Within this work, I put forward my experimental studies of the interactions in ultracold atomic mixtures, and their applications in quantum thermodynamics. I detail the key features and the progress made towards building a system which is capable of cooling quantum gases to absolute zero. In order to quantify the heat exchange of a system, knowledge of its temperature is required. Precise temperature measurements of systems with low numbers of ultracold atoms are of great importance in quantum technologies, but they can be quite resource intensive. We first utilised the experimental system to develop an adaptive Bayesian strategy, which processes data from both real and simulated release-recapture thermometry experiments of a single atom of 41K in an optical tweezer at microkelvin temperatures. By adaptively choosing the release-recapture times, this strategy produces more reliable estimates, while converging faster to the real temperature compared to conventional methods. A simpler, non-adaptive a priori method produced competitive results when applied to real experimental data. In addition to this, the underlying Bayesian framework is not specific to our platform and can be adapted for various experiments. Following this, I move onto the studies of a quantum impurity system, which involved immersing the single atom in a bath of ultracold 87Rb atoms. The K atom is trapped within a species-selective dipole potential, allowing for independent manipulation of the bath and impurity. I explore the characterisation and control of the interactions between the two subsystems. This was performed using Feshbach spectroscopy, where I detected several interdimensional confinement induced Feshbach resonances for the KRb interspecies scattering length. In addition to the underlying free-space s-wave interactions, I detected a series of p-wave resonance features. I further determine how the resonances behave as the temperature of the bath and the dimensionality of the interactions change. Finally, I was able to screen the quantum impurity by tuning the wavelength of the species selective tweezer, which adds another tool for control of the system. These results have a range of applications in quantum simulations of quantum impurity models and quantum thermodynamics.
Within this work, I put forward my experimental studies of the interactions in ultracold atomic mixtures, and their applications in quantum thermodynamics. I detail the key features and the progress made towards building a system which is capable of cooling quantum gases to absolute zero. In order to quantify the heat exchange of a system, knowledge of its temperature is required. Precise temperature measurements of systems with low numbers of ultracold atoms are of great importance in quantum technologies, but they can be quite resource intensive. We first utilised the experimental system to develop an adaptive Bayesian strategy, which processes data from both real and simulated release-recapture thermometry experiments of a single atom of 41K in an optical tweezer at microkelvin temperatures. By adaptively choosing the release-recapture times, this strategy produces more reliable estimates, while converging faster to the real temperature compared to conventional methods. A simpler, non-adaptive a priori method produced competitive results when applied to real experimental data. In addition to this, the underlying Bayesian framework is not specific to our platform and can be adapted for various experiments. Following this, I move onto the studies of a quantum impurity system, which involved immersing the single atom in a bath of ultracold 87Rb atoms. The K atom is trapped within a species-selective dipole potential, allowing for independent manipulation of the bath and impurity. I explore the characterisation and control of the interactions between the two subsystems. This was performed using Feshbach spectroscopy, where I detected several interdimensional confinement induced Feshbach resonances for the KRb interspecies scattering length. In addition to the underlying free-space s-wave interactions, I detected a series of p-wave resonance features. I further determine how the resonances behave as the temperature of the bath and the dimensionality of the interactions change. Finally, I was able to screen the quantum impurity by tuning the wavelength of the species selective tweezer, which adds another tool for control of the system. These results have a range of applications in quantum simulations of quantum impurity models and quantum thermodynamics.
Abstract We implement an experimental architecture in which a single atom of K is trapped in an optical tweezer, and is immersed in a bath of Rb atoms at ultralow temperatures. In this regime, the motion of the single trapped atom is confined to the lowest quantum vibrational levels. This realizes an elementary and fully controllable quantum impurity system. For the trapping of the K atom, we use a species-selective dipole potential, that allows us to independently manipulate the quantum impurity and the bath. We concentrate on the characterization and control of the interactions between the two subsystems. To this end, we perform Feshbach spectroscopy, detecting several inter-dimensional confinement-induced Feshbach resonances for the KRb interspecies scattering length, that parametrizes the strength of the interactions. We compare our data to a theory for inter-dimensional scattering, finding good agreement. Notably, we also detect a series of p-wave resonances stemming from the underlying free-space s-wave interactions. We further determine how the resonances behave as the temperature of the bath and the dimensionality of the interactions change. Additionally, we are able to screen the quantum impurity from the bath by finely tuning the wavelength of the light that produces the optical tweezer, providing us with a new effective tool to control and minimize the interactions. Our results open a range of new possibilities in quantum simulations of quantum impurity models, quantum information, and quantum thermodynamics, where the interactions between a quantized system and the bath is a powerful yet largely underutilized resource.
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.
We investigate quantum-mechanical interactions between ultracold rubidium and cesium in an optical trap at temperatures of a few microkelvin. Our results provide, on the one hand, an experimental key to understand the collisional properties and, on the other hand, a tool to control the interspecies interactions. By performing loss measurements we locate several Feshbach resonances, which provide insight into the energy structure of weakly bound RbCs molecules near the dissociation threshold and allow for the production of such heteronuclear Feshbach molecules. In the future we will transfer these loosely-bound molecules into the absolute internal ground state. The availability of ultracold heteronuclear ground state molecules will open the door to investigate phenomena associated with ultracold polar quantum gases. In our new experimental set-up we are able to trap and cool rubidium and cesium atoms in their lowest internal states. First we load both species into a two-color magneto-optical trap, having full control over the single-species atom number. We extend the technique of degenerate Raman-sideband cooling to a two-color version, which is able to simultaneously cool and polarize both rubidium and cesium. Thereafter we load the atoms into a levitated crossed optical dipole trap. Because of the presence of the gradient magnetic field the trap is highly state selective and consequently provides perfect spin-polarization of the sample. Furthermore, a coincidence of the magnetic-moment-to-mass ratios of the two species allows for simultaneous levitation of both, which assures an almost perfect spatial overlap between the species. We perform Feshbach spectroscopy in two different spin channels of the mixture within a magnetic field ranging from 20 to 300G. In the lowest spin combination of the species we locate 23 interspecies Feshbach resonances, while in a higher spin mixture we find 2 resonances. The high number of resonances found within this range of magnetic field is unusual for alkali mixtures. The presence of many resonances points to scattering properties of the mixture, which include higher-order coupling mechanisms. The obtained data on the Feshbach spectroscopy provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identification of possible starting points for the association of ultracold heteronuclear RbCs molecules. In addition we show preliminary results on spectroscopy of the binding energy of RbCs dimers, based on a modulation of the magnetic field. The recently obtained Bose-Einstein condensate of Cs atoms, which represents a benchmark for the performance of the present apparatus, will be discussed as well as the potential pathways towards a double-degenerate mixture. We follow two main goals, first the production of a double Bose-Einstein condensate and, second, the transfer of shallow bound Feshbach molecules to the absolute internal ground state. The production of heteronuclear ground state molecules allows to enter the world of ultracold polar quantum systems.
This thesis presents theoretical research on microscopic measuring devices and thermal machines constructed from single trapped atoms. We study several variants on this theme, with a particular emphasis on their application to ultracold atom physics. The first part of the thesis is about quantum refrigerators powered by thermal absorption rather than by external work. We propose and detail how such a machine may be practically constructed with a trapped atom placed inside an optical cavity, and employed to cool the atom close to absolute zero temperature using only collimated sunlight as an energy source. We then show that quantum absorption refrigerators in the strong-coupling regime exhibit coherent oscillations, thus enabling one to outperform a typical classical refrigerator by reaching lower temperatures than the steady state in a finite time using quantum coherence. The second part of the thesis studies the use of impurity atoms as probes of ultracold atomic gases. We discuss two different thermometry methods using impurities which are sensitive to temperature differences on the order of nanokelvin or less. We characterise the precision of the proposed thermometers by explicit calculations in the context of both weakly and strongly interacting atomic Bose gases. Finally, we study impurities immersed in a cold atomic Fermi gas realising a superfluid. We show that the impurities' energy dissipation rate probes the spectrum of density fluctuations in the gas, providing nondestructive access to various properties of the superfluid order parameter along the crossover from a Bardeen-Cooper-Schrieffer (BCS) state to a Bose-Einstein condensate (BEC).
Trapped ultracold gases are highly controllable experimental systems with unique quantum properties, making them a brilliant medium for research in solid state physics, quantum computation, and precision measurements. The standard technique for observing a cold atomic sample is time-of-flight absorption imaging. While this provides valuable information regarding the distribution and quantum state of atoms, the process destroys the sample via spontaneous emission and recoil heating, allowing for acquisition of just one data point per experimental run. In this thesis we develop and implement a complementary hyperfine state sensitive, off-resonant, optical dispersive interrogation system that uses frequency modulation spectroscopy to monitor the temporal dynamics of atomic population during coherent dynamical processes in cold atom systems. The applicability of this robust and powerful tool is expanded to simultaneously measure both the F = 2 hyperfine state population of 87Rb and the F = 9/2 hyperfine state population of 40K. This dual-species probe is used to monitor the sympathetic cooling process, whereby 40K is cooled indirectly via collision with a sample of 87Rb, which itself is undergoing forced RF evaporative cooling. The dispersive probing technique is then used to investigate a range of magnetic Feshbach resonances in 87Rb. Feshbach resonances allow the atomic interaction strength to be precisely tuned via an external magnetic field. Rapid and effcient location and identification of four such resonances were demonstrated between the |F=1, mF=1> and |F=2, mF=0> states. Despite the resonance-induced loss features being <~0.1 G wide, only a small number of experimental runs was sufficient to explore a magnetic field range >18 G. The resonances consist of two known s-wave features in the vicinity of 9 G and 18 G, and two previously unobserved p-wave features near 5 G and 10 G. The dispersive detection approach is further used to monitor the atomic loss in real-time, and directly characterise the two-body loss dynamics for each Feshbach resonance. We further characterise the four observed Feshbach resonances by dispersively measuring the 2-body loss rate coefficient, K21, as a function of magnetic field. Finally, the dispersive probe method is paired with an FPGA-based feedback algorithm and used to perform reliable transfer of 87Rb atoms to a target quantum state in the presence of an unknown, random magnetic field. The algorithm processes off-resonant optical measurements of state populations during a microwave-induced adiabatic rapid passage in real time, and terminates the process when a certain threshold in population transfer is achieved.
Within the scope of this thesis, comprehensive experimental studies on interspecies interactions between 87Rb and various isotopes of Yb at temperatures in the µK regime have been performed. The interactions between Yb and 87Rb manifested themselves through interspecies thermalization and - in the case of 174Yb and 87Rb - through phase separation.
The experimental investigations described in this thesis were carried out within a research project, whose long term goal is the creation of ultracold YbRb molecules. The experimental apparatus is designed for combined cooling and trapping of gaseous Yb and Rb atomic samples to the µK and nK regime.
Due to the particular magnetic properties of Rb and Yb, a unique combination of trapping potentials, which allows for independent manipulation of ultracold 87Rb and Yb atoms, was used for the described measurements.
The combined trap setup consisted of a Ioffe-Pritchard-type magnetic trap (MT) for 87Rb and a bichromatic optical dipole trap (BIODT) for Yb. The main scientific result of this PhD work is the observation of phase separation in a thermal mixture of 174Yb and 87Rb in the µK regime and the detailed experimental study and theoretical analysis of this unexpected effect. Analysis of the experimental observations required accurate modeling of the trapping potentials in order to extract quantitative results on the interspecies interaction. These results can be related to the 174Yb 87Rb interspecies s-wave scattering length, using an approach, which not only describes mean field effects but also includes multiple-particle interactions.
This analysis yields a value for the s-wave scattering length of a_(87-174)=4710 (+3970) (-1520) a0, which is extraordinarily large. Indeed, this is to our knowledge the largest interspecies scattering length that has been observed to date at zero magnetic field. In contrast to observations of phase separation in other systems that have been made so far, phase separation between 174Yb and 87Rb as described in this PhD work is observed in purely thermal and not quantum degenerate samples. In this thesis, the study of phase separation in a 174Yb87Rb mixture was supplemented by an investigation of elastic collision rates for several of the Yb isotopes. The most notable finding was that for the combination \Yb{170}87Rb the interspecies scattering length has a value of a_(87-174)=6.6 (+3.5) (-2.9)a0 and is thus extremely small. The results of our studies on interspecies interactions between 87Rb and Yb can be used for model calculations on the YbRb molecular ground state potential: E. Tiesinga and S. Maxwell from the atomic theory group at NIST determined a Lennard-Jones potential, which quantitatively reproduces the present experimental observations. This knowledge will significantly help to provide a strategy for the controlled transfer of excited state YbRb* molecules to a selected ground state.