Initialization and read-out of intrinsic spin defects in a van der Waals crystal at room temperature

At a Glance

Metadata	Details
Publication Date	2020-02-24
Journal	Nature Materials
Authors	Andreas Gottscholl, Mehran Kianinia, Victor Soltamov, Sergei Orlinskii, Georgy Mamin
Institutions	Universidade Federal de Minas Gerais, Kazan Federal University
Citations	422
Analysis	Full AI Review Included

Executive Summary

This research establishes hexagonal Boron Nitride (hBN) as a viable, room-temperature platform for scalable quantum technologies by identifying and characterizing an intrinsic, optically addressable spin defect.

Platform Validation: hBN, a 2D van der Waals crystal, is confirmed as a host for robust, optically addressable spin defects, overcoming previous limitations in 2D materials.
Defect Identification: The active spin center is conclusively identified as the negatively charged boron vacancy (V_B^-), possessing a triplet (S=1) ground state.
Room Temperature Operation: Optically Detected Magnetic Resonance (ODMR) and spin polarization are successfully demonstrated at room temperature (300 K).
Key Spin Parameters: The zero-field splitting (ZFS) is determined to be approximately 3.5 GHz, suitable for microwave manipulation schemes. The defect exhibits high symmetry (almost uniaxial).
Spin Initialization: Optical pumping using a 532 nm laser induces population inversion in the spin ground state, fulfilling the prerequisite for coherent spin manipulation.
Scalability Advantage: The 2D nature of hBN allows for seamless integration with nanophotonic components, cavities, and heterogeneous opto-electronic devices, facilitating nanoscale quantum sensing realizations.

Technical Specifications

Parameter	Value	Unit	Context
Material Platform	Hexagonal Boron Nitride (hBN)	Crystal	2D van der Waals crystal
Spin Defect Identified	Negatively Charged Boron Vacancy (V_B^-)	Defect	Intrinsic S=1 triplet ground state
Operating Temperature	300	K	Room Temperature ODMR
Zero Field Splitting (D/h)	3.48	GHz	ODMR measurement (300 K, B=0)
Zero Field Splitting (D/h)	3.6	GHz	EPR measurement (5 K)
Off-Axial Splitting (E/h)	50	MHz	Small off-axial component of ZFS
Landé Factor (g)	2.000	Dimensionless	Almost isotropic
Hyperfine Splitting (A/h)	47	MHz	Interaction with 3 equivalent ¹⁴N nuclei
Photoluminescence (PL) Peak	850	nm	Strong room temperature emission
Excitation Wavelength (λ_exc)	532	nm	Green laser excitation
EPR Frequency (X-band)	9.4	GHz	Fixed frequency for EPR/High-field ODMR
ODMR Microwave Frequency Range	3.0 to 4.0	GHz	Range studied for ZFS determination
ODMR Laser Power (Zero-field)	10	mW	Power at sample surface
ODMR Laser Spot Diameter	10	µm	Approximate spot size

Key Methodologies

The research employed a combination of advanced spectroscopic techniques to characterize the spin defect and its optical properties.

Sample Preparation:
- hBN single crystals and multilayered flakes were used.
- Defects were created via irradiation methods, including fast neutron irradiation and ion implantation (Lithium or Gallium), confirming the intrinsic nature of the resulting emitters.
Zero-Field ODMR Setup (Room Temperature):
- A confocal microscope setup was used for optical excitation (532 nm laser) and PL collection (>650 nm).
- Microwaves were applied via a 0.5 mm wide copper-stripline, generated by a signal generator and amplified.
- ODMR signal (change in PL intensity, ΔPL/PL) was detected using a lock-in amplifier referenced to the on-off modulation of the microwaves.
Electron Paramagnetic Resonance (EPR) Spectroscopy:
- Measurements were performed in the X-band regime (9.4 GHz) at low temperature (T = 5 K).
- The sample was placed in an optically-accessible microwave cavity on a rotatable quartz rod to study angular dependence (polar and azimuthal rotations).
- EPR signals were recorded both with and without 532 nm optical excitation (50 mW power) to observe optically induced population effects.
Spin Hamiltonian Analysis:
- Experimental ODMR and EPR spectra were fitted using the standard S=1 spin Hamiltonian (Equation 1) to extract the Zero Field Splitting parameters (D and E) and the Landé factor (g).
- EPR spectral simulations (using EasySpin software) were performed to analyze the hyperfine structure.
Defect Confirmation via Hyperfine Structure:
- Analysis of the hyperfine splitting revealed seven observed lines, consistent with a spin S=1 system interacting with three equivalent Nitrogen nuclei (¹⁴N, I=1).
- This result unambiguously confirmed the defect structure as the negatively charged boron vacancy (V_B^-), ruling out the nitrogen vacancy (V_N) defect.

Commercial Applications

The successful demonstration of room-temperature, optically controlled spin defects in 2D hBN opens pathways for several advanced engineering and commercial applications:

Application Area	Technical Advantage / Product Relevance
Quantum Sensing	The 2D nature grants nanoscale proximity of the spin probe to target materials, enabling high-resolution magnetometry, electrometry, and thermometry, similar to NV-diamond sensors but in an ultrathin format.
Scalable Quantum Computing	hBN is a prime platform for scalable quantum technologies, allowing seamless integration of solid-state qubits (V_B^-) with heterogeneous opto-electronic devices, resonators, and nanophotonic circuits.
Quantum Networks	The realization of a robust spin-photon interface in hBN is crucial for developing basic two-node quantum networks and distributing entanglement between solid-state nodes.
Spin-Optomechanics	The ability to host spin defects in hBN membranes accelerates research into spin-mechanical coupling schemes, potentially leading to novel quantum transducers and sensors.
Hybrid Quantum Systems	The technology supports the engineering of hBN-NV diamond hybrid structures, combining the benefits of 3D and 2D quantum platforms for advanced sensing and information processing.
High Coherence Qubits	Future control over isotopic purity (e.g., enriching ¹¹B) is predicted to yield potentially high coherence times, making V_B^- a candidate for robust, long-lived qubits.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41563-020-0619-6