Weakly interacting atomic gases at ultra-low temperatures are superfluid. Owing to the high degree of control they offer, these quantum gases give access to a wonderful playground to explore superfluid dynamics. In particular, quantum gases flowing inside waveguides can mimic the behavior of electrons in superconducting circuits, opening the new field of 'atomtronics.' The elementary circuit is the ring, where the atomic circulation is expected to be quantized. I will discuss two opposite limits: the production of a small current of a few quanta, and the fast rotation regime where the velocity exceeds the speed of sound.
Weakly interacting atomic gases at ultra-low temperatures are superfluid. Owing to the high degree of control they offer, these quantum gases give access to a wonderful playground to explore superfluid dynamics. In particular, quantum gases flowing inside waveguides can mimic the behavior of electrons in superconducting circuits, opening the new field of 'atomtronics.' The elementary circuit is the ring, where the atomic circulation is expected to be quantized. I will discuss two opposite limits: the production of a small current of a few quanta, and the fast rotation regime where the velocity exceeds the speed of sound.
Weakly interacting atomic gases at ultra low temperatures are superfluid. Owing to the high degree of control they offer, these quantum gases give access to a wonderful playground to explore superfluid dynamics. In particular, quantum gases flowing inside waveguides can mimick the behavior of electrons in superconducting circuits, opening the new field of `atomtronics’. The elementary circuit is the ring, where the atomic circulation is expected to be quantized. I will discuss two opposite limits: the production and decay of a small current of a few quanta, and the fast rotation regime where the velocity exceeds the speed of sound.
Superfluidity is a remarkable phenomenon. Superfluidity was initially characterized by flow without friction, first seen in liquid helium in 1938, and has been studied extensively since. Superfluidity is believed to be related to, but not identical to Bose-Einstein condensation, a statistical mechanical phenomena predicted by Albert Einstein in 1924 based on the statistics of Satyendra Nath Bose, where bosonic atoms make a phase transition to form a Bose-Einstein condensate (BEC), a gas which has macroscopic occupation of a single quantum state. Developments in laser cooling of neutral atoms and the subsequent realization of Bose-Einstein condensates in ultracold gases have opened a new window into the study of superfluidity and its relation to Bose-Einstein condensation. In our atomic sodium BEC experiment, we studied superfluidity and dissipationless flow in an all-optical toroidal trap, constructed using the combination of a horizontal ``sheet''-like beam and vertical ``ring''-like beam, which, like a circuit loop, allows flow around the ring. On inducing a single quantum of circulation in the condensate, the smoothness and uniformity of the toroidal BEC enabled the sustaining of a persistent current lasting 40 seconds, limited by the lifetime of the BEC due to background gas pressure. This success set the stage for further experiments studying superfluidity. In a first set of experiments, we studied the stability of the persistent current by inserting a barrier in the flow path of the ring. The superflow stopped abruptly at a barrier strength such that the local flow velocity at the barrier exceeded a critical velocity, which supported decay via the creation of a vortex-antivortex pair. Our precise control in inducing and arresting superflow in the BEC is a first step toward studying other aspects of superfluidity, such as the effect of temperature and dimensionality. This thesis discusses these experiments and also details partial-transfer absorption imaging, an imaging technique developed in the course of this work.
The hallmark of superfluidity is the appearance of "vortex states" carrying a quantized metastable circulating current. Considering a unidirectional flow of particles in a ring, at first it appears that any amount of scattering will randomize the velocity, as in the Drude model, and eventually the ergodic steady state will be characterized by a vanishingly small fluctuating current. However, Landau and followers have shown that this is not always the case. If elementary excitations (e.g. phonons) have higher velocity than that of the flow, simple kinematic considerations imply metastability of the vortex state: the energy of the motion cannot dissipate into phonons. On the other hand if this Landau criterion is violated the circulating current can decay. Below we show that the standard Landau and Bogoliubov superfluidity criteria fail in low-dimensional circuits. Proper determination of the superfluidity regime-diagram must account for the crucial role of chaos, an ingredient missing from the conventional stability analysis. Accordingly, we find novel types of superfluidity, associated with irregular or chaotic or breathing vortex states.
Persistent currents in annular geometries have played an important role in disclosing the quantum phase coherence of superconductors and mesoscopic electronic systems.Ultracold atomic gases in multiply connected traps also exhibit long-lived supercurrents and have attracted much interest both for fundamental studies of superfluid dynamics and as prototypes for atomtronics circuits.Here, we report on the realization of supercurrents in homogeneous, tunable fermionic rings.We gain exquisite, rapid control over quantized persistent currents in all regimes of the BCS-BEC crossover through a universal phase-imprinting technique, attaining on-demand circulations w as high as 9. High-fidelity readout of the superfluid circulation state is achieved by exploiting an interferometric protocol, which also yields local information about the superfluid phase around the ring.In the absence of externally introduced perturbations, we find the induced metastable supercurrents to be as long-lived as the atomic sample.Conversely, we trigger and inspect the supercurrent decay by inserting a single small obstacle within the ring.For circulations higher than a critical value, the quantized current is observed to dissipate via the emission of vortices, i.e., quantized phase slips, which we directly image, in good agreement with numerical simulations.The critical circulation at which the superflow becomes unstable is found to depend starkly on the interaction strength, taking its maximum value for the unitary Fermi gas.Our results demonstrate fast and accurate control of quantized collective excitations in a macroscopic quantum system and establish strongly interacting fermionic superfluids as excellent candidates for atomtronics applications.
Persistent current, or "flow without friction", as well as quantum vortices are the hallmarks of superfluidity. Recently a very long-lived persistent flow of atoms has been experimentally observed in Bose-Einstein condensates trapped in a ring-shaped potential. This enables fundamental studies of superfluidity and may lead to applications in high-precision metrology and atomtronics. We overview our recent theoretical studies of the generation of the persistent current in a stirred toroidal atomic Bose-Einstein condensate, and discuss our new investigation of the hysteresis in the atomtronic circuit.
This is a chapter for a book. The first paragraph of this chapter is as follows: "Ultracold quantum gases offer a wonderful playground for quantum many body physics, as experimental systems are widely controllable, both statically and dynamically. One such system is the one-dimensional (1D) Bose gas on a ring. In this system binary contact interactions between the constituent bosonic atoms, usually alkali metals, can be controlled in both sign and magnitude; a recent experiment has tuned interactions over seven orders of magnitude, using an atom-molecule resonance called a Feshbach resonance. Thus one can directly realize the Lieb-Liniger Hamiltonian, from the weakly- to the strongly-interacting regime. At the same time there are a number of experiments utilizing ring traps. The ring geometry affords us the opportunity to study topological properties of this system as well; one of the main properties of a superfluid is the quantized circulation in which the average angular momentum per particle, L/N, is quantized under rotation. Thus we focus on a tunable 1D Bose system for which the main control parameters are interaction and rotation. We will show that there is a critical boundary in the interaction-rotation control-parameter plane over which the topological properties of the system change. This is the basis of our concept of \textit{metastable quantum phase transitions} (QPTs). Moreover, we will show that the finite domain of the ring is necessary for the QPT to occur at all because the zero-point kinetic pressure can induce QPTs, i.e., the system must be finite; we thus seek to generalize the concept of QPTs to inherently finite, mesoscopic or nanoscopic systems."