Once it’s explained to us, it sounds pretty obvious: instead of using ordinary light to see and photograph a scene, MIT researchers uses a femtosecond laser, which emits bursts of light so short that their duration is measured in quadrillionths of a second. An interesting property of femtosecond laser is that, unlike ordinary light, it is easily reflected by walls, doors and floors — non-reflective surfaces — that can be thought of as acting like mirrors. Want to see what’s behind a wall? Just aim the laser at the door opening and it is reflected inside the room and all around until it finds its way out the door again and is captured and measured by a sensor. By sending the laser into the room at different angles, the camera can build an image of what’s behind that wall.
I’ve probably simplified this more than I should and to get the correct technical explanation, please watch the video and read the article at: MIT.