Enlarge / A Bose-Einstein condensate of rubidium atoms forms. Similar clouds of atoms were used to study entanglement. (credit: NIST/JILA/CU-Boulder)
To say the quantum world is unintuitive is a staggering understatement. Particles end up in more than one place at a time, and the instances interact with each other. Decisions made after a photon has traversed an obstacle course determine the path it took through it. Entangled quantum objects can be in separate galaxies, yet measuring one will instantly set the fate of the second. Obviously, things like this don’t take place in the world of our common experience.
So where’s the boundary that separates the quantum world from ours? While the experiments above were first demonstrated with individual particles, researchers have revisited some of them with ever-larger objects, showing that entire molecules will act just like an electron does. Now, the limit’s been pushed back even further, as a collection of papers describes the entanglement of objects that consist of thousands of atoms.
A quantum cloud
Three of the papers grace the pages of Science, and all rely on a similar material: clouds of ultra-cold atoms, up to 20,000 of them. These rely on yet another quantum quirk: if two particles become physically indistinguishable, they start to behave like a single system of entangled particles. (As one of the more lucid teams of researchers write, “The entanglement generation relies on the fundamental particle-exchange symmetry in ensembles of identical particles.”) Conveniently, if you pick the right atoms to make a cloud, they’ll naturally form a Bose-Einstein condensate, in which all of them adopt the same state.
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