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Physicists Create Coldest Quantum Gas of Molecules

Physicists at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, have produced a gas of potassium-rubidium molecules at temperatures as low as 50 nanokelvin (nK) — that’s 50 billionths of a Kelvin, or just a smidge above absolute zero. The two atoms involved are in 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).

|SOURCE| In a quantum gas, all of the molecules’ properties are restricted to specific values, or quantized, like rungs on a ladder or notes on a musical scale.

Chilling the gas to the lowest temperatures gives physicists maximum control over the molecules.

“The basic techniques for making the gas are the same ones we’ve used before, but we have a few new tricks such as significantly improving the cooling of the atoms, creating more of them in the lowest-energy state. This results in a higher conversion efficiency so we get more molecules,” said Dr. Jun Ye, a fellow of NIST/JILA.

Before now, the coldest two-atom molecules were produced in maximum numbers of tens of thousands and at temperatures no lower than a few hundred nK.

Dr. Ye and colleagues produced 100,000 molecules at 250 nK and as many as 25,000 molecules at 50 nK.

“This gas temperature record is much lower than (about one-third of) the level where quantum effects start to take over from classical effects, and the molecules last for a few seconds,” Dr. Ye said.

The new gas is the first to get cold and dense enough for the matter waves of these molecules to be longer than distances between them, making them overlap with each other to create a new entity. Physicists call this quantum degeneracy.

Quantum degeneracy also means an increase in the repulsion among fermionic particles, which tend to be loners 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,” Dr. Ye said.

“And this is also the first quantum degenerate gas of stable molecules in bulk, and the chemical reactions are suppressed — a result that nobody had predicted.”

The molecules created in this experiment are called polar molecules because they have a positive electric charge at the rubidium atom and a negative charge at the potassium atom. Their interactions vary by direction and can be controlled with electric fields.

Polar molecules thus offer more tunable, stronger interactions and additional control ‘knobs’ compared with neutral particles.

These new ultralow temperatures will enable scientists to compare chemical reactions in quantum versus classical environments and study how electric fields affect the polar interactions.

Eventual practical benefits could include new chemical processes, new methods for quantum computing using charged molecules as quantum bits, and new precision measurement tools such as molecular clocks.

The results are published in the journal Science.

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