Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Quantum simulator and supercomputer at the crossroads

04.10.2010
MPQ-LMU scientists in an international collaboration measure for the first time a many-body phase diagram with ultracold atoms in optical lattices at finite temperatures.

Transitions between different phases of matter are a phenomenon occurring in everyday life. For example water – depending on its temperature – can take the form of a solid, a liquid or a gas. The circumstances that lead to the phase-transition of a substance are of fundamental interest in understanding emergent quantum phenomena of a many-particle system.


Matter-wave interference patterns across the BEC transition in the lattice: The image shows interference patterns of ultracold atoms released from an optical lattice at temperatures ranging from 10nK to 50nK (increasing from left to right). The experimental results (front row) perfectly match numerical quantum Monte Carlo simulations (back row) performed without free parameters. As the sample becomes colder sharp interference peaks appear (center), indicating the transition from a normal gas to a so-called superfluid. MPQ

In this respect, the ability to study phase transition between novel states of matter with ultracold atoms in optical lattices has raised the hope to answer open questions in condensed matter physics. MPQ-LMU scientists around Prof. Immanuel Bloch in collaboration with physicists in Switzerland, France, the United States and Russia have now for the first time determined the phase-diagram of an interacting many-particle system at finite temperatures (Nature Physics, AOP, 3.10.2010, DOI:10.1038/NPHYS1799). Employing state-of-the art numerical quantum “Monte Carlo” methods implemented on a supercomputer, it was possible to validate the measurements and the strategies used to extract the relevant information from them. This exemplary benchmarking provides an important milestone on the way towards quantum simulations with ultracold atoms in optical lattices beyond the reach of numerical methods and present day super computers.

In the experiments, a sample of up to 300.000 “bosonic” rubidium atoms was cooled down to a temperature close to absolute zero – approximately minus 273°C. At such low temperatures, all atoms in the ultracold gas tend to behave exactly the same, forming a new state of matter known as Bose-Einstein condensate (BEC). Once this state is reached, the researchers “shake” the atoms to intentionally heat them up again, thereby controlling the temperature of the gas to better than one hundredth of a millionth of a degree. The so-prepared ultracold – yet not as cold – gas is then loaded into a three-dimensional optical lattice. Such a lattice is created by three mutually orthogonal standing waves of laser light, forming “a crystal of light” in which the atoms are trapped. Much like electrons in a real solid body, they can move within the lattice and interact with each other repulsively. It is this analogy that has sparked a vast interest in this field, since it allows for the study of complex condensed matter phenomena in a tunable system without defects.

When being loaded into the optical lattice, the atoms can arrange in three different phases depending on their temperature, their mobility and the strength of the repulsion between them. If the strength of the repulsion between the atoms is much larger than their mobility, a so-called Mott-insulator will form at zero temperature in which the atoms are pinned to their lattice sites. If the mobility increases, a quantum phase transition is crossed towards a superfluid phase in which the wave functions of the atoms are delocalized over the whole lattice. The superfluid phase exists up to a transition temperature above which a normal gas is formed. This temperature tends to absolute zero as the phase transition between the superfluid and the Mott-insulator is approached – a feature which is typical in the vicinity of a quantum phase transition.

In order to determine the phase of the atoms in the experiments, they are instantaneously released from the optical lattice. Now, according to the laws of quantum mechanics, a matter wave expands from each of the lattice sites, much like electromagnetic waves expanding from an array of light sources. And as in the latter case, an interference pattern emerges that reflects the coherence properties of the array of sources. It is this information of the coherence properties that the scientists are looking at in order to read out the many-body phase of the atoms in the artificial crystal: The normal gas in the lattice shows little coherence and almost no interference pattern would be visible after releasing the atoms. The superfluid, however, does exhibit long-range phase coherence which results in sharp interference peaks. By determining the temperature of the onset of these defined structures for various ratios of interaction strength and mobility, the researchers could map out the complete phase boundary between the superfluid and the normal gas.

Given the large number of particles and the size of the artificial crystal, it is extremely demanding to simulate the physics of the present systems on a classical computer. Only recently, suitable quantum Monte Carlo methods have been developed, that allow for the direct simulation of the experiments on up to ten billion lattice sites without significant simplification of the problem. They have been implemented at the ETH in Zurich on the “Brutus” computer cluster. With the simulation results, it was for the first time possible to directly determine the temperature of the lattice gas, to quantify heating rates in the optical lattice and to validate the strategies employed to determine the phase diagram. The numerical calculations, however, could last several days, up to weeks, where the experiments could be performed within one or two hours. This difference in the timescales shows the value of the experimental setup as a “quantum simulator” of numerous, more complex problems beyond the reach of state-of-the-art numerical methods. [Stefan Trotzky]

Original publication:
Stefan Trotzky, Lode Pollet, Fabrice Gerbier, Ute Schnorrberger, Immanuel Bloch, Nikolay V. Prokof’ev, Boris Svistunov and Matthias Troyer
"Suppression of the critical temperature for superfluidity near the Mott transition"

Nature Physics, Advance Online Publication, DOI: 10.1038/NPHYS1799

Dipl.-Phys. Stefan Trotzky
LMU Munich, Fakultät für Physik
Schellingstr. 4
80799 München
Phone: +49 89 2180 6133
Fax: +49 89 2180 63851
e-mail: stefan.trotzky@lmu.de
Prof. Dr. Immanuel Bloch
Chair of Experimental Physics LMU Munich,
Schellingstr. 4
80799 München, Germany, and
Max Planck Institute of Quantum Optics
Phone: +49 89 32905 138
Fax: +49 89 32905 313
e-mail: immanuel.bloch@mpq.mpg.de
Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Phone: +49 - 89 / 32905 - 213
e-mail: olivia.meyer-streng@mpq.mpg.de

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

More articles from Physics and Astronomy:

nachricht Smallest transistor worldwide switches current with a single atom in solid electrolyte
17.08.2018 | Karlsruher Institut für Technologie (KIT)

nachricht Protecting the power grid: Advanced plasma switch for more efficient transmission
17.08.2018 | DOE/Princeton Plasma Physics Laboratory

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: It’s All in the Mix: Jülich Researchers are Developing Fast-Charging Solid-State Batteries

There are currently great hopes for solid-state batteries. They contain no liquid parts that could leak or catch fire. For this reason, they do not require cooling and are considered to be much safer, more reliable, and longer lasting than traditional lithium-ion batteries. Jülich scientists have now introduced a new concept that allows currents up to ten times greater during charging and discharging than previously described in the literature. The improvement was achieved by a “clever” choice of materials with a focus on consistently good compatibility. All components were made from phosphate compounds, which are well matched both chemically and mechanically.

The low current is considered one of the biggest hurdles in the development of solid-state batteries. It is the reason why the batteries take a relatively long...

Im Focus: Color effects from transparent 3D-printed nanostructures

New design tool automatically creates nanostructure 3D-print templates for user-given colors
Scientists present work at prestigious SIGGRAPH conference

Most of the objects we see are colored by pigments, but using pigments has disadvantages: such colors can fade, industrial pigments are often toxic, and...

Im Focus: Unraveling the nature of 'whistlers' from space in the lab

A new study sheds light on how ultralow frequency radio waves and plasmas interact

Scientists at the University of California, Los Angeles present new research on a curious cosmic phenomenon known as "whistlers" -- very low frequency packets...

Im Focus: New interactive machine learning tool makes car designs more aerodynamic

Scientists develop first tool to use machine learning methods to compute flow around interactively designable 3D objects. Tool will be presented at this year’s prestigious SIGGRAPH conference.

When engineers or designers want to test the aerodynamic properties of the newly designed shape of a car, airplane, or other object, they would normally model...

Im Focus: Robots as 'pump attendants': TU Graz develops robot-controlled rapid charging system for e-vehicles

Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.

Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

LaserForum 2018 deals with 3D production of components

17.08.2018 | Event News

Within reach of the Universe

08.08.2018 | Event News

A journey through the history of microscopy – new exhibition opens at the MDC

27.07.2018 | Event News

 
Latest News

A materials scientist’s dream come true

21.08.2018 | Materials Sciences

Quantum bugs, meet your new swatter

20.08.2018 | Information Technology

A novel synthetic antibody enables conditional “protein knockdown” in vertebrates

20.08.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>