Non-Locality Bursts Into Life

NEWS AND VIEWS

Non-locality bursts into life

The past few years have seen a resurgence of interest in the foundations of quantum mechanics, not least because there has been a succession of generations.

THAT we seem to be in for another bout of components of a single well-defined quan­ with a nearly perfect memory of where argument about the interpretation of tum state may be physically separated yet they have come from, but that they then quantum mechanics is plain, to judge from still bound together by the overall proper­ evolve independently. The numerical ine­ what has been appearing in the physics ties of the quantum state. quality is derived by a comparison of the journals, but why now, more than half a A simple example is that of two elec­ two states, represented by the directions a century after Nils Bohr is believed to have trons more or less at rest in a singlet state, and b, with a third, say c. The correlation settled everything? There are several which means that their net spin momen­ (or anticorrelation) is less strong. explanations. Bohr did not quieten all tum is zero, or that (if the term has Daniel I. Fivel from the University of doubters, among whom Einstein was one meaning) the two electron spins are op­ Maryland at College Park has now (most often represented by the gedanken positely directed. So what happens if the brought many of these arguments to­ experiment due to Einstein, Podolski and two electrons are flung diametrically apart gether in what seems to be a splendid Rosen, or EPR for short). Nor was de by some agency or another? Although the piece of rationalization by generalization Broglie ever reconciled to the Copenha­ random direction in which the two spins (Phys. Rev. Lett. 61, 286; 1991). All these gen view; there remains an active school cancel is still undetermined, the cancella­ discussions seem to hang on correlating of researchers devoted to his view. tion persists indefinitely. And if, at some what happens at one measuring device Much more recently, it has been rec­ stage, the projection of the angular with what happens at another. So the ognized that Bohr plucked from thin air momentum of one operator in some expectation value of a result with orienta­ the assumption that the square of the direction is measured, one can confidently tion a at one magnet and orientation b at (generally complex) amplititude of a solu­ infer the exact opposite for the cor­ the other is a number that might be tion to the wave equation represents the responding component of the other. thought to connect together all pairs of probability distribution of the particle or Einstein's protest was that this state of points on the surface of a sphere. (It turns other entity concerned, however success­ affairs cannot make sense. By what phy­ out to be simplest to get rid of negative ful interpretations based on it may be. The sical means can one electron "know" how numbers by adding + 1 to the expectation recent resurgence of interest thus at least to satisfy the overall requirements of the value and dividing the result by two.) partly springs from the drive to make such quantum state once a decision has been The function that results has all the leaps in the dark intelligible. made to measure the angular momentum properties of a metric (for measuring This seems the spirit in which people of its partner? Whence (in part) the distances on the surface of a sphere) and have been testing the old principle that doctrine of hidden variables - the asser­ Bell's inequality turns out to be just the quantities or "observables" represented tion that each of the two separating triangle inequality (that the sum of two by mathematical operators that do not electrons must carry the information sides must be greater than the third). But commute with each other cannot be mea­ determining its behaviour, consistently Fivel is also able to show that the assump­ sured simultaneously. From that it follows with that of its partner, when it encounters tion that only local issues matter, perhaps that it is possible to know only two a piece of equipment that requires the helped along by hidden variables, lead to components of the angular momentum of direction of its spin to be declared. the construction of a metric incompatible a particle, perhaps the components in two Attempts to investigate this behaviour with the first. It looks as if the outlook for distinct directions, but more usually the experimentally are bound, of course, to be the EPR way of working may be doomed. component in one direction and the total frustrating. With the same singlet state of But that will not prevent further exper­ angular momentum (which is a number). the electron pair, one can think (most of iment. Indeed the same issue of the same The now-standard drill for doing that this is still gedanken stuff) of using a strong journal contains an argument (by J. D. stems from the fact that any relationship magnetic field to allow electrons to pass if Franson of Johns Hopkins University) between operators representing physical they are polarized in the direction of the that non-locality can be demonstrated by observables must be mirrored in an iden­ field, but which rejects them if they are intereference between the photons of two tical relationship between values assigned oppositely polarized. One can be sure that light beams manipulated so that they (as if by measurement) to the same both members of a singlet electron pair reach the detectors at slightly different quantities, when it becomes a matter (not will each pass through a magnet in its path times, and an intriguing experiment with a always simple) of showing that the allow­ only if the fields are in opposite direction. pair of nonlinear crystals (lithium iodate) able numbers cannot be consistently as­ Most simply, the outcome of such an by a group from the University of Ro­ signed to all of a set of physical variables. experiment can be represented by the chester in which unexplained non-locality But a more immediate stimulus of product of two numbers, one for each is destroyed by preventing the stimulation interest among experimentalists in the magnet, in which one scores + 1 for of one crystal by light from the other. For interpretation of quantum mechanics has passage and -1 for failure. Allowing for piquancy, there is also a protest by P.R . been the publication (itself now more than arbitrary directions of the magnetic field, Holland and J.P.Vigier from the Univers­ a quarter of a century ago) of the late J. S. say a and b, the general quantum result is ity of Paris at what they consider to be a Bell's now-famous inequality - a nume­ that the expectation value of the product misrepresentation of de Broglie. rical relation between the probability that of these two numbers (or the average over There is ample evidence that the physically distant events will be correlated many individual measurements) is simply preoccupation will not go away. This does with each other. the negative of the scalar product a. b. not mean that people are worrying about Non-locality is the new game, as the Bell's tongue-in-cheek inequalities the foundations, but that they wish to EPR experiment dictates. Einstein was arise from supposing that the two elec­ understand them better. A new genera­ not the only one offended that the several trons leave the tightly bound singlet state tion has arrived. John Maddox NATURE · VOL 352 · 25 JULY 1991 277