Researchers in Singapore have built a refrigerator that’s just three atoms big. This quantum fridge won’t keep drinks cold, but it’s cool proof of physics operating at the smallest scales.
Researchers have built tiny heat engines before, but quantum fridges existed only as proposals until the team at the Centre for Quantum Technologies at the National University of Singapore chilled with their atoms. The device is an absorption refrigerator. It works without moving parts, using heat to drive a cooling process. The first absorption refrigerators, introduced in the 1850s, cycled the evaporation and absorption of a liquid, with cooling happening during the evaporation stage. They were widely used to make ice and chill food into the 20th Century. Albert Einstein even held a patent on an improved design.
Today’s fridges and air conditioners more often use a compressor, but absorption refrigerators still have their uses — science experiments included. “Our device is the first implementation of the absorption refrigeration cycle on the nanoscale,” says Stefan Nimmrichter, co-author of a paper published in Nature Communications.
To create an absorption fridge with just three atoms took exquisite control. “As an experimental scientist, it’s a pure joy to be able to manipulate individual atoms,” says Gleb Maslennikov, the paper’s first author. First, the researchers caught and held three atoms of the element Ytterbium in a metal chamber from which they’d removed all the air. They also pulled one electron off each atom to leave them with a positive charge. The charged atoms — called ions — can then be held in place with electric fields. Meanwhile, the researchers nudge and zap the ions with lasers to bring them into their lowest energy state of motion. The result is that the ions are suspended almost perfectly still, strung out in a line.
Another laser zap then injects some heat, making the ions wiggle about. The ions interact with each other because of their like charges. The result is three patterns of wiggle — squishing and stretching along the line, like a slinky, rocking like a seesaw pivoting about the central atom, and zig-zagging out from the line like a waving skipping rope.
The energy in each wiggling mode is quantised with the energy carried by a number of ‘phonons’.
