Two tiny dust grains hover at the crossing of laser beams. Each is held by its own 'tweezer'—a focused beam that keeps the particle from falling. Inside a mirrored cavity, light bounces between the walls, and its oscillations are coupled to the trembling of the grains.
Usually, such particles constantly jitter due to thermal motion, and this jitter obscures quantum effects. To stop the jitter, researchers use light: it drains energy from the motion, like a hand calming a swinging swing. But when there are two particles, they have 'dark' motional modes — ones that light cannot reach and cool. Then the swing keeps rocking.
The authors found a way to control the coupling between light and particles by rotating the polarization. At the right angle, all six motional modes of the two grains are simultaneously cooled to their quantum ground state — the lowest energy allowed by quantum mechanics. It's like stopping six swings at once by pulling a single rope at the right angle.
Now the two grains can behave as a single quantum object. Such systems will help study how quantum laws manifest in the world we see with the naked eye.
Particles cooled to the ground state have temperatures in the millionths of a kelvin — colder than interstellar space.