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1. Hyperfine structure and collisions in three-photon Rydberg electromagnetically induced transparency

   https://doi.org/10.1103/PhysRevA.111.032821  

   Duspayev, Alisher; Raithel, Georg (March 2025, Physical Review A)

   Multiphoton electromagnetically induced transparency (EIT) of atomic vapors involves several intermediate atomic levels. The substructure of these levels and their collisional interactions can drastically alter experimental EIT signals. Here we report on hyperfine structure and collision effects in three-photon Rydberg EIT on the cascade 5S1/2 → 5P1/2 → 5D3/2 → 25F5/2 in a room-temperature 85Rb vapor cell. In our measurements of EIT spectra, we identify two types of EIT signatures that correspond with distinct excitation pathways and atomic velocity classes in the atomic vapor. The 5D3/2 hyperfine structure and Autler-Townes splittings lead to complex patterns in the EIT spectra, which we analyze with the aid of 10-level EIT simulations. Adding 50 mTorr of Ar gas alters the EIT spectra and induces an additional, third EIT mode. Based on our simulation results, we attribute these changes to hyperfine collisions in the Rb 5D3/2 level. Our study may become useful in quantum technologies involving Rydberg EIT and hyperfine collisions in vapor cells, including noninvasive spatiotemporally resolved electric-field sensing of electric fields in low-pressure plasmas. 

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2. High-angular-momentum Rydberg states in a room-temperature vapor cell for dc electric-field sensing

   https://doi.org/10.1103/PhysRevResearch.6.023138  

   Duspayev, Alisher; Cardman, Ryan; Anderson, David A; Raithel, Georg (May 2024, Physical Review Research)

   We prepare and analyze Rydberg states with orbital quantum numbers $\ell \le 6$using three-optical-photon electromagnetically induced transparency (EIT) and radio frequency (rf) dressing, and employ the high- $\ell$states in electric-field sensing. Rubidium-85 atoms in a room-temperature vapor cell are first promoted into the $25F_{5/2}$state via Rydberg-EIT with three infrared laser beams. Two rf dressing fields then (near-)resonantly couple the $25F, 25H(\ell =5)$, and $25I(\ell =6)$Rydberg states. The dependence of the rf-dressed Rydberg-state level structure on rf powers, rf and laser frequencies is characterized using EIT. Furthermore, we discuss the principles of dc-electric-field sensing using high- $\ell$Rydberg states and experimentally demonstrate the method using test electric fields of $\lesssim 50$V/m induced via photo-illumination of the vapor-cell wall. We measure the highly nonlinear dependence of the dc-electric-field strength on the power of the photo-illumination laser. Numerical calculations, which reproduce our experimental observations well, elucidate the underlying physics. Our paper is relevant to high-precision spectroscopy of high- $\ell$Rydberg states, Rydberg-atom-based electric-field sensing, and plasma electric-field diagnostics. Published by the American Physical Society2024 

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3. High accurate and efficient electrical impedance tomography for fast brain imaging

   https://doi.org/10.1117/12.2659172  

   Tran, Hoang Van; Kim, Min Hyuck; Le, Thong Chi; Yoon, Hargsoon (January 2023, Nano-, Bio-, Info-Tech Sensors, and Wearable Systems 2023)

   Kim, Jaehwan; Oh, Ilkwon; Yoon, Hargsoon; Porfiri, Maurizio (Ed.)

   Electrical Impedance Tomography (EIT) is a medical imaging technique that reconstructs impedance distribution inside a target object by injecting electrical currents into pairs of electrodes and measuring induced voltages on the remaining electrodes. Since neural signals result from the activity of ion channels causing impedance changes in the cell membrane, EIT can image these neural activities for understanding brain function and medical purposes. In our research, our self-developed electronic prototype board was used to generate high-quality electrical current and collect the data on electrodes with a high sampling rate and bit-resolution. In image reconstruction, a preprocessing data analysis algorithm was newly developed and applied to improve the accuracy of our EIT imaging. The human head has complex anatomical geometry and non-uniform resistivity distribution along with the highly resistive skull, which makes brain-EIT remains challenging inaccurate image reconstruction. To mimic the human head, a multi-layered human head phantom was designed and tested to investigate the effect of the skull structure on imaging. In this presentation, comparison studies for measurements and simulation results will be introduced to discuss the source of errors and improve the accuracy and efficiency of our brain-EIT system. 

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4. Hidden non-collinear spin-order induced topological surface states

   https://doi.org/10.1038/s41467-024-47340-2  

   Huang, Zengle; Yi, Hemian; Kaplan, Daniel; Min, Lujin; Tan, Hengxin; Chan, Ying-Ting; Mao, Zhiqiang; Yan, Binghai; Chang, Cui-Zu; Wu, Weida (December 2024, Nature Communications)

   Abstract Rare-earth monopnictides are a family of materials simultaneously displaying complex magnetism, strong electronic correlation, and topological band structure. The recently discovered emergent arc-like surface states in these materials have been attributed to the multi-wave-vector antiferromagnetic order, yet the direct experimental evidence has been elusive. Here we report observation of non-collinear antiferromagnetic order with multiple modulations using spin-polarized scanning tunneling microscopy. Moreover, we discover a hidden spin-rotation transition of single-to-multiple modulations 2 K below the Néel temperature. The hidden transition coincides with the onset of the surface states splitting observed by our angle-resolved photoemission spectroscopy measurements. Single modulation gives rise to a band inversion with induced topological surface states in a local momentum region while the full Brillouin zone carries trivial topological indices, and multiple modulation further splits the surface bands via non-collinear spin tilting, as revealed by our calculations. The direct evidence of the non-collinear spin order in NdSb not only clarifies the mechanism of the emergent topological surface states, but also opens up a new paradigm of control and manipulation of band topology with magnetism. 

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5. Elasto-Inertial Turbulence

   https://doi.org/10.1146/annurev-fluid-032822-025933  

   Dubief, Yves; Terrapon, Vincent E.; Hof, Björn (January 2023, Annual Review of Fluid Mechanics)

   The dissolution of minute concentration of polymers in wall-bounded flows is well-known for its unparalleled ability to reduce turbulent friction drag. Another phenomenon, elasto-inertial turbulence (EIT), has been far less studied even though elastic instabilities have already been observed in dilute polymer solutions before the discovery of polymer drag reduction. EIT is a chaotic state driven by polymer dynamics that is observed across many orders of magnitude in Reynolds number. It involves energy transfer from small elastic scales to large flow scales. The investigation of the mechanisms of EIT offers the possibility to better understand other complex phenomena such as elastic turbulence and maximum drag reduction. In this review, we survey recent research efforts that are advancing the understanding of the dynamics of EIT. We highlight the fundamental differences between EIT and Newtonian/inertial turbulence from the perspective of experiments, numerical simulations, instabilities, and coherent structures. Finally, we discuss the possible links between EIT and elastic turbulence and polymer drag reduction, as well as the remaining challenges in unraveling the self-sustaining mechanism of EIT. 

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