Entanglement as an Element-of-Reality

Entanglement—according to Schrödinger (1935) the essential prop-
ertyofquantummechanics—teachesusthatthepropertiesofindividualquantum
systems cannot be considered to be (local) elements of physical reality before
and independent of observation. Yet it is a widespread point of view that the way the observations on, say, two particles are correlated, i.e. the specific type of their entanglement, can still be considered as a property of the physical world.
Here I discuss a previous experiment (Walther et al., 2006) showing that this is
explicitly not the case. The correlations between a single particle property, the
polarizationstateofaphoton,andajointpropertyoftwoparticles,theentangled
state of a photon pair in a three-photon entangled state, have been measured. It
is shown that the correlations between these properties can obey a cosine rela-
tion in direct analogy with the polarization correlations in one of the triplet Bell
states (Bell, 1964). The cosine correlations between the polarization and entan-
gled state measurements are too strong for any local-realistic explanation and
are experimentally exploited to violate a Clauser-Horne-Shimony-Holt (CHSH)
Bell inequality (Bell, 1964; Clauser et al., 1969). Thus, entanglement itself can
be an entangled property leading to the notion of entangled entanglement.