  Numerous work had been done to quantify the entanglement of a two-qubit
quantum state, but it can be seen that previous works were based on joint
measurements on two copies or more than two copies of a quantum state under
consideration. In this work, we show that a single copy and two measurements
are enough to estimate the entanglement quantifier like entanglement negativity
and concurrence. To achieve our aim, we establish a relationship between the
entanglement negativity and the minimum eigenvalue of structural physical
approximation of partial transpose of an arbitrary two-qubit state. The derived
relation make possible to estimate entanglement negativity experimentally by
Hong-Ou-Mandel interferometry with only two detectors. Also, we derive the
upper bound of the concurrence of an arbitrary two-qubit state and have shown
that the upper bound can be realized in experiment. We will further show that
the concurrence of (i) an arbitrary pure two-qubit states and (ii) a particular
class of mixed states, namely, rank-2 quasi-distillable mixed states, can be
exactly estimated with two measurements.
