Two entangled photons fly apart in opposite directions but continue to behave as a single whole: measuring one instantly determines the state of the other. That's quantum entanglement. But sometimes it resembles a seasoned spy — leaving no traces detectable by ordinary checks. Such 'bound' entanglement isn't useful for ultra-secure communication, but detecting it is an important task for understanding the quantum world.

The authors found a way to expose this entanglement. They noticed that the difference between two mathematical characteristics of a state, partial transpose (swapping parts of the matrix) and purity, splits into parts, one of which is related to chirality. Chirality is the property of an object not matching its mirror image, like left and right hands. In quantum physics, it's measured by a quantity that describes, for example, the behavior of electrons in special magnetic materials. It turns out that chirality is what gives away the presence of hidden entanglement.

Based on this idea, the physicists built a multichannel spectral classifier. It works like a sorting machine: it takes a quantum state, decomposes it along several features (including chirality), and delivers a verdict — whether hidden entanglement is present. On all known families of bound entangled states of dimension 3×3, the classifier achieved 99.9% correct answers and zero false positives. For comparison, the old method based solely on the CCNR criterion gave about 40%.

Such an approach isn't just solving an academic problem. Bound entanglement is the 'dark matter' of quantum information theory: it exists, affects systems, but isn't directly visible. Being able to detect it will help better control quantum computers and might open new ways to protect information. Chirality, familiar from the spirals of shells and sugar molecules, becomes a key to the mysteries of the quantum world.