Periodic Reporting for period 4 - TransQ (Mass, heat and spin transport in interlinked quantum gases)
Reporting period: 2022-04-01 to 2022-09-30
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. In these synthetic quantum many-body systems all parameters can be controlled and the constituents individually measured. Often this is termed analogue quantum simulation.
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, which has obvious applications.
Our experiments are carried out at such low temperatures that quantum effects play a major role. For example, how many particles can be transported through the channel within a given period of time depends on their quantum statistics, whether they are bosons or fermions. Starting with a two component Fermi gas and switching on interactions between the spin components will lead to pairing and eventually to the formation of a superfluid that bypasses Pauli’s principle. How this is exactly happening in a transport setting is one of the key questions of the project. To get understanding, we devised schemes to address the spin components of the Fermi gas individually and to break pairs locally. The latter induces dissipation and we found a remarkable robustness of the flow through the channel. Our understanding is that high order pair-tunneling is responsible.
In other settings, we investigated functionality in time periodic potentials and could create different forms of topological pumps, which lead to adiabatic and dissipation free transport of particles.
In conclusion, the project created a fundament for the understanding of directed transport in an ultra-cold gas of atoms. We could very clearly distinguish regimes in which the quantum nature and the presence of strong interactions were governing the transport physics. In terms of relevance for applications of our findings, the situation has changed substantially since the beginning of our project, when the ultracold gases were mainly considered a toolbox for analog quantum simulation of strongly interacting quantum many-body systems. More recently, neutral atoms in arrays of optical tweezers have become a leading platform for qubit quantum simulations, with a huge potential for scalability beyond a thousand qubits. The question of how to efficiently move and sort atoms – ideally without or only minimal motional excitations – has become an urgent theme in the quest for such quantum machines. The methods and understanding, that we have developed in this project have therefore gained considerable additional relevance.
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, which has obvious applications.
Our experiments are carried out at such low temperatures that quantum effects play a major role. For example, how many particles can be transported through the channel within a given period of time depends on their quantum statistics, whether they are bosons or fermions. Starting with a two component Fermi gas and switching on interactions between the spin components will lead to pairing and eventually to the formation of a superfluid that bypasses Pauli’s principle. How this is exactly happening in a transport setting is one of the key questions of the project. To get understanding, we devised schemes to address the spin components of the Fermi gas individually and to break pairs locally. The latter induces dissipation and we found a remarkable robustness of the flow through the channel. Our understanding is that high order pair-tunneling is responsible.
In other settings, we investigated functionality in time periodic potentials and could create different forms of topological pumps, which lead to adiabatic and dissipation free transport of particles.
In conclusion, the project created a fundament for the understanding of directed transport in an ultra-cold gas of atoms. We could very clearly distinguish regimes in which the quantum nature and the presence of strong interactions were governing the transport physics. In terms of relevance for applications of our findings, the situation has changed substantially since the beginning of our project, when the ultracold gases were mainly considered a toolbox for analog quantum simulation of strongly interacting quantum many-body systems. More recently, neutral atoms in arrays of optical tweezers have become a leading platform for qubit quantum simulations, with a huge potential for scalability beyond a thousand qubits. The question of how to efficiently move and sort atoms – ideally without or only minimal motional excitations – has become an urgent theme in the quest for such quantum machines. The methods and understanding, that we have developed in this project have therefore gained considerable additional relevance.
At the beginning of the project we studied the transport of atoms through a mesoscopic lattice. 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.
In subsequent 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. 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.
Thermoelectric transport is extremely sensitive to quantum confinement and interactions, which are however challenging to study and pin down in real-life materials. In our quantum simulator, we engineered the thermoelectric response of a two-dimensional system of ultracold fermions. Upon increasing interactions to the strongly-correlated regime, we can turn our system from a heat engine into a heat pump.
The project has made it possible to perform quantum simulations of devices created from cold atoms in structured potentials. We studied mass, heat and spin transport in regimes of weak and strong interactions and in the presence of local dissipation. Our work represented a new milestone in the quantum simulation of thermoelectric phenomena.
The physics and the results of the project were published in scientific journals, journals for broader audience and presented on numerous occasions at conferences, workshops as well as in public talks, such as Nacht der Physik at ETH Zurich.
In subsequent 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. 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.
Thermoelectric transport is extremely sensitive to quantum confinement and interactions, which are however challenging to study and pin down in real-life materials. In our quantum simulator, we engineered the thermoelectric response of a two-dimensional system of ultracold fermions. Upon increasing interactions to the strongly-correlated regime, we can turn our system from a heat engine into a heat pump.
The project has made it possible to perform quantum simulations of devices created from cold atoms in structured potentials. We studied mass, heat and spin transport in regimes of weak and strong interactions and in the presence of local dissipation. Our work represented a new milestone in the quantum simulation of thermoelectric phenomena.
The physics and the results of the project were published in scientific journals, journals for broader audience and presented on numerous occasions at conferences, workshops as well as in public talks, such as Nacht der Physik at ETH Zurich.
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.
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)
Thermoelectric phenomena are actively investigated in condensed matter systems, while only few studies with cold atoms are dedicated to this topic. We observed a striking interaction-assisted reversal of the thermoelectric current, a novel effect never observed with cold atoms before.
In ongoing experiments, we introduce complex light structures to dissipate specific spin components or to create dark states in individual spin components of paired particles in the channel. This leads us to a new level of local control in strongly interacting quantum many-body systems.
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.
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)
Thermoelectric phenomena are actively investigated in condensed matter systems, while only few studies with cold atoms are dedicated to this topic. We observed a striking interaction-assisted reversal of the thermoelectric current, a novel effect never observed with cold atoms before.
In ongoing experiments, we introduce complex light structures to dissipate specific spin components or to create dark states in individual spin components of paired particles in the channel. This leads us to a new level of local control in strongly interacting quantum many-body systems.