Measuring entanglement and entanglement measures

Summary form only given. Entangled states of multi-particle systems are arguably the quintessential feature of quantum mechanics. In addition to their central role in discussions of non-local quantum correlations, they form the basis of quantum information, and enable such phenomena as quantum cryptography, quantum dense coding, teleportation, and quantum computation. Via the process of spontaneous parametric downconversion in a pair of nonlinear crystals, it is now possible to produce, with high efficiency, pairs of photons entangled in their polarisation degree of freedom. We can also control both the extent of entanglement and the purity of the state. Density matrices for states generated by this method, and measured using tomographic techniques, are shown.