离和靠近OWP的动力学退耦脉冲序列对中心施主量子比特在核自旋浴中退相干的影响。理解这种行为的关键是分析多体量子浴中独立的翻转自旋对通常占主导地位的贡献的抑制程度。我们发现，最近测量的哈恩回波衰减在OWP（最低阶动力学去耦），必须考虑集群的三个相互作用的自旋，因为独立的对甚至不给有限的$T_2$衰减时间。我们发现，在OWP附近工作时，动态去耦序列需要数百个脉冲的一个数量级的增强$T_2$，相反，制度远离OWP，只需要约10个脉冲。
英文摘要：
There are two distinct techniques of proven effectiveness for extending the coherence lifetime of spin qubits in environments of other spins. One is dynamical decoupling, whereby the qubit is subjected to a carefully timed sequence of control pulses; the other is tuning the qubit towards 'optimal working points' (OWPs), which are sweet-spots for reduced decoherence in magnetic fields. By means of quantum many-body calculations, we investigate the effects of dynamical decoupling pulse sequences far from and near OWPs for a central donor qubit subject to decoherence from a nuclear spin bath. Key to understanding the behavior is to analyse the degree of suppression of the usually dominant contribution from independent pairs of flip-flopping spins within the many-body quantum bath. We find that to simulate recently measured Hahn echo decays at OWPs (lowest-order dynamical decoupling), one must consider clusters of three interacting spins, since independent pairs do not even give finite $T_2$ decay times. We show that while operating near OWPs, dynamical decoupling sequences require hundreds of pulses for a single order of magnitude enhancement of $T_2$, in contrast to regimes far from OWPs, where only about ten pulses are required.
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