Thermometry and Thermodynamics within an Ultracold Mixture of 87Rb and 41K

Hewitt, Thomas (2025). Thermometry and Thermodynamics within an Ultracold Mixture of 87Rb and 41K. University of Birmingham. Ph.D.

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Abstract

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.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):