Scientists at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder (CU-Boulder), NIST and Temple University in Philadelphia have produced the first high-density gas of ultracold "polar" molecules—molecules with a positive electric charge at one end and a negative charge at the other—that are both stable and capable of strong interactions. The long-sought milestone in physics has potential applications in quantum computing, precision measurement and designer chemistry.
Described in a recent paper,* the ultracold polar molecules could be used to study controlled interactions of molecules separated by relatively long distances, offering a richer selection of features than is possible with individual atoms and potentially leading to new states of matter.
The JILA group combined potassium and rubidium to create molecules that have a stable and measurable separation of electric charge. That, along with the ultracold temperatures and high density —the gas has a density of 10 quadrillion molecules per cubic centimeter and a temperature of 350 nanoKelvin above absolute zero (about minus 273 degrees Celsius)—allows the molecules to exert strong forces on each other. They also have what is considered a long lifespan for the quantum world, lasting about 30 thousandths of a second.
After creating an initial large, weakly bound potassium-rubidium molecules using a magnetic field technique pioneered by NIST/JILA Fellow Deborah Jin, one of the co-authors, the scientists faced the considerable challenge of efficiently converting atoms that are far apart into tightly bound molecules without allowing the released binding energy to heat the gas. In a process that Jin describes as "chemistry without explosions," the team used two lasers, each tuned to a different energy jump in the molecules, to convert the binding energy into light instead of heat. The molecules absorb near-infrared laser light and release red light. In the process, more than 80 percent of the molecules are converted to the lowest and most stable energy level.
The research, which was supported by the National Science Foundation, NIST, the Air Force Office of Scientific Research and the W.M. Keck Foundation, is part of a larger NIST/JILA effort to develop techniques to understand and control the complex features of molecules and their interactions. Practical benefits could include new chemical reactions and processes for making designer materials and improving energy production, new methods for quantum computing using charged molecules as quantum bits, new tools for precision measurement such as optical molecular clocks or molecular systems that enable searches for new theories of physics beyond the Standard Model, and improved understanding of condensed matter phenomena such as colossal magnetoresistance and superconductivity.
For more details and illustrations, see "JILA scientists create first dense gas of ultracold 'polar' molecules."
* K.K. Ni, S. Ospelkaus, M.H.G. de Miranda, A. Pe'er, B. Neyenhuis, J.J. Zirbel, S. Kotochigova, P.S. Julienne, D.S. Jin, J. Ye. A high phase-space-density gas of polar molecules. Science Express. Sept. 18, 2008.