arXiv subjects

Milos Toth

Publications and source records attributed to Milos Toth.

At least 19 recordsLinked to original sources

arXiv2026

Twist-Controlled Modulation of Quantum Emitters in a Van der Waals Bilayer

Stacking and twisting two dimensional materials has garnered enormous attention across the condensed matter and the nanophotonic communities. The surge of interest stems from the emergence of novel photophysical phenomena that arise due to the interlayer coupling of the individual layers. Here, we demonstrate that the twist degree of freedom can modulate a single quantum emitter at room temperature. We employ a van der Waals homobilayer of hexagonal boron nitride (hBN) and model the emission properties of quantum emitters as a function of the twist angle. Density functional theory results show that the embedded emitters are strongly influenced by the twist angle and the stacking of the top hBN layer. We consequently engineer these systems experimentally, and demonstrate in-situ tuning of embedded quantum emitters by mechanically twisting the top hBN layer, achieving tunability of over 30 nm (~ 100 meV). Our work demonstrates that mechanical twisting can be harnessed to modulate the embedded quantum emitters in a vdW material, marking a crucial step towards a programmable on-chip quantum circuitry.

cond-mat.mtrl-sci↗

arXiv2025

Tip-enhanced quantum-sensing spectroscopy for bright and reconfigurable solid-state single-photon emitters

Atom-like defects in hexagonal boron nitride (hBN) provide room-temperature single-photon emission and coherent spin states, making them attractive for quantum-computing and -sensing applications. However, their random spatial and spectral characteristics hamper deterministic coupling with nano-optical cavities, limiting their use as bright single-photon sources and sensitive quantum sensors. Here, we present tip-enhanced quantum-sensing spectroscopy of single-photon emitters in hBN. Through precise spatial positioning of individual emitters within tip-cavities with different plasmon resonances, we adaptively control the enhancement rates of both excitation and emission, as well as the single-photon purity. In this way, optimal selection of their relative contributions can effectively reconfigure solid-state single-photon sources, with simultaneous nano-spectroscopic space- and time-resolved analyses. Furthermore, we demonstrate tip-enhanced quantum-sensing with single spin defects through optically detected magnetic resonance (ODMR) experiments in tip-coupled hBN nanoflakes. Our approach provides a unique pathway toward highly-sensitive and deterministic quantum-sensing with room-temperature single-photon emitters.

quant-ph↗

arXiv2025

Quantum Emitters in Rhombohedral Boron Nitride

Rhombohedral boron nitride (rBN) is an emerging wide-bandgap van der Waals (vdW) material that combines strong second-order nonlinear optical properties with the structural flexibility of layered 2D systems. Here we show that rBN hosts optically-addressable spin defects and single-photon emitters (SPEs). Both are fabricated deterministically, using site-specific techniques, and are compared to their analogues in hexagonal boron nitride (hBN). Emission spectra in hBN and rBN are compared, and computational models of defects in hBN and rBN are used to elucidate the debated atomic structure of the B-center SPE in BN. Our results establish rBN as a monolithic vdW platform that uniquely combines second-order nonlinear optical properties, optically addressable spin defects, and high-quality SPEs, opening new possibilities for integrated quantum and nonlinear photonics.

physics.optics↗

arXiv2025

Quantum Emitters in Flux Grown hBN

Hexagonal boron nitride (hBN) is an emerging material for use in quantum technologies, hosting bright and stable single photon emitters (SPEs). The B-center is one promising SPE in hBN, due to the near-deterministic creation methods and regular emission wavelength. However, incorporation of B-centers in high-quality crystals remains challenging, typically relying on additional post-growth methods to increase creation efficiency. Here, we have demonstrated controlled carbon doping of hBN during growth, using a metal flux based method to increase the efficiency of B-center creation. Importantly, single B-centers with $g^{(2)}(0) < 0.5$ were able to be generated in the as-grown hBN when carbon additions during growth exceeded 2.5 wt.% C. Resonant excitation measurements revealed linewidths of 3.5 GHz with only moderate spectral diffusion present, demonstrating the applicability of the as-grown hBN as a host for high quality B-centers.

cond-mat.mtrl-sci↗

arXiv2025

A Van der Waals Moir\'e Bilayer Photonic Crystal Cavity

Enhancing light-matter interactions with photonic structures is critical in classical and quantum nanophotonics. Recently, Moir\'e twisted bilayer optical materials have been proposed as a promising means towards a tunable and controllable platform for nanophotonic devices, with proof of principle realisations in the near infrared spectral range. However, the realisation of Moir\'e photonic crystal (PhC) cavities has been challenging, due to a lack of advanced nanofabrication techniques and availability of standalone transparent membranes. Here, we leverage the properties of the van der Waals material hexagonal Boron Nitride to realize Moir\'e bilayer PhC cavities. We design and fabricate a range of devices with controllable twist angles, with flatband modes in the visible spectral range (~ 450 nm). Optical characterization confirms the presence of spatially periodic cavity modes originating from the engineered dispersion relation (flatband). Our findings present a major step towards harnessing a two-dimensional van der Waals material for the next-generation of on chip, twisted nanophotonic systems.

physics.optics↗

arXiv2025

Fabrication of ultra-smooth, high-aspect ratio, sub-10 nanometer nanostructures

Deterministic and versatile approaches to sample preparation on nanoscopic scales are important in many fields including photonics, electronics, biology and material science. However, challenges exist in meeting many nanostructuring demands--particularly in emerging optical materials and component architectures. Here, we report a nanofabrication workflow that overcomes long-standing challenges in deterministic and top-down sample preparation procedures. The salient feature is a carbon mask with a low sputter yield that can be readily shaped using high resolution electron beam processing techniques. When combined with focused ion beam processing, the masking technique yields structures with ultra-smooth, near-vertical side walls. We target different material platforms to showcase the broad utility of the technique. As a first test case, we prepared nanometric gaps in evaporated Au. Gap widths of 7 plus/minus 2 nm, aspect ratios of 17, and line edge roughness values of 3sigma = 2.04 nm are achieved. Furthermore, the gap widths represent an order of magnitude improvement on system resolution limits. As a second test case, we designed and fabricated dielectric resonators in the ternary compounds MnPSe3 and NiPS3; a class of van der Waals material resistant to chemical etch approaches. Nanoantenna arrays with incrementally increasing diameter were fabricated in crystalline, exfoliated flakes. The optical response was measured by dark field spectroscopy and is in agreement with simulations. The workflow reported here leverages established techniques in material processing without the need for custom or specialized hardware. It is broadly applicable to functional materials and devices, and extends high speed focused ion beam milling to true sub-10 nm length scales.

physics.optics↗

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↗

arXiv2025

Electrical Generation of Colour Centres in Hexagonal Boron Nitride

Defects in wide band gap crystals have emerged as a promising platform for hosting colour centres that enable quantum photonic applications. Among these, hexagonal boron nitride (hBN), a van der Waals material, stands out for its ability to be integrated into heterostructures, enabling unconventional charge injection mechanisms that bypass the need for p-n junctions. This advancement allows for the electrical excitation of hBN colour centres deep inside the large hBN bandgap, which has seen rapid progress in recent developments. Here, we fabricate hBN electroluminescence (EL) devices that generate narrowband colour centres suitable for electrical excitation. The colour centres are localised to tunnelling current hotspots within the hBN flake, which are designed during device fabrication. We outline the optimal conditions for device operation and colour centre stability, focusing on minimising background emission and ensuring prolonged operation. Our findings follow up on the existing literature and mark a step forward towards the integration of hBN based colour centres into quantum photonic technologies.

physics.optics↗

arXiv2024

Decoherence of Quantum Emitters in hexagonal Boron Nitride

Coherent quantum emitters are a central resource for advanced quantum technologies. Hexagonal boron nitride (hBN) hosts a range of quantum emitters that can be engineered using techniques such as high-temperature annealing, optical doping, and irradiation with electrons or ions. Here, we demonstrate that such processes can degrade the coherence, and hence the functionality, of quantum emitters in hBN. Specifically, we show that hBN annealing and doping methods that are used routinely in hBN nanofabrication protocols give rise to decoherence of B-center quantum emitters. The decoherence is characterized in detail, and attributed to defects that act as charge traps which fluctuate electrostatically during SPE excitation and induce spectral diffusion. The decoherence is minimal when the emitters are engineered by electron beam irradiation of as-grown, pristine flakes of hBN, where B-center linewidths approach the lifetime limit needed for quantum applications involving interference and entanglement. Our work highlights the critical importance of crystal lattice quality to achieving coherent quantum emitters in hBN, despite the common perception that the hBN lattice and hBN SPEs are highly-stable and resilient against chemical and thermal degradation. It underscores the need for nanofabrication techniques that are minimally invasive and avoid crystal damage when engineering hBN SPEs and devices for quantum-coherent technologies.

quant-ph↗

arXiv2024

Quantum Optics Applications of Hexagonal Boron Nitride Defects

Hexagonal boron nitride (hBN) has emerged as a compelling platform for both classical and quantum technologies. In particular, the past decade has witnessed a surge of novel ideas and developments, which may be overwhelming for newcomers to the field. This review provides an overview of the fundamental concepts and key applications of hBN, including quantum sensing, quantum key distribution, quantum computing, and quantum memory. Additionally, we highlight critical experimental and theoretical advances that have expanded the capabilities of hBN, in a cohesive and accessible manner. The objective is to equip readers with a comprehensive understanding of the diverse applications of hBN, and provide insights into ongoing research efforts.

quant-ph↗

arXiv2024

Quantum Efficiency the B-centre in hexagonal boron nitride

B-centres in hexagonal boron nitride (hBN) are gaining significant research interest for quantum photonics applications due to precise emitter positioning and highly reproducible emission wavelengths. Here, we leverage the layered nature of hBN to directly measure the quantum efficiency (QE) of single B-centres. The defects were engineered in a 35 nm flake of hBN using electron beam irradiation, and the local dielectric environment was altered by transferring a 250 nm hBN flake on top of the one containing the emitters. By analysing the resulting change in measured lifetimes, we determined the QE of B-centres in the thin flake of hBN, as well as after the transfer. Our results indicate that B-centres located in thin flakes can exhibit QEs higher than 40%. Near-unity QEs are achievable under reasonable Purcell enhancement for emitters embedded in thick flakes of hBN, highlighting their promise for quantum photonics applications.

physics.optics↗

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

Hydrogen plasma inhibits ion beam restructuring of materials

Focused ion beam (FIB) techniques are employed widely for nanofabrication, and processing of materials and devices. However, ion irradiation often gives rise to severe damage due to atomic displacements that cause defect formation, migration and clustering within the ion-solid interaction volume. The resulting restructuring degrades the functionality of materials, and limits the utility FIB ablation and nanofabrication techniques. Here we show that such restructuring can be inhibited by performing FIB irradiation in a hydrogen plasma environment via chemical pathways that modify defect binding energies and transport kinetics, as well as material ablation rates. The method is minimally-invasive and has the potential to greatly expand the utility of FIB nanofabrication techniques in processing of functional materials and devices.

physics.app-ph↗

arXiv2024

Electron Beam Restructuring of Quantum Emitters in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) holds promise as a solid state, van der Waals host of single photon emitters for on-chip quantum photonics. The B-centre defect emitting at 436 nm is particularly compelling as it can be generated by electron beam irradiation. However, the emitter generation mechanism is unknown, the robustness of the method is variable, and it has only been applied successfully to thick flakes of hBN (>> 10 nm). Here, we use in-situ time-resolved cathodoluminescence (CL) spectroscopy to investigate the kinetics of B-centre generation. We show that the generation of B-centres is accompanied by quenching of a carbon-related emission at ~305 nm and that both processes are rate-limited by electromigration of defects in the hBN lattice. We identify problems that limit the efficacy and reproducibility of the emitter generation method, and solve them using a combination of optimized electron beam parameters and hBN pre- and post-processing treatments. We achieve B-centre quantum emitters in hBN flakes as thin as 8 nm, elucidate the mechanisms responsible for electron beam restructuring of quantum emitters in hBN, and gain insights towards identification of the atomic structure of the B-centre quantum emitter.

physics.app-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↗

arXiv2024

Engineering Quantum Light Sources with Flat Optics

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, precise sensing and imaging. Recent advancements have witnessed a significant shift towards the utilization of ``flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, we explore the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, as well as single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. We discuss the diverse application range of these sources and highlight the current challenges and perspectives in the field.

physics.optics↗

arXiv2024

Near-coherent quantum emitters in hexagonal boron nitride with discrete polarization axes

Hexagonal boron nitride (hBN) has recently gained attention as a solid state host of quantum emitters. However, hBN emitters reported to date lack the properties needed for their deployment in scalable quantum technologies. Here we employ spectral hole burning spectroscopy and resonant polarization measurements to observe nearly-coherent hBN quantum emitters, both as singles and in ensembles, with three discrete polarization axes indicative of a C2v symmetry defect. Our results constitute an important milestone towards the implementation of hBN quantum emitters in integrated quantum photonics.

quant-ph↗

arXiv2023

On-Demand Quantum Light Sources for Underwater Communications

Quantum communication has been at the forefront of modern research for decades, however it is severely hampered in underwater applications, where the properties of water absorb nearly all useful optical wavelengths and prevent them from propagating more than, in most cases, a few metres. This research reports on-demand quantum light sources, suitable for underwater optical communication. The single photon emitters, which can be engineered using an electron beam, are based on impurities in hexagonal boron nitride. They have a zero phonon line at ~ 436 nm, near the minimum value of water absorption and are shown to suffer negligible transmission and purity loss when travelling through water channels. These emitters are also shown to possess exceptional underwater transmission properties compared to emitters at other optical wavelengths and are utilised in a proof of principle underwater communication link with rates of several kbits/s.

physics.optics↗