Yuki Nakamura

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.

Placing a sensor close to the target at the nano-level is a central challenge in quantum sensing. We demonstrate magnetic field imaging with a boron vacancy (VB−) defects array in hexagonal boron nitride with a few 10 nm thickness. VB− sensor spots with a size of (100 nm)2 are arranged periodically with nanoscale accuracy using a helium ion microscope and attached tightly to a gold wire. The sensor array allows us to visualize the magnetic field induced by the current in the straight micro wire with a high spatial resolution. Each sensor exhibits a practical sensitivity of 73.6 μT/Hz0.5, suitable for quantum materials research. Our technique of arranging VB− quantum sensors periodically and tightly on measurement targets will maximize their potential.

Geometric effects can play a pivotal role in streamlining quantum manipulation. We demonstrate a geometric diabatic control, that is, perfect tunneling between spin states in a diamond by a quadratic sweep of a driving field. The field sweep speed for the perfect tunneling is determined by the geometric amplitude factor and can be tuned arbitrarily. Our results are obtained by testing a quadratic version of Berry's twisted Landau-Zener model. This geometric tuning is robust over a wide parameter range. Our work provides a basis for quantum control in various systems, including condensed-matter physics, quantum computation, and nuclear magnetic resonance.

For quantum sensing, it is vital to develop an efficient technique for determining the quantum state of the sensor. We optimize the weighting of the photoluminescence intensity for readout of the spin state of the nitrogen-vacancy (NV) center in diamond. We find that adopting a physical model that considers the optical transitions and relaxations of the NV center allows for an efficient readout. Our method improves the signal-to-noise ratio of the readout by 5.4% in a short time of 3 s, while the existing methods typically require 1 min of integration time. We also show that our technique enhances the readout of the nuclear spin memory. The demonstrated way is helpful for a wide range of measurements, from a few minutes to several days.