  In a recent work [Phys. Rev. Lett. 120, 170502 (2018)], Pallister et al.
proposed an optimal strategy to verify non-maximally entangled two-qubit pure
states under the constraint that the accessible measurements being locally
projective and non-adaptive. Their nice result leads naturally to the question:
What is the optimal strategy among general LOCC measurements? In this Letter,
we answer this problem completely for two-qubit pure states. To be specific, we
give the optimal strategy for each of the following available classes of
measurements: (i) local operations and one-way classical communication (one-way
LOCC) measurements; (ii) local operations and two-way classical communication
(two-way LOCC) measurements; and (iii) separable measurements. Surprisingly,
our results reveal that for the two-qubit pure state verification problem,
two-way LOCC measurements remarkably outperforms one-way LOCC measurements and
has the same power as the separable measurements.
