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Final Report Summary - SINGLEATOMS (Quantum engineering of ultracold atoms in optical lattices)

Optical lattices are one of the most important experimental tools that permit the convergence between quantum information science and condensed matter physics. They consist of a periodic potential obtained by interfering several counter-propagating laser beams. The potential wells can be tailored such that the cold atoms occupy only the lowest energy state, thus realising effective low-dimensional systems. One can also allow for tunnelling between the sites. In this case, optical lattices constitute an almost perfect implementation of the so-called Hubbard model, which describes among others the transition between strongly correlated insulating and conducting systems. By tuning the lattice parameters, it is possible for example to make these interactions of either ferromagnetic or antiferromagnetic type, which gives access to strongly correlated magnetic phases. These interactions are also believed to play an important role in the context of high-Tc superconductivity. Last but not least, the use of spin-dependent lattices offers an ideal mean to parallelise 2-qubits operations, an essential step towards complex quantum computing.
Experiments with optical lattices have so far suffered from an important drawback: because of the dense periodic structure, it has been impossible to probe or to manipulate the atoms at the level of a single lattice site. For quantum processing, this means that only simple algorithms have been implemented, in which the same operation was applied to all pairs of qubits. In condensed matter-like experiments, this lack of a local probe also constitutes a severe limitation. Dilute gases are intrinsically inhomogeneous, as they have to be trapped by an external potential; this means that any measurement performed on a macroscopic scale inevitably leads to a blurred, incomplete characterisation of the system.
The main objective of the project was to develop and demonstrate new experimental techniques that allow imaging and manipulating ultracold atoms in an optical lattice with single-atom and single-site resolution. This objective could be reached even beyond our most optimistic expectations and, with these novel techniques at hand, we were able to perform several spectacular experiments illustrating fundamental aspects of the behaviour of quantum many-body systems that could never be observed before.

The principle of the imaging technique that we developed in the frame of the project works as follows: After the system has been prepared in the state that we want to observe, the atoms are pinned in place by an extremely deep three-dimensional optical lattice. We then illuminate the atoms with near resonant light such that the atoms continuously fluoresce. Using a particular configuration of laser beams to excite the fluorescence, we achieve a so-called optical molasses and make sure that the atoms remain pinned at their original position as they fluoresce. We then image the gas through a specially designed high-numerical aperture microscope objective to obtain a picture of the system with a diffraction-limited resolution of 700 nm. The possibility to let the atom fluoresce for a very long time and the use of a high detection efficiency CCD detector ultimately provide the sensitivity to detect every single atom present in the system.
One of the main challenges we faced when developing this technique was to combine the opposite requirements of an experimental setup capable of efficiently producing ultracold gases with those of a high-resolution optical imaging system. This was only possible with the design and construction of a new type of experimental setup, which Prof. Stefan Kuhr and his team started in 2008. As this project began, the last pieces of the apparatus, including the microscope objective, had to be put together. It took the first 8 months of the project to produce the first strongly correlated ultracold gases and to get the imaging system to work.
The second main objective of the project was to demonstrate the manipulation of the spin of individual atoms localized in an optical lattice. This required further developments of the experimental setup, which took us another 6 months. The achievement of this part of the projected marked the end of the “development phase” of the project. The following 10 months were devoted to the “exploitation phase”, during which we performed fundamental experiments on strongly correlated ultracold gases by taking full advantage of the new possibilities offered by our imaging and manipulation techniques.

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