Nature Phys. 9, 19–22 (2012)

The generation of correlated particles and a means of assessing their states are both essential techniques for investigating quantum mechanical phenomena. Although researchers have entangled discrete variables of up to 14 ions and continuous variables between three intense optical beams, entangling the continuous properties of three or more individual particles remains a challenge. Now, Krister Shalm and co-workers from the University of Waterloo and the University of Calgary in Canada have not only generated tripartite continuous-variable entanglement between three separate particles, but also proposed a scheme to verify the correlated state. The researchers generated three entangled photons from two periodically poled potassium titanyl phosphate crystals by cascaded spontaneous parametric downconversion. First, they used a 404 nm pump laser to generate a pair of daughter photons at 776 nm and 842 nm. They then used the photon at 776 nm to generate a pair of granddaughter photons at 1,530 nm and 1,570 nm. To verify the correlated states of the photons, the researchers used a continuous-variable entanglement criterion; instead of measuring the frequencies of each individual photon, they measured the frequency of the pump, thus providing a direct measurement of the total frequency of the three daughter photons required by the inequalities. By putting the uncertainty in the arrival times between two of the three photons into the inequalities, they verified that the three photons strongly violated inequalities and were therefore tripartite continuous-variable entangled.