Atoms in Optical Lattices

Superfluid to MI

Cold bosonic or fermionic atoms in optical lattices, i.e. a periodic array of microtraps generated by counterpropagating laser beams, provide an ideal laboratory system for the study of new complex quantum phenomena due to their high tunability and control. This feature makes atoms in optical lattices an ideal tool in the study of strongly correlated condensed matter systems and in the implementation of quantum information processing.
In the first of these two points, the focus is on the atomic quantum simulation of complex (condensed matter) systems via the implementation of their Hamiltonians with cold atomic gases. The thermodynamic properties and the dynamics of the strongly correlated quantum phase are then extracted by a proper measurement and study of the atomic system. Such quantum simulators provide a novel approach to the study of so far unsolved problems in physics. This opens the new possibilities of studying interesting aspects of many body systems, e.g. quantum phase transitions [1], and strongly correlated atomic systems.
Parallel QIP In view of the implementation of quantum information processing, arrays of a large numbers of identifiable qubits are obtained in a Mott insulator phase of cold atoms, which is obtained by loading an atomic BEC in an optical lattice [1]. These qubits can be entangled in massively parallel operation with spin-dependent lattices [2] (see also [3,4]). A large amount of the research activity of our group has focused on the investigation of atoms in optical lattices in recent years.

[1] D. Jaksch, C. Bruder, J.I. Cirac, C.W. Gardiner, and P. Zoller, Cold bosonic atoms in optical lattices, Phys. Rev. Lett. 81, 3108 (1998).

[2] D. Jaksch, H.-J. Briegel, J.I. Cirac, C. W. Gardiner, and P. Zoller, Entanglement of Atoms via Cold Controlled Collisions, Phys. Rev. Lett. 82, 1975 (1999).

[3] D. Jaksch, P. Zoller, The cold atom Hubbard toolbox , Annals of Physics 315 1, 52-79 (2005),

[4] Maciej Lewenstein, Anna Sanpera, Veronica Ahufinger, Bogdan Damski, Aditi Sen De, Ujjwal Sen, Ultracold atomic gases in optical lattices: Mimicking condensed matter physics and beyond, cond-mat/0606771 (2006,

Repulsively bound pairs

Repulsively Bound Pairs

Throughout physics, stable composite objects are usually formed by way of attractive forces, which allow the constituents to lower their energy by binding together. Repulsive forces separate particles in free space. However, in a structured environment such as a periodic potential and in the absence of dissipation, stable composite objects can exist even for repulsive interactions. In collaboration with the experimental group of Rudi Grimm these exotic bound states have been observed and analyzed theoretically [1]. These repulsively bound pairs exhibit long lifetimes, even under conditions when they collide with one another. Signatures of the pairs are also recognized in the characteristic momentum distribution and through spectroscopic measurements. There is no analogue in traditional condensed matter systems of such repulsively bound pairs, owing to the presence of strong decay channels.

[1] K. Winkler, G. Thalhammer, F. Lang, R. Grimm, J. Hecker Denschlag, and A. J. Daley, A. Kantian, H. P. Büchler, P. Zoller, Repulsively bound atom pairs in an optical lattice, Nature 441, 853-856, (2006).

Press release: "Verhängnissvolle Affären" in der Quantenwelt

Controlled dissipation in an optical lattice

Cooling Schemes

While previous work in optical lattices has primarily focused on studies of Hubbard Hamiltonians, we have recently begun to investigate to what extent “controlled dissipation” can be engineered in optical lattices via interactions with an external reservoir gas. Dissipative processes in this system involve the spontaneous emission of an excitation in the cold reservoir gas (either a BEC or a de-generate Fermi gas), combined with the decay of an atom’s motional state to a lower level. The primary motivation here is to apply ideas familiar from laser cooling involving spontaneous emissions of photons in this new context, where the excitations created in the reservoir (phonons) have similar energy scales and momentum recoil to the scales on which other processes occur in the lattice. These ideas have been explored in recent work of our group, including a fault tolerant dissipative loading scheme for fermionic registers [1], as well as for a novel dark state cooling scheme of atoms within the lowest Bloch band of an optical lattice [2].

[1] A. Griessner, A. J. Daley, D. Jaksch, P. Zoller, Fault-tolerant dissipative preparation of atomic quantum registers with fermions, Phys. Rev. A 72, 032332 (2005).

[2] A. Griessner, A. J. Daley, S. R. Clark, D. Jaksch, P. Zoller, Dark state cooling of atoms by superfluid immersion, cond-mat/0607254.

A single atom transistor

Single Atom Transistor

Modern everyday life would be unimaginable without transistors, since they are the elementary building blocks of almost every electronic device such as cell phones or computers. The basic task of a transistor is to switch large electrical currents depending on a weak input signal. A quantum mechanical analogue can be implemented with a single control atom in an optical pipeline which regulates the flow of an atomic current [1]. The control atom can exist in two different internal states. One state is engineered to completely block the current of atoms flowing past it (“off”), and the other to be transparent (“on”). In contrast to a conventional transistor the switch can exist in the states “on” and “off” simultaneously, a consequence of the quantum nature of the single control atom. A superposition state of a single atom can thus produce a superposition of a reflected and transmitted current. Since the internal state of a single atom is carried over to the motional state of many atoms this feature can be used to implement applications such as quantum state readout or the generation of highly entangled many particle states. In an adapted version it can also serve as a component for the construction of a quantum computer.

[1] A. Micheli, P. Zoller, A Single Atom Mirror for 1D Atomic Lattice Gases, Phys. Rev. A 73, 043613 (2005).

Atomic quantum simulator for lattice gauge theories

Ring echange interactions

Of special interest are also models with large ring exchange interaction due to their potential to exhibit novel and exotic quantum phases. Atoms in optical lattices can provide the tools available for controlling and manipulating cold gases in order to drive the atomic gas in an optical lattice with such an additional ring exchange interaction. The strength of the ring exchange inter-action can be tuned independently on the hopping energy and interaction energy of the bosons. In a recent work [1] we have identified a promising system where this coupling may allow for the realization and observation of an exotic quantum insulator described by the Coulomb phase of a three-dimensional U(1) lattice gauge theory; in quantum magnetism this phase is known as a U(1) spin liquid.

[1] H.P. Büchler, M. Hermele, S.D. Huber, M. P.A. Fisher, P. Zoller, Atomic quantum simulator for lattice gauge theories and ring exchange models, Phys. Rev. Lett. 95, 040402 (2005).