Physicists at the University of Basel have observed the paradox of quantum mechanics Einstein-Podolsky-Rosen in a system of several hundred atoms that interact for the first time. The phenomenon goes back to a famous mental experiment of 1935. It allows to accurately predict the results of measurements and could be used in new types of sensors and imaging methods for electromagnetic fields. The findings were recently published in the journal Science .
How accurate can we predict the results of measurements in 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 accuracy of such predictions. This limit is expressed by the Heisenberg uncertainty relation, which states that it is impossible to predict simultaneously, for example, the measurements of the position and momentum of a particle, or of two components of a turn, with arbitrary precision.
A Paradoxical Decline in Uncertainty
In 1935, however, Albert Einstein, Boris Podolsky and Nathan Rosen published a famous article in which they showed that accurate predictions are theoretically possible under certain circumstances. To do so, they considered two systems, A and B, in what is known as a "tangled" state, in which their properties are strongly correlated.
In this case, the results of the measurements in system A can be used to predict the results of the corresponding measurements in 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 measures in system A to make more precise claims about system B than an observer who has direct access to system B (but not to A).
First observation in a particle system
In the past, experiments have used light or single atoms to study the EPR paradox, which takes its initials from the scientists who discovered it. Now, a team of physicists led by Professor Philipp Treutlein of the Physics Department of the University of Basel and the Swiss Institute of Nanoscience (SNI) has successfully observed the EPR paradox using a multi-particle system of several hundred interacting atoms for the first time .
The experiment used lasers to cool atoms to only a few billionths of a degree above absolute zero. At these temperatures, atoms behave completely according to the laws of quantum mechanics and form what is known as a Bose-Einstein condensate, a state of matter that Einstein predicted in another pioneering document in 1925. In this cloud ultra cold, the atoms collide constantly with each other, causing their spins to get tangled.
Next, the researchers took measurements of the spin in regions spatially separated from the condensate. Thanks to the high resolution images, they were able to measure the spin correlations between the separated regions directly and, at the same time, locate the atoms in precisely defined positions. With their experiment, the researchers managed to use 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," says PhD student Matteo Fadel, lead author of the study. "It is fascinating to observe such a fundamental phenomenon of quantum physics in increasingly larger systems, and at the same time, our experiments establish a link between two of Einstein's most important works."
On the road to quantum technology
In addition to their basic research, scientists are already speculating on possible applications for their discovery. For example, the correlations that are at the heart of the EPR paradox could be used to improve atomic sensors and imaging methods for electromagnetic fields. The development of quantum sensors of this type is one of the objectives of the National Center for Competence in Research in Quantum Science and Technology (NCCR QSIT), in which the research team participates actively.