This work is part of a Defense Advanced Research Projects Agency (DARPA) sponsored project to build an optical lattice emulator, where models of strongly correlated electrons in condensed matter physics are simulated with ultracold atoms moving in an optical lattice. Recently, members of our team have been able to form dense clouds of dipolar fermionic molecules from mixtures of (fermionic)40K and (bosonic)87Rb. Here, we use high performance computing (HPC) resources to find a way to improve the efficiency of molecule formation from the current 20% to almost 100% and thereby show how to create a much denser cloud of dipolar molecules. Our code scales nearly linearly on up to 4,000 (or more) processors, and runs at an efficiency that is almost at 100% of the speed for one arithmetic operation per clock cycle. We have not been able to get the code to run with multiple operations per clock cycle in spite of using highly efficient and optimized libraries for BLAS and LAPACK.
This work is part of a Defense Advanced Research Projects Agency (DARPA) sponsored project to build an optical lattice emulator, where models of strongly correlated electrons in condensed matter physics are simulated with ultracold atoms moving in an optical lattice. Recently, members of our team have been able to form dense clouds of dipolar fermionic molecules from mixtures of (fermionic)40K and (bosonic)87Rb. Here, we use high performance computing (HPC) resources to find a way to improve the efficiency of molecule formation from the current 20% to almost 100% and thereby show how to create a much denser cloud of dipolar molecules. Our code scales nearly linearly on up to 4,000 (or more) processors, and runs at an efficiency that is almost at 100% of the speed for one arithmetic operation per clock cycle. We have not been able to get the code to run with multiple operations per clock cycle in spite of using highly efficient and optimized libraries for BLAS and LAPACK.
Defense Advanced Research Projects Agency (DARPA) is running a program to create an analog quantum-mechanical simulator for strongly interacting quantum particles. It is called an optical lattice emulator, and it involves ultracold neutral atoms moving in a corrugated periodic potential created by an optical lattice. The atoms interact with each other via collisions. In this work, we show how one can apply inhomogenenous dynamical mean-field theory (IDMFT) as an approximate computational tool to study the behavior of just such a system. Ultimately, conventional computation will be employed to benchmark the optical lattice emulator on a numerically tractable problem (a verification and validation study), and then the emulator will be applied to problems that cannot be solved with conventional computation. Here, we demonstrate an efficient massively parallel implementation of the IDMFT algorithm that is transportable, scales to many thousands of processors, and runs at approximately 50% of the theoretical peak speed of the machine.
Cooling atoms, both bosons and fermions, to ultra-low temperatures has provided the opportunity to probe distinct quantum phases of matter. The ability to then trap these atoms, or even molecules, in optical lattices and tune the interactions between them presents the possibility of an ideal and clean quantum system that can act as a surrogate for complex condensed-matter systems in which the tuning parameters may be limited. To date, most boson-boson and boson-fermion mixtures in this context have been formed with atoms of the same element. Forming more complex molecules with multiple elements offers the prospect of adding an anisotropic electric dipole interaction into the mix, thereby opening up a number of applications in quantum information processing and metrology. Voigt et al. demonstrate the formation and trapping of ultracold diatomic heteronuclear molecules using 6Li and 40K fermionic atoms. Forming long-lived bosonic molecules from two fermionic species with such different masses offers a rich parameter space in which to study many-body correlated systems with asymmetric interactions. — ISO
Ultracold ground-state dipolar 40K87Rb molecules recently have been produced in a loose harmonic trap by employing a magnetic field sweep across a Feshbach resonance followed by stimulated Raman adiabatic passage [K.-K. Ni et al., Science 322, 231 (2008)]. The overall experimental efficiency for molecule formation was around 20. We show that the efficiency can be increased to nearly 100 if one first loads the atomic gases into an optical lattice of the appropriate depth and tunes the interspecies attraction to have exactly one atom of each species at an occupied lattice site. Our proposed scheme provides a large enhancement to the dipolar molecule density, even at relatively high temperatures, and avoids three-body recombination loss by preventing lattice sites from being triply occupied.
During my PhD, I participated in the full setup from scratch of an experiment that aims at cooling down and manipulating two fermionic alkaline atomic species, 6Li and 40K. Our goal is to study ultracold Fermi mixtures with mass imbalance, and to realize a flexible quantum analog simulator. In fact, for certain quantum many-body problems, such as high critical temperature superconductivity and frustrated anti-ferromagnetism, there is neither complete analytical nor numerical solution. Therefore, cold atoms systems, due to their purity and their high degree of tunability, even on their interaction itself, offer an interesting complementary point of view in the study of those phenomena. As far as the experimental setup is concerned, we built an ultra-high vacuum system, a complete and stable laser source for each species, as well as two performing atomic sources, a 6Li Zeeman slower and a 40K bidimensionnal magneto-optical trap; most of the optical and electrical quantities, as well as the imaging diagnostics, being computer controlled. Once those preliminary steps have been performed and optimized, we could obtain an efficient double magneto-optical trap, containing typically 5x10^9 atoms of 6Li and 8x10^9 atoms of 40K. In this configuration, we produced the first 6Li40K* heteronuclear molecules by photoassociation, for which we identified 70 rovibrational lines. In a second part, I describe in detail the magnetic transport of the atomic mixture, between the magneto-optical trap chamber and a science cell, located in an ultra-high vacuum region with large optical access. The complete setup, from its design to its experimental implementation and optimization, as well as the development of the program and the useful numerical diagnostics, are the heart of my work. The transport efficiency could be tested and optimized, allowing for an efficient transfer of the mixture in the science cell. Thus, after this thesis, all the experimental tools are operational in order to allow for evaporative cooling of the mixture in an optically plugged magnetic trap. Therefore, the field is open for quantum simulation and understanding of many-body problems in ultracold Fermi mixtures.
Ultracold atomic gases have proven to be a versatile tool for the study of many-body quantum physics thanks to the high degree of control that they offer. However, interactions between neutral atoms are usually short-ranged and isotropic, which limits their applicability as a simulator of real condensed matter systems. One of the most common methods of introducing stronger interactions into an ultracold gas is associating pairs of atoms of two different elements into molecules. Such molecules possess a body-fixed electric dipole moment, which can be addressed by an external electric field or microwave radiation to engineer long-ranged, tunable dipolar interactions. In addition to constituting an analogue quantum simulator, ultracold dipolar molecules show potential as a platform for digital quantum computation, precision measurements, and the study of quantum chemistry. The main result presented in this thesis is the first successful synthesis of ultracold 39K133Cs molecules in their vibrational and rotational ground state. Using a novel simultaneous cooling scheme for potassium and caesium, our team has been able to produce mixtures that are cold enough for efficient magnetoassociation into weakly bound molecules to take place. By performing high-resolution laser spectroscopy of these molecules, I have identified a two-photon pathway to the ground state via an exceptionally narrow intermediate excited state. I was then able to perform Stimulated Raman Adiabatic Passage (STIRAP) along that pathway with one-way efficiencies up to 71%, producing trapped samples of up to 3500 ground-state molecules at temperatures of 1 µK. The lifetime of these samples is currently limited by a near-universal two-body loss process, a further study of which could shed light on similar loss mechanisms in other molecular species. I have also carried out preliminary rotational spectroscopy, which paves the way towards using KCs molecules as a controlled interacting spin system and a refinement of their hyperfine coupling constants. The thesis ends with a detailed outline of further steps that should be taken to improve the efficiency of STIRAP, as well as the population and coherence lifetimes in KCs samples. It is followed by a list of suggested experiments that could be carried out using our system after it undergoes the described technical upgrades. Some of these experiments are directly inspired by the results I have obtained during my six-month research stay in the KRb laboratory at JILA.
We report on the creation of ultracold heteronuclear molecules assembled from fermionic 40K and bosonic 87Rb atoms in a 3D optical lattice. Molecules are produced at a heteronuclear Feshbach resonance on both the attractive and the repulsive sides of the resonance. We precisely determine the binding energy of the heteronuclear molecules from rf spectroscopy across the Feshbach resonance. We characterize the lifetime of the molecular sample as a function of magnetic field and measure lifetimes between 20 and 120 ms. The efficiency of molecule creation via rf association is measured and is found to decrease as expected for more deeply bound molecules.