Joint Quantum Institute Creates Synthetic Magnetic Fields by Tricking Neutral Atoms with Lasers

Joint Quantum Institute Synthetic magnetic fields:  A pair of laser beams (red arrows) impinges upon an ultracold gas cloud of rubidum atoms (green oval) to create synthetic magnetic fields (labeled Beff). (Inset) The beams, combined with an external magnetic field (not shown) cause the atoms to "feel" a rotational force; the swirling atoms create vortices in the gas.

 Source: JQI

Synthetic Magnetic Fields:  Vortices Description: A harbinger of the synthetic magnetic fields is the formation of vortices (spots). These spots, the number of which increases with increasing synthetic field, mark the points about which atoms swirled with a whirlpool-like motion. The measurement units in each panel indicate the size of the external magnetic field gradient applied to the gas of atoms, with larger external fields producing more vortices.

 Source: JQI

Achieving an important new capability in ultracold atomic gases, researchers at the Joint Quantum Institute, a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland, have created “synthetic” magnetic fields for ultracold gas atoms, in effect “tricking” neutral atoms into acting as if they are electrically charged particles subjected to a real magnetic field.

The demonstration, described in the latest issue of the journal Nature, not only paves the way for exploring the complex natural phenomena involving charged particles in magnetic fields, but may also contribute to an exotic new form of quantum computing.

As researchers have become increasingly proficient at creating and manipulating gaseous collections of atoms near absolute zero, these ultracold gases have become ideal laboratories for studying the complex behavior of material systems. Unlike usual crystalline materials, they are free of obfuscating properties, such as impurity atoms, that exist in normal solids and liquids.

However, studying the effects of magnetic fields is problematic because the gases are made of neutral atoms and so do not respond to magnetic fields in the same way as charged particles do. So how would you simulate, for example, such important exotic phenomena as the quantum Hall effect, in which electrons can “divide” into quasiparticles carrying only a fraction of the electron’s electric charge?

The answer Ian Spielman and his colleagues came up with is a clever physical trick to make the neutral atoms behave in a way that is mathematically identical to how charged particles move in a magnetic field.

A pair of laser beams illuminates an ultracold gas of rubidium atoms already in a collective state known as a Bose-Einstein condensate. The laser light ties the atoms' internal energy to their external (kinetic) energy, modifying the relationship between their energy and momentum. Simultaneously, the researchers expose the atoms to a real magnetic field that varies along a single direction, so that the alteration also varies along that direction.

In a strange inversion, the laser-illuminated neutral atoms react to the varying magnetic field in a way that is mathematically equivalent to the way a charged particle responds to a uniform magnetic field. The neutral atoms experience a force in a direction perpendicular to both their direction of motion and the direction of the magnetic field gradient in the trap. By fooling the atoms in this fashion, the researchers created vortices in which the atoms swirl in whirlpool-like motions in the gas clouds. The vortices are the “smoking gun,” Spielman says, for the presence of synthetic magnetic fields.

Previously, other researchers had physically spun gases of ultracold atoms to simulate the effects of magnetic fields, but rotating gases are unstable and tend to lose atoms at the highest rotation rates. In their next step, the JQI researchers plan to partition a nearly spherical system of 20,000 rubidium atoms into a stack of about 100 two-dimensional “pancakes” and increase their currently observed 12 vortices to about 200 per-pancake.

At a one-vortex-per-atom ratio, they could observe the quantum Hall effect and control it in unprecedented ways. In turn, they hope to coax atoms to behave like a class of quasiparticles known as “non-abelian anyons,” a required component of “topological quantum computing,” in which anyons dancing in the gas would perform logical operations based on the laws of quantum mechanics.

* Y.J. Lin, R.L. Compton, K. Jimenez-Garcia, J.V. Porto and I.B. Spielman. Synthetic magnetic fields for ultracold neutral atoms. Nature, Dec. 3, 2009.

Source: NIST, Media Contact: Ben Stein, bstein@nist.gov, (301) 975-3097

Scientists See Single Atoms in a Crystalline Structure Made Solely of Light with Quantum Gas Microscope

Image of individual super cold rubidium atoms in an optical lattice. 

Image credit:  Markus Greiner/Department of Physics, Harvard University.

 Harvard physicists created a quantum gas microscope that can be used to observe single atoms at temperatures so low the particles follow the rules of quantum mechanics. The quantum gas microscope developed by Greiner and his colleagues is a high-resolution device capable of viewing single atoms -- in this case, atoms of rubidium -- occupying individual, closely spaced lattice sites. The rubidium atoms are cooled to just 5 billionths of a degree above absolute zero (-273 degrees Celsius).

The work, published in the journal Nature, represents the first time scientists have detected single atoms in a crystalline structure made solely of light, called a Bose Hubbard optical lattice. It's part of scientists' efforts to use ultracold quantum gases to understand and develop novel quantum materials.

"Ultracold atoms in optical lattices can be used as a model to help understand the physics behind superconductivity or quantum magnetism, for example," says senior author Markus Greiner, an assistant professor of physics at Harvard and an affiliate of the Harvard-MIT Center for Ultracold Atoms. "We expect that our technique, which bridges the gap between earlier microscopic and macroscopic approaches to the study of quantum systems, will help in quantum simulations of condensed matter systems, and also find applications in quantum information processing."

"At such low temperatures, atoms follow the rules of quantum mechanics, causing them to behave in very unexpected ways," explains first author Waseem S. Bakr, a graduate student in Harvard's Department of Physics. "Quantum mechanics allows atoms to quickly tunnel around within the lattice, move around with no resistance, and even be 'delocalized' over the entire lattice. With our microscope we can individually observe tens of thousands of atoms working together to perform these amazing feats."

In their paper, Bakr, Greiner, and colleagues present images of single rubidium atoms confined to an optical lattice created through projections of a laser-generated holographic pattern. The neighboring rubidium atoms are just 640 nanometers apart, allowing them to quickly tunnel their way through the lattice.

Confining a quantum gas -- such as a Bose-Einstein condensate -- in such an optically generated lattice creates a system that can be used to model complex phenomena in condensed-matter physics, such as superfluidity. Until now, only the bulk properties of such systems could be studied, but the new microscope's ability to detect arrays of thousands of single atoms gives scientists what amounts to a new workshop for tinkering with the fundamental properties of matter, making it possible to study these simulated systems in much more detail, and possibly also forming the basis of a single-site readout system for quantum computation.

"There are many unsolved questions regarding quantum materials, such as high-temperature superconductors that lose all electrical resistance if they are cooled to moderate temperatures," Greiner says. "We hope this ultracold atom model system can provide answers to some of these important questions, paving the way for creating novel quantum materials with as-yet unknown properties."

Greiner's co-authors on the Nature paper are Waseem S. Bakr, Jonathon I. Gillen, Amy Peng, and Simon Foelling, all of Harvard's Department of Physics and the Harvard-MIT Center for Ultracold Atoms. Their work was supported by the National Science Foundation, the Air Force Office of Scientific Research, the Army Research Office, the Defense Advanced Research Projects Agency , and the Alfred P. Sloan Foundation.