  Detecting and quantifying quantum entanglement of a given unknown state poses
problems that are fundamentally important for quantum information processing.
Surprisingly, no direct (i.e., without quantum tomography) universal
experimental implementation of a necessary and sufficient test of entanglement
has been designed even for a general two-qubit state. Here we propose an
experimental method for detecting a collective universal witness, which is a
necessary and sufficient test of two-photon polarization entanglement. It
allows us to detect entanglement for any two-qubit mixed state and to establish
tight upper and lower bounds on its amount. A different element of this method
is the sequential character of its main components, which allows us to obtain
relatively complicated information about quantum correlations with the help of
simple linear-optical elements. As such, this proposal realizes a universal
two-qubit entanglement test within the present state of the art of quantum
optics. We show the optimality of our setup with respect to the minimal number
of measured quantities.
