  A simplified expression of concurrence for two-qubit mixed state having no
more than three non-vanishing eigenvalues is obtained. Basing on SU(2) coherent
states, the amount of entanglement of two-qubit pure states is studied and
conditions for entanglement are calculated by formulating the measure in terms
of some new parameters (amplitudes of coherent states). This formalism is
generalized to the case of two-qubit mixed states using the simplified
expression of concurrence.
