Discovery. These quantum particles are basically "immortal"

In the late 1950s, the Soviet physicist Lev Landau developed the theory of Fermi liquids - the state of matter observed at low temperatures for crystalline solids - and introduced the concept of quasiparticles. The latter make it possible to describe complex physical systems in terms of particle groups and interactions, taking the form of vibrations and excitations. Recently, physicists have discovered that unlike ordinary particles that eventually disintegrate and disappear, quasiparticles can escape this inexorable fate, becoming "immortal".

The second law of thermodynamics is clear: the Universe systematically evolves towards disorder, entropy being brought to continually grow in any isolated system. Disintegrated objects can not reform. However, quantum mechanics is known to contravene certain well-established physical rules. In an article published in the journal Nature Physics , physicists have shown that quasiparticles that disintegrate can "be reborn from their ashes".

" Until now, the hypothesis was that the quasiparticles of interacting quantum systems decay after a while, " says physicist Frank Pollman of Munich Technical University. " We now know that the opposite is happening: strong interactions can even completely stop disintegration ." 

Three examples of quasiparticles: a) A polaron, that is to say an electron in a solid interacting with the crystal lattice; b) an exciton, that is, an electron-hole bound state; c) An angulon, that is to say a quantum rotor formed by a phonon field. Credits: Mikhail Lemeshko

Quasiparticles are not ordinary particles, like electrons and quarks. Rather, it is the disturbances or excitations in a solid caused by electrical or magnetic forces that collectively behave like particles. Phonons - discrete units of vibratory energy in a crystal lattice, for example - are classified as quasi-particles, as are polarons, electrons trapped in a network surrounded by a polarization cloud.

Quasiparticles: they disintegrate ... then reform

The researchers involved in this latest study have developed numerical methods to compute the complex interactions of these quasiparticles and have run simulations on a powerful computer to observe their disintegration.

" The result of the simulation: of course, the quasiparticles disintegrate, but new entities of identical particles emerge from the debris, " says physicist Ruben Verresen from the Technical University of Munich and the Max Planck Institute for Complex Systems Physics . " If this degradation occurs very quickly, a reverse reaction will occur after a while and debris will converge again. This process can be repeated at infinity and a sustained oscillation between disintegration and rebirth appears.

This does not violate the second law of thermodynamics because oscillation is a wave transformed into matter, which is covered by the concept of quantum mechanics of the wave-particle duality. Their entropy does not decrease but remains constant. In fact, the discovery solved two other puzzles. For example, there is a magnetic compound, Ba3CoSb2O9, used in experiments whose unexpected stability had previously been found.

It now seems that the key lies in the quasi-magnetic particles it contains, called magnons. According to the simulation, they reorganize after the degradation. Helium is another potential example: it becomes a superfluid without resistance at a temperature close to absolute zero, and this particular property could be explained by the fact that this gas is filled with quasi-particles called rotons.

For the moment, the work only concerns theory, but researchers believe that this immortality of quasi-particles offers a strong potential for sustainable data storage in quantum computing systems.


 Avoided quasiparticle decay from strong quantum interactions
Ruben Verresen, Roderich Moessner, Frank Pollmann
 Nature Physics 
DOI: 10.1038 / s41567-019-0535-3

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