Boron Nitride Defects Boosted for Quantum Sensing with Threefold Signal Enhancement

Scientists are increasingly focused on optically addressable solid-state spin defects as crucial components for quantum sensing and information processing. Ivan Zhigulin, Nicholas P. Sloane, and colleagues from the University of Technology Sydney, alongside Benjamin Whitefield, Jean-Philippe Tetienne, Mehran Kianinia, and Igor Aharonovich, have now investigated the excitation dynamics of recently discovered single spin defects in hexagonal boron nitride (hBN). Their research reveals that manipulating the excitation wavelength significantly enhances both the optically detected magnetic resonance (ODMR) contrast and magnetic field sensitivity threefold. This finding provides valuable mechanistic insights into the behaviour of spin complex emitters within hBN and demonstrates the critical role of excitation wavelength in optimising their performance for advanced quantum technologies.

Recent discoveries have revealed single spin defects in hBN exhibiting both S = 1 and S = 1/2 spin transitions, presenting a novel platform for quantum technologies.

This work details the excitation dynamics of these unique spin defects, specifically investigating how varying the excitation wavelength impacts their spin-dependent fluorescence and overall dynamics. Researchers found that precise control over the excitation wavelength dramatically alters the photodynamics of these spin complex emitters, offering valuable mechanistic insights into their behaviour.
The study focuses on spin complexes within hBN, which possess both strongly coupled (S = 1) and weakly coupled (S = {1,0}) spin manifolds. These complexes feature energetic levels including optically active ground and excited states, alongside triplet states within a metastable regime. Transitions between these states, influenced by microwave frequencies, modulate fluorescence and create detectable ODMR signals.

Prior observations indicated no ODMR signal at zero field, suggesting a cascade through the weakly coupled manifold before relaxation to the ground state. Continuous wave ODMR spectra revealed transitions corresponding to both spin manifolds, including a forbidden double-quantum transition at 2.9GHz exhibiting a surprisingly strong signal.

By systematically altering the excitation wavelength, the research team demonstrated a significant increase in ODMR contrast, approaching nearly 100 percent. This improvement directly translates to enhanced magnetic field sensitivity, crucial for applications in quantum sensing. Furthermore, the excitation wavelength profoundly influences the stability of photoluminescence emitted by the spin complex, indicating a strong connection between light absorption and spin dynamics.

These findings highlight the critical importance of optimising excitation wavelength to maximise the performance of hBN spin defects in emerging quantum technologies and sensing applications. The work provides a deeper understanding of these spin complex emitters, paving the way for improved design and implementation in future quantum devices.

Co-excitation characterisation of hBN spin defects via confocal photoluminescence microscopy

A confocal microscope configuration underpinned the study of excitation dynamics in hexagonal boron nitride (hBN) spin defects. Researchers employed a setup allowing simultaneous co-excitation with 532nm and 633nm wavelengths to investigate the impact on spin-dependent fluorescence and dynamics. Optical signals were delivered via a 1×2 single-mode optical fibre, directing both wavelengths through a dichroic mirror into a high-numerical aperture objective.

Collected photoluminescence (PL) was then routed for analysis. To characterise the photodynamics, PL count traces were recorded at a fixed 633nm excitation power of 80 μW while systematically varying the 532nm power from 0 to 150 μW. Extracted count histograms, plotted on a logarithmic scale for enhanced visibility, revealed a suppression of blinking behaviour with increasing 532nm power.

Total PL intensities were also extracted as a function of 532nm power, fitting a power saturation model of the form I(P) = I₀ + I∞P/(P + Pₛ) where I₀ represents the constant PL contribution from the 633nm excitation, I∞ denotes luminescence counts at saturation, and Pₛ is the saturation power of 8.33 μW. The researchers defined a threshold value, derived from the minimum overlapping point of two distinct count distributions observed under 633nm excitation alone, to separate “dark” and “bright” emission states.

This threshold, scaled as a counts intensity ratio, enabled consistent extraction of dark state counts at varying co-excitation powers. Under sole 633nm excitation, approximately 11% of counts were in the dark state, decreasing to around 4% with the addition of only 2 μW of 532nm excitation. Furthermore, ODMR contrast measurements were performed, demonstrating a reduction from approximately 30% for both -1/2 ↔ +1/2 and 0 ↔ +1 transitions with increasing 532nm power, revealing a trade-off between stabilised emission and ODMR contrast.

Excitation Wavelength Optimisation of Spin Contrast and Sensitivity in Hexagonal Boron Nitride

Optical measurements reveal a threefold enhancement in both optically detected magnetic resonance (ODMR) contrast and magnetic field sensitivity through manipulation of excitation wavelength. Specifically, the ODMR contrast for the -1/2 ↔ +1/2 transition increased from 36% to 98% when switching excitation from 532nm to 633nm.

These findings stem from investigations into the excitation dynamics of single spin defects possessing combined S = 1 and S = 1/2 spin transitions within hexagonal boron nitride. The research details the impact of excitation wavelength on spin-dependent fluorescence and the spin dynamics of these defects.

The spin complex emitter exhibits two distinct spin manifolds: a localised, strongly coupled spin pair and a delocalised, weakly coupled spin pair. Energetic levels include optically active S = 0 ground and excited states, alongside triplet states within a metastable regime. Transitions within the strongly coupled manifold, specifically -1 ↔ 0, 0 ↔ +1, and -1 ↔ +1 resonances, were observed at approximately 0.7, 2.1, and 2.9GHz, respectively, under an applied out-of-plane magnetic field of ~50 mT.

A peak at ~1.1GHz corresponds to the second harmonic of the 0 ↔ +1 transition. Confocal microscopy was used to locate emitters, and PL spectra under 532nm and 633nm excitations showed no discernible differences. Autocorrelation measurements exhibited comparable antibunching behaviour (g(2)(0) However, the spin-dependent fluorescence demonstrated a strong dependence on excitation wavelength, with the -1/2 ↔ +1/2 transition displaying the substantial increase in ODMR contrast. The 2.9GHz -1 ↔ +1 transition, a double-quantum transition, produced a strong ODMR signal despite being forbidden under standard spin-selection rules.

Wavelength manipulation optimises spin defect performance in hexagonal boron nitride

Researchers have demonstrated a threefold enhancement in both optically detected magnetic resonance (ODMR) contrast and magnetic field sensitivity by manipulating the excitation wavelength of spin defects in hexagonal boron nitride. Detailed investigations into the spin and photodynamics of these spin complexes reveal a strong correlation between the excitation wavelength and the population of distinct excited states with varying coupling strengths to metastable states.

This understanding allowed for a co-excitation scheme, utilising both 532nm and 633nm wavelengths, to stabilise the photoluminescence of the emitter and optimise performance. The study establishes that 633nm excitation wavelengths are particularly effective at promoting intersystem crossing into these metastable states, thereby improving ODMR contrast and sensitivity.

Spectrally resolved ODMR measurements performed at cryogenic temperatures further elucidated the spin-dependent photoluminescence of the spin complex. These findings are significant because they highlight the critical role of excitation wavelength in controlling the behaviour of these emitters, offering new strategies for optimising their use in quantum sensing and photonic applications.

Limitations acknowledged by the researchers include the complexity of fully mapping the excited state landscape and the need for further investigation into the precise nature of the optically inactive states. Future work may focus on refining the co-excitation scheme and exploring other wavelength combinations to further enhance emitter performance and realise quantum sensing at nanometre resolution, potentially exceeding the limitations of standard confocal microscopy.