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In quantum mechanics, interactions between particles can
give rise to entanglement, which is a strange type of connection that could
never be described by a non-quantum, classical theory. These connections,
called quantum correlations, are present in entangled systems even if the
objects are not physically linked (with wires, for example). Entanglement is at
the heart of what distinguishes purely quantum systems from classical ones; it
is why they are potentially useful, but it sometimes makes them very difficult
to understand.