Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Breakdown of correlated tunneling

18.10.2010
MPQ-LMU scientists together with scientists from the Weizmann Institute in Israel show, how quantum-mechanical tunneling through a barrier can be altered drastically due to the interplay of many particles in low dimensions

Quantum systems do often behave differently from what we would expect intuitively and from our daily experience.

One example is provided by the so-called Landau-Zener problem. It describes e.g. the tunneling of a quantum particle between two potential wells with an initial difference in potential energy that is gradually reversed over time.

The Russian physicist Lew Landau and the American physicist Clarence Zener have tackled this problem in a more general context in 1932. They found that the quantum particle would be transported from one well to the other, provided that the reversal in potential energy is carried out slow enough.

This is true no matter whether the particle started out in the well which was higher or lower in potential energy. In this respect, the result differs much from the expectation for a classical fluid, which will always find its way into the well lower eventually in energy. MPQ-LMU scientists around Prof. Immanuel Bloch in collaboration with theoreticians from the Weizmann Institute of Science in Rehovot, Israel have experimentally investigated a Landau-Zener scenario in a system of one-dimensional quantum gases in two coupled potential tubes. They found that the collective tunneling of the many particles is drastically altered from the single-particle case, ultimately leading to a breakdown of the transfer to the opposite side (Nature Physics, AOP, 17.10.2010, DOI: 10.1038/NPHYS1801).

In their experiments, the scientists cool down a small cloud of “bosonic” rubidium atoms to only a few nanokelvin above absolute zero – that is about minus 273 degree Celsius. At such low temperatures, all atoms gather in a single quantum state, forming a new state of matter known as the Bose-Einstein condensate. This ultracold quantum gas is subsequently loaded into a so-called two-dimensional “optical lattice”. This optical lattice is created by the interference of two orthogonal pairs of counter-propagating laser beams which results in a two dimensional pattern of bright “lines of light”. The interaction between light and atoms forces the latter to arrange in a matching regular pattern of thousands of elongated quantum gases, each consisting of up to 110 atoms.

Now, the scientists add another pair of counter-propagating laser beams forming an optical lattice with just half the period along one of the initial directions. This leads to a splitting of each of the potential tubes trapping the orthogonally elongated quantum gases into two. The full control of the relative phase between this additional optical lattice and the initial one allows the scientists to only populate one of the potential tubes of each pair by lifting the other one higher in potential energy during the splitting. Furthermore, they are able to change the difference in potential energy between the tubes of each pair by controlling the relative phase in real-time. Therefore, they can rapidly set an arbitrary initial difference in potential energy and then gradually reverse the potential energy difference as in the original Landau-Zener problem. The physicists are especially interested in the question, how the one-dimensional geometry and the many-body nature of the quantum gas would alter the findings from those for a single quantum particle.

Already in the conceptually simple case where all atoms start in the potential well initially lower in energy showed some significant deviations from the single particle physics. For slow enough changes of the potential energies – or “sweeps” – all particles ended up in the opposite tube as it would also be the case for a single quantum particle. The speed of the sweep, however, could be faster in the case of the one-dimensional quantum gases. Here, the repulsive interaction forces the atoms to line up for the transfer and to move to the opposite potential tube one-by-one, rather than all at once. Since each of these single-particle transfers can be carried out faster in the many-body system, the rate at which the potential energy is changed can be higher. The total time needed for a successful sweep however, is approximately the same as in the case of a single particle, since all particles have to move over and the advantage of the faster transfer of a single particle is lost.

Even more striking is the difference if all particles start out in the potential tube which is initially higher in potential energy. Here, the scientists found that not all particles would reach the opposite tube, no matter how slow the sweep is carried out. Actually, the transfer efficiency was decreasing for slower and slower Landau-Zener sweeps. This result strongly resembles the expectation for a classical fluid which would always flow into the lower tube – here the one that the atoms started from. In the classical case, the potential energy is converted into kinetic energy. Eventually, this kinetic energy will be given to the environment as the fluid comes to rest.

In a closed quantum system, such an energy exchange with the environment is not possible: the excess energy cannot leave the system and the relaxation of the quantum gas into the lower-energy tube is blocked. In a one-dimensional system, however, there are low-energy excitations which are often referred to as “phonons” due to their similarity to the lattice vibrations in solids. These phonon excitations provide an “inner environment” in which the excess energy can be stored in arbitrary portions. As a result, the quantum gas will relax towards the potential tube lower in energy while heating up. This relaxation mechanism is strongest for small differences in the potential energy of the two tubes. The slower the Landau-Zener sweep is carried out, the longer the system will reside in this region and the more effective will be the relaxation.

The work described here provides for the first time an experimental study of a Landau-Zener problem generalized to a one-dimensional many-body setup. The dynamics of such systems still holds a number of open questions. Especially the understanding of whether and how low-dimensional quantum gases far from equilibrium relax and eventually thermalize in closed quantum systems remains an unsolved problem. Furthermore, such generalized Landau-Zener experiments hold the prospect of detecting many-body quantum phase transitions at which the excitation spectra of the quantum gases are drastically altered. The measurements described here allow for a deep analysis of non-equilibrium phenomena in coupled one-dimensional systems and opens the way for the detailed study of their intrinsic sound-wave like excitations. Stefan Trotzky/Olivia Meyer-Streng

Original publication:
Yu-Ao Chen, Sebastian D. Huber, Stefan Trotzky, Immanuel Bloch and Ehud Altman
Many-body Landau–Zener dynamics in coupled one-dimensional Bose liquids
Nature Physics, AOP, 17.10.2010, DOI: 10.1038/NPHYS1801
Contact:
www.quantum-munich.de
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 | Max-Planck-Institut
Further information:
http://www.quantum-munich.de
http://www.mpq.mpg.de

More articles from Physics and Astronomy:

nachricht Magnetic nano-imaging on a table top
20.04.2018 | Georg-August-Universität Göttingen

nachricht New record on squeezing light to one atom: Atomic Lego guides light below one nanometer
20.04.2018 | ICFO-The Institute of Photonic Sciences

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: Spider silk key to new bone-fixing composite

University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.

Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.

Im Focus: Writing and deleting magnets with lasers

Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...

Im Focus: Gamma-ray flashes from plasma filaments

Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.

The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...

Im Focus: Basel researchers succeed in cultivating cartilage from stem cells

Stable joint cartilage can be produced from adult stem cells originating from bone marrow. This is made possible by inducing specific molecular processes occurring during embryonic cartilage formation, as researchers from the University and University Hospital of Basel report in the scientific journal PNAS.

Certain mesenchymal stem/stromal cells from the bone marrow of adults are considered extremely promising for skeletal tissue regeneration. These adult stem...

Im Focus: Like a wedge in a hinge

Researchers lay groundwork to tailor drugs for new targets in cancer therapy

In the fight against cancer, scientists are developing new drugs to hit tumor cells at so far unused weak points. Such a “sore spot” is the protein complex...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Invitation to the upcoming "Current Topics in Bioinformatics: Big Data in Genomics and Medicine"

13.04.2018 | Event News

Unique scope of UV LED technologies and applications presented in Berlin: ICULTA-2018

12.04.2018 | Event News

IWOLIA: A conference bringing together German Industrie 4.0 and French Industrie du Futur

09.04.2018 | Event News

 
Latest News

Magnetic nano-imaging on a table top

20.04.2018 | Physics and Astronomy

Start of work for the world's largest electric truck

20.04.2018 | Interdisciplinary Research

Atoms may hum a tune from grand cosmic symphony

20.04.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>