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A team of researchers in Switzerland has observed the quantum mechanical Einstein-Podolsky-Rosen paradox in a system of interacting ultracold atoms. Their work appears in the journal Science.
A cloud of atoms is held above a chip by electromagnetic fields; the EPR paradox was observed between the spatially separated regions A and B. Image credit: Department of Physics, University of Basel.
“How precisely can we predict the results of measurements on a physical system? In the world of tiny particles, which is governed by the laws of quantum physics, there is a fundamental limit to the precision of such predictions,” said team leader Professor Philipp Treutlein from the University of Basel and colleagues.
“This limit is expressed by the Heisenberg uncertainty relation, which states that it is impossible to simultaneously predict, for example, the measurements of a particle’s position and momentum, or of two components of a spin, with arbitrary precision.”
In 1935, however, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper in the journal Physical Review in which they showed that precise predictions are theoretically possible under certain circumstances.
“To do so, they considered two systems, A and B, in what is known as an ‘entangled’ state, in which their properties are strongly correlated,” the physicists explained.
“In this case, the results of measurements on system A can be used to predict the results of corresponding measurements on system B with, in principle, arbitrary precision. This is possible even if systems A and B are spatially separated.”
“The paradox is that an observer can use measurements on system A to make more precise statements about system B than an observer who has direct access to system B (but not to A).”
In the past, experiments have used light or individual atoms to study the Einstein-Podolsky-Rosen (EPR) paradox.
Now, Professor Treutlein and co-authors have successfully observed the paradox using a Bose-Einstein condensate — a many-particle system of several hundred interacting atoms — for the first time.
The team used lasers to cool atoms to just a few billionths of a degree above absolute zero.
In this ultracold cloud, the atoms constantly collide with one another, causing their spins to become entangled.
The physicists then took measurements of the spin in spatially separated regions of the condensate.
Thanks to high-resolution imaging, they were able to measure the spin correlations between the separate regions directly and, at the same time, to localize the atoms in precisely defined positions.
With their experiment, they succeeded in using measurements in a given region to predict the results for another region.
“The results of the measurements in the two regions were so strongly correlated that they allowed us to demonstrate the EPR paradox,” said Matteo Fadel, a Ph.D. student at the University of Basel.
“It’s fascinating to observe such a fundamental phenomenon of quantum physics in ever larger systems. At the same time, our experiments establish a link between two of Einstein’s most important works.”
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Matteo Fadel et al. 2018. Spatial entanglement patterns and Einstein-Podolsky-Rosen steering in Bose-Einstein condensates. Science 360 (6387): 409-413; doi: 10.1126/science.aao1850
