The collective behavior of an ensemble of strongly interacting fermions is central to many physical systems, and its theoretical description is challenging due to the many-body nature of the problem. The ultracold Fermi gas is an ideal model system to shed light on this issue, as it provides excellent controllability, reproducibility, and unique detection methods. One of the problems, however, which complicates the interpretation of such experiments is the inherent density inhomogeneity of the gas due to harmonic confinement. We have developed a technique to overcome this difficulty by selectively probing atoms near the center of a trapped gas while still retaining momentum resolution. In this contribution to the 21th International Conference on Laser Spectroscopy (ICOLS 2013), we give an overview of this technique and some of the observations that have resulted from its implementation.
The collective behavior of an ensemble of strongly interacting fermions is central to many physical systems, and its theoretical description is challenging due to the many-body nature of the problem. The ultracold Fermi gas is an ideal model system to shed light on this issue, as it provides excellent controllability, reproducibility, and unique detection methods. One of the problems, however, which complicates the interpretation of such experiments is the inherent density inhomogeneity of the gas due to harmonic confinement. We have developed a technique to overcome this difficulty by selectively probing atoms near the center of a trapped gas while still retaining momentum resolution. In this contribution to the 21th International Conference on Laser Spectroscopy (ICOLS 2013), we give an overview of this technique and some of the observations that have resulted from its implementation.
The ideal (i.e., noninteracting), homogeneous Fermi gas, with its characteristic sharp Fermi surface in the momentum distribution, is a fundamental concept relevant to the behavior of many systems. With trapped Fermi gases of ultracold atoms, one can realize and probe a nearly ideal Fermi gas; however, these systems have a nonuniform density due to the confining potential. We show that the effect of the density variation, which typically washes out any semblance of a Fermi surface step in the momentum distribution, can be mitigated by selectively probing atoms near the center of a trapped gas. With this approach, we have directly measured a Fermi surface in momentum space for a nearly ideal gas, where the average density and temperature of the probed portion of the gas can be determined from the location and sharpness of the Fermi surface.
This thesis presents experiments probing the physics of strongly interacting fermionic atoms in the BCS-BEC crossover. Ultracold atom experiments bring the ability to arbitrarily tune interatomic interactions, which allows for unprecedented access to the regime of strongly interacting physics. The majority of cold atom experiments, however, are carried out in an atom trap that imprints an inhomogeneous density on the cloud of atoms. Many phenomena, especially the signatures of phase transitions, are significantly modified by this non-uniform density. In this thesis, I present a novel imaging technique that allows us to probe a region of nearly homogeneous density within a larger, inhomogenenous cloud. Using this technique, I present new results for strongly interacting fermionic atoms, including the first measurements of the contact and the occupied spectral function of a homogeneous Fermi gas, and the first direct observation of the "textbook" momentum distribution of an ideal Fermi gas.
The nature of the normal phase of an ultracold Fermi gas in the BCS-BEC crossover regime is an interesting and unresolved question. As interactions increase, while the many-body ground state remains a condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules. In this thesis, I present a technique to spatially select a homogeneous sample from the center of a trapped gas to explore this crossover. Combining this technique with RF spectroscopy, we locally measure Tan's contact as a function of temperature and compare to various many-body theories. In another experiment, we measure the distribution of single-particle energies and momenta for a normal gas across the BCS-BEC crossover. We find that the data fit well to a two-part function that includes a peak corresponding to fermionic quasiparticles and an "incoherent background" that is modeled using the dispersion of thermal molecules. I also describe the construction of a new-generation Fermi gas apparatus.
Ultracold atoms in optical lattices provide an extremely clean and controllable setting to explore quantum many-body phases of matter. Here we give a brief review of the strong-correlation physics that has been realized using such ultracold atoms in optical lattices ranging from the realization of Hubbard models to studies of quantum magnetism and the detection of single atoms with lattice site resolution. All this has opened up fundamentally new opportunities for the investigation of quantum many-body systems. Introduction Over the past years, ultracold atoms in optical lattices have emerged as versatile new systems to explore the physics of quantum many-body systems. On the one hand, they can be helpful in gaining a better understanding of known phases of matter and their dynamical behavior; on the other hand, they allow one to realize completely novel quantum systems that have not been studied before in nature [1, 2, 3]. Commonly, the approach of exploring quantum many-body systems in such a way is referred to as “quantum simulations.” Examples of some of the first strongly interacting many-body phases that have been realized both in lattices and in the continuum include the quantum phase transition from a superfluid to a Mott insulator [4, 5, 6], fermionic Mott insulators [7, 8], the achievement of a Tonks- Girardeau gas [9, 10], and the realization of the Bose-Einsten condensate (BEC)– Bardeen-Cooper-Schrieffer (BCS) crossover (see also Chapter 12) in Fermi gas mixture [11] using Feshbach resonances [12]. In almost all of these experiments, detection was limited to time-of-flight imaging or more refined derived techniques that mainly characterized the momentum distribution of the quantum gas [2]. For several years, researchers in the field have therefore aspired to employ in situ single-particle detection methods for the analysis of ultracold quantum gases. Only recently has it become possible to implement such imaging techniques, marking a milestone for the characterization and manipulation of ultracold quantum gases [13, 14, 15, 16, 17]. In our discussion, we will focus on one of the most successful of these techniques based on high-resolution fluorescence imaging. Despite being a rather new technique, such quantum gas microscopy has already proven to be an enabling technology for probing and manipulating quantum many-body systems. For the first time, controllable and strongly interacting many-body systems, as realized with ultracold atoms, could be observed on a local scale [17, 16].
We report on the experimental realization of homogeneous two-dimensional (2D) Fermi gases trapped in a box potential. In contrast to harmonically trapped gases, these homogeneous 2D systems are ideally suited to probe local as well as nonlocal properties of strongly interacting many-body systems. As a first benchmark experiment, we use a local probe to measure the density of a noninteracting 2D Fermi gas as a function of the chemical potential and find excellent agreement with the corresponding equation of state. We then perform matter wave focusing to extract the momentum distribution of the system and directly observe Pauli blocking in a near unity occupation of momentum states. Finally, we measure the momentum distribution of an interacting homogeneous 2D gas in the crossover between attractively interacting fermions and bosonic dimers.
Engineered ultracold atomic systems are a valuable platform for fundamental quantum mechanics studies and the development of quantum technologies. At near zero absolute temperature, atoms exhibit macroscopic phase coherence and collective quantum behavior, enabling their use in precision metrology, quantum simulation, and even information processing. This review provides an introductory overview of the key techniques used to trap, manipulate, and detect ultracold atoms, while highlighting the main applications of each method. We outline the principles of laser cooling, magnetic and optical trapping, and the most widely used techniques, including optical lattices and tweezers. Next, we discuss the manipulation methods of atomic internal and external degrees of freedom, and we present atom interferometry techniques and how to leverage and control interatomic interactions. Next, we review common ensemble detection strategies, including absorption and fluorescence imaging, state-selective readout, correlation and quantum non-demolition measurements and conclude with high-resolution approaches. This review aims to provide newcomers to the field with a broad understanding of the experimental toolkit that underpins research in ultracold atom physics and its applications across quantum science and technology.