Expérimental
http://tiny.cc/QG What happens to the ground state of a quantum system when we add dissipation? How is the lifetime of excited states affected by dissipation? How is quantum coherence destroyed by dissipation? The student will measure the lifetime and coherence of a bad qubit: a Josephson junction shunted by an on-chip resistance and embedded in a superconducting microwave cavity. The student is expected to aid in the design of devices using microwave simulation software; fabricate samples in a clean room using techniques such as microlithography and electron beam evaporation; cool samples using a cryogen free dilution cryostat; and make sensitive microwave measurements at low temperatures

Théorique, numérique
During this M2 internship, we propose to study theoretically and numerically the dynamics of Bose gases subjected to both an oscillating driving force and a spatially disordered potential. This scenario, recently realized experimentally, gives rise to an original mechanism of sub-diffusion in energy space, whose quantitative description for realistic models of disorder remains to establish. This is the task that will be accomplished during this internship. More generally, this internship will be an opportunity to become familiar with the modern research fields of non-equilibrium quantum physics and ultracold Bose gases.

Expérimental
Cold atom inertial sensors have many applications in fundamental physics (testing the laws of gravitation, gravitational astronomy), geosciences (measuring the Earth's gravity field or rotation) and inertial navigation. The operation of these sensors is based on atomic interferometry, taking advantage of superpositions between quantum states of different momentum in an atom, generated by optical transitions with two (or more) photons. To broaden their range of applications, it is necessary to constantly push back their performance in terms of sensitivity, stability, precision, dynamic range, compactness or robustness, ease of use and cost. The aim of this Master project will be to study and improve our state-of-the-art SYRTE's cold atom gyroscope by one order of magnitude compared with the current state of the art to reach the interferometer's detection limit, which is intrinsically linked to quantum projection noise. It will use new methods like successive joint measurements without dead time. Obtaining this regime requires some modifications to the existing experiment, in particular to the Raman lasers used to manipulate the atomic wave packet, but also to the preparation and the detection of the atomic samples. This method is very general and could also be applied to more common three-pulse interferometers such as accelerometers and gravimeters.

Théorique, numérique
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.

Théorique, numérique
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.

Expérimental
Developing a broadband, high-sensitivity accelerometer operating at cryogenic temperatures is a key challenge in many cutting-edge experimental physics domains, from quantum technologies (including near-field microscopy, quantum memories, etc.) to gravitational wave detection. To realize such a sensor, a promising approach is hybrid optomechanics, which couples quantum and mechanical degrees of freedom in a single physical system. Rare-earth ion-doped crystals, known for their extremely narrow optical transitions at low temperature (~3K), exhibit natural optomechanical coupling through the piezospectroscopic sensitivity of the ion’s energy levels to mechanical stress. These crystals have recently emerged as strong candidates for quantum-enabled, low-temperature accelerometry, and we recently demonstrated continuous optical measurement of cryostat vibrations with such crystals, with an already promising sensitivity and bandwidth [1,2]. However, significant work is needed to obtain an ultra-sensitive, unidirectional and calibrated accelerometer. During this internship, we will investigate the fundamental and technical limitations of the method (in terms of sensitivity and bandwidth in particular), using emulated or real vibrations. Additionally, we aim to extend the operational range to higher temperatures (up to 10K), which will be key for expanding the potential applications of our sensor

Expérimental
In optical levitation, a nanoparticle is trapped in vacuum using tightly focused light. The light produces a force akin to a mechanical spring and the system reduces to a simple mass-spring resonator with kHz oscillations. Despite its simplicity, a levitated object provides remarkable interactions between light and its motion that can be harnessed to display quantum properties. In that regard, the particle must be cooled down to its quantum ground state, which requires to monitor its motion with optimal precision. Typically, this is achieved using the classical light produced by a laser. Yet, lasers are intrinsically shot-noise limited, thus making cooling challenging. Recently, some works have emphasized that quantum light can outperform classical light. For instance, entangled photons can serve to suppress shot noise. A photon of the pair images a target (signal), while a second acts as a reference (idler). As shot noise identically affects both photons, it is suppressed from the signal by subtracting the idler. During this internship, the candidate will experimentally harness entangled photons to perform sub-shot noise monitoring of levitated objects. He or she will develop an entangled-photon source, later on deployed on a levitation setup. To characterize the source, the student will visit the team of Pr. Molina in San Sebastian (Spain). Following the internship, he or she will be offered a PhD in cotutelle between Prs. Bachelard and Molina’s teams.