Schematic illustration of the experiment. An initial density modualtion is imprinted onto the ultracold atoms held in the optical lattice potential (1). Without any disorder, the density modulation is washed out completely in the ensuing dynamics, indicating relaxation towards a thermal equilibrium state (2). In the presence of sufficiently strong disorder, the researchers find that even for long evolution times the system retains memory of the initial state, indicating a non-thermal state in which the system remains stuck (3). Credit: M. Schreiber, LMU
Using ultracold atoms trapped in light crystals, scientists have observed a novel state of matter that never thermalizes. What happens if one mixes cold and hot water? After some initial dynamics, one is left with lukewarm water – the system has thermalized to a new thermal equilibrium. In this thermal state, the system typically behaves very classically, any initial quantum effects would have been diluted over the whole system and typically cannot be detected anymore.
A team has for the first time created and analyzed a Many-Body Localized state, where despite interactions the many-body state fails to act as its own heat bath and does not thermalize. In this insulating state the system retains a quantum memory of its initial quantum state, even for long times.
Basko, Aleiner & Altshuler predicted in 2005 under special circumstances, a many-body localized state of matter should remain stable up to a critical temperature in the system, above which, or for weak enough disorder, particles are delocalized and system thermalizes. Today, it is understood this exotic transition presents a sharp boundary between a macroscopic system that shows strong quantum behaviour and one in which quantum effects are washed away in the dynamics.
MBL is of potential interest for applications in quantum information science as a means to protect the quantum information from decoherence. Munich and Weizmann present an experimental observation of many-body localized states for ultracold K+ atoms in an artificial crystal of light, optical lattice (overlapping and interfering several laser beams where the atoms can be trapped along 1 direction of motion) Both strength of the disorder and of the interaction between the atoms can be precisely controlled.
>>They directly tested whether intrinsic dynamics of interacting atoms in the optical lattice brings them to thermal equilibrium. Thus they prepare the system in a state with an imprinted density ripple and measure how such an imprint evolves with time. If dynamics is thermalizing, the density modulation is rapidly lost as thermal equilibrium must bear no memory of the initial state. Conversely, a persistent density modulation after the system has relaxed indicates localization.
The experimental findings were supported with theoretical calculations and simulations. While the non-interaction problem can be solved on any home computer, the Weizmann Institute had to use a supercomputer to simulate the behavior of only 40 interacting particles even for short times. “We were astonished to see the lifetimes of this novel state” … “Even though it is very quantum in nature, it is also much more stable than any typical many-body state we have looked at in the past.” http://www.en.uni-muenchen.de/news/newsarchiv/2015/bloch_mbl.html
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