Seminars

Seminars will be held in room S-141 in the Physics and Astronomy Department building on Mondays at 4:00 PM, unless noted otherwise. When necessary, virtual seminar Zoom login instructions will be sent out via email.

Fall 2022

 September 8, 2022, 4:30 PM Prof. Klaus D. Jöns, Institute for Photonic Quantum Systems (PhoQS), Center for Optoelectronics and Photonics Paderborn (CeOPP), and Department of Physics, Paderborn University The Quest for the Ideal Quantum Light Source (Host: Eden Figueroa)

 With the advent of the second quantum revolution, striving for real-world applications of quantum technologies, enormous efforts have been made to develop and optimize the necessary building blocks. For photonic quantum technologies, which uses light particles –photons– as quantum information carriers (qubits), one crucial technology are quantum light sources generating these qubits. In recent years, epitaxial semiconductor quantum dots have made substantial progress, bringing us closer to practical quantum light sources for photonic quantum technology applications. In particular, quantum dots exhibit the lowest multi-photon emission probability [1], strongly polarization entangled photon pairs at telecom frequencies [2], and two-photon interference raw visibility close to unity [3]. However, no ideal quantum light source has yet been developed, addressing all prerequisites at once. I will discuss our efforts improving source properties such as purity, brightness, indistinguishability, and on-demand entangled photon pair generation. I will put these achievements in relation to the different excitation methods used: resonant excitation of a quantum mechanical 2-level system and two-photon excitation of a 3-level quantum ladder system [4]. Finally, I will discuss solutions to overcome these fundamental challenges and show our recent efforts to simultaneously achieve high purity and indistinguishability from a quantum dot 3-level quantum ladder system [5]. References: [1] L. Schweickert et al., Appl. Phys. Lett. 112, 093106 (2018). [2] K.D. Zeuner at al., ACS Photonics 8, 8, 2337–2344 (2021). [3] E. Schöll et al., Nano Lett. 19(4), 2404–2410 (2019). [4] E. Schöll et al., Phys. Rev. Lett. 125, 233605 (2020). [5] F. Sbresny et al., Phys. Rev. Lett. 128, 093603 (2022).

 October 6, 2022 [Thursday, SCGP-102] Prof. Dr. Gerard Meijer, Fritz Haber Institute of the Max Planck Society Molecular physics studies at the Fritz Haber Institute (Host: Jesús Pérez Ríos)

 It is now exactly twenty years ago that the Molecular Physics department at the Fritz Haber Institute (FHI) in Berlin has been established. Cold Molecules have been an important theme of the ongoing research from the beginning and our specific contributions to the field have in the first decade been centred around the manipulation and control of molecular beams with electric fields, as reviewed in 2012 [1]. In that year I left the FHI to serve a term as president of the Radboud University in Nijmegen, The Netherlands, my alma mater. Since my return to the FHI in 2017, we have started new research activities, and here I report on experiments that are currently being performed in our department to study and control chiral molecules and to laser cool and trap diatomic molecules. In the study of chiral molecules, photo-electron circular dichroism (PECD) [2] - a forward-backward asymmetry in the photoemission from a non-racemic sample induced by circularly polarized light - and microwave three wave mixing (M3WM) [3] have emerged as powerful new techniques during the last decades. We have demonstrated that PECD can be observed for chiral molecules in solution [4] as well as for anions in the gas phase. We have performed M3WM experiments on a jet-cooled beam of 1-indanol, in a scheme that has enabled the first quantitative comparison of experiment and theory for the transfer efficiency in what is the simplest triangle for enantiomer-specific state transfer (ESST) for any chiral molecule, that is, the one involving the absolute ground state level [5]. In our search for "the most ideal molecule" for laser cooling and trapping, that is, yielding the highest densities of ultracold molecular samples, we have identified and focused in on aluminum monofluoride. The AlF molecule has a binding energy of almost 7 eV and a bright beam of AlF can be produced, either pulsed or cw. The photon scattering rate on the $A^\left\{1\right\}\Pi -X^\left\{1\right\}\Sigma^\left\{+\right\}$band around 227 nm is very high, the Franck-Condon matrix is highly diagonal, all Q-lines of a $\left\{\right\}^\left\{1\right\}\Pi \leftarrow \left\{\right\}^\left\{1\right\}\Sigma^\left\{+\right\}$ transition are rotationally closed and the hyperfine splitting in the $\left\{\right\}^\left\{1\right\}\Sigma^\left\{+\right\}$ state is within the natural linewidth of the optical transition. The distance needed for laser slowing a beam of AlF molecules to rest will therefore be only several centimeters and the capture velocity of a MOT will be exceptionally large. We have used pulsed beams of jet-cooled AlF and beams from a buffergas source in combination with radio-frequency, microwave and optical fields to experimentally determine the fine and hyperfine structure of the lowest rotational levels in the $X^\left\{1\right\}\Sigma^\left\{+$, $A^\left\{1\right\}\Pi$, and $a^\left\{3\right\}\Pi$ states of AlF [6]. We have also determined how the hyperfine levels split and shift in external electric and magnetic fields and we have performed a detailed investigation of the singlet-triplet interaction, as this could cause a loss channel [7]. We have experimentally verified the predicted high scattering rate and we have demonstrated efficient optical cycling on the $A^\left\{1\right\}\Pi-X^\left\{1\right\}\Sigma^\left\{+\right\}$ band [8]. References [1] S.Y.T van de Meerakker, H.L. Bethlem, N. Vanhaecke, and G. Meijer, Chem. Rev. 112, 4828 (2012) [2] L. Nahon, G.A. Garcia, C.J. Harding, E. Mikajlo, and I. Powis, J. Chem. Phys. 125, 114309 (2006) [3] D. Patterson, M. Schnell, and J.M. Doyle, Nature 497, 475 (2013) [4] S. Malerz, et al., Rev. Sci. Instrum. 93, 015101 (2022); M. Pohl, et al., Phys. Chem. Chem. Phys. 44, 8081 (2022) [5] J. Lee, J. Bischoff, A.O. Hernandez-Castillo, B. Sartakov, G. Meijer, and S. Eibenberger-Arias, Phys. Rev. Lett. 128, 173001 (2022) [6] S. Truppe, et al., Phys. Rev. A 100, 052513 (2019); N. Walter, et al., J. Chem. Phys. 156, 124306 (2022) [7] M. Doppelbauer, et al., Mol. Phys. 119, e1810351 (2021); N. Walter, et al., J. Chem. Phys. 156, 184301 (2022) [8] S. Hofsäss, M. Doppelbauer, S.C. Wright, S. Kray, B. Sartakov, J. Pérez-Ríos, G. Meijer, and S. Truppe, New J. Phys. 23, 075001 (2021)

 October 27, 2022 (YITP seminar room, 2:30pm) Prof. David Pekker Dept. of Physics & Astronomy University of Pittsburgh Masers with linewidth well below the standard quantum limit and an unrelated discussion of machine learning electronic structure (Host:Tzu-Chieh Wei)

 The standard quantum limit on coherence of laser light was first obtained by Schawlow and Townes in 1958. Except for a small modification in 1999, which decreased this limit by a factor of two, the Schawlow-Townes limit has stood as the ultimate theoretical bound on laser linewidth for 62 years. I will present our theoretical blueprint for building a microwave laser (a maser) with coherence that is better than the standard quantum limit by a factor equal to the number of photons in the laser cavity. The key to our design is a pair of non-linear couplers made of an inductively shunted Josephson junctions that regulate the flow of photons from the gain media (made of a pair of superconducting qubits) to the resonator and out into the transmission line. (This work is related to a concurrent work by Baker, Saadatmand, Berry, and Wiseman). I will also describe my latest work on machine learning electronic structure of molecules. Over the past few years there has been significant progress toward figuring out how to use machine learning to predict electronic structure, thus avoiding expensive electronic structure calculations. However, all these efforts have focused on single-electron properties. I will argue that 1- and 2-electron reduced density matrices (RDMs) are sensible objects for encoding electronic structure for machine learning. The 2-RDM is especially interesting as it contains sufficient information on electron-electron correlations to compute many observables, including energy, with no additional approximation. I will demonstrate the feasibility of learning 1-and 2- RDMs on the toy problem of linear polymers.

 November 7, 2022 Prof. Susanne Ullrich, Department of Physics and Astronomy, University of Georgia The Effect of Sulfur Substitution on the Photoprotective Properties of Uracil (Host: Tom Weinacht)

 The canonical nucleobases, which form the building blocks of our genetic coding material, are known to protect themselves against photodamage through ultrafast internal conversion processes that dissipate potentially harmful UV energy into heat. However, seemingly minor changes such as single atom substitutions can profoundly alter the photophysics of the nucleobases. In thiobases, where an oxygen is replaced by sulfur, these internal conversion pathways are inaccessible and crossing onto the triplet manifold becomes highly efficient. While long-lived, reactive triplet states, as observed in some of the thiobases, negate their photoprotection, these properties are highly desirable for pharmacological applications, e.g. as photosensitizers in cancer treatments. Using time-resolved photoelectron spectroscopy the response of a series of thiouracils to UV irradiation has been investigated to unravel the mechanistic details governing their unique ultrafast intersystem crossing dynamics. Remarkable differences are observed for 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil when the degree of thionation and position of sulfur atom is altered.

 November 28, 2022 Prof. Milan E. Delor, Department of Chemistry, Columbia Universty Imaging and controlling ballistic polarons and polaritons (Hosta: Tom Allison & Tom Weinacht)

 Achieving long-range ballistic (coherent) electron flow in materials at room temperature is a long-standing goal that could unlock lossless energy harvesting and wave-based information technologies. The key challenge is to overcome short-range scattering between electrons and lattice vibrations (phonons). I will describe two avenues to achieve ballistic transport by harnessing strong interactions between coherent and incoherent excitations in solid-state lattices. The first is to leverage polaritons, part-light part-matter quasiparticles resulting from hybridization between microcavity photons and semiconductor excitons. Associated challenges in maintaining light-like coherence in the presence of strong matter-like interactions will be discussed. A second avenue is to leverage strong interactions between electrons and delocalized acoustic phonons. We discovered that in lattices with weak inter-atom electronic coupling, these interactions lead to the formation of acoustic polarons that are intrinsically shielded from phonon scattering. In both cases, we develop ultrafast optical imaging capabilities enabling us to track the propagation of these quasiparticles with femtosecond resolution and few-nanometer sensitivity, providing a precise measurement of quasiparticle velocity, scattering pathways, and transition from coherent to incoherent (diffusive) transport.

2022