  We experimentally measure the lower and upper bounds of concurrence for a set
of two-qubit mixed quantum states using photonic systems. The measured
concurrence bounds are in agreement with the results evaluated from the density
matrices reconstructed through quantum state tomography. In our experiment, we
propose and demonstrate a simple method to provide two faithful copies of a
two-photon mixed state required for parity measurements: Two photon pairs
generated by two neighboring pump laser pulses through optical parametric down
conversion processes represent two identical copies. This method can be
conveniently generalized for entanglement estimation of multi-photon mixed
states.
