Google has developed a new interaction senor using radar technology. The project is called: soli with the purpose of using radar for motion tracking of the human hand.
How does this work?
Soli sensor technology works by emitting electromagnetic waves in a broad bream. Objects within the beam scatter this energy, reflecting some portion back towards the radar antenna. Soli tracks and recognizes dynamic gestures expressed by fine motions of the fingers and hand. Here you can see the prototype developed in between 2014-2015: