Entanglement of a Pair of Quantum Bits

The ``entanglement of formation'' of a mixed state ρ of a bipartite quantum system can be defined as the minimum number of singlets needed to create an ensemble of pure states that represents ρ. We find an exact formula for the entanglement of formation for all mixed states of two qubits having no more than two nonzero eigenvalues, and we report evidence suggesting that the formula is valid for all states of this system.