University of Michigan Physics Professor Georg Raithel, graduate student Andrew Schwarzkopf, and Research Fellow Rachel Sapiro have developed an atom-imaging technique that allows for the detection of the positions of individual Rydberg atoms, which are atoms with a highly excited outer electron. Using this technique, the team provides the first spatially resolved images that demonstrate the “Rydberg blockade.” This effect is at the core of proposals for a quantum computer architecture based on neutral atoms.

A Rydberg atom has such a tenuous grasp on its excited electron that the atom is extremely sensitive to external electric and magnetic fields, and interacts very strongly with other Rydberg atoms. The interaction between Rydberg atoms is so strong that a Rydberg atom can “block” the laser-excitation of another Rydberg atom by shifting the energy levels of the second atom out of resonance with the laser. This is termed the Rydberg blockade effect. This process leads to quantum entanglement, which can be used in quantum computation algorithms.

A second Rydberg atom can only be excited if it is farther than a “blockade radius” from the first atom. Professor Raithel and team directly measured this blockade by laser-exciting Rydberg atoms in a cold atomic vapor and measuring the Rydberg atom positions. They observed a blockade radius of about 10 microns, which is about 100,000 times larger than the radius of a ground state atom.

The research was funded by the National Science Foundation and by the Air Force Office of Scientific Research.

The complete article, Imaging Spatial Correlations of Rydberg Excitations in Cold Atom Clouds, can be found in Phys. Rev. Lett. 107, 103001 (2011) [5 pages].

Figure Caption:
The image shows the probability of having a Rydberg atom as a function of position if there already is one at the center (black spot). The bright ring around the black spot demonstrates that it is unlikely to find a second Rydberg atom close to the one at the center. The distance bar in the image corresponds to 10 microns. The image shown is generated from several thousand individual snapshots of Rydberg atom clouds. The specific Rydberg level used to obtain the image is the 70D level of rubidium-85.