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.
