Far-from-equilibrium Quantum Dynamics Group


  • Dynamics of ultracold atomic quantum gases

    The term `Ultracold atomic and molecular quantum gases' includes both, Bose and Fermi gases, at temperatures typically below 1 Microkelvin, where degeneracy sets in, i.e., Bose-Einstein condensation and the formation of a Fermi sea, respectively. The atomic and molecular species involved so far include alkalis, soon also earth alkalis, and simple, i.e., diatomic molecules made out of these.

    The amazing phenomenon which we now know as Bose-Einstein condensation of particles with mass was predicted almost 80 years ago by Albert Einstein (1879-1955), stimulated by the work of Satyendra Nath Bose (1894-1974) on the statistical properties of light (Bose 1924, Einstein 1924 & 1925). Nonetheless, physicists had to wait for another seven decades before their general technical abilities had developed to a level which allowed them to produce and observe such a condensate in a nearly ideal gas of atoms. The very natural propensity of the particles to interact with each other had caused this delay and had left physicists to be satisfied with phenomena like superfluidity of helium or superconductivity of a solid. Bose-Einstein condensation as the fundamental basis for bosonic superfluid helium is obscured by the eminent strength of the collisional interactions and has been widely accepted as such only after decades of scientific controversy. On the contrary, it is evident that a Bose-Einstein condensate may never be realized in laboratory without interactions when starting from a thermal gas.

    The pioneering achievements of 1995, of the groups around Carl Wieman at Boulder (Colorado), Wolfgang Ketterle (MIT, Cambridge, Massachusets), and Randall Hulet (Rice, Houston, Texas), have been crowned with the Nobel Price of 2001. For an overview of present-day experiments worldwide go to the atom traps page at Innsbruck. In recent years, the centre of momentum of the community's activities has gradually shifted to degenerate atomic Fermi gases. Much of the developed techniques could be transferred to study different exciting phenomena including the formation of a Bose-Einstein condensate of diatomic molecules consisting of fermions and the crossover to a Bardeen-Cooper-Schrieffer-(BCS)-like superfluid which more resembles the electron gas in a superconducting solid.

    Despite the fact that the condensation itself is entirely independent of the interactions and occurs also in an ideal gas, it is to a large part thanks to the interactions that one can realize it in the laboratory, observe it, and apply it for useful purposes. The very last aspect of atomic collisions in a Bose-Einstein condensate (BEC) and of the atoms' interactions with light lies at the heart of the kind of correlations which we are studying. Any many-body system can be fully characterized by correlations, e.g., between different locations in the sample.

    Our research is focused on the many-body dynamics of ultracold quantum gases, in particular for systems where strong correlations between the particles are important. We are currently extending recent substantial innovations in non-equilibrium quantum field theory for ultracold atomic gases. These methods are based on non-perturbative approximation schemes which recently have shown, by comparison with exact results, to excellently describe the far-from-equilibrium long-time dynamics of systems, even for strong interparticle forces. With this step a whole new class of phenomena can be accessed. Our work is strongly linked to presently performed experiments, in particular at Heidelberg, by the group of Prof. Dr. M. Oberthaler.

    Topics we are focusing on include

    • Lower-dimensional systems, in particular ultracold gases in (quasi) one-dimensional traps.
    • Ultracold quantum gases in lattice trapping potentials.
    • Molecules in degenerate Fermi gases, the dynamics of the transition to a BCS superfluid, and the crossover to a Bardeen-Cooper-Schrieffer (BCS) superfluid.
    • Molecule formation in Bose-Einstein condensates and `superchemical' reactions.

    These key phenomena have important potential applications, e.g., in quantum information technology, for precision tests of many-body quantum theories, for precision measurements in atomic and molecular physics, for atom interferometry, and for the definition of new time standards. Moreover, making the advanced quantum-field theoretical methods accessible to precise atomic physics experimental verification, has important potential impact on other areas like heavy ion collisions or cosmology, where such methods are needed and much more difficult to check experimentally.

    Recent Publications.

    Some more details:

    Molecule formation in ultracold gases and the quest for a `Molecular Bose-Einstein condensate' (MBEC) has enjoyed, in the past few years, a hot rain of attention and effort within the community of ultracold atomic gases. After the atomic condensate had been `done', the next natural step to tackle was the formation of a BEC of molecules. This is still an important goal, as a condensate of molecules in their rovibrational ground state has not been observed yet. A start has been made in 2002 with a condensate of homonuclear alkali dimers in vibrationally highly excited levels, and several successful experiments have followed making the search for molecule condensation to one of the strongest subfields within the community. While the direct cooling of molecules is very difficult due to their rich internal level structure, the association of already condensed atoms succeeded. Basic techniques applied include the coupling of colliding atoms into a molecular state by means of external light fields or by using resonantly enhanced transitions mediated by the internal atomic forces.

    A further new world to be explored is what has been named `superchemistry'. There is undoubtedly a big potential behind the idea to bring the atoms sufficiently close to each other to enable chemical bondings, however, not by their thermal motion but rather by dynamically controlling their collisional interactions. We have seen that close to a scattering resonance the binding length of the dimers is big enough such that the molecules formed from pairs of condensed alkali atoms represent the largest ever produced in physics. Hence, the association of atoms to molecules and the dissociation of these by approaching or crossing a scattering resonance may be seen as a new form of `wavefunction chemistry'. Pictorially, this chemistry happens through the simultaneous adiabatic joining-together of `neighbouring' pairs of atoms within the gas, like at a party, where the music suddenly fades over from pop to waltz or quick step. (The latter often causes also a big burst fraction.) Nowadays, only the rather `boring' family of alkali atoms has found to this exciting new social behaviour. But let physicists proceed to carbon.

    Novel experimental techniques allow to trap and manipulate ultracold atomic gases in an unprecedented manner, including the formation of Bose-Einstein condensates (BEC) and degenerate Fermi gases. Unlike most other many-particle systems, the atom clouds can be confined in almost arbitrarily shaped traps, and the forces between the atoms can be be made strong by applying external electromagnetic fields. In this way, they can be brought far out of equilibrium, and their dynamics can be studied in detail. The theory of this remains still in its beginnings. Our research work addresses a shortage of dynamical theories used in the ultracold so far, which are based on low-order perturbation expansions in parameters measuring the interatomic forces. Such approximations are no longer valid for many of today's experimental situations.

    The development of concrete applications exploiting the quantum correlations in atomic many-body ensembles, in particular within special trapping geometries like surface, micro and lattice traps, is an important task on the way towards quantum-information technology. From the present viewpoint it is not clear whether there will be a quantum computer, and, if yes, what it will look like. Even if a `digital' quantum computer based on gates which operate on qubits should not become real in one of the forms imagined today, the control of ultracold gases still bears the huge potential to lead to the development of an `analog' quantum computer. The idea behind such a device is to simulate one physical system with another physical system. For such a simulation task one might use a system which, e.g., allows to a greater extent to adjust its defining properties like interaction strengths and external boundary conditions, or which can be realized in the laboratory at all, in contrast to its maybe cosmological counterpart. At first sight, such ideas seem to be rather science fiction than a serious attempt to apply cold atomic gases. But the development of, e.g., a dynamical quantum field theory which gives reliable answers also far from equilibrium and at large times is a presently forefront subject not only in the field of ultracold quantum gases, but also - and already for a longer time - in the context of, e.g., cosmology or heavy-ion collision physics. Joining together efforts to proceed in the further development of such theories not only gives new tools to the different subject areas but constitutes links between the fields which might be exploited to gain new insight by combining seemingly unrelated experiments.