Two taut strings plucked at the same time sound different if they are connected by a rubber band. Energy starts to flow from one to the other, and the pitch of the combined sound shifts slightly. Two nanomechanical bridges, microscopic beams etched into a crystal, behave much the same way. They vibrate at frequencies millions of times higher than the ear can hear, and they are coupled not by a rubber band but by electric forces.
To measure the strength of this coupling, the authors used a trick reminiscent of the game hot-and-cold, but in reverse. Instead of searching for a hidden object, they watched how quickly the two oscillations fell out of sync. The method proved so precise that just four interference fringes, four short signal bursts, were enough. That's like tuning a guitar by the first four picks of the plectrum, without waiting for the string to die down.
Why such precision? Vibrating bridges make excellent scales for electric charges. If even a single electron appears nearby, it will tug on the bridge, and the coupling will change. The new method can detect this shift, approaching the limit where a single charge becomes visible. Such sensors could search for weak electric fields in biological cells or check the purity of materials.
Two strings connected by a rubber band can now be tuned with the precision of a single electron's jitter. And this is not the limit.
Nanomechanical bridges vibrate at frequencies on the order of hundreds of megahertz — millions of times faster than a mosquito flaps its wings.