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Quantum simulation of novel many-body phenomena with Ytterbium atoms in optical lattices

Periodic Reporting for period 1 - YbQuantumSim (Quantum simulation of novel many-body phenomena with Ytterbium atoms in optical lattices)

Reporting period: 2015-07-01 to 2017-06-30

The experimental study of materials and their description using physical models have been core issues in modern physics since its foundation. In spite of the remarkable achievements, many important questions have been eluding condensed matter physicists for many decades. A famous example are high-temperature superconducting materials, which were discovered in the 1980s, but whose underlying superconductivity mechanism still remains to be clarified. These materials have nowadays important technological applications, such as in transportation and medicine. Reaching a full understanding could guide chemists to develop new and much more efficient superconductors, with a major economic impact.
The difficulties that hinder the progress of our understanding in condensed matter physics are both experimental and theoretical. On the one hand, materials have complex chemical structures, which are hard to describe, and their quality and variability are limited. On the other, even the most simple models can be too difficult to solve analytically or numerically. Probing ultra-cold fermionic atoms trapped in optical lattices is a novel approach to tackle the difficult problems in condensed matter physics. These systems have a very high degree of controllability and versatility (the atomic potentials can be tailored almost at will) and are free of imperfections. Therefore they provide a powerful platform to simulate open condensed matter problems and probe the relevant many-body quantum states.
This project aims to explore multi-orbital physics using fermionic ytterbium atoms and investigate its role in quantum magnetism and electric conduction. In a first part, we were able to quantum simulate the Fermi-Hubbard model with SU(N)-extended symmetry. Secondly, we investigated the inter-orbital interactions and discovered an original orbital-induced Feshbach resonance. Thirdly, we prepared a system capable of simulating Kondo and Kondo-lattice physics.
Three studies were performed as part of this project and each represent a step forward towards the study of multi-orbital strongly interacting systems in lattices (e.g. Kondo lattice model).
Firstly, we were able to load ytterbium atoms into an optical lattice at very low temperature and set up a tool to probe them locally. By varying the lattice strength, we studied the cross-over between the metallic and the Mott insulating phases by directly measuring the equation of state. This specifically allowed to probe the role of the extended SU(N)-symmetry present in ytterbium atoms. Although the SU(N) Fermi-Hubbard model is theoretically still mostly not understood, our equation of state measurements match those special limiting cases for which which theories exist, such as in certain weak and strong interaction regimes. We hope our results will serve as benchmark for theories addressing the challenging intermediate interaction case. Moreover, we show that our results pave the way to study interatomic magnetic correlations.
In our other line of research, we studied in detail the microscopic interactions between atoms in different orbital states, which is a necessary step towards the simulation of many-body orbital quantum magnetism. Based on our early results, a novel type of Feshbach resonance in ytterbium-173 was predicted by a group of theorists, which we subsequently could indeed identify for the first time. This resonance allows to tune the interaction strength between orbitals. By changing the magnetic field strength, we measured a variation in the interatomic collision rate by almost two orders of magnitude in the bulk.
Continuing the multi-orbital physics research, we constructed an orbital-dependent lattice and loaded ytterbium atoms into it. The atoms are distributed in two atomic orbitals (of type g and e) with distinct internal spin (down and up, respectively) and are trapped in one-dimensional tubes. We superimposed an orbital-dependent optical lattice along the longitudinal axis of the tubes, creating a strong lattice for atoms in the e orbital, but with a weak effect on atoms in the g orbital. Due to the spin-exchange nature of interactions, this system is predicted to display features of Kondo (lattice) physics, with the e orbital playing the role of impurity and the g orbital as the conducting electron. We are analyzing the many-body physics of this model system and have submitted a manuscript with our results.
The results obtained in the framework of this project demonstrated the remarkable potential and uniqueness of the ytterbium-173 isotope to simulate interesting many-body problems. Our first two studies resulted in publications in world-class peer-reviewed scientific journals and a third manuscript is submitted for publication.
We measured for the first time the thermodynamic properties of the SU(N)-symmetric Fermi-Hubbard Hamiltonian, in particular its equation of state. We showed evidence that our system may be partially correlated, which is a fundamental step towards probing exotic SU(N) quantum magnetism.
The discovery of a fundamentally new inter-orbital Feshbach resonance introduces the interaction degree of freedom to the rich experimental toolbox of ytterbium-173. This discovery triggered much attention from the community, with theoretical proposals of creating the first two-band superfluid with ultra-cold atoms and of observing the long-sought Leggett mode.
The experiments realised in the context of the project directly probe fundamental condensed-matter systems and our results serve as guide and benchmark for further advancement of our understanding of decade-lasting problems in condensed matter physics. Providing a new testbed for multi-orbital electron physics in these materials such as the Kondo effect will enable new inroads to understanding some important classes of materials. A better understanding of the hallmark properties of such materials in electrical conduction and thermal properties both in equilibrium and non-equilibrium is a significant goal for fundamental material science and condensed matter physics.
SU(N) Mott insulator in optical lattice. Measured density pattern is shown, with sketch of geometry.