Session R13: Atomtronics

Magnon optics and thermodynamics in a degenerate spinor Bose gas

At low temperature, spinor Bose gases form magnetically ordered superfluids. Like other magnetic materials, such a fluid supports magnons, the Nambu-Goldstone bosons corresponding to the spontaneous breaking of rotational symmetry. We have developed methods to produce and detect such excitations in a $^{87}$Rb $F=1$ spinor Bose gas. I will discuss precise measurements of the magnon recoil energy using coherent magnon interferometry, the use of thermalized magnons to measure and lower the temperature of quantum gases, and the phenomenon of magnon condensation in a quantum gas.   

Atomtronics with Ultracold Bose Gases

Neutral atom systems can exhibit similar transport properties like solid state devices. For instance, a neutral atom current is induced by a difference in chemical potential very much in the same way as a voltage drives an electric current. Employing Bose-Einstein condensed atomic gases allows observing superfluid transport phenomena, thus drawing connections to superconductivity. With help of light fields, the atomic current can additionally be guided in engineered potential landscapes in which one can also incorporate tunneling junctions. Eventually, the different components and elements can be integrated in atomtronic circuits which shed light on fundamental transport properties of many-body quantum systems. In this talk, I will present two fundamental atomtronic devices. The first is the observation of negative differential conductivity, which occurs at a multimode tunneling junction for ultracold atoms [1]. The second is the appearance of a DC Josephson current in a biased tunneling junction [2], which features bistable transport characteristics. I will discuss the prospects of these basic elements for more complex atomtronic circuits. References [1] R. Labouvie, B. Santra, S. Heun, S. Wimberger, and H. Ott ``Negative Differential Conductivity in an Interacting Quantum Gas'' Phys. Rev. Lett. 115, 050601 (2015). [2] R. Labouvie, B. Santra, Simon Heun, and H. Ott ``Nonequilibrium steady states in a driven-dissipative superfluid'' arXiv:1507.05007   

Dynamics of quantum impurities in many-body systems of ultracold atoms

Recent theoretical and experimental progress in the study of dynamics of quantum impurities in ensembles of ultracold atoms will be reviewed. Examples include RF spectroscopy of Bose and Fermi polarons, interferometric measurements of ultrafast dynamics of impurities in an atomic Fermi sea, exploring crossover from few- to many-body dynamics in Rydberg excitations. New directions including realizations of Kondo type models and Shiba states in Fermi superfluids will be discussed.   

Transport dynamics and dissipation in polariton ring condensates and cold atoms

Recent progress in experiments has opened new contexts in which to observe and explore out-of-equilibrium quantum transport dynamics. On the side of cold atoms, significant advances are made possibile by the ability to control and measure atomic dynamics time-dependently, as well as to explore the effects of strong interactions. This is especially true in recent experiments with quantum gas microscopes, which now provide single-site and single atom measurement and control. At the same time, new methods for control and longer coherence times have been realised in condensates of exciton polaritons. This has enabled the development of ring geometries for these systems, and corresponding quantised circulation. I will discuss our recent theoretical work looking at the interplay between coherent dynamics and dissipation in these systems. For polaritons in a ring trap, half-quantum vortices are allowed in which there is a phase rotation of $\pi$ and a corresponding polarisation vector rotation of $\pi$ around the ring. The observed half-quantum state in current experiments is novel, in that the handedness of the spin flips from one side of the ring to the other side in addition to the rotation of the linear polarization component. This type of state is not possible for vortices in a simply connected geometry, and we investigate how the interplay between the polariton production and dissipation can give rise to this state in a ring trap. I will also discuss the transport dynamics of cold atoms in tilted optical lattices, in the presence of decoherence from sources including spontaneous emissions.   

Driving transitions between quantized flow states in an atomtronic circuit

Superfluidity, or flow without resistance, is a macroscopic quantum effect that is present in a multitude of systems, including liquid helium, superconductors, and ultra-cold atomic gases. In superconductors, flow without resistance has led to the development of a number of useful devices. Here, I will present our work studying a superfluid analog to the rf-superconducting interference device (SQUID). Our atomtronic analog is formed in a ring-shaped Bose-Einstein condensate (BEC) of sodium atoms. Ring condensates are unique in that they can support persistent currents that are quantized. We drive transitions between persistent current states using a rotating perturbation, or weak link. Here, rotation acts as the analog to magnetic field in superconductors. In our system, a current (as viewed in the frame co-rotating with the perturbation) develops to oppose any change in rotation. If the rotation rate is sufficiently large, the critical current of the superfluid is exceeded in the weak link region, causing a transition to a state of larger persistent current. The strength of the perturbation tunes the critical rotation rates. Like the rf-SQUID, the transitions show hysteresis – rotation rates that increase the quantized current are different from those that decrease the current. The size of the hysteresis loop allows us to explore the microscopic mechanisms responsible for the transitions. In a more recent experiment, we have observed the time it takes for the first persistent current state to decay in the presence of a stationary perturbation. The measured timescales depend strongly on temperature, but in a way that suggests that other physical effects, like quantum coherence, could also play a role in the transitions between current states.