Report on the Study of Quantum Coherent Spins in Hexagonal Boron Nitride at Room Temperature

Introduction

The realization of quantum networks and sensors requires solid-state spin-photon interfaces that possess single-photon emission capabilities and long-lived spin coherence, which can be integrated into scalable devices. Ideally, these devices should operate at room temperature. However, despite rapid progress in multiple candidate systems, systems that can maintain quantum-coherent single spins at room temperature are still very rare. This study aims to fill this research gap and explore the feasibility of achieving quantum coherent control at room temperature in the layered van der Waals material—hexagonal boron nitride (hBN).

Paper Source

This paper, titled “A quantum coherent spin in hexagonal boron nitride at ambient conditions,” was written by Hannah L. Stern et al. The research institutions include the Cavendish Laboratory, University of Cambridge, and the University of Technology Sydney, among others. This study was accepted by the journal Nature Materials on April 2, 2024, and is about to be published online.

Research Process

Research Object and Sample Preparation

The research object is single-photon emission defects in hexagonal boron nitride (hBN). The samples were grown on a sapphire substrate using metal-organic chemical vapor deposition (MOVPE), with defects introduced using a carbon source and ammonia. By controlling the flow rate of the carbon source, defects with single-spin activity can be introduced into hBN.

Optical Measurements

Optical measurements were performed using a homemade confocal microscope system under room temperature and ambient conditions. Continuous wave 532-nanometer lasers were used for excitation, and fluorescence was detected using avalanche photodiodes (APD) or photoluminescence spectra were measured using a spectrometer. Additionally, a Hannbury Brown and Twiss interferometer was used for intensity correlation measurements.

ODMR Measurements

Optically detected magnetic resonance (ODMR) was used to probe the spin states of the defects. Continuous wave and pulsed ODMR measurements were performed by applying an external magnetic field to the defects while maintaining a constant magnetic field, obtaining parameters such as spin resonance frequency and spin-lattice relaxation time (T1).

Experiment and Data Analysis

A series of angle-resolved magneto-optical measurements and microwave interference measurements, including Rabi oscillations and Ramsey interference, were employed to detect the dynamical behavior and coherence of defect spins. Dynamic decoupling pulse experiments were used to extend the spin coherence time.

Main Research Results

Ground State Spin Triplet

Through angle-resolved magneto-optical measurements, it was determined that the defects in hexagonal boron nitride have a ground state spin triplet (S=1) with a zero-field splitting of 1.96 GHz. The ODMR signals of multiple defects showed significant contrast, and two distinct resonance frequencies were observed at zero field, 1.87 GHz and 1.99 GHz, respectively. Further confirmation that the Z-axis of the defect lies in the plane of the hBN layer was obtained through vector magnetic field-dependent ODMR measurements.

Measurement of Spin Coherence Time

Microwave Ramsey interference measurements determined the bare inhomogeneous dephasing time (T*_2) of a single defect to be approximately 100 ns. Surprisingly, under zero magnetic field, the Rabi coherence time (T_Rabi) exceeded 1 μs, indicating that the electron spin can effectively decouple from its reversible decoherence environment.

Dynamic Decoupling and Protection

Further dynamic decoupling pulse experiments showed a spin echo coherence time (T_SE) of approximately 200 ns, and the spin coherence time could be extended to over 1 μs with the increase in decoupling pulses. The relationship between coherence time and the number of decoupling pulses exhibited a power-law relationship close to 0.67, consistent with theoretical predictions for the situation where the central electron spin couples with a few slowly evolving neighboring nuclei. The fine structure of the ODMR signal confirmed the hyperfine coupling with a few inequivalent nitrogen and boron atoms.

Chemical Structure of Carbon-Related Defects

By combining magnetic field direction and strength-dependent ODMR spectra with a hyperfine coupling model of the electron spin and two inequivalent nuclei, the chemical structure of the defects was further elucidated. These results provide important references for theoretical research aimed at determining the microscopic structure of this carbon-based spin triplet defect.

Research Conclusions and Significance

This study achieved quantum coherent spin control with long-lived spin coherence and single-photon emission capabilities at room temperature in hexagonal boron nitride. This discovery provides a new material platform for constructing scalable quantum network devices and sensors. Particularly in the area of quantum sensing, the room-temperature spin coherence and flexibility of coupling with neighboring nuclei make this defect system a highly promising nanoscale sensor. Through dynamic decoupling to extend spin coherence time, it is expected that efficient quantum information processing and sensing can be achieved under zero magnetic field and room temperature conditions.

Research Highlights

1. Achievement of Spin Triplet under Zero Magnetic Field: This study is the first to achieve quantum coherent control of spin triplet defects in hexagonal boron nitride under zero magnetic field.
2. Long-Lived Spin Coherence Time: Through dynamic decoupling pulse experiments, a spin coherence time exceeding 1 μs was achieved, revealing a protection mechanism of the system in decohering environments.
3. High ODMR Contrast: The ODMR signal showed a high contrast close to 50%, providing high sensitivity for practical applications.
4. Potential for Nanoscale Quantum Sensing: The distance of the identified defects from the surface, up to 15 nm, shows great potential as a nanoscale sensor.

Directions for Future Research

Future studies should further optimize the optical quality of the defects, integrate quantum photonic systems, and embed hBN defects into nanostructures to achieve large-scale deployment of quantum networks and sensors. At the same time, exploring and elucidating the chemical structure and charge state dynamics of this carbon-based spin triplet defect will help to further enhance the performance and application prospects of the system.

This study has pioneered a new direction in the field of quantum technology, providing a material platform that maintains quantum coherence under room temperature and zero magnetic field conditions, to achieve large-scale quantum networks and high-sensitivity quantum sensors.