Direct measurement of the Schmidt number for biphotons

01.01.2014, 00:00

Newsletter 7

Spontaneous parametric down-conversion is the most widely used source of entangled photons.  Nevertheless, quantifying the degree of entanglement still proves a challenging task. The only operational entanglement measure that is routinely employed is the Fedorov ratio, namely the ratio of the joint state spectral width to the width of the correlations between the twin photons. It turns out, however, that this method of determining the degree of angular entanglement only works in the near- and far-field regions of the source and will fail otherwise.  We have recently implemented an interferometric scheme that allows direct access to the spatial Schmidt number, which is a more fundamental entanglement quantifier and does not depend on propagation. In our set-up we exploit the fact that the more entangled a bipartite system is, the more incoherent are its subsystems. The Schmidt number is measured as the inverse visibility of interference observed in a modified Mach-Zehnder interferometer (see figure). 

Contact: felix.just(at)mpl.mpg(dot)de
Group: Leuchs Division
Reference: F. Just et al., New J. Phys. 15 083015 (2013).