A laser beam usually brings heat to mind, but it can do the opposite: cool atoms almost to a halt. If you shine light against an incoming atom, it slows it down, much like a headwind slows a cyclist. This trick is called Doppler cooling. Thanks to it, atoms slow down so much that their temperature drops to millionths of a kelvin, colder than outer space.

Light can also hold atoms in place. In an optical lattice created by intersecting laser beams, atoms sit like balls in an egg carton. This allows building ultra-precise atomic clocks that lose only one second over billions of years. Such clocks are already running in laboratories today, and in the future they will help search for gravitational waves and test fundamental laws of nature.

So light, which once only heated matter, has become the primary tool for cooling and controlling atoms. This has opened the door to quantum computers and new sensors that will change our lives.