Spooky quantum connection quantified for multiple particles

‘Entanglement entropy’ measurements provide clues to properties of complicated systems

rubidium atoms

EVERYWHERE ENTANGLEMENT  Each green dot in this cloud is a rubidium atom. Harvard physicists isolated sets of four atoms and measured the degree of entanglement between the atoms.

Markus Greiner

A first-of-its-kind measurement has quantified a mysterious quantum bond shared by several particles rather than just two. The experiment, reported in the Dec. 3 Nature, brings physicists closer to understanding the true scope of this link, known as quantum entanglement.

Entanglement interweaves particles’ fates so that some of each particle’s properties, which are inherently uncertain according to quantum mechanics, are tied to those of its partners. Each particle essentially sacrifices its individuality to become part of an umbrella entangled state. While physicists have developed reliable methods for detecting entanglement between pairs of particles, the measurements get tricky when three or more particles are involved.

A team of quantum physicists from Harvard University measured a property called entanglement entropy, which quantifies the apparent randomness that comes with observing just a portion of an entangled whole. Markus Greiner and colleagues used lasers to create an optical cage with four compartments, each of which held a rubidium atom chilled to nearly absolute zero. The researchers could tweak the laser settings to adjust the height of the walls between compartments. If the walls were low enough, atoms could exploit their strange quantum ability to occupy multiple compartments at once. As the four atoms jumped around, they interacted and established a state of entanglement.

Greiner’s team created a pair of four-compartment systems and confirmed that they were identical using a technique developed for comparing photons. Then the researchers compared portions of the two cages — say, two of the four compartments where atoms could reside. The partial system of one cage differed from the corresponding partial system of the other cage. A difference between parts when the wholes are indistinguishable “only happens if there is entanglement within each system,” Greiner says.

Peter Zoller, a theoretical quantum physicist at the University of Innsbruck in Austria, says that while studying entangled particle pairs is interesting, the real world is dominated by entangled states that encompass much larger sets of particles. Analyzing particles in collections similar to those in Greiner’s experiment could help physicists understand the complex entanglement-rich interactions between electrons in superconductors, which conduct electrical current with no resistance. 

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