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March 21, 2011

Controlling the spin state of single atoms using lasers is a step towards quantum computing

With the addressing scheme arbitrary patterns of atoms in the lattice can be prepared. The atomic patterns each consist of 10 - 30 single atoms that are kept in an artificial crystal of light.

Using a laser beam, scientists could address single atoms in a lattice of light and change their spin state. They managed to exert total control over individual atoms and 'write' arbitrary two-dimensional patterns.

The laser-cooled rubidium atoms were loaded into an artificial crystal of light, created by superimposing several laser beams together. They were then kept in the lattice of light in a manner described by the team as being akin to keeping marbles in the hollows of an egg carton. The lattice of light slightly deforms the electron shell of an atom and as a result changes the energy difference between its two spin states.

Nature - Single-spin addressing in an atomic Mott insulator

Quantum optics group in Munich



Ultracold atoms in optical lattices provide a versatile tool with which to investigate fundamental properties of quantum many-body systems. In particular, the high degree of control of experimental parameters has allowed the study of many interesting phenomena, such as quantum phase transitions and quantum spin dynamics. Here we demonstrate how such control can be implemented at the most fundamental level of a single spin at a specific site of an optical lattice. Using a tightly focused laser beam together with a microwave field, we were able to flip the spin of individual atoms in a Mott insulator with sub-diffraction-limited resolution, well below the lattice spacing. The Mott insulator provided us with a large two-dimensional array of perfectly arranged atoms, in which we created arbitrary spin patterns by sequentially addressing selected lattice sites after freezing out the atom distribution. We directly monitored the tunnelling quantum dynamics of single atoms in the lattice prepared along a single line, and observed that our addressing scheme leaves the atoms in the motional ground state. The results should enable studies of entropy transport and the quantum dynamics of spin impurities, the implementation of novel cooling schemes, and the engineering of quantum many-body phases and various quantum information processing applications.

By taking advantage of the versatility of ultracold atoms in optical lattices, the researchers were able to bring a high level of control to the experiment. The scientists demonstrated how such control can be implemented at the most fundamental level of a single spin at a specific site of an optical lattice.

Starting from an arrangement of 16 atoms that were strung together on neighbouring lattice sites like a necklace of beads, the scientists studied what happens when the height of the lattice is ramped down so far that the particles are allowed to 'tunnel' according to the rules of quantum mechanics. The results show that they move from one lattice site to the other, even if their energy is not sufficient to cross the barrier between the lattice wells.

By stringing the atoms along a line they were able to directly observe their tunnelling dynamics in what could be described as a 'racing dual' of atoms. 'As soon as the height of the lattice has reached the point where tunnelling is possible, the particles start running as if they took part in a horse-race,' described researcher Christof Weitenberg.
'By taking snapshots of the atoms in the lattice at different times after the "starting signal", we could directly observe the quantum mechanical tunnelling-effect of single massive particles in an optical lattice for the first time.'

This research builds on previous work carried out by the team. Several months ago they showed that each site of the optical lattice can be filled with exactly one atom. Now they have succeeded in individually addressing each atom in the lattice and changing its energy state. 'We have shown that we can individually address single atoms. In order for the atom to suit as a quantum bit, we need to generate coherent superpositions of its two spin states,' explains researcher Stefan Kuhr. 'A further step is to realise elementary logical operations between two selected atoms in the lattice, so-called quantum gates.'

The overall objectives of the AQUTE project are to develop quantum technologies based on atomic, molecular and optical (AMO) systems for both scalable quantum computation and entanglement-enabled technologies like metrology and sensing. The project also hopes to establish and exploit new interdisciplinary connections coming from AMO physics.


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