One of two indentical decay channels used by Luiz Davidovich and colleagues in their search for entanglement sudden death. (Courtesy: Luiz Davidovich).
The researchers studied pairs of photons that were entangled in two different ways: one type had a certain combination of horizontal and vertical polarizations, while the other type had a different combination of these polarizations. Both initial states were created with the same degree of entanglement and both were subjected to the same gradual decay of vertical polarization. It turned out that the entangled pairs that were more vertically than horizontally polarized underwent ESD, whereas the pairs that where the opposite was true decayed relatively slowly as expected. Davidovich reckons that the vertically-rich entanglement suffered ESD because in this experiment, vertical polarization is a higher energy state and is therefore more sensitive to decay via interactions with the environment than is the lower-energy state of horizontal polarization.
Source: Entanglement Dies A Sudden Death [ http://physicsweb.org/articles/news/11/5/1/1#esd ], Hamish Johnston, PhysicsWeb.org, 07/05/01