Molecules cooled to ultralow temperatures provide fundamental new insights to molecular interaction dynamics in the quantum regime. In recent years, researchers from various scientific disciplines such as atomic, optical, and condensed matter physics, physical chemistry, and quantum science have started working together to explore many emergent research topics related to cold molecules, including cold chemistry, strongly correlated quantum systems, novel quantum phases, and precision measurement. The exceedingly low energy regimes for ultracold molecules are a completely new playground for chemical physics where quantum behavior will emerge as dominant mechanism for molecular interaction and dynamics. Unique and complex molecular energy structure provides new opportunities for sensive probe of fundamental physics. The anisotropic and long-range dipolar interactions add new ingredients to strongly correlated and collective quantum dynamics in many-body systems.
Complete control of molecular interactions by producing a molecular gas at very low entropy and near absolute zero has long been hindered by their complex energy level structure. Our lab has recently developed a number of technical tools to laser cool and magneto-optically trap polar molecules, as well as to cool molecules via evaporation. Another recent experiment, in collaboration with Debbie Jin, has brought polar molecules into the quantum regime, in which ultracold molecular collisions and chemical reactions must be described fully quantum mechanically. We control chemical reaction via quantum statistics of the molecules, along with their long-range and anisotropic dipolar interactions. Further, molecules can be confined in reduced spatial dimensions and their interactions are precisely manipulated via external electric fields. Those efforts have started to yield observations on strongly interacting and collective quantum effects in an ultracold gas of molecules.
Our group is investigating several different species of cold and ultracold molecules (including KRb, OH, YO, ND3 and H2CO) with scientific goals including: (1) studying quantum physics and many-body systems with dipolar molecules, (2) performing precision measurement to test fundamental physics, and (3) controlling ultracold chemical reactions at the quantum level. We have developed new technologies for both direct cooling and trapping of ground-state molecules and coherent magneto-photo association of ultracold bi-alkali molecules such as KRb.
In 2008, the Jun Ye and Deborah Jin groups crafted an entirely new form of matter consisting of tens of thousands of ultracold potassium-rubidium (KRb) molecules in a low energy state in which both vibrations and rotations were as low as allowed by the laws of quantum mechanics. The creation of these novel molecules has made possible unprecedented explorations of the quantum world and launched the new field of ultracold chemistry.
The Jin and Ye groups have learned to manipulate, observe, and control the ultralow-temperature KRb molecules. In 2009, the Jin/Ye team was able to create KRb molecules in their lowest quantum mechanical state, which included being in the lowest of 36 possible nuclear spin states. Once the molecules were in this lowest (or ground) state, the experimentalists were able to observe the molecules colliding as well as breaking and forming chemical bonds. At ultralow temperatures KRb molecules manifest mostly as quantum mechanical waves, rather than as ordinary particles. These waves extend long distances inside the gaseous cloud of molecules, and chemical reactions are governed by the laws of quantum mechanics. How this plays out depends on whether the particles are bosons or fermions. Fermions like KRb molecules cannot occupy the same lowest energy state (or place). So when these standoffish molecules approach each other, they can only get so close before they start circling around each other. The amazing thing is, some pairs of KRb molecules still manage to slowly form new chemical bonds!
While the KRb molecules are dancing around each other, they may quantum mechanically tunnel through the barrier between them and undergo a chemical reaction. Another way to get the KRb molecules to chemically react is to prepare them with different nuclear spin states. Once they are no longer identical, they can collide head on, breaking and forming chemical bonds. The reaction rates for these reactions are 10–100 times faster than expected because of the wave nature of the ultracold molecules.
The team also tried a strategy for increasing the reaction rates of the KRb molecules: applying a modest electric field. In the electric field, the original p-wave barrier for the fermionic molecules is strongly modified, allowing the two-body reaction rate to rise very steeply as a function of the E field. If our goal is to cool the gas of KRb molecules down to the temperature where all their quantum states occupy the lowest possible energy levels — a state known as quantum degeneracy, then we have to reduce the rate for this lossy collisions. Following suggestions by theorists, our team squeezed the KRb molecules into a two-dimensional pancake trap. The trap forced the molecules to line up side by side with identical ends of the molecules next to each other. This configuration mostly prevents the molecules from aligning head-to-tail, which enhances chemical reactions because opposite ends of dipoles attract one another. We also made sure the KRb molecules were in the same quantum state. Then they turned on an electric field to increase the repulsion between the side-to-side molecules. The whole process has allowed the molecular reaction rate to remain modestly low even under large electric fields.
In the latest work, we can produce and trap ultralow temperature (< 100 nK) KRb molecules in a 3-D optical lattice. The lifetime of these molecules, now individually trapped in separate harmonic potentials, now extends to beyond 20 s. We can also precisely control the polarizability of individual molecular states inside the lattice, permitting highly coherent manipulation of quantum states for the trapped molecules. We start to observe exotic quantum behaviors of a dipolar quantum gas loaded in an optical lattice. We are excited to explore an array of interesting quantum dynamics mediated by the anisotropic and long-range dipolar interactions on a lattice, and many intellecturally intriguing theory proposals have emerged on this subject.
In 2002, our group began cooling studies of highly reactive OH radicals with the goal of making precision measurements of four specific transition frequencies that have different dependences on the fine structure constant (α). The hope was that precise laboratory measurements could then be compared to observations of similar OH transitions in outer space that took place billions of years ago in the early universe (but which were only recently detected on Earth). Provided both astronomers and laboratory scientists could measure at least two of the four transitions precisely enough to detect small differences in α, the value of α today could be compared with its value 10 billion years ago. This allow a test for various extensions to the Standard Model. Our group succeeded in making this measurement by using a Stark decelerator (invented by G. Meijer's group and developed for OH in the JILA lab in 2003) to put the brakes on fast-moving OH molecules until they traveled suffciently slowly through a microwave measurement cavity. The Stark decelerator could slow these interesting dipolar molecules to "lukewarm" temperatures of about 50 mK.
From the very beginning, our group has focused on the possibility of trapping OH molecules in a magnetic trap, and leaving the external electric field freely available to tune possible molecular interactions. This has turned out to be a very fruitful scientific direction. First, we developed a magneto-electrostatic trap for studying cold collisions. In the process, We optimized the Stark decelerator to produce molecules moving slowly enough to trap. Then we coupled the end of the decelerator into a magnetic trap surrounded by electrodes capable of creating both uniform and nonuniform electric fields. In studies of the new trap, we discovered that an applied electric field not only affected the number of molecules trapped, but also modified the trap itself, making it deeper so fewer molecules could escape. The electric field also changed the oscillation frequency of the OH molecules inside the trap. To enhance the spatial density of trapped molecules, we then invented a permanent magnetic trap that is perfectly mode-matched to the Stark decelerator. This allowed 50% of the entire decelerated molecular packet to be loaded and trapped in the magnetic trap.
Using this trap, we have studied a number of collisions between OH molecules and other atoms and molecules. The trap loss mechanism allows accumulation of weak collisional signals so that they are measurable. We have also explored dipolar collisions between trapped OH and electric field guided ND3 molecules to observe how such collisions depend on an applied external electric field — a key step in using electric fields to control cold molecule collisions and possibly even chemical reactions.
In the meantime, our group has also begun exploring innovative experimental techniques for cooling cold OH molecules to ultracold temperatures (<1 mK). Until recently, many physicists considered it difficult, if not impossible, to evaporatively cool molecules, because their collision behavior is far more complex than that of atoms. The conventional wisdom was that it was unlikely that cold molecules would collide and bounce off one another often enough to facilitate the kind of stepwise cooling that works so well with atoms. In atomic “collide-and-bounce” collisions (known as elastic collisions), some atoms are left much hotter than the others. By removing the hotter atoms, researchers are able to get the remaining atom gas to cool rapidly to ultralow temperatures. In 2012, our group, along with theoretical collaboration from Goulven Quéméner and John Bohn, succeeded with an innovative evaporative cooling experiment with a molecular gas of OH. Even though the initial demonstration cooled the OH molecules down to only 4 mK, It was so successful that the group is optimistic about being able to eventually cool an OH gas to reach quantum degeneracy. This will be an exciting step to open the door to studying the ultracold chemistry of familiar molecules that exist in the natural world.
The evaporative cooling required a neat trick to work. We applied an electric field that opens up some little gateways in the trap to let the hot molecules out. This process removed the most energetic OH molecules, lowering the temperature of the remaining molecular gas. The experiment could be speeded-up by lowering the height of the trap holding the molecules. As the molecular gas got colder, evaporative cooling worked better and better, Not only did the temperature fall precipitously, but also the density of the remaining molecules increased.
Magneto-Optical Trapping of Molecules
Magneto-optical traps use a combination of laser light and precise adjustments of the magnetic field to rapidly produce ultracold trapped atoms. Such traps were an essential part of the process of producing Bose-Einstein condensates and Fermi gases in their lowest quantum state from clouds of ultracold atoms. Researchers have long debated whether a similar mechanism could also be used to produce dense clouds of simple diatomic molecules.
In 2012 our group successfully laser cooled and magneto-optically trapped the polar molecule yttrium oxide, or YO. First the researchers laser cooled a YO molecular beam in the transverse direction to a temperature of 5 mK. Then, we added in an oscillating magnetic field, creating a magneto-optical trap that further cooled the molecules to 2 mK. This achievement was the first-ever successful magneto-optical trapping experiment with a molecule. However, the demonstration has so far been limited to 1-D and 2-D configurations. The group is working intensively to achieve a molecular magneto-optic trap in 3D. The new technique will become an excellent tool for producing ultracold diatomic molecules, much as it already is for atoms. The group anticipates that it will lead to advances in the study of strongly interacting quantum systems, precision measurement, and ultracold chemistry.