Anomalous broadening of driven Rydberg atoms Rydberg atoms are a popular choice for quantum device proposals because they interact strongly with each other and their individual and collective behavior is easy to manipulate. While experimenting with rubidium, one of the most popular Rydberg systems, PFC-supported researchers discovered an anomalous behavior that could be problematic for these proposals. For a single atom, the spacing between quantum energy levels is sharply defined, and off-resonant photons will excite the atom weakly or not at all. When many atoms interact, this definite spacing gets smeared out—an example of spectral broadening. This broadening allows a wider band of energies to excite the atoms. Researchers observed a signature of such broadening in a cold cloud of rubidium atoms trapped in a 3D optical lattice. When driving a transition from a low-energy state to a high-energy Rydberg state, they found that they created more Rydberg atoms than expected. The anomaly persisted even when varying the intensity of the excitation laser and the density of the atoms. Typical causes of broadening, such as short-range interactions between atoms or imperfections in laser beams, did not capture the magnitude of the effect. The team suspects that a small fraction of thermal atoms in different Rydberg levels broadened the transition to the target Rydberg state. This would cause the atoms to sample different shifts to their energy level structure as the configuration of contaminant atoms changed with time. The broadening could challenge proposals that would use Rydberg atoms for quantum information processing tasks, since there is currently no way to control when and where the contaminants appear. Illustration of a contaminant Rydberg atom in an optical lattice. Image credit, Kelley/JQI “Anomalous Broadening in Driven Dissipative Rydberg Systems,” E.A. Goldschmidt, T. Boulier, R. C. Brown, S. B. Koller, J. T. Young, A. V. Gorshkov, S. L. Rolston, and J. V. Porto, Phys. Rev. Lett., 116, 113001 (2016)