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Mass, heat and spin transport in interlinked quantum gases

Periodic Reporting for period 2 - TransQ (Mass, heat and spin transport in interlinked quantum gases)

Reporting period: 2019-04-01 to 2020-09-30

How is functionality influenced by quantum effects? This is the central question underlying the research of the ERC project TransQ. To address it, we use a gas of ultracold atoms as a highly controlled quantum system and put it into settings of functionality that has counterparts in electronic devices or fluids in tubing.

The control of gases by laser cooling, evaporative cooling and trapping has led to the observation of Bose-Einstein condensation, a state of matter in which the particles lose their individuality, as predicted by many-body quantum mechanics. Following the observation of Bose-Einstein condensation, researchers have created a previously unimaginable toolbox to create different states of matter in quantum gases. In these synthetic quantum many-body systems all parameters can be controlled and the constituents individually measured. Often this is termed analogue quantum simulation, as key models to describe the electronic properties of materials can be created with ab-initio knowledge of the parameters and probed with high precision.

In this project, we explore the basic principles of functionality in synthetically created quantum many-body systems. The starting point is to design configurations which allow the flow of particles, heat or other entities, like the spin of particles, between reservoirs. In this regime, where the laws of quantum mechanics govern the physics, such out-of-equilibrium situations are far from being understood. For example, one of our research questions was, how does heat flow through a narrow channel, that is connected to two reservoirs containing quantum gases of different temperatures. The underlying physics is the same as in the thermoelectric effect. The latter makes it possible to generate an electronic current from a temperature gradient. Since temperature gradients are ubiquitous, the hope is to exploit waste heat or body heat to power devices. The more efficient thermoelectric materials are discovered, the broader will be the range of applications. In turn, this could allow society to reduce its use of fossil fuel. In our project we study this effect for strongly interacting superfluid reservoirs and for structured channels. In our ideal systems the figure of merit for the thermoelectric response indeed exceeds that of the best materials.
At the beginning of the project we studied the transport of atoms through a mesoscopic lattice. Ultracold fermions in optical lattices provide a rich playground to study many-body phenomena in condensed-matter, but characterizing their transport properties as directly as in solid-state two-terminal setups had remained challenging. We probed the insulating character of a lattice engineered inside a quantum wire by measuring its conductance. As attractive interactions were increased, we showed first indication of the previously unobserved Luther-Emery liquid. This work opened up the avenue for quantum simulations of mesoscopic physics problems involving non-equilibrium, interferences and interactions.

In a more recent work we investigated heat and matter currents in a strongly interacting Fermi gas of atoms. Heat and matter currents are required to relax an out-of-equilibrium system with temperature and chemical potential gradients to thermodynamical equilibrium. The ratio of heat to particle conductance characterizes this response and takes a universal value for typical electronic materials, known as the Wiedemann–Franz law, originating in the quasi-particle nature of the excitations contributing to transport. Investigating the transport dynamics between two reservoirs of ultracold and strongly interacting Fermi gases, connected by a quantum point contact, we observed a nonequilibrium steady state, strongly violating the Wiedemann–Franz law. This cold atom version of the fountain effect, previously observed in superfluid helium superleaks, is characterized by a weak coupling between heat and particle currents that results in a nonvanishing Seebeck coefficient.
The transport setting for quantum gases using two reservoirs, which was pioneered by my group, is unique worldwide.

Very recently we demonstrated simultaneous control over transport and spin properties of cold atoms, and thus establishes a framework for exploring concepts in spintronics and solid-state physics. One of the more unexpected things that can be done with charge-neutral atoms is to use them to emulate the fundamental behaviour of electrons. We used our our platform in which atoms cooled to temperatures close to absolute zero are transported through one- and two-dimensional structures, driven by a potential difference. In a pair of papers published in Physical Review Letters and Physical Review A, we reported that we have mastered in our transport experiments control over another quantum property of the atoms — their spin. We added to the transport channel a tightly focussed light beam, which induces local interactions that are equivalent to exposing the atoms to a strong magnetic field. As a consequence, the degeneracy of the spin states was lifted, which in turn serves as the basis for an efficient spin filter: atoms of one spin orientation are repelled, whereas those of another orientation are free to pass (see the figure). Importantly, even though the application of an additional light field leads to the loss of atoms, these dissipative processes do not destroy the quantization of conductance. We replicated this experimental finding in numerical simulation and substantiated its validity through an extension of the Landauer–Büttiker model, the key formalism for quantum transport. The efficiency of the atomic spin filter matches that of the best equivalent elements for electronic systems. This, together with the extraordinary ‘cleanness’ and controllability of the cold-atom platform, opens up exciting new perspectives for exploring the dynamics of quantum transport. In particular, as the interaction between the atoms can be tuned, the platform provides access to spin transport of strongly correlated quantum systems. This regime is difficult to study otherwise, but is of considerable fundamental and practical interest, not least for applications in spintronic devices and to explore fundamental phases of matter.

Figure: An optical beam (red) introduces an effect equivalent to applying a magnetic field inside an optically defined structure in which the atoms move (green). Atoms in the energetically lower spin state (orange) can flow while atoms in a higher spin state (blue) are blocked. (Adapted from doi: 10.1103/PhysRevLett.123.193605)

In upcoming experiments, we plan to explore complex structures in the channel, as well as driven and dissipative channel configurations.