BEC

Magnets, whether in the form of a bar, horseshoe or electromagnet, always have two poles. If you break a magnet in half, you’ll end up with two new magnets, each with its own magnetic north and south.But some physics theories predict the existence of single-pole magnets—a situation akin to electric charges, which come in either positive or negative chunks. One particular incarnation—called the Yang monopole after its discoverer—was originally predicted in the context of high-energy physics, but it has never been observed. Now, a team at JQI led by postdoctoral researcher Seiji Sugawa and JQI Fellow Ian Spielman have succeeded in emulating a Yang monopole with an ultracold gas of rubidium atoms. The result, which provides another example of using cold quantum gases to simulate other areas of physics, was reported in the June 29 issue of Science.

Researchers playing with a cloud of ultracold atoms uncovered behavior that bears a striking resemblance to the universe in microcosm. Their work, which forges new connections between atomic physics and the sudden expansion of the early universe, was published April 19 in Physical Review X and featured in Physics. "From the atomic physics perspective, the experiment is beautifully described by existing theory," says Stephen Eckel, an atomic physicist at the National Institute of Standards and Technology (NIST) and the lead author of the new paper. "But even more striking is how that theory connects with cosmology." In several sets of experiments, Eckel and his colleagues rapidly expanded the size of a doughnut-shaped cloud of atoms, taking snapshots during the process. The growth happens so fast that the cloud is left humming, and a related hum may have appeared on cosmic scales during the rapid expansion of the early universe—an epoch that cosmologists refer to as the period of inflation. The work brought together experts in atomic physics and gravity, and the authors say it is a testament to the versatility of the Bose-Einstein condensate (BEC)—an ultracold cloud of atoms that can be described as a single quantum object—as a platform for testing ideas from other areas of physics. "Maybe this will one day inform future models of cosmology," Eckel says. "Or vice versa. Maybe there will be a model of cosmology that’s difficult to solve but that you could simulate using a cold atomic gas."

When your heart beats, blood courses through your veins in waves of pressure. These pressure waves manifest as your pulse, a regular rhythm unperturbed by the complex internal structure of the body. Scientists call such robust waves solitons, and in many ways they behave more like discrete particles than waves. Soliton theory may aid in the understanding of tsunamis, which—unlike other water waves—can sustain themselves over vast oceanic distances.Solitons can arise in the quantum world as well. At most temperatures, gas atoms bounce around like billiard balls, colliding with each other and rocketing off into random directions. Near absolute zero, however, certain kinds of atoms suddenly start behaving according to the very different rules of quantum mechanics, and begin a kind of coordinated dance. Under pristine conditions, solitons can emerge inside these ultracold quantum fluids, surviving for several seconds.Curious about how solitons behave in less than pristine conditions, scientists at NIST’s Physical Measurement Laboratory, in collaboration with researchers at the Joint Quantum Institute (JQI), have added some stress to a soliton’s life. They began by cooling down a cloud of rubidium atoms. Right before the gas became a homogenous quantum fluid, a radio-frequency magnetic field coaxed a handful of these atoms into retaining their classical, billiard ball-like state. Those atoms are, in effect, impurities in the atomic mix. The scientists then used laser light to push apart atoms in one region of the fluid, creating a solitary wave of low density—a “dark” soliton.

When is a traffic jam not a traffic jam? When it's a quantum traffic jam, of course. Only in quantum physics can traffic be standing still and moving at the same time. A new theoretical paper from scientists at the National Institute of Standards and Technology (NIST) and the University of Maryland suggests that intentionally creating just such a traffic jam out of a ring of several thousand ultracold atoms could enable precise measurements of motion. If implemented with the right experimental setup, the atoms could provide a measurement of gravity, possibly even at distances as short as 10 micrometers—about a tenth of a human hair's width.

Theoretical physicists studying the behavior of ultra-cold atoms have discovered a new source of friction, dispensing with a century-old paradox in the process. Their prediction, which experimenters may soon try to verify, was reported recently in Physical Review Letters.The friction afflicts certain arrangements of atoms in a Bose-Einstein Condensate (BEC), a quantum state of matter in which the atoms behave in lockstep. In this state, well-tuned magnetic fields can cause the atoms to attract one another and even bunch together, forming a single composite particle known as a soliton.

The idea of a pump is at least as old as the ancient Greek philosopher and scientist Archimedes. More than 2000 years ago, Archimedes allegedly invented a corkscrew pump that could lift water up an incline with the turn of a handle. Versions of the ancient invention still bear his name and are used today in agriculture and industry.Modern pumps have achieved loftier feats. For instance, in the late 1990s, NIST developed a device that could pump individual electrons, part of a potential new standard for measuring capacitance.While pumps can be operated mechanically, electrically or via any other source of energy, they all share the common feature of being driven by a periodic action. In the Archimedean pump, that action is a full rotation of the handle, which draws up a certain volume of water. For the NIST electron pump, it is a repeating pattern of voltage signals, which causes electrons to hop one at a time between metallic islands.But physicists have sought for decades to build a different kind of pump—one driven by the same kind of periodic action but made possible only by the bizarre rules of quantum mechanics. Owing to their physics, these pumps would be immune to certain imperfections in their fabrication.Now, a team of physicists working in collaboration with JQI Fellow Ian Spielman and NIST postdoctoral researcher Hsin-I Lu has created just such a pump. By periodically jostling many individual atoms, the researchers were able to shift an entire atomic cloud without any apparent overall motion by its constituents. The team is the first to test this predicted behavior, which arises in what they call a geometric charge pump. The work follows close on the heels of two recent papers that examined topological charge pumps, which demonstrate a distinct but related effect. The new result was published May 20 in Physical Review Letters.

The swirling field of a magnet—rendered visible by a sprinkling of iron filings—emerges from the microscopic behavior of atoms and their electrons. In permanent magnets, neighboring atoms align and lock into place to create inseparable north and south poles. For other materials, magnetism can be induced by a field strong enough to coax atoms into alignment.In both cases, atoms are typically arranged in the rigid structure of a solid, glued into a grid and prevented from moving. But the team of JQI Fellow Ian Spielman has been studying the magnetic properties of systems whose tiny constituents are free to roam around—a phenomenon called “itinerant magnetism." “When we think of magnets, we usually think of some lattice,” says graduate student Ana Valdés-Curiel. Now, in a new experiment, Valdés-Curiel and her colleagues have seen the signatures of itinerant magnetism arise in a cold cloud of rubidium atoms.

Particles can be classified as bosons or fermions. A defining characteristic of a boson is its ability to pile into a single quantum state with other bosons. Fermions are not allowed to do this. One broad impact of fermionic anti-social behavior is that it allows for carbon-based life forms, like us, to exist. If the universe were solely made from bosons, life would certainly not look like it does. Recently, JQI theorists* have proposed an elegant method for achieving transmutation--that is, making bosons act like fermions. This work was published in the journal Physical Review Letters.This transmutation is an example of emergent behavior, specifically what’s known as quasiparticle excitations—one of the concepts that make condensed matter systems so interesting. Particles by themselves have mostly well-defined characteristics, but en masse, can work together such that completely distinctive, even exotic phenomena appear. Typically collective behaviors are difficult to study because the large numbers of real particles and all of their interactions are computationally challenging and in many cases prohibitive.

For many years rubidium has been a workhorse in the investigation of ultracold atoms.  Now JQI scientists are using Rb to cool another species, ytterbium, an element prized for its possible use in advanced optical clocks and in studying basic quantum phenomena.   Yb shows itself useful in another way: it comes in numerous available isotopes, some of which are bosonic in nature and some fermionic.
Yb-171 has proven satisfactorily amenable to cooling in the atom trap lab of Steve Rolston and Trey Porto.  First Rb-87 atoms are loaded into a magneto-optic trap---an enclosure where magnetic fields and laser beams are used to confine atoms---and then cooled until they form a Bose-Einstein condensate (BEC).  Slow-moving Yb atoms, in contact with the Rb atoms, are cooled right along with them.  Thus Yb atoms lose excess energy to warming the colder Rb atoms.

From NIST-PML--JQI scientists have achieved a major milestone in simulating the dynamics of condensed-matter systems – such as the behavior of charged particles in semiconductors and other materials – through manipulation of carefully controlled quantum-mechanical models.
Going beyond their pioneering experiments in 2009 (the creation of “artificial magnetism”), the team has created a model system in which electrically neutral atoms are coaxed into performing just as electrons arrayed in a two-dimensional sheet do when they are exposed to a strong magnetic field.
The scientists then showed for the first time that it is possible to tune the model system such that the atoms (acting as electron surrogates) replicate the signature “edge state” behavior of real electrons in the quantum Hall effect (QHE), a phenomenon which forms the basis for the international standard of electrical resistance.* The researchers report their work in the 25 September issue of the journal Science.