Scanning nano-SQUID with single electron spin sensitivity

Abstract: One of the critical milestones in the intensive pursuit of quantitative nanoscale magnetic imaging tools is achieving the level of sensitivity required for detecting the field generated by the spin magnetic moment {\mu}B of a single electron. Superconducting quantum interference devices (SQUIDs), which were traditionally the most sensitive magnetometers, could not hitherto reach this goal because of their relatively large effective size (of the order of 1 {\mu}m). Here we report self-aligned fabrication of nano-SQUIDs with diameters as small as 46 nm and with an extremely low flux noise of 50 n{\Phi}0/Hz^1/2, representing almost two orders of magnitude improvement in spin sensitivity, down to 0.38 {\mu}B/Hz^1/2. In addition, the devices operate over a wide range of magnetic fields with 0.6 {\mu}B/Hz^1/2 sensitivity even at 1 T. We demonstrate magnetic imaging of vortices in type II superconductor that are 120 nm apart and scanning measurements of AC magnetic fields down to 50 nT. The unique geometry of these nano-SQUIDs that reside on the apex of a sharp tip allows approaching the sample to within a few nm, which paves the way to a new class of single-spin resolved scanning probe microscopy.

• The paper demonstrates a novel fabrication technique for self-aligned nano-SQUIDs-on-tip achieving 0.38 μB/Hz^1/2 spin sensitivity.
• It employs Nb and Pb to create devices as small as 46 nm, allowing scanning probe microscopy at a few nanometers from the sample.
• The work achieves unprecedented performance by operating under high magnetic fields up to 1 Tesla with flux noise as low as 50 nΦ0/Hz^1/2.

Insights into Scanning Nano-SQUID with Single Electron Spin Sensitivity

The paper in question discusses the development of ultra-small nano-SQUIDs-on-tip (SOTs) fabricated from Nb and Pb, demonstrating significant advancements in sensitivity and spatial resolution for magnetic imaging. These devices reach single electron spin sensitivity, marking a significant improvement from previously attainable metrics, and extending the operational range to much higher magnetic fields and temperatures.

Technical Advances and Methodology

By leveraging the shorter coherence length and penetration depth of Nb and Pb, the research successfully fabricates nano-SQUIDs on the apex of sharp tips with diameters as small as 46 nm. These spatial dimensions are critical as they allow the SOTs to approach the sample within a few nanometers, enabling single-spin resolved scanning probe microscopy. The fabrication process entails a self-aligned stepwise deposition of superconducting films onto hollow quartz pipettes, which creates the ideal geometry for these nano-SQUIDs.

Significant Findings

A standout feature of the research is its demonstration of more than two orders of magnitude improvement in spin sensitivity over previous devices, achieving sensitivity as low as 0.38 μB/Hz^1/2 with the 46 nm Pb SOT. This extraordinary sensitivity is paired with a device architecture that allows the SOTs to operate at high applied fields up to 1 Tesla, which is unprecedented given the prior limitations of SQUIDs operating only in low magnetic fields.

The low flux noise levels attained, with the Pb SOTs reaching 50 nΦ0/Hz^1/2, further emphasize the advancements in device performance. This is among the lowest flux noise levels recorded, potentially reshaping the landscape of nanoscale magnetic imaging.

Implications and Future Directions

From a practical perspective, the demonstrated capabilities allow for real-time, high-resolution imaging of vortices in type II superconductors and AC magnetic fields at nanoscale levels. The SOTs' extremely low spin noise positions them as versatile tools for a broad array of applications in condensed matter physics, particularly in studying quantum magnetism and elucidating the complexities of superconducting materials.

Theoretically, this research paves the way for exploring quantum limit sensitivities in magnetometry and suggests that further optimization could shrink the kinetic inductance, pushing noise limits even lower. Potential future applications could involve even smaller SOTs and enhance the robustness and sensitivity to extend their use in more diverse and challenging environments.

This study is a quintessential example of how advances in materials science and nanofabrication techniques can significantly elevate the performance thresholds of measurement devices, thus enabling novel explorations at the frontier of quantum mechanics and material science. The work holds promise for transformative applications in fundamental research and the development of future technology in quantum sensing and imaging.