Stefano Marti

Inside a tiny crystal, sound is trapped like light in a mirrored room. It bounces off the walls and forms standing waves — a multitude of harmonic oscillations, each with its own frequency. Physicists have learned to use these oscillations as qubits — the basic units of quantum information.

Two pendulums swing in sync, even though there is not a single shared string between them. One swings left, the other at the same instant goes right, and so it repeats, without a single touch. In the quantum world, such coordination is called an Einstein–Podolsky–Rosen correlation, or EPR steering.