Robust Nuclear Spin Polarization via Ground-State Level Anti-Crossing of Boron Vacancy Defects in Hexagonal Boron Nitride

Abstract

Nuclear spin polarization plays a crucial role in quantum information processing and quantum sensing. In this work, we demonstrate a robust and efficient method for nuclear spin polarization with boron vacancy ($\mathrm{V_B^-}$) defects in hexagonal boron nitride (h-BN) using ground-state level anti-crossing (GSLAC). We show that GSLAC-assisted nuclear polarization can be achieved with significantly lower laser power than excited-state level anti-crossing, making the process experimentally more viable. Furthermore, we have demonstrated direct optical readout of nuclear spins for $\mathrm{V_B^-}$ in h-BN. Our findings suggest that GSLAC is a promising technique for the precise control and manipulation of nuclear spins in $\mathrm{V_B^-}$ defects in h-BN.

Figures (4)

• Figure 1: (a) Scheme for performing nuclear polarization with the assistance of GSLAC. Magnetic field is not shown. (b) Diagram of energy levels of $\mathrm{V_B^-}$ with optical transitions. Green, red, and grey arrows represent laser excitation, radiative recombination, and nonradiative intersystem crossing, respectively. (c) Simplified diagram of dynamics of nuclear polarization via GSLAC. Excitation and radiative decay are depicted by green arrows, while red arrows show electron spin polarization through intersystem crossing. Grey arrows illustrate flip-flop process. For simplicity, more energy levels and transitions are omitted.
• Figure 2: ((a) A diagram depicting the ODMR of $\mathrm{V_B^-}$ defects under an external magnetic field aligned with the c-axis. The bottom and top shaded regions correspond to the magnetic fields of (c, d) and (e, f), respectively. (b) The pulse sequence used for pulsed ODMR measurement, featuring crucial parameters labeled accordingly. Laser, microwave, and readout window are arranged from top to bottom, respectively. (c, d) Pulsed ODMR spectra captured at the magnetic field indicated by the bottom shaded area in (a), with laser powers of 150 mW (c) and 1.5 mW (d). The ODMR of $m_I$=0 is highlighted by green shaded areas for easy reference. (e, f) Pulsed ODMR spectra captured at the magnetic field indicated by the top shaded area in (a), replicating the conditions in (c, d).
• Figure 3: The degree of nuclear spin polarization obtained from CW ODMR spectra is plotted against magnetic field in the range of 28 to 200 mT. The experimental results are represented by dots, while theoretical results are presented using curves. Laser power used in experiments and pump rate used in simulations have been labeled.
• Figure 4: (a), (b) Pulse sequence of ODNMR measurements for $m_s = 0$ (a) and $m_s = -1$ (b) branches. The laser, RF/MW pulses, and readout window are arranged from top to bottom. RF has a duration of 0.5 $\mathrm{\mu}$s, while each microwave has a duration corresponding to the $\pi$ pulse length. (c), (d) ODNMR spectra of $m_s = 0$ (c) and $m_s = -1$ (d) branches at 80 mT. Simulated spectra are shown in the top panels, whereas experimental results are displayed in the bottom panels. Experimental data have been fitted using multiple Lorentz peaks. Inset shows a simplified diagram of the transitions. (e), (f) Magnetic field dependent ODNMR spectra of $m_s = 0$ (e) and $m_s = -1$ (f) branches. Due to our diplexer's limited microwave frequency range, the 113 mT to 134 mT range could not be reached in (f).