A flashlight beam in a dark room can hold a dust mote almost still: that's how optical tweezers work. But light doesn't just push the particle; it also jostles it with random kicks. Each absorbed photon strikes it like an invisible hammer. The authors of the new study have devised a light trap of a special shape. It resembles a rotating saddle, woven from two different light threads — a smooth Gaussian one and a vortex Laguerre-Gaussian one, whose frequencies differ slightly.
The rotating saddle is cleverly designed. The particle doesn't roll into a pit but constantly balances on the ridge. It's like a ball on the back of a galloping carousel horse, forever seeking equilibrium. As a result, the dust mote barely absorbs light. That means it heats up less and receives fewer random jolts. Moreover, it can spread out in space far more than in a regular tweezer. This opens the door to quantum experiments with macroscopic objects. The particle can be in two places at once, and its motion can be squeezed in terms of uncertainty, as if we reduced the jitter of a pendulum in one direction by amplifying it in another.
The most practical promise of the work is a force sensor of unprecedented sensitivity. If you suspend a particle in this light saddle and watch its trembling, you can detect a nudge of tens of zeptonewtons. For comparison, that's roughly the force needed to move a single atom on a crystal surface. Such a device could hear the whisper of individual molecules or detect the gravitational pull of tiny masses. And all this in an ordinary lab, without cosmic cold or multi-ton setups.
A zeptonewton is the force one bacterium exerts on another when they barely touch in a water droplet.
In Vernor Vinge's novel A Fire Upon the Deep, hypersensitive gravitational detectors track the movement of fleets — here, we're talking about a device that could sense the gravity of a dust mote.