An orchestra of a hundred violinists plays strictly from the score. If the musicians cannot hear one another, the melody is predictable. But once they start listening to their neighbors, each one adds a tiny inaccuracy, and the music turns into an unpredictable improvisation. Physicists have seen a similar transition from order to chaos in a crystal of a hundred beryllium ions cooled almost to absolute zero.

The trapped ions are linked to one another: their internal states form a shared "spin," and their collective vibrations form a single shared "voice." The coupling between them can be made weak or strong. In the weak regime, the system behaves simply: all the spins point in the same direction until the external field becomes too large. Then they start to flip back and forth, and this transition happens twice as early as it would in equilibrium.

This chaos causes every ion to become entangled with all the others. The physicists measured the Renyi entropy, a measure of entanglement, and saw that it reaches a maximum. What is more, in this state the quantum fluctuations of one quantity shrink to 2.6 dB below the standard quantum limit, at the cost of growing fluctuations in another. This property is already helping to build more sensitive sensors today. At the end of the experiment, the hundred ions found a common rhythm again, showing that even in chaos the quantum orchestra remains a single whole.