[论文解读] Using Andreev bound states and spin to remove domain walls in a Kitaev chain

Wietze D. Huisman, Sebastiaan L. D. ten Haaf+7 more|arXiv (Cornell University)|Jan 19, 2026

Topological Materials and Phenomena被引用 1

一句话总结

论文表明量子点的自旋配置和Andreev束缚态的电荷能在没有外部磁通的情况下移除三位Kitaev链中的域墙，尽管由此诱导的相移可能偏离严格的π并随ABS能量平滑变化。

ABSTRACT

Quantum dot-superconductor hybrids have been established as a suitable platform for realizing Kitaev chains hosting Majorana bound states. Implementing these structures in a qubit architecture is expected to result in coherence times that scale exponentially with the lengths of the chains. To scale to longer systems, the phase differences between all superconducting segments in the chain need to be controlled. While this control has been demonstrated by using an external magnetic flux, ideally it can be achieved with control over intrinsic system parameters. In this work, we investigate whether the relevant phase differences can be tuned through the spin degree of freedom in each QD, or the chemical potential of the discrete bound states in the hybrid sections. We confirm that both these tuning knobs allow for controlling the phase difference in the couplings between neighbouring QDs, bypassing the requirement to tune an external flux. However, we find that the amplitude of the phase shifts can deviate from a discrete $π$-shift. We introduce a spatial variation in the spin-orbit field as a possible mechanism to explain the observed behaviour and comment on the consequences for experimentally creating long Kitaev chains.

研究动机与目标

• 通过避免超导段之间π相差导致的域墙，推动Kitaev链向更长阵列的扩展。
• 研究自旋配置和ABS能量是否能调节分子间相位以实现均匀耦合。
• 在三位InSbAs QD–超导异质链中实验证明自旋和ABS调谐如何影响激发隙。
• 分析与理想π相位的偏离并提出平滑相演化的机制。
• 评估为Majorana量子比特设计无磁通可扩展Kitaev链的意义。

提出的方法

• 用一般哈密顿量对Kitaev链进行建模，其中配对项的超导相位被分配并可通过自旋配置和ABS能量进行调谐。
• 利用通过Andreev束缚态耦合的自旋极化量子点实现相邻量子点之间的t和Delta耦合。
• 使用可控磁通的超导环来控制相邻耦合之间的相差并测量中间量子点的激发隙。
• 调节ABS能量（和自旋配置），找出|t| = |Delta|的甜点并观测相应的相移。
• 进行射频反射测量以探测局部态密度和链上微分电导的快速测量。

Figure 1: Proposed mechanisms for controlling phase in the Kitaev chain and device layout of a three-site chain. a) Schematic of ECT and CAR sub-gap processes in a two-site Kitaev chain setup, controlled by an ABS with energy $E_{\mathrm{ABS}}$ . b) ECT ( $t$ ) and CAR ( $\Delta$ ) amplitudes as a f

实验结果

研究问题

• RQ1 外部量子点的自旋极化是否通过在耦合之间诱导π相移来移除域墙？
• RQ2 通过调整ABS电荷是否能提供无磁通的方法来控制Kitaev链中分子间的相差？
• RQ3 观测到的相移是否始终严格为0或π，还是可以随ABS能量或其他微观细节平滑变化？
• RQ4 哪些微观机制能解释非π、平滑变化的相移，以及它对扩展到更长链的影响？

主要发现

• 改变外部量子点的自旋配置会引入相位移，能够移除域墙并重新打开中间量子点的激发隙。
• 改变混合段的ABS能量可产生相似的相移，提供第二个无磁通控制旋钮。
• 相位移随ABS电荷平滑演化，在某些区域导致甜点之间出现非π差值。
• 一种涉及自旋轨道场在空间上变化的机制能够解释平滑相演化和偏离离散0/π移位的现象。
• 从严格的π相位偏离会降低中间隙，但只要相位偏移不为零，该方法就有助于避免隙关闭并支持无磁通扩展。
• 结果强调在设计更长的无磁通相控Kitaev链以实现Majorana基量子比特时需要考虑的实际因素。

Figure 2: Spin-induced phase shifts. a) Bias spectroscopy measurement for the middle quantum dot as a function of the out-of-plane magnetic field $B_{\mathrm{z}}$ for an $\uparrow\uparrow\uparrow$ spin configuration. b) A higher resolution $G_{\mathrm{MM}}$ linetrace along $V_{\mathrm{M}}$ = 0, expr