A Quantum Pen for Single Atoms

The ability to observe and control single atoms individually in a lattice of light, to change their spin states and to arrange them in arbitrary patterns has been demonstrated – a discovery that scientists say will advance quantum information processing.

Using a microscope, Stefan Kuhr and Immanuel Bloch at Max Planck Institute of Quantum Optics focused a laser beam down to a diameter of about 600 nm, which is just above the lattice spacing, and directed it at individual atoms with high precision.

With the help of a laser beam, the scientists could address single atoms in the lattice of light and change their spin state. In this way, they succeeded in having total control over the single atoms and in 'writing' arbitrary two-dimensional patterns. (Image: I. Bloch, MPQ)

The laser beam slightly deforms the electron shell of the addressed atom, changing the energy difference between its two spin states. Atoms with a spin – i.e., an intrinsic angular momentum – behave like little magnetic needles that can align in two opposite directions. If the atoms are irradiated with microwaves that are in resonance with the modified spin transition, only the addressed atoms absorb a microwave photon, which causes their spin to flip. All other atoms in the lattice remain unaffected by the microwave field.

The scientists demonstrated the high fidelity of this addressing scheme in a series of experiments. For this purpose, the spins of all atoms along a line were flipped one after the other, by moving the addressing laser from lattice site to lattice site. After removing all atoms with a flipped spin from the trap, the addressed atoms are visible as holes, which easily can be counted. In this way, the physicists deduced that the addressing worked in 95 percent of the cases. The method provides the possibility to generate arbitrary distributions of atoms in the lattice.

Starting from an arrangement of 16 atoms that were strung together on neighboring 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. They move from one lattice site to the other, even if their energy is not sufficient to cross the barrier between the lattice wells.

With the addressing scheme, arbitrary patterns of atoms in the lattice can be prepared. The atomic patterns each consist of 10 to 30 single atoms, which are kept in an artificial crystal of light (Image: I. Bloch, C. Weitenberg, P. Schauß, MPQ)

“As soon as the height of the lattice has reached the point where tunneling is possible, the particles start running as if they took part in a horse race,” doctoral candidate Christof Weitenberg said. “By taking snapshots of the atoms in the lattice at different times after the ‘starting signal,’ we could directly observe the quantum mechanical tunneling effect of single massive particles in an optical lattice for the first time.”

The new addressing technique allows many interesting studies of the dynamics of collective quantum states as they appear in solid-state systems. It also opens new perspectives in quantum information processing.

“A Mott isolator with exactly one atom per lattice site acts as a natural quantum register with a few hundred quantum bits, the ideal starting point for scalable quantum information processing,” Kuhr said. “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. A further step is to realize elementary logical operations between two selected atoms in the lattice, so-called quantum gates.”

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