Inseparable Two Spin- $\frac{1}{2}$ Density Matrices Can Be Distilled to a Singlet Form

A quantum system is called inseparable if its density matrix cannot be written as a mixture of product states. In this Letter we apply the separability criterion, local filtering, and Bennett et al. distillation protocol [Phys. Rev. Lett. 76, 722 (1996)] to show that any inseparable $2\times 2$ system represents the entanglement which, however small, can be distilled to a singlet form.