An entanglement witness is a mathematical tool used in quantum mechanics to detect whether a given quantum state exhibits entanglement. Entanglement is a fundamental phenomenon in quantum physics where the states of two or more particles become correlated in such a way that the state of one particle cannot be described independently of the state of the other(s), no matter the distance between them.
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An entanglement witness is a Hermitian operator such that for every separable quantum state , but for at least one entangled state .