New quantum entanglement record but still no Schrödinger’s cat

Swiss researchers have successfully entangled 16 million atoms with a single photon, but macroscopic results are a long way off. Michael Lucy reports.

A cat keeping an eye on developments in laser technology.
A cat keeping an eye on developments in laser technology.
Benjamin Torode / Getty

Your average cat contains more than a hundred million billion billion atoms. So that’s how many atoms would need to be ‘entangled’ via quantum mechanics to realise Schrödinger’s famous thought experiment, in which the random decay of a single atom could cause a cat in a box to be somehow alive and dead at the same time.

We’re not quite there yet, but researchers at the University of Geneva in Switzerland have announced a dramatic step in this direction by creating and verifying a single quantum state of 16 million atoms. While the entanglement of pairs of particles is a common feat in laboratories, multi-particle entanglement is more complex and the previous record was a mere 2900 atoms.

The achievement was “a side project” according to Florian Fröwis, the lead author of a paper describing the research in Nature Communications. It grew from work to develop a ‘quantum memory’ device that can store single photons for use in futuristic communications networks.

“We always had the suspicion that the storage of a single photon should create entanglement between a large number of atoms,” he says.

The quantum memory device is based on a special crystal made of the exotic-sounding yttrium orthosilicate, doped with atoms of neodymium. When the crystal absorbs photons, information about their energy and direction is stored in the relationships between the atoms in the crystal. This means it can later re-emit photons that are identical to those that came in.

When the crystal absorbs only a single photon, however, things get strange. The crystal will still re-emit a photon that is exactly the same as the one that went in, which still means that information about the photon must be stored in the relationship between the atoms. However, a single photon can only be absorbed by a single atom, not by a many at once.

What’s going on? The atoms in the crystal have become entangled: it no longer makes sense to talk about them as individual particles, but only to talk about the system as a whole.

Technically speaking, physicists describe the situation as a combination (called a ‘superposition’) of all the cases in which the photon is absorbed and emitted by a specific one of the atoms. This property is vital to the function of a quantum memory device, which must be able to store and re-emit photons without measuring their properties, which would destroy vital quantum information.

With the aid of some fancy maths, Fröwis and his fellow physicists were able to use data about the way the crystal emitted those single photons to calculate how many atoms were interacting with the photons, and the minimum number of those atoms that must have been entangled.

Their result? Around 40 billion atoms interacted with the photons, of which at least 16 million were entangled.

The number is only a lower bound. “I guess it is more in the order of billions, but this is pure speculation,” notes Fröwis.

The Swiss team are not the only people studying this problem. Physicists at the University of Calgary have also just published research using a similar technique, in which they demonstrate the entanglement of more than 200 groups of atoms, with each group itself containing more than a billion atoms.

So should we start worrying about the existential status of our cats? Not just yet. While 16 million atoms might sound like a lot, there are a million million times as many in even a single grain of sand.

Anyway, says Fröwis, there are some fundamental differences between Schrödinger’s cat and his experiment.

“A so-called Schrödinger-cat state is ideally a superposition of two (or more) ‘macroscopically distinct’ states of a many-body system,” he says, meaning that the different states are easily distinguishable to an observer, just as the cat will be either alive or dead when the box is opened. “Superpositions of macroscopically distinct states imply high fragility of the underlying entanglement, which is not present in our case.”

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