arXiv subjects

Benjamin Whitefield

Publications and source records attributed to Benjamin Whitefield.

13 recordsLinked to original sources

arXiv2026

Correlating spin and optical properties of quantum emitters in hBN

Optically addressable spin defects in hexagonal boron nitride (hBN) are well suited for near-surface quantum sensing. They offer bright, wavelength-tunable single-photon emission with high optically detected magnetic resonance (ODMR) contrast at room temperature. Here we controllably synthesise a high density of spin-complex defects in carbon-doped hBN flakes. We find that the zero-field splitting parameter D is directly correlated with the zero-phonon line of an emitter, while the ODMR contrast shows no such correlation. We further analyse an individual narrowband defect showing 68% ODMR contrast and enhance its photon collection by ~40% using a solid immersion lens. By advancing both the practical synthesis of the spin complexes and the understanding of their microscopic origin, our results move us toward the deterministic creation of ODMR-active quantum emitters

quant-ph↗

arXiv2026

On chip, multifunctional quantum sensing using single spins in a van der Waals crystal

Nanoscale thermometry and magnetometry are in high demand across a wide range of scientific and technological applications. In this context, optically addressable spins in solids have emerged at the forefront of on-chip quantum sensing. However, simultaneous quantum sensing of multiple parameters (e.g., temperature and magnetic field) using the same spin sensor remains challenging due to cross-sensitivity to multiple physical quantities. Here, we demonstrate independent dual sensing of temperature and magnetic field using single quantum emitters in hexagonal boron nitride (hBN). We experimentally verify the independent response of the zero-phonon line (ZPL) position to temperature and of optically detected magnetic resonance (ODMR) to magnetic fields. Furthermore, we demonstrate local temperature sensing of a microcircuit while simultaneously measuring an external magnetic field. Our results establish quantum emitters in hBN as a robust platform for multifunctional quantum sensing under realistic operating conditions.

quant-ph↗

arXiv2026

Multi-wavelength Spin Dynamics of Defects in Hexagonal Boron Nitride

Optically addressable solid-state spin defects are essential platforms for quantum sensing and information processing. Recently, single spin defects with combined S = 1 and S = 1/2 spin transitions were discovered in hexagonal boron nitride (hBN). In this work we unveil their excitation dynamics. In particular, we study the effects of the excitation wavelength on the spin-dependent fluorescence and the spin dynamics of these peculiar quantum spin defects. We find that changing the excitation wavelength leads to a threefold enhancement in both the optically detected magnetic resonance (ODMR) contrast and the corresponding magnetic field sensitivity. In addition, we find that the excitation wavelength has a strong impact on the photodynamics of spin complex emitters. Our work presents valuable insights to the mechanistic understanding of spin complex emitters in hBN and highlights the importance of excitation wavelength for optimising their performance in quantum sensing and quantum technologies.

cond-mat.mtrl-sci↗

arXiv2026

Deterministic integration of quantum emitters and optical cavities in a van der Waals crystal

Single-photon emitters in hexagonal boron nitride (hBN) combine bright optical emission with optically addressable spin states, offering a promising platform for integrated quantum photonics. However, their stochastic creation and spectral variability have prevented deterministic integration with photonic cavities. Here we demonstrate a fabrication protocol that enables precise, deterministic coupling of pre-selected visible emitters to circular Bragg grating (CBG) cavities in hBN. By patterning etched alignment markers and performing prefabrication confocal mapping, we locate emitters with sub-micron accuracy and design cavity geometries matched to their zero-phonon line wavelengths. The resulting devices show enhanced emission and reliable spectral alignment between emitter and cavity mode. This work establishes a deterministic cavity-emitter integration scheme in a van der Waals material and provides a scalable route towards on-chip quantum photonic and spin-based platforms using hBN.

physics.optics↗

arXiv2025

Photodynamics and Temperature Dependence of Single Spin Defects in Hexagonal Boron Nitride

Quantum emitters in hexagonal boron nitride (hBN) that exhibit optically detected magnetic resonance (ODMR) signatures have recently garnered significant attention as an emerging solid-state platform for quantum technologies. However, the underlying spin dynamics, and the mechanisms determining the spin-dependent fluorescence in these defects are still poorly understood. In this work we perform detailed photodynamical studies of the spin complexes in hBN. In particular, we show that spin transitions are located within the metastable manifold which can be explained by the rate model, populating in a cascading manner. In addition, we perform temperature dependent measurements on these defects and show that the spin-lattice relaxation and coherence times increase as the temperature reduces. Furthermore, we find that the ODMR frequencies of the S=1 transition show only a marginal frequency shift as a function of temperature, which makes them a robust sensor at cryogenic temperatures. These insights are crucial for further understanding of the spin dynamics of quantum emitters in hBN and their practical implementation in quantum sensing.

cond-mat.mtrl-sci↗

arXiv2025

Generation of narrowband quantum emitters in hBN with optically addressable spins

Electron spins coupled with optical transitions in solids stand out as a promising platform for developing spin-based quantum technologies. Recently, hexagonal boron nitride (hBN) - a layered Van der Waals (vdW) crystal, has emerged as a promising host for optically addressable spin systems. However, to date, on-demand generation of isolated single photon emitters with pre-determined spin transitions has remained elusive. Here, we report on a single step, thermal processing of hBN flakes that produces high density, narrowband, quantum emitters with optically active spin transitions. Remarkably, over 25% of the emitters exhibit a clear signature of an optical spin readout at room temperature, surpassing all previously reported results by an order of magnitude. The generated spin defect complexes exhibit both S = 1 and S = 1/2 transitions, which are explained by charge transfer from strongly to weakly coupled spin pairs. Our work advances the understanding of spin complexes in hBN and paves the way for single spin - photon interfaces in layered vdW materials with applications in quantum sensing and information processing.

quant-ph↗

arXiv2024

Double Etch Method for the Fabrication of Nanophotonic Devices from Van der Waals Materials

The integration of van der Waals (vdW) materials into photonic devices has laid out a foundation for many new quantum and optoelectronic applications. Despite tremendous progress in the nanofabrication of photonic building blocks from vdW crystals, there are still limitations, specifically with large-area devices and masking. Here, we focus on hexagonal boron nitride (hBN) as a vdW material and present a double etch method that overcomes problems associated with methods that employ metallic films and resist-based films for masking. Efficacy of the developed protocol is demonstrated by designing and fabricating a set of functional photonic components including waveguides, ring resonators and photonic crystal cavities. The functionality of the fabricated structures is demonstrated through optical characterization over several key spectral ranges. These include the near-infrared and blue ranges, where the hBN boron vacancy (VB-) spin defects and the coherent B center quantum emitters emit, respectively. The double etch method enables fabrication of high-quality factor optical cavities and constitutes a promising pathway toward on-chip integration of vdW materials.

physics.optics↗

arXiv2024

A charge transfer mechanism for optically addressable solid-state spin pairs

Optically detected magnetic resonance (ODMR) with no resolvable zero-field splitting has been observed from emitters in hexagonal boron nitride across a broad range of wavelengths, but so far an understanding of their microscopic structure and the physical origin of ODMR has been lacking. Here we perform comprehensive measurements and modelling of the spin-resolved photodynamics of ensembles and single emitters, and uncover a universal model that accounts, and provides an intuitive physical explanation, for all key experimental features. The model, inspired by the radical-pair mechanism from spin chemistry, assumes a pair of nearby point defects -- a primary optically active defect and a secondary defect. Charge transfer between the two defects creates a metastable weakly coupled spin pair with ODMR naturally arising from selection rules. Using first-principle calculations, we show that simple defect pairs made of common carbon defects provide a plausible microscopic explanation. Our optical-spin defect pair (OSDP) model resolves several previously open questions including the asymmetric envelope of the Rabi oscillations, the large variability in ODMR contrast amplitude and sign, and the wide spread in emission wavelength. It may also explain similar phenomena observed in other wide bandgap semiconductors such as GaN. The presented framework will be instrumental in guiding future theoretical and experimental efforts to study and engineer solid-state spin pairs.

cond-mat.mes-hall↗

arXiv2024

Fast characterization of optically detected magnetic resonance spectra via data clustering

Optically detected magnetic resonance (ODMR) has become a well-established and powerful technique for measuring the spin state of solid-state quantum emitters, at room temperature. Relying on spin-dependent recombination processes involving the emitters ground, excited and metastable states, ODMR is enabling spin-based quantum sensing of nanoscale electric and magnetic fields, temperature, strain and pressure, as well as imaging of individual electron and nuclear spins. Central to many of these sensing applications is the ability to reliably analyze ODMR data, as the resonance frequencies in these spectra map directly onto target physical quantities acting on the spin sensor. However, this can be onerous, as relatively long integration times -- from milliseconds up to tens of seconds -- are often needed to reach a signal-to-noise level suitable to determine said resonances using traditional fitting methods. Here, we present an algorithm based on data clustering that overcome this limitation and allows determining the resonance frequencies of ODMR spectra with better accuracy (~1.3x factor), higher resolution (~4.7x factor) and/or overall fewer data points (~5x factor) than standard approaches based on statistical inference. The proposed clustering algorithm (CA) is thus a powerful tool for many ODMR-based quantum sensing applications, especially when dealing with noisy and scarce data sets.

quant-ph↗

arXiv2024

Fibre-integrated van der Waals quantum sensor with an optimal cavity interface

Integrating quantum materials with fibre optics adds advanced functionalities to a variety of applications, and introduces fibre-based quantum devices such as remote sensors capable of probing multiple physical parameters. However, achieving optimal integration between quantum materials and fibres is challenging, particularly due to difficulties in fabrication of quantum elements with suitable dimensions and an efficient photonic interface to a commercial optical fibre. Here we demonstrate a new modality for a fibre-integrated van der Waals quantum sensor. We design and fabricate a hole-based circular Bragg grating cavity from hexagonal boron nitride (hBN), engineer optically active spin defects within the cavity, and integrate the cavity with an optical fibre using a deterministic pattern transfer technique. The fibre-integrated hBN cavity enables efficient excitation and collection of optical signals from spin defects in hBN, thereby enabling all-fibre integrated quantum sensors. Moreover, we demonstrate remote sensing of a ferromagnetic material and of arbitrary magnetic fields. All in all, the hybrid fibre-based quantum sensing platform may pave the way to a new generation of robust, remote, multi-functional quantum sensors.

quant-ph↗

arXiv2023

Manipulating the Charge State of Spin Defects in Hexagonal Boron Nitride

Negatively charged boron vacancies ($\small{V_B^-}$) in hexagonal boron nitride (hBN) have recently gained interest as spin defects for quantum information processing and quantum sensing by a layered material. However, the boron vacancy can exist in a number of charge states in the hBN lattice, but only the -1 state has spin-dependent photoluminescence and acts as a spin-photon interface. Here, we investigate charge state switching of $\small{V_B}$ defects under laser and electron beam excitation. We demonstrate deterministic, reversible switching between the -1 and 0 states ($\small{V_B^- \rightleftharpoons V_B^0 + e^-}$), occurring at rates controlled by excess electrons or holes injected into hBN by a layered heterostructure device. Our work provides a means to monitor and manipulate the $\small{V_B}$ charge state, and to stabilize the -1 state which is a prerequisite for optical spin manipulation and readout of the defect.

physics.optics↗

arXiv2023

Framework for engineering of spin defects in hexagonal boron nitride by focused ion beams

Hexagonal boron nitride (hBN) is gaining interest as a wide bandgap van der Waals host of optically active spin defects for quantum technologies. Most studies of the spin-photon interface in hBN focus on the negatively charged boron vacancy (VB-) defect, which is typically fabricated by ion irradiation. However, VB- fabrication methods often lack robustness and reproducibility when applied to thin flakes (less than 10 nm) of hBN. Here we identify mechanisms that both promote and inhibit VB- generation and optimize ion beam parameters for site-specific fabrication of optically active VB- centers. We emphasize conditions accessible by high resolution focused ion beam (FIB) systems, and present a framework for VB- fabrication in hBN flakes of arbitrary thickness for applications in quantum sensing and quantum information processing.

cond-mat.mtrl-sci↗

arXiv2023

Coupling spin defects in hexagonal boron nitride to a microwave cavity

Optically addressable spin defects in hexagonal boron nitride (hBN) have become a promising platform for quantum sensing. While sensitivity of these defects are limited by their interactions with the spin environment in hBN, inefficient microwave delivery can further reduce their sensitivity. Hare, we design and fabricate a microwave double arc resonator for efficient transferring of the microwave field at 3.8 GHz. The spin transitions in the ground state of VB- are coupled to the frequency of the microwave cavity which results in enhanced optically detected magnetic resonance (ODMR) contrast. In addition, the linewidth of the ODMR signal further reduces, achieving a magnetic field sensitivity as low as 42.4 microtesla per square root of hertz. Our robust and scalable device engineering is promising for future employment of spin defects in hBN for quantum sensing.

physics.app-ph↗