Thèse Capteurs Quantiques Fondés sur des Défauts de Spin dans un Matériau Bidimensionnel H/F

Doctorat.Gouv.Fr

Établissement : Université de Montpellier École doctorale : I2S - Information, Structures, Systèmes Laboratoire de recherche : L2C - Laboratoire Charles Coulomb Direction de la thèse : Vincent JACQUES ORCID 0000000154716061 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-07-15T23:59:59 Les capteurs quantiques fondées sur des défauts de spin dans les solides ont déjà démontré un potentiel considérable pour répondre au besoin croissant de capteurs de haute précision, aussi bien pour la recherche fondamentale que pour les applications industrielles. Les méthodes de détection quantique les plus avancées reposent actuellement sur des défauts de spin hébergés dans des matériaux tridimensionnels (3D), mais elles sont confrontées à plusieurs limitations, notamment : (i) une sensibilité limitée résultant de la faible proximité pouvant être atteinte entre le capteur quantique et l'échantillon à sonder, et (ii) l'impossibilité de concevoir des capteurs quantiques ultraminces et flexibles pouvant être facilement transférés sur les échantillons étudiés ou intégrés dans des dispositifs multifonctionnels complexes. L'objectif de ce projet de recherche est de surmonter ces limitations grâce à la conception d'une membrane flexible de détection quantique reposant sur un matériau bidimensionnel (2D) d'épaisseur atomique.

Notre approche consiste à utiliser des défauts de spin optiquement actifs hébergés dans du nitrure de bore hexagonal (hBN) bidimensionnel, un matériau à large bande interdite qui constitue un élément essentiel des hétérostructures de van der Waals. Plus précisément, les travaux porteront sur l'étude des propriétés de spin et des propriétés optiques du défaut lacune de bore (VB) dans le hBN, afin d'évaluer son potentiel pour des applications de détection quantique, avec un accent particulier sur la détection de champs magnétiques. L'objectif de la thèse sera d'analyser les performances de feuillets ultraminces de hBN (jusqu'à la limite de la monocouche) dopés par des centres VB pour l'imagerie quantitative de champs magnétiques à température cryogénique (4 K). Le capteur quantique bidimensionnel sera ensuite intégré dans des hétérostructures de van der Waals afin de sonder in situ les phénomènes physiques se produisant aux interfaces entre des matériaux 2D d'épaisseur atomique assemblés en empilements verticaux, avec un intérêt particulier pour les aimants bidimensionnels et les supraconducteurs bidimensionnels. Quantum sensing based on optically addressable spin defects has become a central pillar of quantum technology. These atomic-scale systems, embedded in solid-state hosts, offer a unique combination of optical readout, spin coherence, and quantum control, providing powerful tools for probing matter at the nanoscale with wide-ranging applications from condensed matter physics to life sciences. A key challenge in quantum sensing lies in achieving sufficient proximity between the sensor and the target system, as the interactions that encode the information are intrinsically short-ranged and decay rapidly with distance. This is particularly crucial for magnetic field sensing, where the coupling between the spin sensor and the target often depends on magnetic dipolar coupling, which scales with the cube of their relative distance (1/d^3). Consequently, even for established quantum sensing platforms, such as NV centers in diamond, where the spin defects are typically embedded few tens of nanometres below the surface, the sample signals often fall below the detection threshold, fundamentally limiting practical applications.

Two-dimensional (2D) van der Waals materials hosting spin defects have recently emerged as a transformative solution to this limitation. Their atomic-scale thickness enables spin sensors to be placed in a higher proximity to the sample, mitigating the distance barrier that constrains sensitivity in bulk materials. Moreover, 2D materials can be readily exfoliated, transferred, and integrated with a wide range of substrates and devices, offering superior versatility with respect to conventional crystals which are often rigid and difficult to interface. Within this family, hexagonal boron nitride (hBN) stands out as a particularly promising host. This material, which can be exfoliated down to few atomic layers while maintaining perfect chemical stability under ambient conditions, is extensively used for encapsulation of van der Waals heterostructures in 2D material research. Moreover, owing to its wide bandgap, hBN hosts a broad diversity of optically active point defects that show strong potential for applications. Among these, the negatively charged boron vacancy (VB) has emerged as one of the most promising spin defect in hBN for the deployment of quantum sensing technologies on a 2D material platform. This defect exhibits a spin-triplet ground state that can be initialized through optical pumping, coherently manipulated with a microwave excitation, and readout by pure optical means by recording its spin-dependent photoluminescence (PL) signal. These combined properties allow optically detected magnetic resonance (ODMR) measurements to be performed in a manner analogous to NV centers in diamond, but now within a truly van der Waals host. Despite a low quantum yield that has so far prevented its optical detection at the single scale, dense ensembles of VB centers have already shown great promise as a multipurpose quantum sensors for magnetic and electric fields, pressure and temperature. However, most quantum sensing studies have so far used thick hBN flakes, with VB centers located tens of nanometers below the surface, i.e. far from atomic-scale sensing proximity. In addition, experiments have often been performed on hBN layers affected by paramagnetic impurities and limited crystal quality, and under low magnetic fields. These non-ideal conditions have resulted in the relatively short coherence times reported so far as well as in sensing performances that remain below what is required for robust, real-world applications. In this context, the host team has recently demonstrated that the optical and spin properties of VB centers are fully preserved in few-atomic-layer thick hBN flakes, despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects, such as NV centers in diamond.

Capitalizing on these preliminary results, the main objectives of the PhD are:

(i) the realization and optimization of an atomically thin (~1 nm thick) quantum sensor based on an ensemble of VB centers in hBN, with a targeted sensitivity of 10µT/Hz for static (DC) magnetic fields for a diffraction-limited laser excitation of about 1 µm2, and
(ii) Once optimized, this platform will be used to study the phase transition of a 2D superconductor and to demonstrate nanoscale nuclear magnetic resonance (NMR) of nuclear spins embedded in materials interfaced with our 2D sensor.

This PhD project will be pursued in close collaboration with the team led by Cédric Robert at the LPCNO in Toulouse, which bring a key expertise on teh fabrication of van der Waals heterostructures.