Quantum physics ‘goes big’ as researchers probe the boundary between the domain of the small and the everyday.
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Two of the most counter-intuitive and troubling aspects of quantum mechanics — the physics of the very small — are entanglement and superposition.
These are the ideas that particles can influence each other instantaneously, and that quantum systems can effectively exist in two contradictory states at the same time until a measurement is made to determine that state.
Clearly, these are not behaviors we see in everyday objects around us, but quite where the line is between the quantum and the ‘conventional’ has not been clear.
New research from scientists at the Technische Universität Dresden (TUD) and the Technische Universität München (TUM) may have blurred the line between the quantum and the everyday even further.
The team of scientists found quantum behavior in much larger systems than ever before.
The researchers discovered a new phase transition in lithium holmium fluoride (LiHoF4), in which the domains that determine magnetism demonstrate quantum mechanical effects that cause their properties to become entangled.
The research could have important applications in technologies that strongly depend on controlling materials and their properties like magnetism and superconductivity.
Superposition, Entanglement, and Cats in Boxes
One way of thinking about superposition is by using the example of Schrödinger’s cat, as devised by Erwin Schrödinger.
Imagine a cat placed in a box with what Schrödinger called a ‘diabolical device’ constructed of a single radioactive atom and a toxic substance released only if that atom decays — a completely random process.
If we treated the cat, the box, and the device as a quantum system, until the sealed box is open it technically exists in a superposition of states. That means the cat is effectively simultaneously ‘dead’ and ‘alive.’
To extend that example to cover entanglement, imagine there are two cats in identical boxes. One particle must decay, and if one particle decays the other is instantaneously prevented from doing so.
That means the second we open first the box and find the cat is dead or alive, we know the other cat must be if the opposite state without having to open the box.
Such quantum effects would usually be precluded in systems like cats in boxes because of how many atoms they contain, and even the new phase transition observed by the team involves systems with just 1,000s of atoms, several magnitudes less than would be in such a system.
Even so, entanglement in the compound LiHoF4 is still a demonstration of quantum effects in a much larger system than has been seen previously.
Quantum Physics and Phase Transitions
Magnetism in materials is determined by magnetic moments in large domains through the material aligning, At extremely low temperatures, LiHoF4 acts as a ferromagnet with all magnetic moments spontaneously pointing in the same direction.
Applying a magnetic field to this preferred direction that is vertical to it will cause fluctuations in the direction of these magnetic moments. The stronger the field, the greater the fluctuations, and this will eventually lead to a quantum phase transition at which the ferromagnetism disappears completely. As this happens, it results in the entanglement of neighboring magnetic moments.
Scientists have witnessed the spontaneous loss of magnetism in LiHoF4 when exposed to a strong magnet for 25 years. However, what they had not been aware of is what happens if the direction of the applied magnetic field is changed.
Previously, it had been thought that a small tilt in the magnetic field would suppress this quantum phase transition. The team now showed that the phase transition actually continues in these circumstances but in the larger domains of thousands of atoms rather than the small magnetic moments.
This means entire ‘islands’ of magnetic moments in a particular domain adopt the same alignment.
The research could find applications in technology like quantum sensors and quantum computers, in which materials have to be used in a controlled way.
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Reference and Further Reading
Wendl. A., Eisenlohr. H., Rucker. F., et al, , ‘Emergence of mesoscale quantum phase transitions in a ferromagnet,’ Nature, [https://www.nature.com/articles/s41586-022-04995-5]
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