Entropy and Particle Transport in Quantum Gases Manipulated with Near-Resonant Light
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Date
2023Type
- Doctoral Thesis
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Abstract
Entropy transport and particle transport and their interplay give insight into the transport processes in a complex systems, often intractable with a microscopic simulation. In this thesis, we study a quantum gas of ultracold fermionic 6Li flowing through a mesoscopic junction. The atomic nature of the quantum gas enables us to shape a two-terminal transport geometry with light. The tunability of interactions between atoms allows exploration of the weakly-interacting Fermi gas and the strongly interacting, unitary Fermi gas. The first part of the thesis details the technical upgrades to the apparatus. Degradation of laser powers led us to replace the laser system used for laser cooling, by a more powerful and stable system based on a fiber amplifier and frequency doubling. In addition, the unstable lock of the resonator trap was replaced and a spin-resolved imaging system implemented. These upgrades led to a continuous operability of the experiment, greatly simplifying measurements. In the second part we measure how the quantum nature of transport, characterized by the quantization of conductance, competes with particle losses imposed by near-resonant light.
This dissipation mechanism is locally controlled inside the quantum point contact (QPC), under which the conductance plateaus are robust. Additionally, we show that our observations can be described with an extended Landauer-Büttiker model. In a second application of near-resonant light we detune the laser frequency to obtain a spin-dependent optical potential. This effective Zeeman shift in the QPC fully spin-polarizes the currents flowing through it, while marginally reducing the atom-number. Exploring weak interactions we observe that attractive (repulsive) interactions reduce (increase) the effective Zeeman shift visibly. In a third application we use the three-level atom light interaction in the strongly interacting regime to at first dissipatively suppress the fast transport and then restore it via electromagnetically induced transparency. In the third part entropy transport between unitary Fermi gases, superfluid in equilibrium, is studied for varying transport geometries from quasi one-dimensional (1D) to quasi two-dimensional (2D). We find that both entropy and particle currents respond non-linearly to a temperature or chemical potential bias.
The transported entropy per particle is much larger than the superfluid entropy per particle in the equilibrium junction. This and the concurrently produced total entropy imply that the current is not a pure supercurrent and that the transport process is irreversible. Varying the geometry changes the timescales of advective and diffusive transport modes while the transported entropy per particle remains constant, independent of geometry. We derive a phenomenological model based on the irreversible, non-linear transport processes fitting the observations. In an equivalent, second experiment we impose dissipation in the junction. We find that dissipation enhances the advective response by counterintuitively reducing the transported entropy per particle. We employ the phenomenological model to demonstrate that dissipation strongly increases the thermal conductance while reducing the excess current in the quasi-1D junction. The extraced Seebeck coefficient reduces with dissipation, confirming our observations. Show more
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https://doi.org/10.3929/ethz-b-000635896Publication status
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ETH ZurichOrganisational unit
03599 - Esslinger, Tilman / Esslinger, Tilman
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