Optically Trapped Fermi Gases A few hundred thousand atoms, chilled to near absolute zero, mimic the physics of other extreme systems, including neutron stars and superconductors John E. Thomas and Michael E. Gehm he bowl is only a millimeter long trons inside an ordinary metal like cop- balloons by using a form of quantum Tand a tenth of a millimeter wide, no per. Even though the metal is solid, the trickery known as “strong interac- bigger than a piece of lint. Its walls are electrons behave very much like a gas: tions” to expand the balloons greatly constructed of pure light, making this They are free to roam around (which in size. These interactions make the “optical bowl” an appropriately ethere- makes the metal conduct electricity), atoms affect one another at a much al container for the stuff sloshing but they have to fit into a strict energy greater distance than they ordinarily around inside: lithium atoms that have hierarchy—the same one found in the would. There are exciting indications, been chilled to less than a millionth of a degenerate Fermi gases we produced. not yet confirmed, that the strong in- degree above absolute zero. But you These gases are first cousins to an- teractions cause the atoms to form shouldn’t think of the sample trapped other strange quantum beast that ap- loose alliances called Cooper pairs. Su- in this vessel of light as just a cloud of pears at ultracold temperatures, called perconductivity and some forms of su- lithium (any more than you’d think of a a Bose-Einstein condensate. The re- perfluidity are the result of Cooper diamond as a mere lump of carbon). Its search group of Eric Cornell and Carl pairing. But before we describe some value lies not in the atoms that make it Wieman at the University of Colorado of the remarkable properties of the re- up but in the special way they are put at Boulder fashioned the first such con- sultant gas, let us take a moment to ex- together—in a remarkable configura- densate in 1995. (In 2001, Cornell and plain the significant technical hurdles tion known as a degenerate Fermi gas. Wieman shared a Nobel prize for their our group and others had to overcome. Such an assemblage constitutes a new work with Wolfgang Ketterle of the state of matter and is possibly the clos- Massachusetts Institute of Technology.) Chilling Out with Lasers est that scientists will ever come to hav- An atomic degenerate Fermi gas is The possibility of creating macroscopic ing on their desktops a neutron star or a even trickier to create, because it pits quantum systems, such as degenerate piece of the quark matter that made up two precepts of quantum mechanics Fermi gases and Bose-Einstein conden- the early universe. against each other. On the one hand is sates, has come about largely because Although physicists had predicted Heisenberg’s famous uncertainty prin- of improvements in the technology of the existence of degenerate Fermi gases ciple, which says that the location of any optical cooling. In most experiments as long ago as the 1930s, nobody had particle becomes more ambiguous as its with ultracold gases, magnetic forces produced a fully independent one in speed becomes less uncertain. In an ul- ensnare the atoms. By contrast, optical the lab until five years ago. The closest tracold gas, the speed of the atoms is bowls use electric forces, which have model we had was the cloud of elec- known with unusual precision: It is the advantage that they can corral any close to zero. Therefore the atoms get kind of atom, whereas magnetic traps smeared out into blobs that are tens of work only for certain types. John E. Thomas was recently named Fritz London Distinguished Professor of Physics at Duke Uni- thousands of times larger than a normal In the simplest case, an optical bowl versity and is a Fellow of the American Physical room-temperature atom. This blurring consists of an intense laser beam that is Society. He received his doctorate from the Massa- is no problem for a Bose-Einstein con- tightly focused into a high-vacuum re- chusetts Institute of Technology in 1979. His pri- densate, because it is made of “sociable” gion. The light draws cold atoms or mary area of research is quantum optics, and his atoms called bosons, which like to over- molecules toward its focal point and research team was the first, in 2001, to achieve all- lap. But degenerate Fermi gases are confines them in a frictionless, heat- optical trapping of degenerate Fermi gases. made from solitary atoms called fermi- free environment, which is ideal for Michael E. Gehm received his Ph.D. in physics in ons (like the lithium in our trap), which studies of fundamental phenomena. 2003, working in Thomas’s research group on Fer- according to Pauli’s exclusion principle Why would a focused beam of light mi gases. He is currently a postdoctoral fellow at cannot share space with their neighbors. attract atoms? The secret is that light is the Duke University Fitzpatrick Center for Pho- tonic and Communications Systems and at Duke’s As a result, making a degenerate Fermi an electromagnetic wave, consisting of Department of Electrical and Computer Engineer- gas is a lot like trying to pack balloons oscillating electric and magnetic fields. ing. Address for Thomas: Duke University De- into a closet. The electric field in a light beam exerts partment of Physics, Box 90305, Durham, NC Recently our group was able to a force on charged particles, such as the 27708-0305. Internet: [email protected] probe the quantum behavior of these electrons and protons inside an atom. © 2004 Sigma Xi, The Scientific Research Society. Reproduction 238 American Scientist, Volume 92 with permission only. Contact [email protected]. Figure 1. Experiments with ultracold Fermi gases help to illuminate the physics of various extreme systems, including the interior of neutron stars, such as the one left over from the supernova explosion that created the Crab Nebula (above). Neutron stars are prevented from collapsing into black holes by virtue of “Fermi pressure,” a consequence of the Pauli exclusion principle of quantum physics. (Image courtesy of the Eu- ropean Southern Observatory.) An atom, however, has an equal num- the beam, a positively charged nucleus attraction that is slightly stronger than ber of electrons and protons, and thus in an atom below the beam will be the repulsion the electron cloud feels, is electrically neutral. A force nonethe- pulled upward, and the negatively and so there is a net upward force on less arises, for a somewhat subtle rea- charged electron cloud will be pushed the atom. If the situation were reversed, son: the field gradient. downward. The nucleus, being closer with the electric field pointing away If, for example, the electric field to the focus of the beam where the elec- from the beam, the nucleus would points upward and toward the focus of tric field is larger, then experiences an move farther away, and the electron © 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2004 May–June 239 should be able to turn off the molasses after loading, and retain the atoms in the optical bowl for a very long time. Or so the theory goes. In practice, optical bowls did not work as expected until the late ‘90s, when our group fig- ured out what had been vexing them. The problem was that the atoms in an optical bowl slosh back and forth at a certain characteristic frequency: in our experiments, 6,600 times per second in the short direction and 230 times per second in the long direction. (The sec- ond frequency is lower because the at- traction to the center of the optical bowl is weaker in the axial direction.) The lasers then in use were not steady, and in particular they had an intensity Figure 2. Trapping neutral atoms in a cold, rarefied gas (orange cloud) can be accomplished using fluctuation at twice the frequency of an “optical bowl,” an intense laser (blue) that draws the atoms into the focus of the beam. The os- the atoms in the trap. cillating electric field of the laser causes the positively charged nucleus and the negatively To see the problem this causes, charged electrons of each atom to become slightly separated. Because the electric field (black ar- imagine an atom oscillating from side rows) is greatest near the focus, the force on the positive side of an atom is not completely bal- anced by the opposing force on the negative side. Below the beam, for example, the upward force to side, with the walls of the bowl vi- exerted on the top of an atom slightly exceeds the downward force on the bottom. Similarly, brating in and out. The atom gets a above the beam the downward force exerted on the bottom of an atom exceeds the upward push from the inward-moving side of force on the top. Thus in either case a net force arises (purple arrows), pulling the atoms inward. the bowl. By the time this atom reaches the other side (after one half of its peri- cloud would move closer. The net force is the second part of our three-step pro- od of oscillation), it encounters the op- would once again be toward the focus. tocol. First, we cool several hundred posite wall of the bowl, which by this For reasons that are too complicated to million lithium atoms down to about time is again moving inward.