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Tuesday, 21 January 2020

Quantum physics: first experimental proof of a link between quantum entanglement and quantum criticality

Illustration showing two entangled particles. | National Institute of Standards and Technology

Quantum physics concerns infinitely small scales (including the interactions between atoms and particles), for which the laws of classical physics are no longer sufficient to describe the different phenomena at play. Quantum entanglement is the phenomenon in which two particles (or groups of particles) form a linked system, with quantum states depending on each other, regardless of the distance between them. Recently, researchers have been able to establish a link between quantum entanglement and quantum criticality (a phenomenon linked to quantum fluctuations ), by entangling billions & billions of electrons through a metallic film.



To carry out their entanglement experiment, the researchers produced a film from a mixture of ytterbium, rhodium and silicon. They then passed billions & billions of tangled electrons through it. Physicists call this kind of material a “strange metal” because it does not act like real metal at very low temperatures.

"With strange metals, there is an unusual link between electrical resistance and temperature, " said physicist Silke Bühler-Paschen, of the Vienna University of Technology in Austria.


" Unlike simple metals such as copper or gold, this does not seem to be due to the thermal movement of atoms, but to quantum fluctuations at absolute zero temperature, " she adds.

Quantum criticality: a phenomenon linked to quantum fluctuations

These fluctuations represent a quantum criticality. It is more precisely a “point” between the quantum states, which are the equivalent of the transition between liquids, solids and gases in classical physics. The team claims that this cascade of electrons (through the metallic film) has produced the best evidence to date of a link between quantum criticality and entanglement. The results of the study were published in the journal Science .

"When we think of quantum entanglement, we think of small scales,  " says physicist Qimiao Si of Rice University. " We don't associate it with macroscopic objects ". " But at quantum critical point, things are so 'collective' that we are fortunate to see the effects of entanglement, even in a metallic film that contains billions & billions of quantum mechanical objects."

The experiments conducted by Bühler-Paschen, Si and his colleagues, were very difficult to carry out on several levels, from the synthesis of very complex materials required to create the strange metal, to the delicate terahertz spectroscopy required to observe the electrons.

The terahertz spectrometer used to measure entanglement. Credits: Jeff Fitlow / Rice University

After a meticulous measurement process, the team identified what they were looking for: the telltale sign of quantum criticality. " Conceptually, it was truly a dream experience ," says Si. " Probe the charge area at the critical quantum magnetic point, to see if it is indeed critical, if it reveals dynamic scaling ."

“ If nothing collective is observed, no scaling, the critical point is rather easy to describe. But if, on the contrary, something singular is observed (which was the case here), then it is a very direct and new proof concerning the nature of quantum entanglement and quantum criticality, ”he adds



A discovery with high potential and multiple benefits

This discovery may lead to potential advances in quantum computing, telecommunications and more. In the past, researchers had hypothesized a link between quantum entanglement and quantum criticality, but this is the first experimental confirmation.

The study of quantum states is still in its infancy, but it may hold the key to all kinds of exotic quantum phenomena, such as high temperature superconductivity, which is also believed to be supported by quantum criticality.

Understanding how these quantum phases commute gives us more opportunities to manage them in the future. And although we are still far from it, we have come a little closer.

" Our results suggest that the same underlying physics (quantum criticality) can lead to a platform for quantum information and high-temperature superconductivity ," says Si. " When considering this possibility, one cannot refrain from marveling at the wonders of nature ”.


Bibliography:

Singular charge fluctuations at a magnetic quantum critical point
L. Prochaska,  X. Li, D. C. MacFarland A. M. Andrews3, M. Bonta. E. F. Bianco, S. Yazdi, W. Schrenk7, H. Detz7,#, A. Limbeck, Q. Si8, E. Ringe6, G. Strasser, J. Kono, S. Paschen

Science
Vol 367, Issue 6475
17 January 2020

https://science.sciencemag.org/content/367/6475/285

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