New microscope traps and images 1000 laser-cooled fermions

May 17, 2015
A team of MIT physicists has built a microscope that is able to see up to 1000 individual fermionic atoms.

A team of Massachusetts Institute of Technology (MIT; Cambridge, MA) physicists has built a microscope that is able to see up to 1000 individual fermionic atoms. The researchers devised a laser-based technique to trap and freeze fermions in place, and image the particles simultaneously.

Examples of fermions are electrons, protons, neutrons, quarks, and atoms consisting of an odd number of these elementary particlesthe building blocks of matter, interacting in a multitude of permutations to give rise to the elements of the periodic table. Because of their fermionic nature, electrons and nuclear matter are difficult to understand theoretically, so researchers try to use ultracold gases of fermionic atoms as stand-ins for other fermions. But imaging individual fermions is nearly impossible because they are extremely sensitive to light; when a single photon hits an atom, it can knock the particle out of place.

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To circumvent these previous difficulties, the new imaging technique uses two laser beams trained on a cloud of fermionic atoms in an optical lattice. The two beams, each of a different wavelength, cool the cloud, causing individual fermions to drop down an energy level, eventually bringing them to their lowest energy statescool and stable enough to stay in place. At the same time, each fermion releases light, which is captured by the microscope and used to image the fermion's exact position in the lattice to an accuracy better than the wavelength of light.

With the new technique, the researchers are able to cool and image over 95% of the fermionic atoms making up a cloud of potassium gas. Martin Zwierlein, a professor of physics at MIT, says an intriguing result from the technique appears to be that it can keep fermions cold even after imaging.

"That means I know where they are, and I can maybe move them around with a little tweezer to any location, and arrange them in any pattern I'd like," Zwierlein says. The findings are published in Physical Review Letters.

For the past two decades, experimental physicists have studied ultracold atomic gases of the two classes of particles: fermions and bosons--particles such as photons that, unlike fermions, can occupy the same quantum state in limitless numbers. In 2010, a boson microscope developed by Immanuel Bloch’s group at the Max Planck Institute of Quantum Optics, revealed the behavior of bosons under strong interactions. However, no one had yet developed a comparable microscope for fermionic atoms.

Techniques to cool atoms ever closer to absolute zero have been devised in recent decades. Carl Wieman, Eric Cornell, and MIT's Wolfgang Ketterle were able to achieve Bose-Einstein condensation in 1995, a milestone for which they were awarded the 2001 Nobel Prize in physics. Other techniques include a process using lasers to cool atoms from 300 degrees Celsius to a few ten-thousandths of a degree above absolute zero.

And yet, to see individual fermionic atoms, the particles need to be cooled further still. To do this, Zwierlein’s group created an optical lattice using laser beams, forming a structure resembling an egg carton, each well of which could potentially trap a single fermion. Through various stages of laser cooling, magnetic trapping, and further evaporative cooling of the gas, the atoms were prepared at temperatures just above absolute zerocold enough for individual fermions to settle onto the underlying optical lattice.

His team decided to use a two-laser approach to further cool the atoms; the technique manipulates an atom’s particular energy level, or vibrational energy. The team shone two laser beams of differing frequencies at the lattice. The difference in frequencies corresponded to the energy between a fermion’s energy levels. As a result, when both beams were directed at a fermion, the particle would absorb the smaller frequency, and emit a photon from the larger-frequency beam, in turn dropping one energy level to a cooler, more inert state. The lens above the lattice collects the emitted photon, recording its precise position, and that of the fermion.

"The Fermi gas microscope, together with the ability to position atoms at will, might be an important step toward the realization of a quantum computer based on fermions," Zwierlein says. "One would thus harness the power of the very same intricate quantum rules that so far hamper our understanding of electronic systems."

Zwierlein says it is a good time for Fermi gas microscopists: Around the same time his group first reported its results, teams from Harvard and the University of Strathclyde in Glasgow also reported imaging individual fermionic atoms in optical lattices, indicating a promising future for such microscopes.

This research was funded in part by the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research, the Army Research Office, and the David and Lucile Packard Foundation.


About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.

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