In this thesis, we present the first experimental results obtained from a newly developed modular quantum gas platform. The platform features exchangeable optical modules that can be individually characterized and maintained, enabling rapid reconfiguration with modules of diverse functionality. We configure the platform to carry out first quantum simulation experiments using fermionic Li-6 in optical tweezers. In particular, we realize motional ground-state systems in a one-dimensional optical tweezer, demonstrate the preparation of spin-imbalanced systems, explore scaling to multiple tweezers, and extend the system to a tunnel-coupled double well, realizing experimental cycle times as short as 800 ms. We further present the development of a novel free-space fluorescence imaging technique, which utilizes the polarization degree of freedom of the emitted photons to encode spin information. This method extends established free-space fluorescence imaging methods both conceptually and technically, and facilitates their application across a broad range of quantum gas experiments. Finally, we report on the first realization of optical shielding in Li-6, with observed light shifts on the GHz scale. We find that, due to the internal structure of Li-6, theoretical descriptions used for other systems are no longer sufficient to describe such light shifts. We therefore develop a novel theoretical model based on the numerical diagonalization of the strongly coupled multilevel Hamiltonian describing states in the 2p_1/2 , 2p_3/2 , and 3s_1/2 manifolds of Li-6. This model allows for a quantitative description of state-dependent light shifts and provides a foundation for the precise engineering of energy levels. We utilize the differential light shift to achieve high-fidelity shielding of quantum ground state systems from imaging, thereby demonstrating the ability to locally address and measure cold atomic quantum systems of Li-6. This capability for local control and measurement enables the implementation of a broad range of quantum simulation experiments.
In this thesis, we present the first experimental results obtained from a newly developed modular quantum gas platform. The platform features exchangeable optical modules that can be individually characterized and maintained, enabling rapid reconfiguration with modules of diverse functionality. We configure the platform to carry out first quantum simulation experiments using fermionic Li-6 in optical tweezers. In particular, we realize motional ground-state systems in a one-dimensional optical tweezer, demonstrate the preparation of spin-imbalanced systems, explore scaling to multiple tweezers, and extend the system to a tunnel-coupled double well, realizing experimental cycle times as short as 800 ms. We further present the development of a novel free-space fluorescence imaging technique, which utilizes the polarization degree of freedom of the emitted photons to encode spin information. This method extends established free-space fluorescence imaging methods both conceptually and technically, and facilitates their application across a broad range of quantum gas experiments. Finally, we report on the first realization of optical shielding in Li-6, with observed light shifts on the GHz scale. We find that, due to the internal structure of Li-6, theoretical descriptions used for other systems are no longer sufficient to describe such light shifts. We therefore develop a novel theoretical model based on the numerical diagonalization of the strongly coupled multilevel Hamiltonian describing states in the 2p_1/2 , 2p_3/2 , and 3s_1/2 manifolds of Li-6. This model allows for a quantitative description of state-dependent light shifts and provides a foundation for the precise engineering of energy levels. We utilize the differential light shift to achieve high-fidelity shielding of quantum ground state systems from imaging, thereby demonstrating the ability to locally address and measure cold atomic quantum systems of Li-6. This capability for local control and measurement enables the implementation of a broad range of quantum simulation experiments.
Exercising microscopic control over quantum systems is a central theme in modern physics. Neutral-atom quantum simulators leverage this control to explore quantum many-body phenomena by realizing paradigmatic Hamiltonians, such as the Hubbard model, in isolated systems. Advances in single-atom-resolved manipulation and detection have allowed measurements of microscopic correlations in strongly interacting quantum systems. Quantum gas microscopes have successfully approached this from the top down, starting from bulk quantum gases, while optical tweezers offer a complementary bottom-up route based on controlled few-particle systems. Combining these approaches opens a pathway towards assembling many-body systems atom by atom, enabling deterministic control over the initial quantum state. This thesis presents the development of a compact quantum gas microscope for programmable ensembles of fermionic lithium-6. The apparatus integrates laser cooling, magnetic field control, quantum-gas preparation, optical tweezers, projected potentials, and high-resolution imaging around a high-numerical-aperture microscope objective. Evaporative cooling in a crossed optical dipole trap produces degenerate two-component Fermi gases and molecular Bose--Einstein condensates, providing the quantum-degenerate reservoir for subsequent experiments. We demonstrate uniform loading of degenerate fermionic ensembles into programmable two-dimensional tweezer arrays, overcoming reservoir-size limitations by translating the tweezer array across the reservoir. Leveraging fermionic statistics and the resolved motional spectrum, we prepare a singlet pair in the three-dimensional motional ground state of each tweezer trap. Across an 8x8 array, this yields an average single-particle preparation fidelity of 98.47%. We develop a novel approach combining the deterministic preparation with spin- and site-resolved control via global magnetic field gradients and locally projected repulsive potentials to produce arbitrary product state configurations in parallel across all sites. In this way, we achieve programmable assembly of spin--charge product states of up to 128 atoms, with fast experimental cycle times of 3 seconds. Furthermore, we realize a new scheme for simultaneous spin-, site-, and density-resolved imaging of individual lithium-6 atoms. Encoding the spin information in the fluorescence polarization, we resolve the two spin components within a single 20 microsecond exposure, achieving detection fidelities exceeding 98.5%. These capabilities establish a fast route to preparing low-entropy fermionic product states with microscopic control over their initial spin, charge, and motional configuration. Coupling such states through optical lattices or tunnel-coupled tweezers provides a route for initializing far-from-equilibrium many-body systems, with applications to strongly correlated physics, quantum transport and thermalization, and fermionic quantum information processing.
The presented thesis reports on the development of a broadly applicable, open-source, modular system architecture for quantum gas experiments, which has been named Heidelberg Quantum Architecture (HQA). In the ongoing rollover of quantum gas apparatuses from mere experiments to usable platforms for quantum simulation, similar to classical computing in the 1950s, such an architecture can be beneficial in the pursuit of improving the usability and raising the technology readiness level (TRL) of these systems. An exemplary quantum gas apparatus for quantum simulation with ultracold 6 Li, follow- ing the guidelines of HQA, is constructed. It is named Heidelberg Quantum Architecture based experiment described in this thesis (HQA-ONE). The modular concept of Heidel- berg Quantum Architecture (HQA), which disentangles the complexity of the quantum gas apparatus into subsystems that are attached to a common mechanical frame of reference (FOR), is demonstrated. A passive interfacing precision of the exchangeable modules mounted to the framework of around ±10µm is measured. The reconfigurable apparatus HQA-ONE is equipped with modules for deterministic preparation of atom number states in the ground state of an optical tweezer to exemplarily demonstrate a core capability of quantum simulators, quantum state initialization, with the apparatus and indirectly with the platform. Further modules to choose from when configuring the apparatus for quantum simulations in the future are developed, constructed, and presented in the thesis, underlining the reconfiguration character of the platform and its versatility.
This thesis reports on quantum simulation experiments conducted on strongly correlated fermionic atoms coupled to light in an optical cavity. To this day, the quantum-mechanical description of strongly correlated many-body systems remains a considerable theoretical challenge. Quantum simulation experiments with ultracold atoms contribute to the understanding of such systems through controlled studies of complex, engineered many-body states. Advances in measurement and control techniques continuously drive the development of this field, enabling the creation, detection, and manipulation of novel states of matter. The experiments presented in this thesis build upon the prior observation of cavity-induced density-wave ordering in a gas of strongly interacting 6Li atoms. In particular, we report on an extension of the cavity quantum electrodynamics setup through the implementation of a high-resolution imaging system. The imaging setup enables the in situ observation of the density waves through resonant absorption imaging within a single experimental realization. Using this technique, we investigate ordering both following adiabatic preparation and after a quench of the cavity-induced long-range interaction. Our observations support the expectation of a density modulation with a single spatial frequency and phase that extends across the entire atomic cloud. We further present a correlation study between atomic and photonic observables, confirming their correspondence and thereby directly testing the theoretical foundation of cavity-mediated interactions. By imaging density waves in the vicinity of the first higher-order transverse cavity mode, we demonstrate the capability of the imaging setup to reveal local order beyond global observables. Additionally, this thesis presents two studies of density-wave ordering based on measurements of the cavity photon leakage. The first study focuses on the dynamics of order formation following an instantaneous quench. We observe an exponential growth of the order parameter, with a growth rate predominantly governed by the quench strength, in agreement with a theoretical description based on a random phase approximation. In the second study, we realize a system of three mutually coupled orders, enabled by the simultaneous coupling of light to atoms and atom pairs via a photoassociation resonance. In combination with the strong correlations present in the gas, this gives rise to three coupled order parameters: the cavity photon field, a density wave, and a pair-density wave. The mutual competition and enhancement between these orders result in a measurable shift of the density-wave order phase boundary. We conclude the thesis by reporting on recent experimental advances in the implementation of a homogeneous trapping potential, leading to a uniform atomic density along two spatial directions. This represents work towards an ideal starting point for a range of exciting future experiments, including spectroscopic measurements, local potential patterning, and correlation studies in long-range interacting Fermi gases, leveraging the numerical aperture of the imaging system.
This dissertation advances optical tweezer platforms for neutral-atom quantum processors by addressing three interconnected bottlenecks: imaging survival, motional ground-state preparation, and narrow-line cooling, through experimental and theoretical work on single lithium and cesium atoms.For Li, the lightest stable alkali atom, this work demonstrates a per-image survival probability of 99.950(2)% and a state-detection fidelity of 99.97(2)% across 2000 consecutive 15 ms images, a record for any trapped neutral atom at the time of publication. This was achieved by applying Λ-enhanced gray-molasses (Λ-GM) cooling continuously on the D1 line while simultaneously imaging with the same beam, and represents the first demonstration of loading lithium into a tweezer directly from a MOT. To explain this performance, the dissertation develops a unified Lindblad master-equation formalism for laser cooling in a tightly confined harmonic trap, encompassing polarization-gradient, gray-molasses, EIT, Λ-GM, and single-photon sideband cooling within a single theoretical framework, with 1D simulations that correctly predict steady-state temperature, excited-state fraction, and cooling efficiency across these techniques.For133133Cs, the work exploits the forbidden electric-quadrupole transition 6S1/2→5D5/26S1/2→5D5/2 at 685 nm (Γ ≈ 2π × 117.6 kHz) to demonstrate background-free imaging and narrow-line sideband cooling for the first time in an alkali atom. By positioning the atom at the intensity node of a Laguerre-Gaussian vortex beam, the drive is enhanced while cascade fluorescence at 852 nm is optically isolated from the excitation light, enabling both high-fidelity background-free detection and single-photon sideband cooling to near the motional ground state.Beyond the core cooling and imaging results, the dissertation includes a comprehensive experimental techniques chapter (fiber coupling, AOM design, polarimetry, laser locking, optical pumping, and a Gaussian-process optimization framework), a chapter on quadrupole transition theory and orbital-angular-momentum selection rules, and a concluding survey of the competitive quantum computing landscape across neutral-atom, superconducting, trapped-ion, photonic, spin, and topological qubit platforms. Together, this work establishes the Cs and Li cooling/imaging toolbox as a foundation for future assembly of ultracold LiCs polar molecules in optical tweezer arrays.
This thesis describes the methods used to achieve single-atom and single-site resolved fluorescence imaging of a quantum gas of fermionic atoms held in an optical lattice. This kind of quantum gas microscopy has been previously applied to bosonic atoms in two experiments, and has allowed for extensive studies of many-body quantum systems. Here we detail methods to extend the techniques of quantum gas microscopy to fermionic species of atoms, opening the door to quantum simulation of electronic models of condensed matter physics. We prepare quantum degenerate gases of fermionic 40K through sympathetic cooling with a gas of \Rb. The gas prepared is trapped less than 1 mm away from a thin vacuum window, an experimental geometry which permits interrogation with a large Numerical Aperture microscope objective. The cloud is loaded into a 527 nm-period cubic optical lattice, ideal for quantum simulation of many-body models of fermionic particles such as the Hubbard model. The optical lattice is designed to allow for potential depths up to V/kB = 250 microKelvin, sufficient to strongly confine 40K atoms into individual lattice sites. EIT cooling --- a technique previously demonstrated in ion-trapping experiments --- is used to cool the fermionic atoms deeply towards the ground motional state of the lattice wells. We collect the photons scattered during continuous application of EIT cooling, while atoms are kept pinned in the lattice for many seconds. We strongly suppress all other sources of light with the use of a modulated cooling scheme and a spinning chopper wheel. Collecting the scattered light produced during 2 s of EIT cooling allows us to resolve single atoms in a fluorescence image. We demonstrate that the high resolution of our imaging apparatus also permits atom positions to be precisely identified on the length scale of the optical lattice. Thus we demonstrate the realisation of a quantum gas microscope for fermionic atoms, allowing for precise investigation of the ground states and dynamics of correlated quantum states of fermionic particles.
Imagine a new generation of computers that dramatically surpasses the most powerful supercomputers, being able to tackle the most challenging of computational problems. These problems arise, for example, in quantum physics where the computational power required to model a many-particle system increases exponentially with the number of its constituents, and as a consequence the calculations become prohibitively time-consuming. Nobel laureate Richard Feynman proposed this new type of computer in the early 1980s: ‘Let the computer itself be built of quantum mechanical elements which obey quantum mechanical laws.’ Such a quantum simulator would allow researchers to tackle challenging problems in condensed-matter physics. In particular many-body fermionic quantum systems are very difficult to compute due to the antisymmetric nature of the wave function, and the resulting sign problem for quantum Monte Carlo methods. This is why a quantum simulator will be an excellent test bed to investigate phenomena and properties of strongly correlated fermionic quantum systems. A powerful experimental platform to realize a quantum simulator is laser-cooled atoms [1] held in an artificial crystal of light, generated by superimposing laser beams. The atoms are kept in this optical lattice similar to marbles in the hollows of an egg carton, and they can mimic the behaviour of electrons in a real crystal. These systems interface atomic physics with condensed matter physics in a new way, as the tools of atomic physics allow for exquisite control of many experimental parameters, which are often inaccessible in solid state experiments. In-situ imaging techniques [2] using ‘quantum-gas microscopes’ constitute a new approach to the study of many-body quantum systems in an optical lattice with unprecedented resolution at the single-particle and single-site level [3,4]. This new method is complementary to earlier optical lattice experiments measuring distributions in momentum space after a time-of-flight expansion. A quantum-gas microscope uses an optical imaging system (Fig. 1a) for collecting the fluorescence light of ultracold atoms in an optical lattice with high spatial resolution comparable to the lattice spacing of ≈ 500 nm. (a) Atoms in optical lattice atoms are imaged with a high-resolution microscope objective [4]. (b) Single-site addressing is achieved by focussing a laser beam through the microscope onto selected atoms, bringing them into resonance with microwave radiation which produces spin flips [12]. (c) and (d) Single-atom-resolved fluorescence images of bosonic Mottinsulators [4,9]. (e) Deterministically prepared atom distribution using single-site addressing [12]. (f) Direct observation of single-atom quantum walks [15]. Fluorescence imaging of atoms in a quantum-gas microscope has made it possible to directly observe bosonic Mott insulators with single-atom resolution [4–6], giving access to in-situ measurements of temperature and entropy distributions. Subsequently, ground-breaking experiments in many-body quantum dynamics have been performed, such as the first direct observation of correlation spreading after quenches of the system to lower lattice depths [7]. Other experiments include the quantum simulation of an antiferromagnetic spin-1/2 chain [8], the realization of algorithmic cooling [9], the direct measurement of correlations across the superfluid-to-Mott-insulator transition [10], or the observation of the ‘Higgs’ amplitude mode [11]. The high-resolution imaging system is also capable of optically addressing individual atoms with sub-diffraction-limited resolution. For this purpose, a laser beam is focussed though the microscope objective onto selected lattice sites, bringing the atoms into resonance with microwave radiation which produces spin flips [12,13]. The preparation of arbitrary atom distributions in the lattice not only opens perspectives for neutral-atom quantum information processing, but also allows the deterministic creation of specific out-of-equilibrium situations, and their ensuing system dynamics can be studied with single-atom resolution. Examples include the creation of spin impurities and the study of their dynamics in a many-body system [14], the observation of quantum walks [15] and magnon-bound states [16], and the measurement of entanglement entropy [17]. The technique used for single-site-resolved detection of individual atoms in the optical lattice is fluorescence imaging. It requires laser cooling of the atoms to sub-Doppler temperatures while detecting the fluorescence photons emitted during this process. A challenge is to achieve a low particle loss rate and to prevent the atoms from hopping between lattice sites. In the case of the first quantum-gas microscopes, this has been implemented with bosonic rubidium-87 atoms, and also recently for ytterbium-174. However, it has proven more challenging for fermionic alkaline species such as potassium-40 and lithium-6, as their low mass and small excited state hyperfine splitting-prevented standard sub-Doppler cooling from working. As a result, it took several years to extend the quantum-gas microscope technique to fermionic species, and in 2015 several groups have demonstrated single-atom-resolved imaging of fermions [18]. In these setups, the direct observation of band insulators [19], fermionic Mott insulators [20,21] and even antiferromagnetic correlations [22] has been reported most recently. These developments provide fascinating opportunities to study many-body fermionic quantum systems with single-particle access. The fermionic nature of the electron is vital to understanding a range of phenomena within solid-state physics, such as electron pairing in superconductivity, and quantum magnetism. In this context, the quantum simulator will allow, e.g. for direct measurement of ordered quantum phases and out-of-equilibrium dynamics, with access to quantities such as spin--spin correlation functions and the growth of many-body entanglement. The high-resolution imaging systems could also be used to project tailored time-varying light fields onto the atoms, allowing the removal of high-entropy regions by locally modifying the confining potential to attain very low-entropy states. Attaining these low-entropy many-particle states is one of the key challenges in our field. In summary, quantum-gas microscopes offer unprecedented access to many-body quantum systems, and they will allow us to gain an even deeper insight into the intriguing world of quantum physics that governs the properties of strongly correlated materials. Using a well-controlled quantum system to simulate other quantum systems could help resolve some of the most challenging problems scientists face in condensed matter physics and materials science, but it will also provide new insights in quantum chemistry, high-energy physics and biology. SK acknowledges funding from the EU (ERC FERMILATT, SIQS, QuProCs) and SUPA.