Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Speed Limit on the Quantum Highway

26.01.2012
Physicists at the Max Planck Institute of Quantum Optics have measured the propagation velocity of quantum signals in a many-body system.

A quantum computer based on quantum particles instead of classical bits, can in principle outperform any classical computer. However, it still remains an open question, how fast and how efficient quantum computers really may be able to work.


Propagation of quantum correlations in an optical lattice. Artist’s view (Graphic by woogie works animation studio).


a) In the initial state, each lattice site is filled by exactly one atom. The height of the barrier between the sites is then abruptly lowered, bringing the system out of equilibrium. b) After the barrier has been lowered, an entangled doublon-holon pair is formed. The correlated doublons and holons move across the system with opposite momenta. (Graphic: MPQ)

A critical limitation will be given by the velocity with which a quantum signal can spread within a processing unit. For the first time, a group of physicists from the Quantum Many-Body Systems division at the Max-Planck Institute of Quantum Optics (Garching near Munich) in close collaboration with theoretical physicists from the University of Geneva (Switzerland) has succeeded in observing such a process in a solid-state like system (Nature, DOI:10.1038/nature10748).

The physicists generated a perfectly ordered lattice of rubidium atoms and then induced a quantum excitation – an “entangled” pair of a doubly occupied lattice site next to a hole. With the aid of a microscope they observed how this signal moved from lattice site to lattice site. “This measurement gives insight into very elementary processes involved in the communication and processing of quantum information”, Professor Immanuel Bloch points out.

The communication and processing of information in a quantum computer is based on concepts that are inherently different from those used in classical computers. This is due to the fundamental differences between quantum particles and classical objects. Whereas the latter are, for example, either black or white, quantum particles can take on both colours at the same time. It is only at the process of measurement that the particles decide on one of the two possible properties. As a consequence of this peculiar behaviour, two quantum objects can form one entangled state in which their properties are strictly connected, i.e. quantum correlated. At present there is no general model for predicting how fast a quantum correlation can travel after it is generated.

Now physicists from the Quantum Many-Body Systems division have been able to directly observe such a process. They start the experiment by generating an extremely cold gas of rubidium atoms. The ensemble is then kept in a light field which divides it into several parallel one-dimensional tubes. Now the tubes are superimposed with yet another light field, a standing laser light wave. By the periodic sequence of dark and bright areas, the atoms are forced to form a lattice structure: exactly one atom is trapped in each bright spot, and is separated from the neighbouring atom by a dark area which acts as a barrier.

Changing the intensity of the laser light controls the height of this barrier. At the beginning of the experiments, it is set to a value that prevents the atoms from moving to a neighbouring site. Then, in a very short time, the height of the barrier is lowered such that the system gets out of equilibrium and local excitations arise: Under the new conditions one or the other atom is allowed to “tunnel” through the barrier and reach its neighbouring site. If this happens, entangled pairs are generated, each consisting of a doubly occupied site, a so-called doublon, and a hole, named holon. According to a model developed by theoretical physicists from the University of Geneva around Professor Corinna Kollath, both doublon and holon move through the system – in opposite directions – as if they were real particles (see figure). “Regarding one entangled pair, it is not defined whether the doublon sits on the right or on the left side of the holon. Both constellations are present at the same time”, Dr. Marc Cheneau, a scientist in the Quantum Many-Body Systems division, explains. “However, once I observe a doubly occupied or an empty site, I exactly know where to find its counterpart. This is the correlation we are talking about.”

Now the scientists observe how the correlations are carried through the system. Using a new microscopic technique, they can directly image the single atoms on their lattice sites. In simplified terms, they make a series of snapshots each showing the position of the doublons and the holons at that very moment. The propagation velocity of this correlation can be deduced from the distance the two partners have moved apart in a certain period of time. The experimental results are in very good agreement with the predictions of the model mentioned above.
“As long as quantum information is communicated with light quanta, we know, that this is done with the speed of light,” Dr. Cheneau points out. “If, however, quantum bits or quantum registers are based on solid-state structures, things are different. Here the quantum correlation has to be passed on directly from bit to bit. Once we know how fast this process can happen, we have the key to understand, what will limit the velocity of future quantum computers.”
[Olivia Meyer-Streng]

Original Publication:
Marc Cheneau, Peter Barmettler, Dario Poletti, Manuel Endres, Peter Schauß, Takeshi Fukuhara, Christian Gross, Immanuel Bloch, Corinna Kollath and Stefan Kuhr
Light-cone-like spreading of correlations in a quantum many-body system
Nature, DOI:10.1038/nature10748

Contact:

Prof. Dr. Immanuel Bloch
Chair of Quantum Optics
LMU Munich, Schellingstr. 4
80799 München, Germany, and
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching b. München
Phone: +49 89 / 32905 -138
E-mail: immanuel.bloch@mpq.mpg.de

Dr. Marc Cheneau
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching b. München
Phone: +49 89 / 32905 -631
E-mail: marc.cheneau@mpq.mpg.de
Prof. Dr. Stefan Kuhr
University of Strathclyde
Department of Physics
107 Rottenrow East
Glasgow G4 0NG, U.K.
Phone.: +44 141 / 548 -3364
E-mail: stefan.kuhr@strath.ac.uk

Prof. Dr. Corinna Kollath
Department of Theoretical Physics
University of Geneva
24, Quai Ernest Ansermet
1211 Genève, Switzerland
Phone.: +41 22 / 37 96 241
E-mail: corinna.kollath@unige.ch
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.mpq.mpg.de

More articles from Physics and Astronomy:

nachricht NASA's James Webb Space Telescope completes final cryogenic testing
21.11.2017 | NASA/Goddard Space Flight Center

nachricht Previous evidence of water on mars now identified as grainflows
21.11.2017 | US Geological Survey

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: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Previous evidence of water on mars now identified as grainflows

21.11.2017 | Physics and Astronomy

NASA's James Webb Space Telescope completes final cryogenic testing

21.11.2017 | Physics and Astronomy

New catalyst controls activation of a carbon-hydrogen bond

21.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>