Scientists are one step closer to etching smaller computer chips, imaging ever-smaller objects, and detecting gravitational waves, thanks to a recent experiment reported in the May 14th issue of Science. Yaron Silberberg and his “Ultrafast Optics Group” at the Wiezmann Institute of Science were able to put five photons into an entangled state, called a “N00N” state which is a superposition of two other states, one with all N photons taking path A, the other with every photon taking path B, |N,0>+|0,N>. Thus, while it is uncertain which path any particular photon will take, it is 100% certain that they will all take the same path. This experiment demonstrates how quantum entanglement, a correlation between distant particles, can exist between many different photons as well.
That such non-local correlations exist was proved by John Bell in 1964, and accounting for the mechanism by which distant particles or photons are correlated remains one of the biggest puzzles in quantum mechanics to this day. In the literature on causation, most philosophers assume that causes cannot occur later than their effects. However, Einstein’s theory of relativity says that earlier and later are only well-defined for events that are time-like related to each other; only those events in one’s past or future light-cone can be said to be temporally related to you.
However, when we put these two constraints together, it would seem that measurements of distant particles or photons cannot be causally related to each other since they are space-like separated and thus not in one another’s light cone. But if they are not causally related to each other, then it is quite puzzling how the photons could exhibit the correlations that they do. This new experiment which entangles not two, but five, photons at once is yet another great example of this quantum mystery.
By Christopher Hitchcock , California Institute of Technology
(Vol. 2, April 2007)
By Ned Hall, Massachusetts Institute of Technology
(Vol. 1, February 2006)