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Speed limit on the quantum highway

Propagation velocity of quantum signals

Munich, 01/26/2012

A quantum computer based on quantum particles instead of classical bits can in principle outperform any classical computer. However, how fast and how efficient realistic quantum computers may actually be remains an open question. One critical parameter in this context is the velocity with which a quantum signal can propagate 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) and LMU 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. 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, the leader of the division, points out. (Nature online, January 25, 2012)

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 colors at the same time. It is the process of measurement itself that compels the particles to adopt one or other of the two possible properties. As a consequence of this peculiar behavior, 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 observe such a process directly. 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 a second light field, a standing laser light wave, is superimposed on the tubes. The periodic sequence of dark and bright areas forces the atoms to form a lattice structure: exactly one atom is trapped in each bright spot, and is separated from the neighboring 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 neighboring site. Then, over a very short time, the height of the barrier is lowered such that the system is no longer in equilibrium and local excitations arise: Under the new conditions one or the other atom is allowed to “tunnel” through the barrier to an adjacent site. If this happens, en-tangled pairs are generated, each consisting of a doubly occupied site, a so-called doublon, and a hole, named a holon. According to a model developed by theoretical physicists from the University of Geneva led by Professor Corinna Kollath, both doublon and holon move through the system – in opposite directions – as if they were real particles (see figure). “For any given 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 know exactly where to find its counterpart. This is the correlation we are talking about.”

Now the scientists observe how the correlations propagate through the system. Using a new microscopic technique, they can directly image the single atoms on their lattice sites. In sim-plified 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 de-duced 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 correla-tion has to be passed on directly from bit to bit. Once we know how fast this process can happen, we have the key to understanding what will limit the velocity of future quantum com-puters.” Olivia Meyer-Streng

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

Contacts:
Prof. Dr. Immanuel Bloch
Chair of Quantum Optics / LMU Munich
Schellingstr. 4
80799 München
Germany

Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching b. München
Phone: +49 89 / 32905 -138
Email: immanuel.bloch@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

Dr. Marc Cheneau
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching b. München
Phone: +49 89 / 32905 -631
Email: marc.cheneau@mpq.mpg.de

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

 

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