Ultracold gases, such as the Bose-Einstein condensates, can behave as fluids that exhibit the unusual rules of the quantum world. One striking example of this is superfluidity: flow without resistance. If a superfluid flows in a closed loop, for example, around a ring, such a flow would never cease.
Superfluid behavior was first observed in superconductors. Similar to the case of a superfluid in a ring, if a current is created in a loop of superconducting wire, the electrons flow without resistance creating a persistent current. Such a geometry has been used to make one of the most sensitive magnetometers ever invented: the superconducting quantum interference devices (SQUIDs).
We make similar atom “circuits” in our lab. The basis for most of our experiments is a ring of about 500,000 sodium atoms. These atoms are sufficiently cold (about 100 nK or about 100 billionths of a degree above absolute zero) to form a superfluid Bose-Einstein condensate. Our atoms are trapped into a ring shape by an arrangement of lasers in a so-called optical trap that confines them to a toroidal, or donut, shape. In this shape, the atom current does not circle the ring at just any velocity, but only at specified values, corresponding in this experiment, to just a single quantum of angular momentum.
To manipulate the flow in our ring, we use another laser that exerts a repulsive force on the atoms, causing them to move away from the laser. The resulting depletion in the density of the atoms is called a “weak link”. We can use this weak link as a spoon – to attempt to stir or otherwise manipulate the flow of atoms in the ring.
Using our ring with one or more weak links, we have been able to study a variety of different superfluid effects. First, we have observed persistent flow - occurring for a record-high 60 seconds in this experiment. Moreover, we have demonstrated that we can stop this flow using our weak link. Second, we have demonstrated that we can change the persistent flow state of the ring by stirring with the weak link, and that such stirring possesses “hysteresis” - a type of memory effect. Lastly, using two weak links, we have demonstrated that the superfluid can exhibit flow with resistance when it is forced to flow at high velocities. All of these experiments have probed some of the underlying, microscopic behavior of these superfluid clouds and have helped to pave the way for practical devices.
Atomic quantum fluids show promise for constructing ultraprecise versions of sensors and other devices such as gyroscopes (which stabilize objects and aid in navigation), and we are currently working towards the creation of an ultracold-gas version of a SQUID, which could detect rotation. Combined with ultracold atomic-gas analogs of other electronic devices and circuits, or "atomtronics" that have been envisioned, such as diodes and transistors, this work could set the stage for a new generation of ultracold-gas-based precision sensors.