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.
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.
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.
Degenerate quantum gases offer an ideal platform to study superfluidity. An annular superfluid has many subtle properties due to its topological geometry. In this manuscript, we present three ring-shaped superfluids, obtained with different formation mechanisms.The starting point of the experiments is a superfluid confined at the bottom of a bubble-shaped trap, generated by dressing atoms in a quadrupole trap with radio-frequency (rf) photons. Taking advantage of the smoothness of such an adiabatic trap allows us to realize these three annular superfluids.The first one is a levitating ring, formed on the surface of the bubble trap by compensating gravity. We study the structure taken by the atomic cloud related to fine effect of the rf polarization and the strong rf coupling. The second one is a ring trap, formed at the equator of the bubble with an additional blue detuned double light sheet. We set the annular gas into rotation by two methods, resulting in a persistent superfluid current and a quantized circulation. The last one is a fast-rotating dynamical ring, which forms under the effect of the centrifugal force. It is a long-lived supersonic superfluid whose linear velocity can reach Mach 18. We also evidence a collective mode of such a rapid rotating superfluid. The experimental realization of the dynamical ring is an important step towards the giant vortex regime.
Degenerate quantum gases offer an ideal platform to study superfluidity. An annular su- perfluid has many subtle properties due to its topological geometry. In this manuscript, we present three ring-shaped superfluids, obtained with different formation mechanisms. The starting point of the experiments is a superfluid confined at the bottom of a bubble- shaped trap, generated by dressing atoms in a quadrupole trap with radio-frequency (rf) photons. Taking advantage of the smoothness of such an adiabatic trap allows us to realize these three annular superfluids.The first one is a levitating ring, formed on the surface of the bubble trap by compensating gravity. We study the structure taken by the atomic cloud related to fine effect of the rf polarization and the strong rf coupling. The second one is a ring trap, formed at the equator of the bubble with an additional blue detuned double light sheet. We set the annular gas into rotation by two methods, resulting in a persistent superfluid current and a quantized circulation. The last one is a fast-rotating dynamical ring, which forms under the effect of the centrifugal force. It is a long-lived supersonic superfluid whose linear velocity can reach Mach18. We also evidence a collective mode of such a rapid rotating superfluid. The experimental realization of the dynamical ring is an important step towards the giant vortex regime
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 irrotational nature of superfluids leads to spectacular rotational properties. For the fluid to rotate, its density must locally vanish at a singular point called a quantum vortex. The annular geometry is of great interest for studying superfluidity as the gas can rotate in this geometry around a central hole without requiring any singularity, allowing the existence of persistent currents along the ring with a quantized circulation. In out experiment, we confine cold atoms dressed by a radiofrequency field in an adiabatic potential based on a quadrupolar magnetic trap. The resulting bubble-shaped potential, both very smooth and easily tunable, allows us to produce two types of ring-shaped Bose-Einstein condensates. A first strategy consists in adding a far-detuned light sheet to confine the atoms at the intersection between the bubble and the horizontal plane imposed by the light field – i.e. a ring. We present the implementation and optimization of this trap and demonstrate the possibility to prepare and observe superfluid currents in the ring. A second path exploits the centrifugal force and the anharmonicity of the adiabatic potential to create an effective Mexican hat potential by rotating the trapped atoms at the bottom of the bubble faster than the trap frequency. After having realized such a system, we probe its quadrupolar modes to characterize the rotation. The study of vortex distribution in the rotating gas also shows a thermal melting effect of the finite temperature vortex lattice.
Department of Physics, Colorado School of Mines, Golden, CO 80401, U.S.A. Rina Kanamoto Division of Advanced Sciences, Ochadai Academic Production, Ochanomizu University, Bunkyo-ku, Tokyo 112-8610 Japan Masahito Ueda Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033 Japan Ultracold quantum gases offer a wonderful playground for quantum manybody physics [1], 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 [2], using an atom-molecule resonance called a Feshbach resonance. Thus one can directly realize the Lieb-Liniger Hamiltonian (LLH) [3,4], from the weakly-to the strongly-interacting regime. At the same time there are a number of experiments utilizing ring traps [5]. 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 [6]. 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 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.1