研究纳米超导量子干涉器件中的磁通噪声起源

Nano-SQUID (superconducting quantum interference device) has drawn many attentions due to its possibility to achieve single-electron-spin detection by using of only electronic readout. Owning to its super sensitivity and easy accessibility, nano-SQUID is a highly desired technique for a variety of research areas requiring nanoscale magnetism characterization and quantum spin detection, such as studying super high density memory, searching for the quantum information storage unit-qubit, and developing nanoscale magnetic resonance imaging technique. The minimium detectable spin number of a nano-SQUID is proportional to the size of its superconducting ring and the magnetic flux noise. With the recent global trend to shrink the size of SQUID using nanotechnology, the sensitivity of current nano-SQUIDs is limited by the flux noise. However, the origins of the flux noise in superconducting device is still an unanswered question in condensed matter physics, and multiple hypotheses still remain at the stage of numerical simulation. Here, we propose to investigate the origins of the nano-SQUID flux noise experimentally by comparing nano-SQUID prepared using different methods. First, we will study the effect of local magnetic moments generated by the metal induced state at the interface between the niobium and the device substrate. The flux noise will be measured for nano-SQUIDs made of single crystal niobium films that were grown using molecular beam epitaxy technique and multi-crystal niobium films grown by the traditional sputtering technique respectively. Second, we will analyze the effect of dangling-bond-spin states at the surface, or inside of the substrate, by comparing the nano-SQUIDs fabricated on substrates of different materials. In addition, we also will inspect the microscopic coupling mechanism between the flux noise and the nano-SQUID, by measuring the flux noise as functions of the probe current frequency, applied magnetic field and ambient temperature, respectively. In conclusion, the proposed investigation into in the origins of nano-SQUIDs will provide scientific insights to further improve the sensitivity of nano-SQUIDs towards the detection of single spins.

纳米超导量子干涉器件(NanoSQUID)作为一种精密的自旋探测器,可以广泛应用于开发超高密度存储器和搜寻量子信息单元等诸多研究领域。目前，NanoSQUID的灵敏度主要受限于其磁通噪声，但对于引起磁通噪声的原因尚不完全清楚。本项目拟通过改变nanoSQUID的三个主要条件，研究其磁通噪声的可能起源和影响因素。条件一，考察超导金属和nanoSQUID衬底界面态上的局部磁偶极子对磁通噪声的影响。条件二，测量nanoSQUID衬底表面或内部缺陷产生的悬空键自旋涨落对磁通噪声的影响。条件三，分析磁通噪声和nanoSQUID的测量参数（测量电流频率、外加磁场以及环境温度）的关系。通过以上研究，系统地分析NanoSQUID磁通噪声的可能起源，可进一步理解磁通噪声的微观机理，为优化NanoSQUID的灵敏度提供科学指导依据。

纳米超导量子干涉器件（nanoSQUID）是基于SQUID发展起来的一种新型超导器件。它通过现代微纳加工技术将SQUID的超导环缩小到纳米级别，构成极端灵敏的微观自旋探测器，理论上可以达到测量单电子自旋的灵敏度，从而成为探索微观世界和研究量子科学的有效手段之一。本项目通过现有工艺优化，研究获得了一套简单而又稳定的平面nanoSQUID制备工艺，并且自主搭建了一套针对nanoSQUID定制的FPGA快速测量系统。更进一步，为了降低nanoSQUID的磁通噪声，项目组发明了一套3D nanoSQUID的制备工艺，大幅度提高了nanoSQUID的磁通调制深度，达到了45.9%。并且将磁通噪音降低到0.34 μФ0/Hz1/2。此项结果对Nb基 nanoSQUID在单电子自旋探测应用迈出了重要的一步。另外，项目在nanoSQUID应用探索中展开初步尝试，成功探测到单颗铟颗粒的超导抗磁信号，为nanoSQUID在研究超小超导体磁属性应用方面打下扎实基础。