Research overview Materials can be studied using computer simulation which enables one to probe the motion of each constituent atoms and to build correlations between the macroscopic properties and the microscopic behaviors. On the one hand, traditional quantum mechanics methods provides particularly accurate results up to the electronic structure of the material. Yet, the drawback of this method concerns its computational cost which prevents from studying large system sizes and long time scales. On the other hand, effective potentials have been developed to mimic atomic interactions thereby reducing those issues. However, these potentials are often built to reproduce bulk properties of the materials and can hardly be employed to study some specific systems including interfaces and nanomaterials. In this context, a new class of interatomic potentials based on machine-learning algorithms is being developed to retain the accuracy of traditional quantum mechanics methods while being able to run simulations with larger system sizes and longer time scales. Simulation project Using computer simulations, the student will construct a database that should be representative of the different interactions occurring in a specific material. Machine-learning potentials based on the least-angle regression algorithm as well as neural network potentials will be trained and their accuracy will be studied as a function of the size and the complexity of the database.

Research overview The formation of a crystal is triggered by the emergence of a nucleation core. Classical nucleation theory (CNT) is widely employed to discuss its nature and its origin. In CNT, the thermodynamically stable phase is always the one that grows first and its size is then driven by the free energy competition between how much it costs to build a liquid-crystal interface and the gain from growing the crystal. Yet, following Ostwald’s rule, another structure may emerge beforehand if it is closer in free energy to the mother phase. Then, structural and also chemical reorganizations happen during the growth. This multi-stage nucleation mechanism already appears in bulk systems but can be amplified in nanocrystal nucleation where surface effects and chemical reactivity are enhanced. For nanoscience to be inspired by the practical applications instead of still being driven by the synthesis possibilities, it is crucial to reach a better understanding of the unique crystallization mechanisms leading to nanocrystals. Simulation project Atomistic simulations will be performed to study crystallization of binary particles. Examples will be taken from well-studied materials including CuZr, NiAl, NaCl, Water... We will investigate the correlation between the thermodynamic conditions and the final nanoparticles. The goal is to ultimately better understand how nucleation theory is affected by downsizing to the nanometric scale.

The frequency (or time-of-arrival) degree of freedom of a single photon represents a continuous variable used for encoding quantum information [1,2,3]. Frequency is less susceptible to noise in optical fibers, waveguides, and free space, making it a strong candidate for future quantum information processing. Unlike polarization, frequency is unaffected by birefringence, does not require phase stabilization, and is immune to nonlinear effects at the single-photon level. Given the high cost and impracticality of deploying new fiber infrastructure, the project explores how quantum communication can be integrated into existing classical networks by having the coexistence of quantum and classical signals within one optical fiber. This project investigates the development of a quantum channel mediated by stimulated Raman scattering (SRS), when there is coexistence of one classical field and a frequency-encoded single photon state. We will employ a light-matter interaction model to analyze how SRS-induced field distortion impacts the frequency encoding of single photons, including two-color and grid-state encodings [3]. The resulting spectral modifications will be quantified by their effect on quantum communication performance, specifically the quantum bit error rate (QBER) and the asymptotic key rate.

The project focuses on mixtures of electrons (fermion) and excitons (electron-hole pair, a boson) in a new class of materials: van der Waals heterostructures. The latter can be seen as a “mille-feuille”, obtained by stacking atomically thin sheets of various materials. They recently became a prominent platform to study many-body physics, after a milestone discovery of superconductivity in bilayer graphene. Our long term ambition is to introduce superconductivity in a controlled manner, using excitons as force-carrier bosons (instead of phonons in conventional superconductors). The internship will pave the way toward this goal. It includes two steps, (i) the fabrication of the heterostructures and (ii) a first characterization with optical spectroscopy.