Quantum systems made of many particles are the basis of future computers and secure networks. But the more particles there are, the harder it is to understand how they are arranged: the number of possible states grows like an avalanche. The usual approach, measuring everything, takes an incredibly long time.
Physicists have come up with a workaround: instead of reconstructing the whole system, just look at its small parts, like judging a cake not by tasting it whole but by taking a few pieces from different spots. In an experiment with light particles (photons), they reconstructed all pairwise correlations between particles using just 25 measurements instead of the 225 that would be needed for a full description.
It turned out that even from these small fragments, one can confidently say that the system is genuinely quantum and entangled. Moreover, this method tolerates noise better than previous criteria. This means complex quantum networks could be verified faster and more reliably.
The experiment used light particles, photons, that were simultaneously in four different states, as if a coin were not heads or tails but several options at once.