Proc. Natl Acad. Sci. USA 111, 6243–6247 (2014)

Increasing the complexity of entangled states by extending their dimensionality allows fundamental tests of quantum behaviour to be performed. However, it remains challenging to generate, detect and verify high-dimensional entanglement. Now, Mario Krenn and co-workers from Austria, the UK and Spain have developed a unique method to extract information regarding the dimensionality of entanglement, as well as to verify high-dimensional entanglement. They performed a proof-of-principle experiment using two orbital angular momentum entangled photons. The entangled photon pair was generated using spontaneous parametric downconversion in a periodically poled potassium titanyl phosphate crystal, and the photons were separated using a polarizing beam splitter. In both arms of the set-up, the spatial modes of the light were transformed by a spatial light modulator to Laguerre–Gaussian modes having angular quantum numbers of up to 11 and radial quantum numbers of up to 13. The team analysed the correlations of 186 modes of two photons and experimentally demonstrated the largest entangled photonic Hilbert-space dimension realized to date of 103 × 103.