KRb Molecules: A New Form of Matter
During 2008 and 2009, 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 their lowest energy, or ground, state. In the ground state, the molecules’ vibrations, rotations, and nuclear spin states were as low as allowed by the laws of quantum mechanics. Once this low-energy state was achieved, the ultracold molecules unexpectedly began colliding as well as breaking and forming chemical bonds.
The collisions occurred when molecules quantum mechanically tunneled through an energy barrier between them, and then collided, and reacted. The collisions were unexpected because ultracold KRb molecules mostly dance around each other because they are fermions. Because the laws of quantum mechanics forbid fermions from occupying exactly the same quantum state, such collisions weren’t supposed to happen.
The researchers discovered they could get the KRb molecules to chemically react by preparing them with different nuclear spin states. Once they were no longer identical, the ultracold molecules readily collided head on, breaking and forming chemical bonds. The rates for such reactions were 10–100 times faster than expected because ultracold molecules exist as quantum mechanical waves rather than discrete particles.
The team also explored another strategy for increasing the reaction rates of the KRb molecules: applying a modest electric field. In the electric field, fast chemical reactions started looking like explosions. The trouble was that explosions are a nightmare for experimental physicists, especially if the 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. To get there, the researchers needed the KRb molecules to collide, but not react.
Fortunately, the John Bohn group developed a theory that suggested a strategy for seriously suppressing the reaction rate of KRb molecules. The experimentalists 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. The researchers 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 increased the lifetime of the KRb molecules to one second, mostly because it resulted in it being nearly a hundred times more likely that the molecules bounced off one another rather than chemically reacted.
This result meant that the team could cool a gas of KRb molecules with evaporative cooling. In evaporative cooling, repeated collisions that don’t change the quantum states of the molecules will result in the gas getting colder and colder, ideally until the gas reaches quantum degeneracy. The ability to create a quantum degenerate gas of KRb molecules will open the door to exploring the quantum nature of dipolar molecules and their reactivity over long distances. It may even one day lead to the creation of other states of matter never before seen in a laboratory.
In addition to investigating evaporative cooling and ultracold chemistry of KRb molecules, the Ye and Jin groups are also using ultracold KRb molecules in a quantum simulator to actively investigate quantum magnetism and other quantum behaviors. In 2013, for example, the Jin and Ye experimental groups used the simulator to observe spin exchanges (predicted by theorist Ana Maria Rey) in ultracold KRb molecules inside an optical lattice (a crystal of light formed by interacting laser beams).
A New Gateway to Ultracold Chemistry
The Jun Ye group has opened the door to the relatively unexplored terrain of ultracold chemistry by building a magneto-optical trap (MOT) for yttrium oxide (YO) molecules. The 2D MOT uses three lasers and carefully adjusted magnetic fields to partially confine, concentrate, and cool the YO molecules to temperatures of ~2 mK. It is the first device of its kind to successfully cool and confine ordinary molecules found in Nature.
Magneto-optical traps for atoms were invented during the 1980s. The traps made it relatively straightforward for scientists to make ultracold trapped atoms, a feat that led to revolutions in the fields of atomic and quantum physics. At JILA, they were use to make the world's first Bose-Einstein condensate and ultracold Fermi gas as well as novel quantum sensors.
Researchers have been working for nearly two decades to replicate the success of magneto-optical trapping with molecules. However, laser cooling and trapping molecules at ultracold temperatures requires an apparatus that can address multiple energy levels in molecules at the same time. To meet this challenge, the Ye group added two more lasers to its MOT, alternated the polarization of the laser light interacting with the molecules, and rapidly reversed the direction of the magnetic field around the molecules. This combination allowed the reserachers to "talk to" many energy levels of the YO molecules at the same time, and, in the process, create a more concentrated gas of much colder molecules.
To make make ultracold molecules, the Ye group is redesigning their MOT to trap YO molecules in three dimensions.