The quantum world is full of strange connections. Two particles can become “entangled”: the state of one instantly affects the other, even if they are kilometers apart. But how do you know if they are truly entangled? It's like a ball of yarn: at first glance, you can't tell whether it's a single tangled ball or just a mess of loose threads.

Previously, checking required enormous computations — exhaustively testing all possible configurations was feasible only for tiny systems. John von Neumann once laid the foundations of quantum mechanics, and the new work proposes an algorithm that solves the problem quickly — in polynomial time. The condition is simple: if entanglement is present, the algorithm says “yes”; if it's almost absent (particles are far from entangled by a margin η), it says “no.” Intermediate cases remain in a gray zone, but in practice that's enough.

This is a step toward making quantum computers more reliable. In them, entanglement is like fuel: without it, the machine won't take off. Fast diagnostics will allow checking the quality of qubits on the fly, without slowing down the computation. So the ball of yarn stops being a mystery: one quick test, and you see whether it's a tangle or chaos.