The concept of quasiparticles was coined by Nobel laureate Lev Davidovich Landau, who used it to describe collective states of lots of particles or rather their interactions due to electrical or magnetic forces. Due to this interaction, several particles act like one single one.

The assumption has been that quasiparticles in interacting quantum systems decay after a certain time but the opposite is the case, according to a recent paper: strong interactions can even stop decay entirely. Collective lattice vibrations in crystals, so-called phonons, are one example of such quasiparticles.

The saying goes that nothing lasts forever and this seems to be true, all processes increase entropy and thus molecular disorder. A broken glass will never put itself back together again.

Except in the world of the really small, that isn't true.

Image: K. Verresen / TUM

Strong quantum interactions prevent quasiparticles from decay 

Quasiparticles decay but new, identical particle entities emerge from the debris, says the lead author, Ruben Verresen. "If this decay proceeds very quickly, an inverse reaction will occur after a certain time and the debris will converge again. This process can recur endlessly and a sustained oscillation between decay and rebirth emerges."

From a physical point of view, this oscillation is a wave which is transformed into matter, which, according to quantum mechanical wave-particle duality, is possible. Therefore, the immortal quasiparticles do not transgress the second law of thermodynamics. Their entropy remains constant, decay has been stopped.

The reality check

The discovery also explains phenomena which were baffling until now. Experimental physicists had measured that the magnetic compound Ba3CoSB2O9 is astonishingly stable. Magnetic quasiparticles, magnons, are responsible for it. Other quasiparticles, rotons, ensure that helium which is a gas on the earth's surface becomes a liquid at absolute zero which can flow unrestricted.

"Our work is purely basic research," emphasizes Frank Pollmann, Professor for Theoretical Solid-State Physics at the Technical University of Munich. However, it is possible that one day the results will even allow for applications, for example the construction of durable data memories for future quantum computers.