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A – Imaging and Control of Quantum Systems

For a full understanding of structural dynamics, one needs information on the electronic as well as the nuclear degrees of freedom. This research focus uses advanced optical imaging techniques to identify key enabling features for controlling quantum state evolution.

Realisation of a sample of ultracold atoms in a spin-dependent optical lattice with hexagonal symmetry. Soltan-Panahi et al., Nature Physics 7, 434 (2011)

We envisage controling chemistry along the ground state electronic surface to open up all classes of molecular systems to atomic level inspection. The system size is scaled up from small molecules to collective effects in solid state or periodic media and includes the systematic study of isolated molecules with small potential barriers separating different structures but also takes into account a variable coupling of a system to the environment. A detailed understanding of electronic coupling to the bath should enable us to control coherence and degree of dissipation to the point of controlling material properties. For the case of highly correlated electron-lattice systems, this knowledge leads to new means to control coherence and macroscopic properties with the prospect to create transient superconducting states at high temperatures. The design of novel materials with unique properties are greatly aided by our capabilities to build fully controllable quantum simulators based on periodic structures formed in ultracold quantum gases. In these analog quantum processors ultracold matter is tailored to mimic magnetism and superconductivity under idealized conditions. Apart from the long range correlation effects governing material properties, there is a deep fundamental issue related to the role of quantum information transport in such highly quantized systems. To this end, we are studying the coherence properties of matter waves escaping from a macroscopic quantum object like a Bose-Einstein condensate.

 

Research Focus A.1: Photo-driven Dynamics coupled to Electronic Excitations

Understanding and controlling chemical reaction dynamics defines a challenging scientific area where in the last decades different instruments have been developed to determine which specific motions, amid the myriad of possibilities, lead a system to the product of the reaction. This marks the miracle of chemical systems; there are usually only a few modes that direct the process by virtue of the exponential dependence on energetics hidden within a complex potential energy landscape. For all but a few simple molecules, we have poorly resolved “maps” of these potential energy surfaces to guide us; yet chemists harness the power of stored chemical potential routinely without full knowledge of the process. Thus, we are advancing new means of following chemical reaction dynamics to refine our maps of the forces at play with the goal of implementing laser based control methodologies.

Due to the introduction of femtosecond VUV to X-ray pulses, we can directly probe the dynamics of the core electrons during photoinduced structural dynamics. This class of experiments provides important information on electron correlation effects and element specificity in following the structural dynamics and enters a new regime in which the photophysics of the probing event needs to be fully understood.

The interaction of pulses of short-wavelength radiation is in many aspects different from irradiation in the visible range; effects including the creation of highly excited electronic states and the corresponding involvement of many electrons. The primary excitation may be linear or non-linear upon absorption of a single or several photons, but will usually leave a hole in an inner shell of the electron system. This event is followed by a cascade of processes involving both electronic and nuclear movement.

As a consequence, matter will be modified from inside, while excitation in the visible attacks first the weakly bound valence electrons. Another aspect of the inner-shell access is its energetic selectivity allowing to address specific atomic species within a larger molecule or solid state crystal and to probe the local electronic environment. Investigations on water clusters are beneficial for structural investigations of liquid water and interesting synergies arise.

Ultrashort X-ray pulses combined with time-resolving detection techniques allow to obtain spatially localized and temporally resolved information on ultrafast electronic and chemical dynamics after an optical stimulus. Light pulses ranging from 3 nm to 300,000 nm serve to follow the movement of charges within molecules or between molecules and a surface on the relevant femtosecond to attosecond time scale. Light also serves to control the alignment of molecules in space. This capability proves to be essential for dynamical diffraction, where any alignment of otherwise randomly oriented molecules is essential for background suppression of the captured images.

 

Research Focus A.2: Real-time Chemical Dynamics in Complex Environments

The central goal of this project is to take critical steps towards the real-time study of chemical dynamics in the presence of a bath. This bath could be, for example, a solvent such as water in a chemical reaction or a solid surface in heterogeneous catalysis. Due to the complexity of the problem, it is hard to understand how the bath influences the reaction chemistry. By systematically investigating with femtosecond resolution reaction dynamics in a range of well-defined, distinct chemical environments, we gain unprecedented insights into the role each bath plays during a chemical reaction. A related problem is the fact that for time-resolved studies of the detailed atomic motions during a chemical reaction, a large fraction of all molecular reactants in the sample must be forced to start undergoing the chemical reaction at the same time. This requirement to provide a trigger that is faster than the dynamical time scale of interest is the reason why femtochemistry nowadays is largely restricted to photochemical reactions that are associated with electronically excited states of the reacting species. We explore possibilities for triggering thermal chemical reactions by employing ultrafast terahertz and infrared pump pulses in combination with ab initio electronic structure theory and quantum molecular dynamics techniques. The ultimate dream driving the research is the direct, time-resolved exploration of transition states of chemical reactions for which the mechanism is still unknown.

 

Research Focus A.3: Imaging Local and Global Coherence of Superfluid Matter
– Dynamics and Control of Quantum Matter

Superfluidity is a fundamentally important phenomenon in physics with important realizations in superfluids, superconductors of different types and artificial quantum gas model systems. Superfluidity is characterized by long range coherence, i.e. spatially separated parts of the matter wave oscillate in phase. Our notion of such a concept is envoked when we consider interrogating a macroscopic quantum object with a light wave that imprints its phase evenly and simultaneously onto it. Obviously, the exchange of information within the object must be limited by the speed of light; on the other hand, we expect all components of a wave function to be entangled, thus responding to a measurement with unlimited speed. Our capability to create quantum fluids with 100 µm sizes, combined with photonic techniques resolving electronic processes evolving at the atomic unit of time now puts us in the position to investigate the distribution and spread of information in superfluids. To this end, we study the interplay between short range and long range coherences in superfluids. The central question is: Can we image the emergence of local coherence and the interplay with global coherence?  We analyze especially ultracold quantum gases due to their ability to create well controlled and defined coherence properties. However, the studies also have direct relevance for any kind of superfluid matter.

 

Research Focus A.4: Coupled Two-dimensional Superconductors

Understanding high-temperature superconductivity is one of the most important and yet most elusive questions of modern physics, puzzling theorists and experimentalists alike. The physical structure of these superconductors is dominated by layers of Cu-O planes. In this picture, the interaction between the two-dimensional physics of planes that incorporate the Cu-O bonds and the three dimensional phenomenology are not well understood. For example: Is the tunnel coupling between the two-dimensional layers essential? Which influence does the coupling to the bath of phononic modes have on the properties of the materials? Orbital degrees of freedom add an additional level of complexity to a puzzle that is characterized by a multitude of competing phases.

In this project, we explore fundamental properties of two-dimensional coupled quantum systems. The recent spectacular advances both in ultrafast control of solid state systems and control of dilute quantum gases – two areas in which we have significant expertise – put us in a unique position to tackle the problem comprehensively.

The central questions addressed by us concern the key issue of competing order in quantum many-body systems, and the associated dynamics. We trigger these dynamics by either a femtosecond laser pulses in condensed matter, or by quenches of external parameters for ultra-cold atom systems. The competing orders and dynamics are not only of fundamental interest in many-body physics, but immediately raise the question whether the critical temperature of superconductors can be raised dynamically, as a transient, metastable state, to a technologically useful regime.

 

Participating Research Groups

  • Prof. C. Bressler
  • Prof. A. Cavalieri
  • Prof. A. Cavalleri
  • Prof. M. Drescher
  • Prof. J. Dalibard
  • Prof. U. Frühling
  • Prof. A. Hemmerich
  • Prof. G. Huber
  • Prof. F. Kärtner
  • Dr. C. Kränkel
  • Prof. J. Küpper
  • Dr. T. Laarmann
  • Prof. A. Lichtenstein
  • Prof. L. Mathey
  • Dr. M. Meyer
  • Prof. H. Moritz
  • Prof. D. Pfannkuche
  • Prof. M. Potthoff
  • Prof. R. Santra
  • Prof. P. Schmelcher
  • Prof. K. Sengstock
  • Prof. M. Thorwart
  • Dr. O. Vendrell
  • Prof. W. Wurth