Kento Sasaki

Negatively charged boron vacancy (VB−) defects in hexagonal boron nitride (hBN) are promising for nanoscale-proximity quantum sensing. To evaluate their performance, it is important to characterize the spin-coherence times T2* and T2. In this study, we realized subgigahertz Rabi oscillations of VB− using an isotopically enriched h10B15N thin film directly stamped onto a narrow gold wire. Using these strong microwave pulses, we performed Ramsey interference and Hahn-echo measurements. The Ramsey interference signal showed Gaussian-like decay, yielding T2*=13.8 ± 0.5 ns. The Hahn-echo measurement gave T2=108.7 ± 5.5 ns and a stretch factor of αHE=1.25 ± 0.11. These results experimentally clarify the spin-coherence properties of VB− and provide an effective method for evaluating the coherence of spin defects in van der Waals thin films with broad resonance linewidths.

Nitrogen-vacancy centers in diamond are widely used as quantum sensors, but creating shallow ensembles with high yield and reasonable spin coherence remains challenging. We show that high-angle nitrogen ion implantation efficiently forms shallow NV ensembles. Implanting ions at angles above 60\ifmmode^\circ\else\textdegree\fi{} gives NV yields approaching 10% with effective depths below 10 nm. The oblique geometry enhances near-surface vacancy generation while keeping nitrogen close to the surface, improving the conditions for NV formation. The resulting ensembles retain stable charge states and useful spin coherence, offering a practical platform for nanoscale NMR and NQR sensing.

Understanding and controlling vortex motion in superconductors are important both for suppressing dissipation in superconducting devices and for device applications that exploit vortices. In this work, we quantitatively imaged the stray magnetic field distribution of vortices in an NbN thin film by wide-field magnetic imaging using a perfectly aligned diamond nitrogen-vacancy ensemble. By continuously measuring while stepwise varying the applied magnetic field under local laser heating, we captured a rearrangement of the vortex configuration in real space and in real time over more than 100 min. The observed vortex rearrangement is consistent with a reduction of the pinning force due to local laser heating and with the Lorentz force exerted by shielding currents induced by the field variation. These results provide insight into vortex dynamics and suggest potential applications, including vortex exclusion from sensitive regions of superconducting devices.

Quantum diamond microscopy (QDM), which employs nitrogen-vacancy (NV) center ensembles, is a promising approach to quantitatively imaging magnetic fields with both high resolution that approaches the diffraction limit and a wide field of view. The commonly adopted setups of QDM capture the photoluminescence through transparent diamonds, which inevitably entail aberrations—optical errors that degrade the optical resolution and contrast of the obtainable image. In this study, we delve into the impact of optical aberrations, focusing on their dependence on diamond thickness. We first introduce a rigorous model [B. Richards et al., Proc. R. Soc. London, Ser. A 253, 358–379 (1959) and J. Braat et al., J. Opt. Soc. Am. A 20, 2281–2292 (2003)] of diffraction that incorporates aberrations, producing the NV center optical image. We confirm that this model accurately reproduces the confocal images of single NV centers obtained at various depths in diamonds. Extending this model to a wide-field microscope, we find that the model also accurately reproduces the USAF 1951 resolution test chart obtained through diamonds of various thicknesses. Based on these investigations, we quantitatively assess the consequent resolution constraints and propose thinning the diamond as a viable solution. We present a robust method to quantitatively ascertain resolution in optical systems influenced by aberrations caused by ray transmission through diamonds. For instance, for a typical microscope with an objective lens of NA = 0.7, the diffraction limit is achievable through diamonds that are 30 μm thick, and a resolution of 1 μm is obtained through diamonds that are 100 μm thick. Those results open up avenues for enhanced performance in QDM.