Two particles can become entangled so completely that a change in one immediately affects the other, no matter how far away it is. Until now, scientists have assumed such a marriage would endure forever.
But in a paper published today in the journal Science, two physicists show that entangled particles can suddenly and irrevocably lose their connection, a phenomenon called Entanglement Sudden Death, or ESD.
Contagion links in magick depend at least in part upon quantum entanglement in order to work, so this gives some new insight into their operation. Most of us have had the experience that magical links reach a point where they don't seem to work as well, usually after some number of years. ESD might be the reason, and the implication is that once a magical link like hair clipping or an object owned by the target of a spell stops working it will likely never work again unless the failure was due to some other problem with the spell itself. As the operant equation shows, once the Link value goes to zero no spell that depends upon that link will ever produce a probability shift.
Similarity links don't depend upon entanglement, but they are not usually as strong. A photograph is a combined similarity/contagion link and if the person targeted still looks the same as in the photograph it may continue to work, but you likely will see a dropoff in effectiveness once the entanglement factor ceases to be present. At that point you should procure a fresh photograph if possible, the best sort being an instant photo like a Polaroid in which the actual photons striking the subject also interact with the instant film.
It is unclear what the actual lifespan of entanglement is, but from my own experience photos work fine for at least a few years. Clearly this is an area where more magical research is required, and wouldn't it be amusing if magicians could shed some light on a current scientific problem?