Sunlight could cool an atom to its coldest possible temperature

Sunlight could be used to cool a charged atom to its lowest temperature allowed by the laws of physics.

In the 1990s, several people won Nobel prizes for working out how to make atoms extremely cold with precisely controlled laser light. Now, Amanda Younes and Wesley Campbell at the University of California, Los Angeles, have found that some parts of the cooling process could be done with light straight from the sun.

The researchers calculated how a single positively charged barium atom, or ion, …