Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Quantum Matter Stuck in Unrest

31.07.2015

Using ultracold atoms trapped in light crystals, scientists from the MPQ, LMU, and the Weizmann Institute observe 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.


Schematic illustration of the experiment. An initial density modulation 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).

This is true not only when we pour cold milk into our hot coffee, but it is also what happens for almost all interacting systems we know in nature: in the long run they all approach a thermal state with an associated temperature. 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 of researchers from Ludwig-Maximilians-Universität Munich (LMU) and the Max Planck Institute of Quantum Optics (MPQ) lead by Prof. Immanuel Bloch and Dr. Ulrich Schneider in collaboration with the theory group of Prof. Ehud Altman at Weizmann Institute, Rehovot, have now for the first time created and analyzed a so-called Many-Body Localized state, where despite the presence of interactions the many-body state fails to act as its own heat bath and does not thermalize.

In this peculiar insulating state the system retains a quantum memory of its initial quantum state, even for long times. Their results will be published this week in Science Express, the advanced online publication venue of Science.

In a metal the particles that carry energy and electric charge can move freely and spread out over the entire sample allowing the mixing that leads to thermal equilibrium. There are however mechanisms that can hinder such transport. For example in a band insulator, every atom in the crystal has completely filled energy levels or shells. Because of the Pauli principle, electrons in a band insulator cannot move to the filled shell of the nearby atom.

Disorder, in the form of impurity atoms, can also localize quantum particles through a mechanism discovered in 1958 by Nobel-prize winner P. W. Anderson. But neither mechanism is completely effective in actual solids. In a band insulator, for example, a fraction of the charge carriers is always thermally excited to empty atomic orbitals of higher energy, thereby allowing free motion. In a disordered Anderson insulator, the electrons, which would ideally be localized, are kicked around by thermal vibrations of the crystal that ultimately spread them out over the entire system.

A fundamental open question

But what if the particles were trapped in a rigid lattice that does not vibrate? Would the system then remain localized at elevated temperature and fail to attain thermal equilibrium? This is still a fundamental open question. Anderson’s theory of localization was originally formulated for non-interacting particles. If the particles interact, as they generally do, then one particle is kicked and should be delocalized by the thermal motion of the neighboring particles.

Remarkably, Basko, Aleiner & Altshuler predicted in 2005 the exact opposite effect: under special circumstances, a many-body localized state of matter should remain stable up to a critical temperature in the system. Above the critical temperature or for weak enough disorder, the particles are delocalized and the system thermalizes under its own intrinsic dynamics. Today, it is understood that 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.

Many body localized systems are of fundamental interest as the only generic exceptions to thermalization; they represent a new class of systems that fail to be described by standard thermodynamics and statistical physics and require new theoretical and experimental approaches to characterize them. At the same time many-body localization is of potential interest for applications in quantum information science as a means to protect the quantum information from decoherence. But in spite of the fundamental interest in the subject, until now an experimental observation and investigation of this intriguing phenomenon has been lacking.

Precisely controlled in the experiment

In their manuscript, the Munich and Weizmann researchers present now an experimental observation of many-body localized states for ultracold potassium atoms in an artificial crystal of light, a so called optical lattice that is created by overlapping and interfering several laser beams. The optical lattice represents a microscopic grid of tiny light spots in which the atoms can be trapped, such that a rigid and random potential for the atoms is realized along one direction of motion. Both the strength of the disorder and of the interaction between the atoms can be precisely controlled in the experiment.

The team then directly tested whether the intrinsic dynamics of the interacting atoms in the optical lattice brings them to thermal equilibrium. To this end, they prepare the system in a state with an imprinted density ripple and measure how such an imprint evolves with time. If the dynamics is thermalizing, then the density modulation is rapidly lost as the thermal equilibrium must bear no memory of the initial state. Conversely, a persistent density modulation after the system has relaxed indicates localization. In this way the researchers mapped out the boundaries of the localized phase with changing disorder and interaction strengths.

The team reproduced the known non-interacting case as a benchmark, while the experimental findings were supported with theoretical calculations and simulations performed by the Weizmann team. The presence of interactions complicates the problem immensely, as joint quantum motion between all particles has to be taken into account. While the non-interaction problem can be solved on any home computer, the theorists at Weizmann Institute had to use a supercomputer to simulate the behaviour of only 40 interacting particles even for short times.

“We were astonished to see the lifetimes of this novel state” says Michael Schreiber, the leading PhD student, “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.”


Original Publication:
Michael Schreiber, Sean S. Hodgman, Pranjal Bordia, Henrik P. Lüschen, Mark H. Fischer, Ronen Vosk, Ehud Altman, Ulrich Schneider, Immanuel Bloch
Observation of many-body localization of interacting fermions in a quasi-random optical lattice
Science Express, 31 July 2015

Contact:

Prof. Dr. Immanuel Bloch
Chair of Quantum Optics, LMU München
Schellingstr. 4, 80799 Munich
Director at Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching, Germany
Phone: +49 (0)89 / 32 905 -138
E-mail: immanuel.bloch@mpq.mpg.de

Dr. Ulrich Schneider
LMU Munich
Phone: +49 (0)89 / 21 80 - 6129
E-Mail: ulrich.schneider@physik.uni-muenchen.de

Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics, Garching, Germany
Phone: +49 (0)89 / 32 905 -213
E-mail: olivia.meyer-streng@mpq.mpg.de

Dr. Olivia Meyer-Streng | Max-Planck-Institut für Quantenoptik
Further information:
http://www.mpq.mpg.de/

More articles from Physics and Astronomy:

nachricht Tangled magnetic fields power cosmic particle accelerators
14.12.2018 | DOE/SLAC National Accelerator Laboratory

nachricht In search of missing worlds, Hubble finds a fast evaporating exoplanet
14.12.2018 | NASA/Goddard Space Flight Center

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Data use draining your battery? Tiny device to speed up memory while also saving power

The more objects we make "smart," from watches to entire buildings, the greater the need for these devices to store and retrieve massive amounts of data quickly without consuming too much power.

Millions of new memory cells could be part of a computer chip and provide that speed and energy savings, thanks to the discovery of a previously unobserved...

Im Focus: An energy-efficient way to stay warm: Sew high-tech heating patches to your clothes

Personal patches could reduce energy waste in buildings, Rutgers-led study says

What if, instead of turning up the thermostat, you could warm up with high-tech, flexible patches sewn into your clothes - while significantly reducing your...

Im Focus: Lethal combination: Drug cocktail turns off the juice to cancer cells

A widely used diabetes medication combined with an antihypertensive drug specifically inhibits tumor growth – this was discovered by researchers from the University of Basel’s Biozentrum two years ago. In a follow-up study, recently published in “Cell Reports”, the scientists report that this drug cocktail induces cancer cell death by switching off their energy supply.

The widely used anti-diabetes drug metformin not only reduces blood sugar but also has an anti-cancer effect. However, the metformin dose commonly used in the...

Im Focus: New Foldable Drone Flies through Narrow Holes in Rescue Missions

A research team from the University of Zurich has developed a new drone that can retract its propeller arms in flight and make itself small to fit through narrow gaps and holes. This is particularly useful when searching for victims of natural disasters.

Inspecting a damaged building after an earthquake or during a fire is exactly the kind of job that human rescuers would like drones to do for them. A flying...

Im Focus: Topological material switched off and on for the first time

Key advance for future topological transistors

Over the last decade, there has been much excitement about the discovery, recognised by the Nobel Prize in Physics only two years ago, that there are two types...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

ICTM Conference 2019: Digitization emerges as an engineering trend for turbomachinery construction

12.12.2018 | Event News

New Plastics Economy Investor Forum - Meeting Point for Innovations

10.12.2018 | Event News

EGU 2019 meeting: Media registration now open

06.12.2018 | Event News

 
Latest News

Data use draining your battery? Tiny device to speed up memory while also saving power

14.12.2018 | Power and Electrical Engineering

Tangled magnetic fields power cosmic particle accelerators

14.12.2018 | Physics and Astronomy

In search of missing worlds, Hubble finds a fast evaporating exoplanet

14.12.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>