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. It is stronger than ordinary entanglement: measuring the first object instantly sets the state of the second, even if they are far apart.

In a Zurich laboratory, the role of the pendulums was played by two bulk acoustic wave resonators, each the size of a grain of sand and with a mass of about 16 micrograms, that is, roughly $10^{17}$ atoms. They were connected not by a string but by a superconducting qubit. Two microwave tones whose frequencies sum to the frequencies of both resonators made the qubit act as a shared metronome: it does not push the pendulums one after the other, but instead creates paired quantum excitations in both at once.

To measure the correlations, the physicists built an SU(1,1) interferometer. Instead of mirrors and beam splitters, it uses parametric amplifiers: the first prepares the state, the second reads it out with a phase shift. Such a device senses not merely coinciding motions, but specifically the quantum link that violates classical inequalities. The measured criterion dropped to 0.90, whereas anything above 1 could still be chalked up to ordinary physics. The confidence level of the result was 99% for one direction and 98% for the other.

Previously, such correlations had been observed only in microscopic systems such as photons or ions. Here, for the first time, two macroscopic pieces of crystal showed that their motions are linked more strongly than any classical model with shared nudges allows. This is not just a laboratory curiosity: such a link could serve as a resource for quantum teleportation of mechanical states, when the vibrations of one resonator are transferred to another without physical contact.

The two pendulums in Zurich swing in sync, and now we know: their coordination is born not from a shared string, but from a shared quantum rule. What remains is to learn how to use this string that does not exist to transmit motion across a distance.