Entangled Atoms in Salt: Uncovering the Surprising Results of a 2003 Experiment

[Varon]

According to Scientific American June 2011:

"A neat experiment in 2003 proved that larger systems, too, can remain entangled when the leakage is reduced or somehow counteracted. Gabriel Aeppli of University College London and his colleagues took a piece of lithium fluoride salt and put it in an external magnetic field. You can think of the atoms in the salt as little spinning magnets that try to align themselves with the external field, a response known as magnetic susceptibility. Forces that the atoms exert on one another act as a kind of peer pressure to bring them into line more quickly. As the researchers varied the strength of the magnetic field, they measured how quickly the atoms became aligned. They found that the atoms responded much faster than the strength of their mutual interactions would suggest. Evidently some additional effect was helping the atoms to act in unison, and the researchers argued that entanglement was the culprit. If so, the 10^20 atoms of the salt formed a hugely entangled state."

In a normal substance. Aren't the atoms entangled? This is because you can describe the whole molecule as superpositions of all atoms. So how does this differ to the above experiment?

 

[SpectraCat]

According to Scientific American June 2011:

"A neat experiment in 2003 proved that larger systems, too, can remain entangled when the leakage is reduced or somehow counteracted. Gabriel Aeppli of University College London and his colleagues took a piece of lithium fluoride salt and put it in an external magnetic field. You can think of the atoms in the salt as little spinning magnets that try to align themselves with the external field, a response known as magnetic susceptibility. Forces that the atoms exert on one another act as a kind of peer pressure to bring them into line more quickly. As the researchers varied the strength of the magnetic field, they measured how quickly the atoms became aligned. They found that the atoms responded much faster than the strength of their mutual interactions would suggest. Evidently some additional effect was helping the atoms to act in unison, and the researchers argued that entanglement was the culprit. If so, the 10^20 atoms of the salt formed a hugely entangled state."

In a normal substance. Aren't the atoms entangled? This is because you can describe the whole molecule as superpositions of all atoms. So how does this differ to the above experiment?

A superposition in the context you are describing does not just mean "a bunch of quantum states of different systems added together". It means "a bunch of quantum states of a single system added together coherently", i.e. with a well-defined phase relationship that is either time-independent or at least long-lived with respect to the relevant measurement timescale.

Evidently, it is possible to create such superpositions of spin states in a macroscopic salt sample using an external magnetic field. If you tried to do the same thing with, say, the phonon modes of the salt lattice, I doubt that it would work unless the crystal was cooled to extremely low temperatures. Using intense laser fields, it is possible to create coherences in the macroscopic polarization of materials, but these generally last only a short time (a few nanoseconds or so).

The reason that it is hard to create coherent superpositions in macroscopic samples probably won't surprise you at this point ... it is decoherence ;). As the quotation you posted suggests, in most systems the interactions between the states of the system, and between the system and the surroundings, cause any coherent superposition to rapidly lose decohere. And as I said at the beginning, if a quantum system is not in a coherent superposition, then it is not entangled .. at least not by the usual definition of quantum entanglement.