Atomic & Molecular Physics

JILA continues its trendsetting research into Bose-Einstein and fermionic condensation, seeking insights into superfluidity, superconductivity, quantum behavior control, the role of quantum processes in our everyday world, and the development of atom lasers. Current cold-atom research focuses on atom-chip technology, atom/surface interactions, Feshbach resonances that allow for the creation of Bose-Einstein condensates (BECs), and atoms in optical lattices (formed when laser cooling causes atoms to become trapped in egg-cartonlike potential wells that resemble a crystalline lattice). For more than a decade, JILA's ultracold research has been characterized by a high degree of collaboration between experimentalists and theorists.

The Institute's preeminence in ultracold matter was recognized in 1995 with the creation of the world's first BEC by Carl Wieman and Eric Cornell. Wieman and Cornell won the 2001 Nobel Prize in physics for their discovery, which verified a 1924 prediction by Albert Einstein that a condensate, or "superatom," would form at extremely cold temperatures when the wave functions of individual atoms began to overlap and behave identically. Einstein's prediction was an extension of Satyendra Nath Bose's development of the quantum statistics of photons.

Building upon this discovery, theorist Murray Holland and his group developed a proposal that pointed out a cross-over relationship between Bose-Einstein condensation and superconductivity (where pairs of electrons flow with no resistance). The proposal also highlighted the intriguing nature of ultracold Fermi gases that are also superfluids. Superfluid Fermi gases were predicted to form under special conditions (known as Feshbach resonances) where atoms switch from being singular entities to becoming part of exotic, weakly bound molecules.

Inspired by this proposal and the Wieman/Cornell BEC experiment, Deborah Jin and her group achieved the world's first "fermionic condensate" in 2003. Because fermions, such as protons, neutrons, and electrons, cannot occupy exactly the same quantum state, creating a fermionic condensate was even more technically challenging than producing a BEC. The laws of quantum mechanics allow bosons, such as photons and atoms or molecules with integer spins, to occupy the same quantum state, which is a prerequisite for forming a condensate. In contrast, the laws of quantum mechanics don't allow fermions to occupy the same quantum state. To get a Fermi gas of potassium atoms to form a condensate, Jin first had to use a magnetic field to get the atoms to form correlated, or entangled, pairs, which can then act like bosons. Once the atom pairs formed, they became a condensate.

In early 2005, the Jin group found visual evidence of correlated ultracold atoms in the noise patterns present in images of an ultracold potassium cloud. After splitting ultracold molecules into entangled pairs of atoms flying apart in opposite directions, they used a laser beam to create a shadow image of the cloud. They discovered pairs of correlated atoms in the noise patterns of the pictures. Jin's group anticipates using similar noisy pictures to look for entangled atom pairs in fermionic condensates. (Entanglement is an important, albeit counterintuitive, prediction of quantum physics. When two atoms become entangled, the properties of either one of them instantaneously affect the properties of the other partner, even when the two are far apart. Einstein called this phenomenon "spooky action at a distance.")

JILA's experiments with Bose-Einstein and fermionic condensates helped launch research into atoms that act like waves rather than particles and opened the door to the atom-chip technology and the development of atom lasers. Current work is expected to offer new insights into superfluidity and superconductivity, the exquisite control of quantum processes, and the role of quantum processes in the macroscopic world.

Experimental Studies

Eric Cornell, Deborah Jin, and Carl Wieman head three experimental groups investigating Bose-Einstein condensates. Dana Anderson and Jun Ye contribute their unique expertise in atom optics, ultrafast optics, and precision measurement to the effort to define and understand this exotic form of matter. John Bohn, Chris Greene, and Murray Holland contribute theoretical analyses that help explain experimental findings and guide future investigations.

Eric Cornell's group has three major research studies underway: (1) an investigation of the peculiar properties of a rotating BEC, (2) an examination of the interactions between BECs and ordinary matter, and (3) the development of an ultrasensitive atom interferometer, or inertial sensor. Because rotating BECs are quantum mechanical superfluids, Cornell's team is using them to examine some fundamental laws of nature. Team members have shown that rotating condensates develop quantum mechanical tornadoes (vortices) that expel atoms from their cores. The faster a condensate rotates, the more vortices appear. The vortices appear in regular, hexagonal lattices similar to the structure of ordinary crystals. Similar structures appear in the center of neutron stars and as magnetic flux lines in some superconductors. Because of these similarities, Cornell's vortex experiments could one day become model systems not only for studying atomic physics, but also for investigating condensed-matter physics and quantum optics.

The team is currently working on installing a rotating, two-dimensional optical lattice in a BEC machine to better understand ultracold physics in a confined environment. Cornell wants to study competition, frustration, and multistability in bulk superfluids with vortices; the impact of rotation on noncoherent many-body wave functions; and circumstances in which the effects of a rotating lattice on a BEC are similar to those of a magnetic field.

A second research team is investigating the interactions between BECs and ordinary matter. When surfaces made of glass, copper, or silicon come into contact with a BEC, the BEC may heat up or eject atoms; some of its atoms may even lose their quantum properties. The team is also studying a mysterious attractive force, called a Casimir-Polder force, which acts between ordinary matter and a BEC across a vacuum. In essence, the force is the change in the quantum behavior of free space.

To better understand this force, the group created a BEC inside a magnetic trap, whose shape (where the condensate forms) resembles a cereal bowl. Then, they moved the BEC in a bowl closer and closer to a glass surface until distortions in the shape of the bowl appeared. The distortions caused measurable changes in the oscillation frequency of the BEC. The researchers were able to measure the changes due to the Casimir-Polder force at a distance of about 5 µm from the surface. They were able to move the BEC in a bowl close enough to the glass surface to rule out the involvement of exotic forces that do not obey the laws of Newtonian gravity. More recently, the group identified a temperature dependence of the force. The characterization of the Casimir-Polder force will be important for developing atom chips and nanotechnologies.

Cornell collaborates with Dana Anderson on developing an atom interferometer. Their proof-of-concept experiment is currently a very large modular system. It is an analog of the laser-based optical gyroscope used in airplane navigational systems. The atom interferometer replaces a laser with coherent wave packets created and maintained in a BEC. Once it is perfected and scaled down to usable size, the atom interferometer should be three to four orders of magnitude more sensitive than its optical counterpart. Other practical devices under development in Anderson's lab include atom waveguides (pipes that can trap and transport atoms), atom optics, and smaller, simpler BEC systems for atom chips. Anderson recently began to explore "atomtronics," which are cold-atom analogs of semiconductors, diode junctions, and transistors.

Deborah Jin's group investigates the connection between BEC and superconductivity. Because two decades of theoretical analysis suggest that the two phenomena are connected, Jin believes it should be possible to smoothly transition between one and the other. Her team members are studying the crossover region between BEC and superconductivity to determine what factors lead to the highest transition temperatures. In conjunction with these efforts, Murray Holland's group is working on developing a new theory to explain the quantum mechanical behavior of fermions in the crossover region.

Recently, Jin's group investigated the velocity spread of potassium atoms throughout the continuum from BEC to superconductivity. Starting at the latter end, the researchers gradually decreased the magnetic field while monitoring changes in the velocity spread of the ultracold atoms. As they approached the crossover region, the velocity spread showed that atom pairs were not only getting much smaller, but that more pairs were forming. This experiment has provided enough information for Holland's group to evaluate the strengths and weaknesses of different crossover theories and develop a better understanding of crossover physics. This work could help determine the quantum mechanical limits of designing high-temperature superconductors, whose properties closely resemble those of fermions in the crossover region.

Jin is part of powerful JILA collaboration developing magnetic-field Feshbach resonances as a powerful tool for studying quantum gases. Together with Wieman and Cornell and theorists Chris Greene, Murray Holland, and John Bohn, she is exploring Feshbach resonance physics in both BECs and Fermi gases. One focus will be on advancing techniques for controlled and efficient molecule creation. Recently Wieman, Jin, and Cornell conducted a detailed study of ultracold molecule formation with magnetic field sweeps across a Feshbach resonance. In the future, this method will be used in the characterization of cold molecules with high-precision spectroscopy, in the formation of molecular condensates, and in the study of optical lattice traps.

Jin and Holland recently initiated a study of optical lattice traps combined with Feshbach resonances. They will explore new strategies for loading cold atoms into an optical lattice. Following this, Jin will use Feshbach resonances to study molecule formation in both two-dimensional and three-dimensional optical lattices. Jin and Holland anticipate that confinement of ultracold atoms in the lattice will enhance the efficiency of molecule production. Eventually, the researchers want to study the characteristics of a Bose-Fermi mixture in an optical lattice. They also plan to apply a broader understanding of Feshbach resonance physics to the investigation of antiferromagnetism and superfluidity.

Carl Wieman's group investigates the use of magnetic fields to control the atomic interactions in condensates of ⁸⁵Rb isotopes. This technique creates molecules that differ significantly from molecules formed in ordinary chemical reactions. The two ⁸⁵Rb atoms in an ultracold rubidium molecule are very far apart (by molecular standards), very weakly bound, and have a binding energy that corresponds to their highest vibrational state. The researchers are studying the formation processes for such molecules under a range of conditions and mixtures of different atomic species. They are also collaborating with Deborah Jin to see how mixing bosons and fermions changes the behavior of condensates.

Jun Ye adds a unique perspective to JILA's ultracold matter research with his study of the novel cooling dynamics of alkaline earth atoms. This research is guiding the development of more precise atomic clocks and the evolution of ultrafast precision lasers. Precisely controlled femtosecond lasers are making it possible to study atomic structure and behavior at the quantum level. The team currently focuses on a unique feature of atomic strontium that provides a resonance quality factor greater than 10¹⁷. The researchers recently demonstrated novel forms of laser cooling dynamics and precision measurements on optical transition frequencies and collisional dynamics of Sr atoms. Ye is also working with theorist Greene to explore ultracold Sr atoms in a carefully engineered optical lattice. This system will permit studies of degenerate Fermi or Bose gases and investigations of atomic behavior.

Theoretical Studies

JILA's theoretical physicists provide crucial insights to experimental investigations of ultracold atoms. John Bohn and his group work closely with Deborah Jin, Jun Ye, and Heather Lewandowski to provide theoretical explanations of how ultracold atoms and molecules collide and approximations of the behavior of untested ultracold atoms and molecules. Bohn's analyses have generated possible mechanisms for controlling ultracold matter, predictions for controlling ultracold chemical reactions, and suggestions for ways to increase the stability of BECs and Fermi gases. The group recently identified the most suitable molecules for experiments being designed in Eric Cornell's lab to detect the electric dipole moment of an electron.

In 2005, Bohn and his group performed a theoretical analysis of ultracold spectroscopy and found that it looked promising for elucidating electron behavior during molecular collisions. The study showed that whatever happens during ultracold molecular collisions gets imprinted on the collision spectra. This happens because the strength of the collisions depends on the electric field. The challenge now is to determine what the resulting spectra mean in terms of what is actually occurring quantum mechanically between the colliding molecules.

Chris Greene and his group study the quantum mechanical behavior of atomic and molecular systems. Research projects include ultracold collisions between two or three atoms at sub-micro Kelvin temperatures, calculations of various atomic properties of ultracold Sr atoms, the (many-body) dynamics of (hundreds to hundreds of thousands of) atoms or molecules in a BEC, novel laser-cooling schemes for creating BECs, conditions that favor molecule formation in a BEC, and the properties of unusually large, weakly bound molecules that form in BECs.

The group recently completed a detailed calculation of atom-light interactions, which they used to analyze strontium and magnesium atoms completely quantum mechanically in three dimensions. The group simulated atom dynamics with a Monte Carlo simulation, which uses random numbers to produce approximate solutions to problems that are otherwise too complex to solve. The theory project was inspired by a laser-cooling experiment in Jun Ye's lab in which more cooling of strontium atoms was observed than was thought possible at the time. The new analysis revealed a complex interplay between the number of atomic excited states, their spacing, and the laser cooling. Most importantly, the analysis explained the results found in Ye's strontium cooling experiment.

In 2000 and 2002, Greene and his collaborators predicted the existence of a bizarre class of ultralong-range Rydberg molecules in BECs. These molecules are characterized by high electronic excitations, but their truly unusual property is their extraordinary size, which is 50-500 times larger than garden-variety molecules such as nitrogen or oxygen. A defining characteristic of the Rydberg molecules is their quantum mechanical probability density, which exhibits a striking resemblance to the trilobite that ruled Earth's seas 300 million years ago. Experimental searches for these molecules are underway at laboratories around the world.

In 2006, Greene saw experimental confirmation of predictions he and his collaborators made in the late 1990s addressing both the characteristics of, and possible ways to observe, Efimov states in three-body recombination. These states, originally predicted by Vitaly Efimov in 1970, derive from a bizarre quantum-mechanical effect occurring in three-particle systems. Under certain circumstances, the three particles can produce an infinite number of bound levels even though no two of them are capable of forming a stable pair.

Murray Holland conducts theoretical studies of Bose-Einstein condensation and quantum Fermi gases. He has shown that the theory of Bose-Einstein condensation is straightforward under conditions where atomic interactions are weak, but it fails rapidly as interactions strengthen. BECs with strongly interacting atoms resemble a variety of intriguing superfluids whose theoretical descriptions are complex and interesting. His current research includes (1) electron resistance in strong magnetic fields at very low temperatures; (2) Fermi gas superfluidity; (3) Feshbach resonances in optical lattices; (4) resonances in systems with two or fewer dimensions, where the quantum behavior of ultracold atoms and atomic particles such as electrons in zero, one, or two dimensions is not yet well understood; and (5) atomtronics.

Recently, Holland's group investigated whether a Bose-Einstein condensate could spontaneously escape from the trap in which it was created. The analysis explored how a many-body wave function like a BEC could theoretically tunnel out of a potential well (in a manner analogous to the escape of an atom or electron from a similar trap). The group discovered that quantum tunneling in a BEC should be observable on time scales of 10 ms to 10 s. However, even though BECs can tunnel through a potential barrier, the end result is different from that of single electrons, where the electron gets completely out of the potential well. With a BEC, three final states are possible: (1) All atoms in the BEC escape from the potential well; (2) Some atoms escape via tunneling, while others remain in the well; or (3) The entire BEC jumps over the top of the well and runs down the outside as a single ring soliton before breaking up into smaller bullets known as bright vortex solitons. The formation of the soliton(s) is high speed and violent, but entirely classical.