The Nobel Prize in Physics has gone this time to Klauser, Aspect and Zeligman, for their pioneering experiments which proved the existence of the quantum phenomenon that has come to be called entanglement, by which the wave function of a system can carry simultaneous information about two particles which could be widely separated in space. This notion, which is the signature notion of quantum mechanics is at variance with the notion of locality, which lies at the heart of classical physics. Hidden variable theories were proposed by David Bohm, by which the wave function could incorporate a 'hidden variable' which could encode the additional information, and thus make the wavefunction compatible with locality. However, Bell showed, in a series of papers that quantum mechanical wave functions, and wave functions which incorporate hidden variables, had differing levels for correlations for which bounds could be quantified. This led the way to experiments where the predictions of the theories could be verified.
Klauser, a really young man those days, put his career on line and devised ingenious experiments from bits and pieces he found lying around in the lab and set up an experiment which measured the polarisations of two photon states where the photons travelled in opposite directions. Unlike what he hoped, the measurements supported the predictions of quantum mechanics. Aspect and Zeligman in later years added further refinements via very sophisticated experiments which incorporated random elements and very large separations into the experiments. Alas, the photons remained stubbornly correlated, i.e. entangled at levels predicted by quantum mechanics, and notions of locality could not be rescued via hidden variable theories.
It is not quite correct to say that these experiments opened the way to current applications ranging from quantum algorithms to quantum computers which rest on entanglement. Entanglement is anyway central to quantum mechanics. However they are extremely important from the point of view of the foundations of quantum mechanics, and notions of locality, measurement and simultaneity which are crucial ingredients of the theory. It is wonderful that theories and experiments which were considered career breakers for physicists have been honoured with the Nobel prize. Salutations to the stubborn physicists!
This blog post is by Neelima Gupte and Sumathi Rao.
More technical references here.
