[Paper Review] Quantum sensing and imaging with spin defects in hexagonal boron nitride

Sumukh Vaidya, Xingyu Gao+3 more|arXiv (Cornell University)|Feb 22, 2023

Diamond and Carbon-based Materials ResearchMaterials Science3 citations

TL;DR

This review explores the use of spin defects in hexagonal boron nitride (hBN), particularly the negatively charged boron vacancy (V_B⁻), for nanoscale and microscale quantum sensing and imaging. Leveraging hBN's 2D structure, high stability, and optically addressable spins, the paper demonstrates high-sensitivity detection of magnetic fields, temperature, strain, nuclear spins, and RF signals using optical initialization, microwave control, and photoluminescence readout, with coherence times extended to 36 µs under cryogenic conditions and 800 ns via dynamic decoupling protocols.

ABSTRACT

Color centers in hexagonal boron nitride (hBN) have recently emerged as promising candidates for a new wave of quantum applications. Thanks to hBN's high stability and 2-dimensional (2D) layered structure, color centers in hBN can serve as robust quantum emitters that can be readily integrated into nanophotonic and plasmonic structures on a chip. More importantly, the recently discovered optically addressable spin defects in hBN provide a quantum interface between photons and electron spins for quantum sensing applications. The most well-studied hBN spin defects, the negatively charged boron vacancy ($V_B^-$) spin defects, have been used for quantum sensing of static magnetic fields, magnetic noise, temperature, strain, nuclear spins, paramagnetic spins in liquids, RF signals, and beyond. In particular, hBN nanosheets with spin defects can form van der Waals (vdW) heterostructures with 2D magnetic or other materials for in situ quantum sensing and imaging. This review summarizes the rapidly evolving field of nanoscale and microscale quantum sensing with spin defects in hBN. We introduce basic properties of hBN spin defects, quantum sensing protocols, and recent experimental demonstrations of quantum sensing and imaging with hBN spin defects. We also discuss methods to enhance their sensitivity. Finally, we envision some potential developments and applications of hBN spin defects.

Motivation & Objective

• To summarize recent advances in quantum sensing and imaging using spin defects in hexagonal boron nitride (hBN), a 2D van der Waals material with high stability and low surface noise.
• To address limitations of traditional quantum sensors like diamond NV centers, particularly surface-related decoherence and integration challenges.
• To highlight the potential of hBN spin defects for in situ, high-resolution sensing in nanophotonic and plasmonic platforms.
• To review methods for enhancing spin coherence time and photoluminescence intensity to improve sensing sensitivity.
• To envision future applications in quantum sensing, spin optomechanics, and integration with 2D heterostructures.

Proposed method

• Utilization of negatively charged boron vacancy (V_B⁻) defects in hBN as spin qubits with optical initialization using green laser excitation.
• Employment of microwave pulses for coherent spin manipulation and readout via photoluminescence intensity changes in optically detected magnetic resonance (ODMR).
• Application of dynamic decoupling sequences such as CPMG and amplitude-modulated concatenated dynamic decoupling (CCD) to extend spin coherence time (T₂).
• Implementation of disorder-robust protocols like DROID to enhance coherence in strongly interacting spin systems.
• Use of isotopic purification (e.g., 10B) and cryogenic cooling to reduce decoherence and improve T₂.
• Fabrication of hBN nanosheets and vdW heterostructures with 2D magnetic or plasmonic materials for in situ sensing and enhanced field coupling.

Experimental results

Research questions

• RQ1 How can hBN spin defects enable high-sensitivity, nanoscale quantum sensing of magnetic fields, temperature, strain, and nuclear spins in ambient conditions?
• RQ2 What are the key mechanisms and protocols that extend the spin coherence time (T₂) of V_B⁻ defects in hBN beyond the intrinsic limits?
• RQ3 How does the 2D nature of hBN enable integration with nanophotonic and plasmonic structures for enhanced sensing and imaging?
• RQ4 What are the prospects for achieving sub-diffraction-limited spatial resolution in quantum imaging using arrays of hBN spin defects?
• RQ5 How can future material engineering and control strategies (e.g., isotopic purification, defect engineering) further enhance the performance of hBN-based quantum sensors?

Key findings

• The V_B⁻ spin defect in hBN exhibits optically addressable spin states with room-temperature T₂ up to 4 µs, extendable to 36 µs under cryogenic conditions (50 K) and strong magnetic fields (3.2 T).
• Amplitude-modulated concatenated dynamic decoupling (CCD) extended the coherence time of unprotected V_B⁻ spins to 800 ns, while protected superposition qubits achieved up to 800 ns T₂.
• The DROID protocol achieved coherent times of up to 500 ns in strongly interacting spin systems, demonstrating robustness against disorder.
• A 100 nm² array of V_B⁻ defects was fabricated using a helium ion microscope, enabling magnetic field imaging beyond the diffraction limit in one dimension.
• Coherence times of 46 ns were improved to 62 ns via isotopic purification with 10B, though the enhancement was modest.
• Theoretical and experimental evidence supports the potential of hBN spin defects for applications in spin optomechanics, high-harmonic generation, and quantum simulation due to their low mass and strong spin-lattice coupling.