Quantum Gates and Small Cluster States

with Trapped Neutral Atoms

Prof. Dr. Dieter Meschede / Dr. Andrea Alberti

We investigate small strings of neutral atoms for applications in quantum information processing. The atoms are stored, one by one, in a standing wave dipole trap and the interaction between the atoms, necessary for the implementation of quantum gates, will be realized through controlled cold collisions. For this, we employ the technique of spin dependent transport. This technique allows us to "manually" split the wave functions of the trapped atoms in a deterministic and fully controlled single atom Stern-Gerlach experiment, where the dipole trap provides the effective magnetic field. By recombining the atomic wave function, we have realized a single atom interferometer and directly measure the coherence properties of the splitting process. A sequence of splitting operations, carried out on a single atom, results in a quantum analogue of the Galton board, where the atom carries out a quantum walk. Such quantum walks have recently been proposed as an alternative approach to quantum computing. Our ultimate goal is the implementation of fundamental quantum gates using controlled cold collisions within a register of 2-10 trapped neutral atoms. A parallel application of such quantum gates should then open the route towards the preparation of small cluster states consisting of up to 10 individually addressable qubits.

So far, the atom interferometer and the quantum walk have been realized with a thermal ensemble. The entanglement experiments, however, require the atoms to be in the motional ground state. For this purpose, we have developed and implemented a side band cooling scheme in the strong confinement direction by exploiting our spin dependent potentials and driving a microwave transition between the hyperfine ground states of cesium. By displacing the spin dependant components of the potentials with respect to each other, the motional states are not orthogonal anymore. Therefore sideband transitions are allowed, which we utilize for our microwave sideband cooling scheme. By comparing the strength of the first red and blue side band we deduce a 1D-ground state population on the order of 98% after cooling. This robust cooling scheme, or the closely-related Raman sideband cooling, can be employed in the weakly confined directions of the 1D optical lattice, providing a cooling to the 3D motional ground state.