Ultracold science finds new method to get even colder

Researchers have developed a clever way to achieve the coldest temperatures ever recorded on Earth. Achieving such temperatures is necessary to study fundamental properties of matter and the strange effects caused by quantum mechanics. The new method relies on "optical lattices" of atoms from which only the hottest atoms are selectively removed. The approach, reported in Nature, may be well-suited to create memory for future quantum computers. The limits of low temperature have been constantly pushed in recent years, and the current best lies somewhere in the nanoKelvin regime - that is, within just billionths of a degree of "absolute zero" at zero Kelvin or -273.15C. That ultimate limit is set formally as the lowest possible entropy, or disorder, that is achievable. Optical lattices are an ideal system in which to attain temperatures ever nearer that limit. The peaks and troughs of intensity in crossed beams of light form a kind of "egg-crate" structure in which atoms are inclined to remain in the troughs - a point of lowest energy. As the atoms are added to each trough - or each point in the lattice - it becomes more difficult to add another, in a situation called a blockade. But researchers from Harvard University have invented a modification to this effect called orbital exchange blockade. It is a way to cool these assemblages of atoms that could be extended to the picoKelvin regime: within trillionths of a degree of the coldest possible temperature. The team carefully adjusted the intensity of the crossed light beams. The trick was to do so in such a way that only the most energetic atoms in each lattice site absorbed energy from the light fields, becoming more energetic again. By adjusting how frequently the light beam intensities were changed, the team was able to remove these "hottest" atoms from the system, leaving only the "coolest" ones behind. The approach removed entropy, or in other words, reduced the overall temperature of the lattice. In an accompanying article in Nature, optical lattice expert Gretchen Campbell from US measurement agency Nist points out that this ability to specifically address single lattice sites, and potentially to cool to never-before-achieved temperatures, may make the approach useful in quantum computers. These devices, still in early developmental stages, would make use of the slippery nature of quantum states to perform computation at incredible speeds. But like any computer, they would need memory, and optical lattices that keep delicate quantum information preserved in cold atoms could be a suitable solution.