We cannot 'see' magnetic fields and yet nobody seems to find magnetism particularly mysterious. That is because we can see its effects, we have all played with fridge magnets and iron filings. It is true that we trust what we can see more. So what about more elusive phenomena, such as quantum entanglement? Would it be less spooky if you could actually see it with your own eyes?


Well, now you can. In the entanglement equivalent of an 'iron filings' demonstration, Robert Fickler and colleagues have used state-of-the-art cameras to image — in real time — the effect of measurement on a pair of entangled photons (Sci. Rep. 3, 1914; 2013. Movie available via http://go.nature.com/84ppZp).

The experiment starts with two polarization-entangled photons. The polarization of one photon is measured directly, whereas the second photon travels through an interferometric set-up that transfers its polarization to a selected spatial optical mode. These modes have clear visual signatures (the first-order Laguerre–Gauss mode is pictured), although their structures become more complex for higher modes. The speed and sensitivity of the ICCD cameras used allow the effect of polarization measurements on the first photon to become visible in the changing pattern of the second photon's spatial mode. In this way it is possible to monitor the probability distribution of the spatial modes while scanning the polarization states of the first photon.

Visualizing the effect of measurement on one photon of an entangled pair is fascinating, but for multiparticle entanglement things become more complicated. The complexity of entangled states scales exponentially with the number of particles, so testing entanglement in these conditions becomes prohibitively difficult. Nevertheless, significant effort has been devoted to characterizing and quantifying multiparticle entanglement, as well as building understanding through the use of mathematical tools.

In line with this, Michael Walter and colleagues have now found that different classes of entangled states can be associated with geometric objects known as polytopes, which contain all possible local eigenvalues of states in the corresponding entanglement class (Science 340, 1205–1208; 2013). Local information alone can therefore be used to determine whether a pure multiparticle state belongs to that polytope. This approach provides a visual and more practical characterization of entanglement, and more importantly, a local witness of global entanglement.