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Presented By: Department Colloquia

Department Colloquium | Graduate Student Showcase

Ryan Cardman, Xiaoyu Guo, and Robert Saskowski (U-M Physics)

Ryan Cardman
Ponderomotive Laser Spectroscopy of Rubidium Rydberg Atoms
Spectroscopy of Rydberg atoms with very large principal quantum numbers, n, has traditionally been performed through coherent absorption and emission of microwaves. In my talk, I describe a newly developed and recently demonstrated method of manipulating an alkali (rubidium) Rydberg atom’s valence electron using phase-modulated laser fields that drive a coherent transition between two Rydberg states. The light-matter interaction for this method originates from the ponderomotive force of the laser field acting on the Rydberg electron, rather than the more commonly observed electric-dipole force (which, along with magnetic-dipole interactions, governs most of modern spectroscopy). Because the ponderomotive interaction fundamentally differs from the electric-dipole force, differences arise in selection rules, which are considerably more relaxed in ponderomotive than in traditional spectroscopy. For instance, high-harmonic transitions can be driven in first-order perturbation theory (i.e. without virtual intermediate states, and without a significant drop in Rabi frequency with increasing order). We also observe and explain a new paradigm for Doppler-free spectroscopy. In my talk, I will describe details of the optical setup, the phase-control of the light, the experimental spectra, our models, and numerical simulations. Applications of ponderomotive Rydberg-atom spectroscopy include site-selective spin manipulations in quantum simulators, high-l-Rydberg-state initialization, and gate operations in Rydberg quantum computers.

Xiaoyu Guo
Ferro-Rotational Domain Walls Revealed by Electric Quadrupole Second Harmonic Generation Microscopy
Domain walls in (multi)ferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts. However, in contrast to ferromagnetism and ferroelectricity, it is extremely challenging to study ferro-rotational (FR) domain walls because the FR order is invariant under both spatial-inversion and time-reversal operations and thus hardly couple with conventional probes. Here, we investigate an FR candidate NiTiO3 with electric quadrupole (EQ) second harmonic generation rotational anisotropy (SHG RA) and probe the point symmetries of its two degenerate FR domain states. We then visualize the real-space FR domains and domain walls using scanning EQ SHG microscopy. By taking local EQ SHG RA measurements, we further show the restoration of the mirror symmetry at FR domain walls and prove their nonpolar nature. Our findings not only provide insight into FR domain walls, but also demonstrate a powerful tool for future studies on domain walls of unconventional ferroics.

Robert Saskowski
Towards Unreasonable Effectiveness in AdS5
Gauge/gravity duality has garnered an enormous amount of interest in the last twenty-five years. The prototypical example is the AdS/CFT correspondence, which relates supergravity with a negative cosmological constant to a conformal field theory in one lower dimension. In particular, this relates gravitational and non-gravitational theories. I will focus on the supergravity story, specifically with higher-derivative corrections. In this talk, I will give a brief introduction to the AdS/CFT correspondence and higher derivatives and discuss some recent work with my advisor Jim Liu regarding the universality of supersymmetric four-derivative corrections to minimal supergravity in five spacetime dimensions.

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