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. Credit: Max Planck Institute of Quantum Optics |
Physicists at the Max Planck Institute of Quantum Optics
succeeded in manipulating atoms individually in a lattice of light and in
arranging them in arbitrary patterns. These results are an important step
towards large-scale quantum computing and for the simulation of condensed
matter systems.
Physicists around the world are searching for the best way
to realize a quantum computer. Now scientists of the team around Stefan Kuhr
and Immanuel Bloch at the Max Planck Institute of Quantum Optics took a
decisive step in this direction. They could address and change the spin of
single atoms with laser light and arrange them in arbitrary patterns. In this
way, the physicists strung the atoms along a line and could observe their tunneling
dynamics in a racing duel of the atoms. A register of hundreds of addressable
quantum particles could serve for storing and processing of quantum information
in a quantum computer.
In the present experiment, the scientists load laser-cooled
rubidium atoms into an artificial crystal of light. These so-called optical
lattices are generated by superimposing several laser beams. The atoms are kept
in the lattice of light similar to marbles in the hollows of an egg carton.
Already a few months ago, the team of Stefan Kuhr and
Immanuel Bloch showed that each site of the optical lattice can be filled with
one atom. With the help of a microscope, the scientists visualized atom by atom
and thereby verified the shell-like structure of this Mott insulator. Now the
scientists succeeded in individually addressing the atoms in the lattice and in
changing their respective energy state. Using the microscope, they 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.
The laser beam slightly deforms the electron shell of the
addressed atom and thereby changes the energy difference between its two spin states.
Atoms with a spin 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
can easily be counted. In this way, the physicists deduced that the addressing
worked in 95% of the cases. Atoms at the neighboring sites are not influenced
by the addressing laser. 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. “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 describes. “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” as Stefan Kuhr explains. “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.”
