Measurement of purity, the simplest nonlinear functional of the density matrix

After reviewing the earlier ideas to measure the purity of a quantum system, a new scheme for the measurement of purity is presented, which aims at providing a simple, direct and efficient procedure without resorting to quantum state tomography. The scheme is also illustrated in the Bloch-vector representation for a two-level quantum system (qubit) and for a general D-level quantum system. As a variant, a simple example implemented in a one-dimensional scattering process to measure the purity of the target qubit is also given.