  Since entanglement is not an observable per se, measuring its value in
practice is a difficult task. Here we propose a protocol for quantifying a
particular entanglement measure, namely concurrence, of an arbitrary two-qubit
pure state via a single fixed measurement set-up by exploiting so-called weak
measurements and the associated weak values together with the properties of the
Laguerre-Gaussian modes. The virtue of our technique is that it is generally
applicable for all two-qubit systems and does not involve simultaneous copies
of the entangled state. We also propose an explicit optical implementation of
the protocol.
