![]() ![]() These “spins” are defined by two hyperfine levels, with the spin of each cloud determined by the number of atoms in one level minus the number of atoms in the other level. Instead of using position and momentum as envisaged by EPR, Colciaghi and colleagues use “pseudospins”-a pair of quantum states that, like spin, constitute a two-level system. In order to test the paradox, it is necessary to measure two noncommuting observables. Once released from the trap, the condensate expands to form two entangled clouds separated by up to 100 µm. They start by preparing a single Bose-Einstein condensate in a trap and engineer an interaction to entangle the condensate’s atoms (Fig. Whereas most Bell tests have been conducted on pairs of individual particles, Colciaghi and colleagues use clouds of several hundred rubidium-87 atoms. EPR proposed to resolve this paradox by suggesting that quantum mechanics is incomplete, implying that a full theory would include what physicists now term local hidden variables-a possibility that Bell tests have since ruled out. But quantum mechanics does not allow simultaneous, precisely defined values for both position and momentum. Importantly, the system is set up so that the particles are “space-like separated,” meaning there can be no disturbance of A because of a measurement at B.Īssuming local realism, EPR concluded that the particles’ positions and momenta are both simultaneously well-defined. This relationship between observables means that an experimentalist should be able to determine the position or momentum of particle A with certainty by making the appropriate measurement of B. The systems are prepared so that the particles’ positions are correlated and their momenta are anticorrelated. Their results bring into question the validity of EPR’s local realism for mesoscopic massive systems.ĮPR considered a system of two spatially separated particles, A and B, that have pairs of noncommuting observables, such as their position and momentum. Now Paolo Colciaghi and colleagues at the University of Basel, Switzerland, have tested EPR’s argument for a larger system comprising clouds of hundreds of atoms. John Bell subsequently proposed a way to experimentally test these “local realism” assumptions, and so-called Bell tests have since invalidated them for systems of a few small particles, such as electrons or photons. ![]() Second, physical processes have effects that act locally rather than instantaneously over a distance. ![]() First, if the value of a physical property of a system can be predicted with certainty, without disturbance to the system, then there is an “element of reality” to that property, meaning it has a value even if it isn’t measured. In 1935, Einstein, Podolsky, and Rosen (EPR) presented an argument that they claimed implies that quantum mechanics provides an incomplete description of reality. When the condensate is released, it forms two separate clouds whose pseudospins are entangled. An interaction is engineered between the atoms while they are trapped as a Bose-Einstein condensate (left), causing them to become entangled (center). adapted by APS Figure 1: Colciaghi and colleagues perform a test of the Einstein-Podolsky-Rosen paradox using the “pseudospins” of two clouds of hundreds of rubidium-87 atoms. ![]()
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