Researchers make the coldest quantum gas molecules

JILA researchers have developed an extremely cold and durable gas of molecules that follow the wave patterns of quantum mechanics instead of being limited to the strictly particulate nature of classical physics. The creation of this gas increases the chances of progress in areas such as designer chemistry and quantum computing.

As indicated on the cover of the February 22 issue of Science, the team produced a gas composed of potassium and rubidium molecules (KRb) at temperatures as low as 50 nanokelvins (nK). That's 50 billion Kelvin, or just a little more than absolute zero, the lowest temperature theoretically possible. The molecules are in the lowest possible states of energy, constituting what is called a degenerate Fermi gas.

In a quantum gas, all properties of molecules are limited to specific, or quantized values, such as bars on a scale or notes on a musical scale. Cooling the gas at the lowest temperatures gives researchers maximum control over the molecules. The two atoms involved belong to different classes: potassium is a fermion (with an odd number of subatomic components called protons and neutrons) and rubidium is a boson (with an even number of subatomic components). The resulting molecules have a Fermi character.

JILA is operated jointly by the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder. NIST researchers at JILA have been striving for years to understand and control ultra-cold molecules, which are more complex than atoms, because they have not only many internal energy levels, but also a rotation and vibration. The JILA team manufactured its first molecular gas 10 years ago.

"The basic techniques for making gas are the same as the ones we used before, but we have some new stuff, such as significantly improving the cooling of atoms, creating more of them in the lowest energy state, "NIST / JILA Comrade Jun Ye said. "This translates into higher conversion efficiency, so we get more molecules."

The JILA team produced 100,000 molecules at 250 nK and up to 25,000 molecules at 50 nK.

Until now, the coldest molecules with two atoms were produced in numbers of tens of thousands and at temperatures not exceeding a few hundred nanoKelvin. JILA's latest gas temperature record is well under a third (about) the level at which quantum effects start to take over the classic effects and the molecules last for a few seconds – a remarkable longevity, Ye said.

The new gas is the first to become sufficiently cold and dense that the material waves of these molecules are longer than the distances between them, which superimposes them to create a new entity. Scientists call this quantum degeneration. (Quantum matter can behave either as a particle or as a matter wave, that is, as a waveform pattern representing the probability of locating a particle. ).

Quantum degeneracy also means an increase in repulsion among fermionic particles, which tend to be solitary anyway, resulting in fewer chemical reactions and a more stable gas. This is the first experiment in which scientists have observed collective quantum effects directly affecting the chemistry of individual molecules, Ye said.

"This is the first quantum degenerate gas of stable bulk molecules, and the chemical reactions are suppressed – a result that no one had predicted," Ye said.

The molecules created in this experiment are called polar molecules because they have a positive electrical charge on the rubidium atom and a negative charge on the potassium atom. Their interactions vary by direction and can be controlled with electric fields. The polar molecules thus offer more adjustable and stronger interactions and additional "buttons" of control compared to neutral particles.

These new ultra-low temperatures will allow researchers to compare chemical reactions in quantum and classical environments and to study how electric fields affect polar interactions. Potential practical benefits could include new chemical processes, new quantum computing methods using charged molecules as quantum bits, as well as new precision measurement tools such as molecular clocks.

The process of making the molecules begins with a gaseous mixture of very cold potassium and rubidium atoms confined by a laser beam. By sweeping the atoms of a precisely tuned magnetic field, scientists create large, loosely bound molecules, containing one atom of each type. The late Deborah Jin, a colleague from Ye, developed this technique in 2003, during the demonstration of the first Fermi condensate in the world.

To convert these relatively soft molecules into closely related molecules without heating the gas, scientists use two lasers operating at different frequencies, each resonating with a different energy jump in the molecules, in order to convert the energy of binding in light instead of heat. Molecules absorb near-infrared laser light and release red light. In the process, 90% of the molecules are converted via a state of intermediate energy, at the lowest and most stable energy level.

Explore further:
Resonances of collision between ultra-cold atom and molecules visualized for the first time

More information:
Luigi De Marco et al. A Fermi gas degenerated from polar molecules, Science (2019). DOI: 10.1126 / science.aau7230