In a chain of eight atoms, a tiny magnet is hidden — like an invisible stone at the bottom of a pond. Usually, to find it, you'd have to reach in and touch it. But if that's impossible — say, the defect is deep inside the material — physicists have devised a clever trick: look at the ripples on the water.

Quantum entanglement (a special connection between particles, causing them to behave as a single whole even at a distance) pervades all the atoms of the chain. Alice, sitting at one end, measures her atom — this creates a disturbance, like a pebble thrown in. She reports the result to Bob, who sits at the other end. Bob, knowing what Alice did, measures the energy of his atom. The waves from the stone reach him, and from their height he figures out whether a stone is at the bottom and how heavy it is.

The energy Bob receives precisely reveals the presence and strength of the impurity. This is akin to echolocation: the object is unseen, but the reflected signal is heard.

In the future, this trick will be tested on real atoms in laboratories.