The paper by William K. Wootters defines the entanglement of formation for a mixed state of two qubits and provides a formula to calculate it. The entanglement of formation is defined as the minimum average entanglement of an ensemble of pure states that represents the mixed state. Wootters extends an earlier conjecture, which was proven for a special class of mixed states, to arbitrary states of two qubits. The formula for the entanglement of formation is derived using the "spin flip" transformation and the concurrence, a measure of entanglement for pure states. The proof involves constructing an optimal decomposition of the density matrix into pure states, ensuring that the average entanglement is minimized. The paper also discusses the interpretation of the entanglement of formation and its potential additivity, which could strengthen its physical interpretation.The paper by William K. Wootters defines the entanglement of formation for a mixed state of two qubits and provides a formula to calculate it. The entanglement of formation is defined as the minimum average entanglement of an ensemble of pure states that represents the mixed state. Wootters extends an earlier conjecture, which was proven for a special class of mixed states, to arbitrary states of two qubits. The formula for the entanglement of formation is derived using the "spin flip" transformation and the concurrence, a measure of entanglement for pure states. The proof involves constructing an optimal decomposition of the density matrix into pure states, ensuring that the average entanglement is minimized. The paper also discusses the interpretation of the entanglement of formation and its potential additivity, which could strengthen its physical interpretation.