JILA scientists create first dense gas of ultracold 'polar' molecules

Scientists at JILA, a joint institute of the National Instituteof Standards and Technology (NIST)and the University of Colorado atBoulder (CU-Boulder), have applied their expertise in ultracold atoms andlasers to produce the first high-density gas of ultracold molecules—twodifferent atoms bonded together—that are both stable and capable of stronginteractions.

The long-sought milestone in physics has potential applications in quantum computing, precisionmeasurement and designer chemistry.

Described in the Sept. 18 issue of Science Express,* JILA's creationof ultracold "polar" molecules—featuring a positive electric charge atone end and a negative charge at the other—paves the way for controlledinteractions of molecules separated by relatively long distances, offering aricher selection of features than is possible with individual atoms andpotentially leading to new states of matter.

"Ultracold polar molecules really represent now one of the hottest frontiers inphysics," says NIST/JILA Fellow Jun Ye, an author of the paper. "They are potentially anew form of matter, a quantum gas with strong interactions that vary by direction and thatyou can control using external tools such as electric fields."

The authors say atoms are like basketballs, round and somewhat featureless,whereas molecules are more like footballs, with angles, and characteristics that vary bydirection.

"This is really a big deal," says NIST/JILA Fellow Deborah Jin, another author of thenew paper. "This is something people have been trying to do for a long time, using allkinds of different approaches."

Jin and Ye are adjoint professors of physics at CU-Boulder and both teachundergraduate and graduate students. Other authors of the paper include a NIST theoristat the Joint Quantum Institute at the University of Maryland and a theorist at TempleUniversity in Philadelphia.

Two types of atoms are found in nature—fermions, which are made of an oddnumber of subatomic components (protons and neutrons), and bosons, made of an evennumber of subatomic components. The JILA group combined potassium and rubidium,which are different classes of atoms (a slightly negative fermion and a slightly positiveboson, respectively). The resulting molecules exhibit a permanent and significantdifferential in electric charge, which, along with the ultracold temperatures and highdensity, allows the molecules to exert strong forces on each other.

The molecules are in the lowest possible vibrational energy state and are notrotating, so they are relatively stable and easy to control. They also have what isconsidered a long lifespan for the quantum world, lasting about 30 milliseconds(thousandths of a second).

JILA's ultracold polar gas has a density of 10 quadrillion molecules per cubiccentimeter, a temperature of 350 nanoKelvin above absolute zero (about minus 273degrees Celsius or minus 459 degrees Fahrenheit), and a measurable separation ofelectric charge.

The process for making the molecules begins with a gas mixture of very coldpotassium and rubidium atoms confined by a laser beam. By sweeping a precisely tunedmagnetic field across the atoms, scientists create large, weakly bound moleculescontaining one atom of each type. This technique was pioneered by Jin in her 2003demonstration of the world's first Fermi pair condensate.

At this stage the molecules are very large and possess a high amount of internalenergy, which allows them to decay and heat up rapidly, both undesirable effects forpractical applications. The scientists faced the considerable challenge of efficientlyconverting atoms that are far apart into tightly bound molecules, without allowing thereleased binding energy to heat the gas.

In a process that Jin describes as "chemistry without explosions," scientists usedtwo lasers operating at different frequencies—each resonating with a different energy jumpin the molecules—to convert the binding energy into light instead of heat. The moleculesabsorb near-infrared laser light and release red light. In the process, more than 80 percentof the molecules are converted, through an intermediate energy state, to the lowest andmost stable energy level.

A key to success was the development of detailed theory for the potassiumrubidiummolecule's energy states to identify the appropriate intermediate state andselect the laser colors for optimal control. In addition, both lasers were locked to anoptical frequency comb, a precise measurement tool invented in part at NIST and JILA,synchronizing the two signals perfectly.

The research described in Science is part of a larger NIST/JILA effort to developtechniques to understand and control the complex features of molecules and theirinteractions. Practical benefits could include new chemical reactions and processes formaking designer materials and improving energy production, new methods for quantumcomputing using charged molecules as quantum bits, new tools for precision measurementsuch as optical molecular clocks or molecular systems that enable searches for newtheories of physics beyond the Standard Model, and improved understanding ofcondensed matter phenomena such as colossal magnetoresistance (for improved datastorage and processing) and superconductivity (for perfectly efficient electric powertransmission).

JILA researchers are now working to improve the efficiency of producing tightlybound polar molecules and extend molecule lifetimes. They also plan to apply the newmolecules to explore new scientific directions.

Source: National Institute of Standards and Technology (NIST)

A dense gas of ultracold polar molecules.

(Photo Credit: G. Kuebler/JILA)