Atom circuit (figures: “ring with barrier”is the left-hand figure; “ring” is the right-hand figure) False color images of an “atom circuit” made of an ultracold sodium gas. Red denotes a greater density of atoms and traces the path of circulating atoms around the ring. A laser-based barrier can stop the flow of atoms around the circuit (left); without the barrier the atoms circulate around the ring (right). Credit: JQI/NIST |
Researchers
from the National Institute of Standards and Technology (NIST) and the
University of Maryland (UM) have created the first nontrivial “atom
circuit,” a donut-shaped loop of ultracold gas atoms circulating in a
current analogous to a ring of electrons in a superconducting wire. The
circuit is “nontrivial” because it includes a circuit element—an
adjustable barrier that controls the flow of atom current to specific
allowed values. The newly published* work was done at the Joint Quantum
Institute, a NIST/UM collaboration.
Ultracold
gases, such as the Bose-Einstein condensate of sodium atoms in this
experiment, are fluids that exhibit the unusual rules of the quantum
world. Atomic quantum fluids show promise for constructing ultraprecise
versions of sensors and other devices such as gyroscopes (which
stabilize objects and aid in navigation). Superfluid helium circuits
have already been used to detect rotation. Superconducting quantum
interference devices (SQUIDs) use superconducting electrons in a loop to
make highly sensitive measurements of magnetic fields. Researchers are
striving to create 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.
To
make their atom circuit, researchers created a long-lived persistent
current—a frictionless flow of particles—in a Bose-Einstein condensate
of sodium atoms held by an arrangement of lasers in a so-called optical
trap that confines them to a toroidal, or donut, shape. Persistent
flow—occurring for a record-high 40 seconds in this experiment—is a
hallmark of superfluidity, the fluid analog of superconductivity.
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. A focused laser beam creates the circuit
element—a barrier across one side of the ring. The barrier constitutes a
tunable “weak link” that can turn off the current around the loop.
Superflow
stops abruptly when the strength of the barrier is sufficiently high.
Like water in a pinched garden hose, the atoms speed up in the vicinity
of the barrier. But when the velocity reaches a critical value, the
atoms encounter resistance to flow (viscosity) and the circulation
stops, as there are no external forces to sustain it.
In
atomic Bose-Einstein condensates, researchers have previously created
Josephson junctions, a thin barrier separating two superfluid regions,
in a single atomic trap. SQUIDs require a Josephson junction in a
circuit. This present work represents the implementation of a complete
atom circuit, containing a superfluid ring of current and a tunable weak
link barrier. This is an important step toward realizing an atomic
SQUID analog.
*
A. Ramanathan, K. C. Wright, S. R. Muniz, M. Zelan, W. T. Hill III, C.
J. Lobb, K. Helmerson, W. D. Phillips and G. K. Campbell. Superflow in a toroidal Bose-Einstein condensate: an atom circuit with a tunable weak link. Physical Review Letters. Published online March 28, 2011.
