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Stirring Up New Physics in Toroidal BECs

September 15, 2008

Quantum effects don’t usually intrude into daily life. That’s a fortunate state of affairs for a person who needs to know, say, the position and speed of his car at the same time.

But in the lab, macroscopic quantum systems are increasingly the subject of intense investigation, and none more so than the eerie entities called Bose- Einstein condensates (BECs). At JQI, BECs will be a focus of special attention as part of the Physics Frontier Center activities.

Researchers first created a BEC in 1995, 70 years after Satyendra Nath Bose and Albert Einstein predicted the exotic state of matter, by supercooling a cloud of trapped atoms to the point at which they all collapse into the same quantum state. In that strange condition, atoms behave in a collective fashion, as if they were a single entity or “super-atom.” But BECs can contain hundreds of thousands, or even millions, of atoms, and the resulting aggregation is big enough to see with the (very acute) naked eye, and to examine with ordinary, large-scale optical devices.

That’s one reason that, 13 years after the initial condensate was made, BECs are still revealing a host of novel phenomena and providing new insights into quantum mechanics. And JQI scientists are poised to take that research to new, ever larger dimensions.

A group headed by JQI Fellows Bill Phillips and Kris Helmerson at NIST has been exploring the characteristics of BECs created in a torus (donut) shape and then exposed to various forces or fields.

Usually, when atoms are trapped and chilled to around 200 billionths of a degree above absolute zero, the resulting condensate is approximately spherical or cigar-shaped.

The JQI team employs a different technique: They use an elliptical trap and shine a high-energy green laser beam through the middle of the trapped cloud, expelling atoms from the center. The result is a BEC in the shape of a torus, which has considerable advantages for certain kinds of study. For example, an ordinary BEC has a substantial density gradient, thinning out from the center to the edges. A toroidal BEC has a much more uniform density. In addition, it provides a closed loop in which atoms can move or forces can propagate, allowing a wide range of effects. That property allowed the researchers to demonstrate, in a paper published last fall, the first evidence for “persistent flow” -- frictionless, superfluid motion of atoms -- around an ultracold BEC ring.

Yet another highly intriguing advantage is that the BEC’s ring shape is the same as that of the superconducting quantum interference device, or SQUID. A SQUID is a loop of superconducting material containing one or more super-thin insulating barriers called Josephson junctions (JJ). Thanks to a quantum phenomenon called tunneling, which results from the fact that a particle’s wave function can extend across classical barriers, a current can flow across a JJ in the absence of any voltage. Applying a voltage causes the current to start alternating. Applying a magnetic field causes the current level to change.

Solid state SQUIDs have enormous potential for quantum physics and the future of information processing, but have decoherence properties that are poorly understood. That’s one reason that the JQI team, which contains exports in BECs and SQUIDS, will direct some of its Physics Frontier Center work to creating and studying atomic SQUID analogues (ASA) in which moving atoms in a BEC
torus play the role of electrical current.

Already JQI researchers have shown that the rotational frequency in an ASA is analogous to the magnetic field in a SQUID, with the field strength proportional to the frequency. Testing such similarities and differences is likely to provide valuable insights.

(And perhaps practical products as well: Some sort of ASA might serve an ultra-sensitive rotation detector, much as SQUIDs are now routinely used to detect extremely faint magnetic fields.)

The group has already conducted a lot of preliminary research. In particular, they developed a novel method of controlling the motion of BEC atoms by whacking them with a specially prepared laser beam, called a Laguerre-Gauss mode, that conveys orbital angular momentum to atoms it strikes. (Other teams have moved the atoms by physically stirring them.) The Laguerre- Gauss technique has many advantages, and it can produce results that are very difficult to imagine from a purely classical perspective.

For example, Helmerson and colleagues have used two carefully correlated laser pulses to generate a coherent superposition of BEC atoms rotating in two opposite directions in the torus. To confirm that half the atoms really are going one way while half are going the other, the group used another light pattern to nudge the two atomic flows so that they interfered with one another. The resulting images clearly demonstrate the expected interference patterns.

“We have the technology now to create any arbitrary rotational state -- but also any superposition of rotational states,” Helmerson says. “We’ve added another tool to the tool box of atom manipulation.” Moreover, because rotation is a topological property -- that is, one that forms a shape over an extended area and hundreds of thousands of atoms -- it tends to be more robust and less prone to external perturbation than many other atomic states, such as spin, that are often manipulated in superposition experiments. In fact, the rotation persists for 10 seconds, and does so even when the BEC atoms make up only 20 percent of the atoms in the trap.

If these and other techniques can be improved, they may lead to macroscopic quantum phenomena never before observed, such as a coherent “Schrodinger cat” state in which every atom in the torus is moving both clockwise and counterclockwise simultaneously. Preparation of an ASA might end up producing a “cat” state, while also allowing scientists to study how momentum changes and dissipates. The team also wants to see how a structure such as a vortex will behave in an ASA. They are familiar in JJs, but have never been observed in a BEC tunneling through a junction.