Single-site addressing in optical lattices
Ultracold quantum gases in optical lattices have evolved in the last years into an interdisciplinary tool for many-body solid state and quantum physics. To fully exploit the possibilities of such a system for quantum computing, we need to detect and to manipulate individual atoms on their lattice sites - a feature that was missing so far in this type of experiments.
Experimental setup
- Addressing single lattice sites
Addressing of individual lattice sites is achieved through a high resolution optical microscope that images the fluorescence light of the trapped atoms onto a CCD camera with single photon detection capability. A specific advantage of this technique is that all lattice sites within the field of view are detected simultaneously. The imaging system has a numerical aperture of NA = 0.69 yielding a diffraction limited resolution of 700 nm for λ = 780 nm.
High resolution images of a Mott insulator
We achieved high resolution imaging of strongly interacting bosonic Mott insulators in an optical lattice with single-atom and single-site resolution [Nature 467, 68 (2010)]. For the first time we could record an in-situ image of a quantum fluid in which each underlying quantum particle is detected. We demonstrated how successive Mott insulator shells are formed for increasing particle numbers, which appear as alternating rings of one and zero atoms due to a parity measurement. From a single image, we fully reconstructed the atom distribution on the lattice and identified individual excitations with high fidelity. Using a simple theoretical model we characterized the average density distribution and the number of fluctuations of the quantum system, and used this for an in-situ temperature measurement.
- Measured atom distribution of an ultracold quantum gas held in a two-dimensional optical lattice. For the BEC (left), a random atom distribution in the lattice is observed, whereas for strong repulsive interactions between the atoms, an ordered Mott insulating state (middle & right) is formed.
Goals
The possibility to detect and to manipulate the 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 be possible to observe and to manipulate density, spin structure, and correlations at the scale of a lattice site. By directly counting the number of atoms on the individual lattice sites, we can measure the statistics of site occupancy, both in the superfluid and in the Mott insulator phases, and detect imperfections such as doubly occupied sites or vacancies.
Using this new tool, we propose to investigate steady-state and dynamical properties of low-dimensional systems, which could not be detected by the conventional time-of-flight images. We plan to engineer fast, high-fidelity, quantum gates using Rydberg states or collisions in a spin-dependent lattice and to create massively entangled systems as a resource for quantum computation.
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- Christof Weitenberg (left picture) and Manuel Endres (right picture) at the experimental setup at MPQ.
