Physicists conduct the most massive test ever on the Einstein-Podolsky-Rosen paradox

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In the most massive test to date, physicists have probed a major paradox in quantum mechanics and found that it still holds true even for clouds of hundreds of atoms.

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Using two entangled Bose-Einstein condensates, each made up of 700 atoms, a team of physicists co-led by Paolo Colciaghi and Yifan Li of the University of Basel in Switzerland showed that the Einstein-Podolsky-Rosen (EPR) paradox increases .

The researchers say this has important implications for quantum metrology, the study of measuring things according to quantum theory.

‘Our results represent the first observation of the EPR paradox with spatially separated massive many-particle systems,’ the researchers write in their paper.

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“They show that the conflict between quantum mechanics and local realism does not disappear as the size of the system increases to more than a thousand massive particles.”

While we’re pretty good at describing the Universe mathematically, our understanding of how things work is fragmentary at best.

One of the tools we use to fill one of the gaps is quantum mechanics, a theory born in the early 20th century, advocated by physicist Niels Bohr, to describe how atomic and subatomic matter behaves. In this tiny realm, classical physics collapses; when the old rules no longer apply, new rules must be made.

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But quantum mechanics is not without its flaws, and in 1935 three famous physicists found a significant hole. Albert Einstein, Boris Podolsky and Nathan Rosen described the famous Einstein-Podolsky-Rosen paradox.

Nothing can travel faster than light, right? But it gets a little tricky with quantum entanglement, what Einstein called “spooky action at a distance.” This is where you relate two (or more) particles so that their properties are related; if one particle, for example, rotates in one direction, the other rotates in the other.

These particles maintain this link even over large distances, and it’s not clear how or why. Scientists know that if you measure the properties of one particle, you can infer the properties of the other, even at that distance.

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However, under quantum mechanics, the particle won’t have those properties until you measure it (a quirk explored by Schrdinger’s cat thought experiment).

And, under quantum mechanics, if you know one property of a particle, like its position, you can’t know another, like its momentum, with certainty. This is Heisenberg’s uncertainty principle.

The classical physics concept of local realism also states that for one object or energy to affect another, the two must interact.

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The EPR paradox, therefore, is complex. When you measure a particle in an entangled system, that measurement affects the other particle in some way, even if the measurement doesn’t happen locally.

You also know more about particles than Heisenberg’s uncertainty principle allows. And somehow, that influence happens instantaneously, defying the speed of light.

The EPR paradox, therefore, suggests that the theory of quantum mechanics is incomplete; it does not fully describe the reality of the Universe we live in. Physicists have mostly tested it on small entangled systems, consisting of just a pair of atoms or photons, often, in what’s known as the Bell test (named after its creator, physicist John Stewart Bell).

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So far, every test Bell has conducted has found that the real world behaves inconsistently with local realism. But how deep is the paradox?

Well, this is where we come to Bose-Einstein condensates, a state of matter created by cooling a cloud of bosons to just a fraction above absolute zero. At such low temperatures, the atoms drop to their lowest possible energy state without stopping completely.

When they reach these low energies, the quantum properties of the particles can no longer interfere with each other; they move close enough to each other to overlap, resulting in a high-density cloud of atoms that behaves like a “super atom” or matter wave.

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Colciaghi, Li and their physicist colleagues Philipp Treutlein and Tilman Zibold, also of the University of Basel, generated two Bose-Einstein condensates using two clouds, each made up of 700 rubidium-87 atoms. They separated these condensates spatially down to 100 micrometres and measured their properties.

They measured the quantum properties of the condensates known as pseudospins, independently choosing which value to measure for each cloud.

They found that the properties of the two condensates appeared to be related in a way that could not be attributed to chance, demonstrating that the EPR paradox held on a much larger scale than Bell’s previous tests.

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The implications of the team’s findings are largely relevant to future quantum research.

“Our experiment is particularly suitable for quantum metrology applications. One of the two systems can, for example, be used as a small sensor to probe fields and forces with high spatial resolution and the other as a reference to reduce the quantum noise of the first system “, write the researchers in their paper.

“The demonstration of EPR entanglement in combination with the spatial separation and individual addressability of the systems involved is therefore not only fundamentally significant, but also provides the necessary ingredients to exploit EPR entanglement in many-particle systems as an asset.”

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Now go make yourself a nice cup of tea and sit down. You’ve earned it.

The research was published in Physical revision X.

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