Open quantum dynamics of spin qubits on graphene nanoribbons

下一代量子计算机将需要利用新材料来实现更高的温度操作和对量子比特的全电气控制。实现这些目标的有力竞争者是原子精密的石墨烯纳米带，其真正的潜力直到最近才随着纳米制造方面的突破性发展而被发现。由于它们的分子精度，这些吉祥的石墨烯纳米带在10K[1]的温度下具有毫秒量级的自旋弛豫时间，为进一步的研究提供了令人兴奋的前景。这个项目将研究一种自旋量子比特框架，它利用邻近效应来实现超快的全电单量子比特控制，使Rabi频率突破GHz势垒[2]。总体目标是通过理论和数值相结合的方法来理解自旋-声子耦合对量子比特稳定性的影响。识别和表征自旋退相干通道的能力将为理解石墨烯纳米带中自旋量子比特编码策略的弹性提供垫脚石。主要目的是确定电子-声子耦合和常见的随机弹性散射源对量子位稳定性的影响，并模拟近邻石墨烯纳米带(GNRs)的开放量子动力学。得益于团队的专业知识，我们将使用多体微扰理论和数值精确方法来获得适用于现实系统的稳健的微观理论。将考虑以下影响。(I)自旋-声子耦合。电子-声子耦合是栅定义石墨烯量子点的主要电荷弛豫机制[3]，脉冲电子顺磁共振实验[4]证明，它很可能是决定自下而上GNR自旋相干时间的主要因素[4]。(Ii)精神紊乱。自下而上的GNR最常见的缺陷是边缘的“咬合缺陷”(即缺失C环)[5]。我们将绘制出(I)和(II)的开放退相通道，计算微观弛豫时间以及随后的T1和T2时间。次要目标是：(1)描述由于自旋-轨道混合引起的自旋-声子弛豫过程；以及(2)确定磁噪声的影响。

The next generation of quantum computers will need to make use of new materials to realise higher temperature operation and all-electrical control of qubits. Strong contenders for achieving these goals are atomically precise graphene nanoribbons, whose true potential has only recently been uncovered with ground-breaking developments in nanofabrication. Due to their molecular precision, these auspicious graphene nanoribbons boast spin relaxation times on the order of milliseconds at temperatures as large as 10 K [1], offering exciting prospects for further investigations. This project will investigate a spin-qubit framework that exploits proximity effects to enable ultra-fast all-electrical single-qubit control with Rabi frequencies breaking the GHz barrier [2]. The overall aim is to understand the effects of spin-phonon coupling upon qubit stability by using a combination of theoretical and numerical methods. The ability to identify and characterise the spin decoherence channels will provide a stepping stone to understand the resilience of spin-qubit encoding strategies in graphene nanoribbons. The main objectives are to determine the effects of electron-phonon coupling and common random sources of elastic scattering upon qubit stability and model the open quantum dynamics of proximitised graphene nanoribbons(GNRs). Benefiting from the team's expertise, we will use many-body perturbation theory and numerically exact methods to obtain a robust microscopic theory applicable to realistic systems. The following effects will be considered. (i) Spin-phonon coupling. Electron-phonon coupling is the main charge relaxation mechanism in gate-defined graphene quantum dots [3] and is likely to be the main factor in setting the spin coherence time in bottom-up GNRs as evidenced by pulse electron paramagnetic resonance experiments [4]. (ii) Disorder. The most common imperfection in bottom-up GNRs are "bite defects" (i.e. missing C rings) at the edge [5]. We shall map out the open dephasing channels for (i) and (ii), calculating the microscopic relaxation times and the ensuing T1 and T2 times. The secondary objectives are: (1) characterise spin-phonon relaxation processes due to spin-orbit admixture; and (2) determine the impact of magnetic noise.