ADDRESSING · Ultracold quantum gases in optical lattices with single site addressability

Ultracold quantum gases in optical lattices with single site addressability

In the past few years ultracold quantum gases in optical lattices have evolved into an interdisciplinary tool for many-body solid state and quantum physics. Periodic optical potentials created by counter-propagating laser beams in one, two or three dimensions form an artificial 'crystal of light'. The use of laser beams allows the control of many relevant parameters, like lattice depth and spacing. In addition, the potential can be changed or entirely switched off during the experiment. Most of these experimental degrees of freedom are typically not tuneable over a wide range in a solid state physics experiment, which makes optical lattices an interesting test bed for condensed matter models such as those related to high-Tc-superconductivity. Due to their high degree of purity and regularity, they could also be a candidate for scalable quantum computation architecture. The possibility to detect and to manipulate atoms individually on their lattice sites allows the conception of an entirely new generation of experiments in the fields of quantum information and quantum simulation. It will become possible not only to probe and observe the density, spin structure and subtle correlations of novel and fundamental quantum phases at the scale of a lattice site, but also to manipulate the particles on that fundamental microscopic length scale. In this project we have designed and successfully implemented a new state-of-the-art experimental setup with the capability of resolving and addressing single lattice sites. The first part of the project was focussed on the assembly and test of the vacuum setup, the production of ultracold atomic samples, and the development and tests of the high-resolution, custom-made imaging system. Using this system, we have pursued and successfully realised one of the major milestones in the community at the interface between cold atoms and condensed matter, namely the single atom resolved imaging of a strongly correlated quantum many-body system. We have done this by imaging a so-called Mott insulating state in optical lattices, where due to the quantum mechanical repulsion between the atoms, they order in states of well defined occupation numbers. This phenomenon is known from condensed matter physics, where, however, the sample cannot be systematically controlled in order to investigate its properties. By using the high flexibility of ultracold atoms as a quantum simulator for this and related phenomena, we hope to be able to contribute with new understanding of these complex phenomena. Our images clearly show each individual atom and we have for the first time been able to observe the individual defects arising from the finite temperature of the sample. The very reliable determination of the temperature in optical lattices is an important step towards the realisation of other elusive quantum phases of great importance. Sending a laser beam backwards through the high resolution imaging system allows us to focus it onto a single lattice site. In this way we can for the first time manipulate the state of individual atoms in the optical lattice. Combined with the single site occupancy of our Mott insulating state this could form the basis of a quantum computation architecture with up to hundreds of individually addressable qubits. The approach also offers many possibilities for the study of out of equilibrium phenomena by for instance creating a local perturbation and observing how the system subsequently evolves. We have performed first proof of principle experiments involving addressing in optical lattices and are currently working on sufficiently refining the addressing of individual sites.

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