Recent experiments performed with individually-controlled, monovalent alkaline atoms trapped in arrays of optical tweezers and excited to Rydberg levels, demonstrated the relevance of this approach to build quantum information processing apparatus. Our team, a pioneer in the study of Rydberg excitation of divalent ytterbium atoms, recently succeeded in the optical manipulation of atoms in a Rydberg state enabled by the remaining core electron, the so-called Ionic Core Excitation (ICE), circumventing the autoionization issue which was the major limitation of this promising technique. The coordinator will capitalize on this success to create and lead a new team dedicated to quantum information processing experiments with ytterbium atoms. To do so, he will take in charge the existing apparatus, presently dedicated to the study of frozen Rydberg gases of ytterbium excited from a magneto-optical trap. Thanks to his expertise in the operation of experiments studying ultracold atoms, he will upgrade this apparatus to meet the best standards of platforms based on atoms trapped in optical tweezers. Relying both on his experience gained in Rydberg physics, and an already fruitful collaboration with the members of his present team with recognized expertise in this domain, he will eventually conduct proof-of concept experiments demonstrating the maturity of exciting prospects offered by alkaline-earth species. Among them, the new and powerful knob provided by ICE will enable new approaches to conduct groundbreaking experiments in the domains of quantum simulation and the ongoing development of quantum computing.

The cold Rydberg atoms are today recognized as a powerful tool for studying many physical situations, combining extreme and exaggerated properties of cold atoms and Rydberg atoms, respectively. Indeed, the electric dipole momenta of the Rydberg atoms can reach values several thousands times those of very polar molecules! Cold Rydberg excitation offers the possibility to entangle physical systems at large distances with many applications for quantum engineering and quantum simulation. The aim of the proposal COCORYM, for COrrelated COld RYdberg Matter, is to study the preparation, the evolution and the control of a cold Rydberg gas in configurations of very long-range interatomic dipole-dipole interactions, which can in a first approximation considered as a frozen gas, the properties of which can present similarities with an amorphous solid. Such atomic ensembles simulate mesoscopic situations at the crossings of condensed matter physics, plasma physics and chemistry, which will be investigated for characterizing properties of coherence of the matter. The proposal COCORYM will illustrate through five ambitious objectives, the different properties of coherence of Rydberg ensembles: (i) the characterization of a few-body in a cold Rydberg gas; (ii) the demonstration of superradiance for an ensemble of entangled pairs of Rydberg atoms; (iii) the Rydberg photoassociation for the formation of macrodimers, constituted by two bounded Rydberg atom; (iv) the quantum or classical diffusion of the Rydberg excitation; (v) the auto-organisation of laser cooled and trapped Rydberg atoms. The three first objectives will be tackled with the beginning of the proposal, using an available setup with Caesium atom. These results will constitute important steps in the understanding of the behavior of an ensemble of Rydberg atoms in strong long-range interaction. They will be real breakthroughs to open further developments of the entanglement of an ensemble of atoms, the control of cooperative emission or absorption, and an ultracold chemistry at mesoscopic distance. The two next objectives concern the properties of correlations and collective many-body effects; they necessitate the development of a new experimental setup, the tasks of which are planned during the first two years of the proposal. The prepared Rydberg ensembles correspond a priori to a disordered medium, but ordered or partially ordered situations can be prepared in one-, two- or three-dimensional space. The use of the dipole blockade of the Rydberg excitation is a way to prepare a correlated ensemble of excited atoms. The created correlations between the atoms are difficult to fully characterize. We need for that to develop a selective, temporally and spatially resolved detection for the Rydberg atoms. To go further, we need also to be able to laser-manipulate cold Rydberg atoms. A purpose of the proposal is to prepare an ensemble of strongly interacting and laser-controllable cold Rydberg Ytterbium atoms. The new Ytterbium setup is an important investment for the future of the cold Rydberg atom subject at the Laboratoire Aimé Cotton. The Ytterbium possesses two optically active electrons. Laser-cooling and Bose-Einstein condensation can be reached. Rydberg Ytterbium atoms can also be doubly-excited by using the second valence electron to perform a non-destructive imaging detection and to manipulate, cool or trap the Rydberg atoms, for controlling the cold Rydberg assembly. A complication of the experiment is the autoionization process of the doubly excited Rydberg atoms. To prevent the autoionization of the doubly excited atoms, an important task will be to initially prepare the Rydberg atoms in a high angular momentum state, which is non autoionizing and also possesses a long radiative lifetime.

Over the two last decades of the twentieth century, physicists completed the cooling of atomic samples initiated with the development of laser techniques. It was then possible to cool atoms at temperatures below 1 µK. Beyond the technological breakthrough, it has allowed further development of a wide variety of studies dealing with degenerate gases, quantum simulation of solid state physics, metrology, quantum chaos, etc. The question of creating molecular samples in the same range of temperature emerged at the end of the nineties. This interest can be summarized by the fact that molecules, compared to atoms, are complex objects that possibly feature a long range and anisotropic interaction. Developing such an experimental skill could then offer new perspectives in fundamental research and technology linked to quantum physics: for example, investigation of collisions and chemical reactions in ultracold and quantum regime, creation of molecular samples in correlated regime, improvement of metrological precision, metrology,etc. Unfortunately, molecular cooling cannot copy the laser techniques used in atomic cooling. Therefore, for more than ten years now, a great variety of methods have been proposed and tested to create cold molecules. Even if there have been impressive successes, an efficient direct molecular cooling is still missing. The main goal of the MolSisCool project is to develop a new technique of molecular cooling by means of a combination of laser techniques and static electromagnetic fields. On the long view, our objective is also to bridge a technological gap, namely cooling molecules at temperatures in the µK range. The heart of our approach is to implement an optical Sisyphus cooling of molecules which, with a single photon, is able to remove much more kinetic energy than standard Doppler cooling does (~1K vs ~1µK). As molecules cannot perform many successive optical transitions, Sisyphus cooling is thus particularly promising. We aim to demonstrate the feasibility of Sisyphus cooling on barium fluoride (BaF) according to the following route: 1- Production and spectroscopic characterization of a molecular beam of BaF; 2- Optical pumping of the molecular sample to a single rotational level; 3- Transverse cooling of the beam/collimation by Sisyphus cooling; 4- Partial beam deceleration by Sisyphus approach. The first step consists in setting up the experiment framework: a supersonic molecular beam based on the well mastered free expansion technique. The second is a broadband optical pumping we have already demonstrated on ultracold Cs2 molecules. It will both show the versatility of such a pumping and prepare molecules for the subsequent experiments. The first and fourth steps constitute the central part of this project and, beyond the experimental skills, will require theoretical support. Bearing in mind that this kind of source is a new tool for fundamental research, we will initiate new studies; thus, we plan to investigate collisional interactions in a mixture of cold BaF molecules and Rydberg atoms. In particular we mention that the development of optical frequency combs provided, through optical fibers, by the SYRTE, should make metrology accessible to any group.

Bose-Einstein condensation in a dilute weakly interacting atomic gas which happens at temperatures only millionth of a degree above absolute zero was first realized in 1995. Because of the deep connections with many important physics phenomena, such as superfluidity and superconductivity, it has since become one of most exciting subjects in physics, awarded by the Nobel Prize in 2001. But many intriguing problems in few-body and many-body physics arise when the particles of the dilute gas interact through strong long-range anisotropic forces, like the so-called polar molecules possessing a permanent electric dipole moment in their own frame. In the proposed joint experimental (CUHK) and theoretical (LAC) project COPOMOL, we will produce a quantum gas of polar 23Na87Rb bosonic molecules to study the ultracold physics with strong and anisotropic interactions, focusing on the precise control of ultracold molecule-molecule collisions with external electromagnetic fields to enter the regime where many-body physics is dominant. Inspired by the amazing achievements of the JILA group with 40K87Rb, we will take advantage of the remarkable properties of the 23Na87Rb ground state molecule, namely a large permanent dipole moment (5 times larger than in KRb) and a chemical stability against mutual collisions, to fully explore dipolar physics with quantum gases. The project will pursue the following objectives planned for the next four years: after the demonstration of the efficient production of Feshbach molecules, we will work out the best strategy for the reliable population transfer to the absolute molecular ground state with stimulated Raman adiabatic passage (STIRAP). Then the control of molecular collisions will be achieved by tuning of the anisotropic long-range dipole-dipole interactions with an external electric field, as a prerequisite to create a molecular quantum degenerate gas. Signatures of many-body physics phenomena in reduced geometries will be searched for by loading the quantum gas in an optical lattice. COPOMOL is timely given the particularly favorable configuration of the consortium. The CUHK group has already successfully prepared binary Na-Rb atomic BECs and investigated their interspecies Feshbach resonances. The LAC team includes several top experts on theoretical molecular structure, spectroscopy, and dynamics. Many high quality calculations related to heteronuclear alkali diatomic molecules, including NaRb, have been carried out while models for elastic, inelastic and reactive collisions between polar molecules have been developed. Another important fact is that the CUHK PI already has a long history of fruitful collaborations and discussions with the LAC PI and co-PI. Mastering these approaches is crucial as many applications proposed with dipolar interactions rely on the collisional properties of the ground-state molecules with and without induced dipole moment. The dangling question, whether trapped polar molecules without chemical reaction channels in the absolute lowest energy level will be really stable, will be answered first. We will measure and model the elastic and inelastic collisions with non-polarized and polarized samples to seek ways of achieving evaporative cooling for the production of a NaRb BEC. We will test the universal model for inelastic collisions with controlled loss channels including nuclear spin flips, rotational quenching, vibrational quenching and chemical reactions. These channels will be created by manipulating the internal state of the molecules with microwaves and lasers. These collisional studies will be further performed with molecules trapped in an optical lattice to explore the anisotropic character of dipolar interactions thus yielding a promising platform for observing several exotic many-body quantum phases.

The research field of ultracold (T<<1mK) molecules involves an increasing number of groups throughout the world, due to their foreseen applications in quantum technologies and cold chemistry. Molecules at ultracold temperatures can be precisely controlled in both their translational motion and their internal quantum states. Unfortunately, until recently, laser cooling could not be applied easily to molecules because they generally do not possess suitable closed optical transitions, like in atomic systems: their complex internal structure, prevents them from transferring cooling and slowing methods known from atomic physics, except for a very restricted class of molecular species . The main goal of the current proposal is to refine laser cooling further, to invent new cooling schemes which take into account the unique features of molecular structure, and produce a dense sample of absolute ground state of cold trapped Rb2 molecules. We intend to study bi-molecular collisions in two directions: first by finding optimal conditions to suppress them by optical shielding, and second to observe molecule-molecule collisions, possibly assisted by light, eventually forming Rb4 polyatomic molecules. The proposal is built upon three main objectives mixing experimental and theoretical aspects: (i) The experimental achievement of laser-cooling and trapping of Rb2 molecules from a supersonic molecular beam, guided by simulations elaborated using molecular structure calculations. For this aim, we will implement a rovibrational optical pumping technique, which will act like a broad-band repumping light source, thus imposing a “closed optical transition” to the molecules. (ii) The investigation of suppression of bi-molecular collisions using optical laser light which is blue-detuned from a suitable electronic molecular transition. Increasing the shielding efficiency will be of great interest for experimentalists whose aim is to obtain high density long-lived samples of ultracold molecules, paving the way to quantum degeneracy. (iii) The search for ultracold inelastic and reactive collisions between Rb2 molecules, possibly assisted by light, including photoassociation and formation of stable Rb4 molecules. Photoionization mass spectroscopy will allow us to identify the species present in the sample (Rb, Rb2, Rb3 , Rb4). Such processes exemplify a novel ultracold chemistry, intuitively assumed to be dominated by the long-range interactions between the reacting particles, but possibly with a complex interplay with short-range dynamics. This is still an open question, since the large amount of resonances of the collisional complex may give rise to a long-lived complex at short distances. The detection of photoassociation lines may provide an experimental insight into the role of these resonances in the dynamics. This work is a continuation of a successful long collaboration between the Brazilian USP-SC experimental group and the French LAC theoretical team. Together they have made a series of achievements including the successful creation of ultracold ground state rubidium diatomic molecules by short range photoassociation. Four joint scientific papers have been published within this collaboration since 2013.