Quantum Entanglement Monogamy

If A is maximally entangled with B, it cannot be entangled with C — entanglement cannot be freely shared

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E(A:B) entanglement
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E(A:C) entanglement
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E(A:B)+E(A:C)
1.00
Monogamy bound (≤1)
0.70
0.30
0.05
4

Monogamy of Entanglement

Coffman-Kundu-Wootters (CKW) inequality (2000): For a 3-qubit state, the squared concurrences satisfy C²(A:B) + C²(A:C) ≤ C²(A:BC). The residual — called 3-tangle — measures genuine tripartite entanglement. If qubit A is maximally entangled with B (C=1), it has zero entanglement with any third party. This monogamy is fundamental to quantum cryptography: an eavesdropper cannot share entanglement with both communicating parties simultaneously. Drag the sliders to explore: when the sum exceeds the bound (red), the state is unphysical.