One of the things that sets the quantum world apart from our everyday classical one is the capacity for entanglement—when two or more objects share an invisible connection that entwines their fates. Entanglement is the most extreme version of a quantum connection, where measuring one particle can tell you everything you need to know about another. Short of that, particles can still sync up in decidedly quantum ways, where measuring one particle will give you some incomplete information about another. Such quantum correlations can be used to make more precise measurements than classical ones. For example, they can help us detect gravitational waves.

Photons of light don’t often naturally connect in this way. But when they do, quantum-correlated photons could potentially be useful to study materials’ quantum features. Generating this quantum light is tricky business, however, and has so far been largely confined to just a few photons.

Keep reading at scientificamerican.com.