arXiv subjects

Nathan Coste

Publications and source records attributed to Nathan Coste.

10 recordsLinked to original sources

arXiv2026

Pulsed coherent spectroscopy of a quantum emitter in hexagonal Boron Nitride

Defects in solid-state systems constitute a promising platform for the realization of deterministic quantum emitters. Among many candidate materials and emitters, point defects in hexagonal Boron Nitride (hBN) have recently emerged as particularly promising. In this work, we probe the coherence of an individual B center with a zero phonon line at 436 nm, under pulsed resonant excitation. We observe power-dependent Rabi oscillations up to 5{\pi}, demonstrating optical coherent control of the transition. We achieve an excellent single photon purity of 93% at {\pi}-pulse. Furthermore, we probe the coherence of the two-level system using Ramsey interferometry, revealing an inhomogeneous coherence time of T_2*=0.60 ns. These results establish B centers in hBN as viable candidates for triggered, coherent quantum emitters and represent an important step towards their integration into quantum photonic platforms.

quant-ph↗

arXiv2026

Engineering and Tuning of high quality hexagonal boron nitride nanophotonic resonators

Van der Waals materials are offering intriguing opportunities as building blocks for advanced quantum information technologies and integrated quantum photonic systems. Critical to their development, is robust and high quality light-matter interactions which can be delivered through the fabrication of optical resonators. Here we demonstrate a robust fabrication of one dimensional photonic crystal cavities (1D PCC) and microdisk resonators from hexagonal boron nitride, exhibiting Quality factors of ~ 4300 and ~ 8300, respectively. With these two classes of devices we demonstrated cavity mode tuning via atomic layer deposition and gas condensation. Cavity resonances were shifted by and ~9 nm for the 1D PCCs and ~16 nm in the microdisk resonators, respectively. Our work opening a promising pathway for a realisation emitter cavity coupling in hBN and eventually to a fully integrated quantum photonic circuitry with hBN.

physics.optics↗

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

Visible Integrated Photonics with Tantalum Pentoxide

Developing chip-scale photonic platforms for the visible spectrum is essential for translating next-generation optical technologies. Here, we demonstrate that tantalum pentoxide Ta2O5 on insulator is a highly promising material framework for these applications, supporting high-Q resonator architectures across the visible spectral range. Utilizing routine and scalable thin-film deposition and fabrication methodologies, we realize Ta2O5 waveguide and resonator layouts compatible with photonic integrated circuit (PIC) architectures. Through a comparison with numerical simulations, we characterise compact racetrack resonators, evaluating their performance in both the green and blue spectral regions. In addition, we fabricate large-scale ring resonators exhibiting record-high quality factors for Ta2O5 in the visible spectrum, exceeding 7.0 x 10^4. These results highlight the immense potential of this platform for visible photonic systems and establish a robust, straightforward baseline for future visible integrated optics.

physics.optics↗

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

An Inverse Design Wavelength Demultiplexer for On-Chip Photoluminescence Sorting in TMDC Heterostructures

Emerging two-dimensional transition metal dichalcogenides (TMDCs) offer a promising platform for on-chip integrated photonics because of their unique optical and electronic properties. Their naturally passivated surfaces make them highly tolerant to lattice mismatch, enabling seamless heterogeneous integration by stacking different van der Waals materials, a crucial step in the development of advanced photonic devices. Here, we demonstrate the use of an inverse design wavelength demultiplexing waveguides for on-chip sorting and routing of distinct photoluminescence from the heterojunction formed by WS2 and WSe2 monolayers. The integrated nanophotonic chip splits and sorts excitonic emission into individual waveguides at room temperature. Our demonstration opens up new perspectives for integrating light sources in van der Waals materials with functional integrated photonics, offering a versatile platform for both fundamental research and practical applications.

physics.optics↗

arXiv2024

Indistinguishability of remote quantum dot-cavity single-photon sources

Generating identical photons from remote emitter-based bright single-photon sources is an important step for scaling up optical quantum technologies. Here, we study the Hong-Ou-Mandel interference of photons emitted from remote sources based on semiconductor quantum dots. We make use of a deterministic fabrication technique to position the quantum dots in a spectrally resonant micropillar cavity and fine tune their operation wavelength electrically. Doing so, we can match four pairs of sources between five distinct sources, study them under various excitation schemes and measure their degree of indistinguishability. We demonstrate remote indistinguishabiltiy between 44$\pm$1% and 69$\pm$1% depending on the pair of sources and excitation conditions, record values for quantum dots in cavities. The relative contribution of pure dephasing and spectral diffusion is then analysed, revealing that the remaining distinguishability is mostly due to low frequency noise

quant-ph↗

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 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↗