Dr. Esteban Goetz, Department of Physics, University of Connecticut

Interferometric Harmonic Spectroscopy for Electron Dynamics Imaging and Attosecond Pulse Train Phase Characterization

The advent of ultrashort light pulses has opened the possibility of investigating atomic and molecular processes on their natural time scales. In particular, Attosecond Transient Absorption Spectroscopy (ATAS) [1], a technique that allows to time-resolve the quantum dynamics of electrons by monitoring the absorption of extreme ultraviolet (XUV) radiation by an atomic or molecular system when the latter is dressed by an infrared (IR) laser source.

Motivated by recent experimental advances in self-referenced interferometric harmonic spectroscopy [2], we theoretically investigate an alternative approach to ATAS for electron dynamics imaging and attosecond pulse train (APT) phase characterization. In contrast to ATAS, which gives access to the imaginary part of the refractive index through an absorption measurement, an interferometric phase measurement gives information of its real part. In this talk, I will discuss the link between the XUV phase measurements of Ref. [2] and the different photoexcitation pathways occurring at the atomic level which are imprinted in the real part of the macroscopic refractive index. As an application, we show how such an interferometric approach can be used for phase retrieval of attosecond pulse trains based on two-arm harmonic spectroscopy and an optimization algorithm. Finally, I will highlight the impact of spin-orbit couplings and macroscopic and field propagation effects on the phase measurements and APT phase retrieval. Our theoretical description is based on numerical solution of the scalar Maxwell equations beyond Beer’s Law for the macroscopic field propagation coupled to the time-dependent Schroedinger equation for the quantum dynamics.

[1] M. Holler et al., Phys. Rev. Lett. 106, 123601 (2011)

[2] G. R. Harrison et al., arXiv:2305.17263 (2023)

Prof. Claudio Chamon, Boston University

Circuit complexity and functionality: a thermodynamic perspective

We explore a link between complexity and physics for circuits of given functionality. Taking advantage of the connection between circuit counting problems and the derivation of ensembles in statistical mechanics, we tie the entropy of circuits of a given functionality and fixed number of gates to circuit complexity. We use thermodynamic relations to connect the quantity analogous to the equilibrium temperature to the exponent describing the exponential growth of the number of distinct functionalities as a function of complexity. This connection is intimately related to the finite compressibility of typical circuits. Finally, we use the thermodynamic approach to formulate a framework for the obfuscation of programs of arbitrary length – an important problem in cryptography – as thermalization through recursive mixing of neighboring sections of a circuit, which can viewed as the mixing of two containers with “gases of gates”. This recursive process equilibrates the average complexity and leads to the saturation of the circuit entropy, while preserving functionality of the overall circuit. The thermodynamic arguments hinge on ergodicity in the space of circuits which we conjecture is limited to disconnected ergodic sectors due to fragmentation. The notion of fragmentation has important implications for the problem of circuit obfuscation as it implies that there are circuits with same size and functionality that cannot be connected via local moves. Furthermore, we argue that fragmentation is unavoidable unless the complexity classes NP and coNP coincide.

Dr. Paul M. Eugenio, University of Connecticut

Tunable moire sublattices in twisted square homobilayers: exploiting fundamental principles for new technologies

Stacking and twisting atomically thin bilayers at small angles produces an approximate periodic pattern, due to the overlap of the crystal layers. These devices, dubbed “moire” bilayers, exhibit a high degree of tunability and variability: through choice of twist angle, constituent layers, and gating. To date, a number of such devices have been built which have demonstrated a plethora of novel phases, including non-trivial topology and Mott physics. Despite this explosion in moire research, moire bilayers have been almost exclusively formed from layers with triangular/hexagonal crystal geometry, and where the valence bands are centered on the Gamma or K/K’ high symmetry points. Here we theoretically demonstrate that moire devices formed from square bilayers can be used to simulate the ground state of the Hubbard model, but where the ratio of the nearest-neighbor (t) and next-to-nearest neighbor (t’) tunneling can be tuned between zero and infinity, in situ via an electric field. If experimentally realized, such a device would be the first of its kind, and would open a pathway toward the testing of a number of proposed exotic phases, such as a spin-liquid and d+id superconductivity. Most importantly, the square Hubbard model is a quintessential model for high-Tc in cuprates, where numerics has demonstrated the absence of superconductivity when t’=0.

Graduate student Geoff Harrison, Department of Physics, University of Connecticut

ITAS: A Technique for Complete Quantum Measurements on a New Timescale

Transient absorption spectroscopy is a well-established technique used to study electron dynamics in atomic and molecular systems but typically can only measure the magnitude of the electronic wavefunction. We have integrated interferometric methods into this technique to allow complete quantum measurements of both the magnitude and phase of electronic wavefunctions. A spatial light modulator (SLM) is used to separate the interferometric arms in an extremely stable way, enabling the measurement of effects on the zeptosecond timescale (with a jitter of 3zs). In this talk, I’ll describe how we’ve utilized SLMs to make these measurements possible and share some initial data we’ve taken looking at phase effects in argon.

Dr. Fatma Aslan, Jefferson National Laboratory and UConn

Hadron structure-oriented approach to TMD phenomenology

We present a first practical implementation of a recently proposed hadron structure oriented (HSO) approach to TMD phenomenology applied to Drell-Yan like processes. We compare and contrast general features of our methodology with other common practices and emphasize the improvements derived from our approach that we view as essential for applications where extracting details of nonperturbative transverse hadron structure is a major goal. These include the HSO’s preservation of a basic TMD parton-model-like framework even while accounting for full TMD factorization and evolution, explicit preservation of the integral relationship between TMD and collinear PDFs, and the ability to meaningfully compare different theoretical models of nonperturbative TMD parton distributions.

Dr. J. I. A. Li, Brown University

The Superconducting Diode Effect And Spontaneous Symmetry Breaking In Multi-Layer Graphene

The superconducting diode effect, defined as nonreciprocity in the critical supercurrent, provides a unique window into the nature of the superconducting phase. It has been argued that a zero-field diode effect in the superconducting transport requires inversion and time-reversal symmetries to be simultaneously broken. Along this vein, the zero-field superconducting diode effect in multi-layer graphene provides direct evidence of the microscopic coexistence between superconductivity and time-reversal symmetry breaking. In this talk, I will discuss our recent efforts that utilize the angle-resolved measurement of transport nonreciprocity to directly probe the nature of spontaneous symmetry breaking in the normal phase. By investigating the interplay between transport nonreciprocity, ferromagnetism, and superconductivity, our findings suggest that the exchange-driven instability in the momentum space plays a key role in the zero-field superconducting diode effect.

Graduate student Debadarshini Mishra, Department of Physics, University of Connecticut

Imaging ultrafast dynamics in molecular systems

Imaging electronic and molecular dynamics at the attosecond and femtosecond timescales is crucial for understanding the mechanisms of chemical reactions, a fundamental aspect in fields ranging from materials science to biochemistry. This in-depth understanding of chemical processes may allow for precise control over reaction dynamics, thereby paving the way for advancements in technology and medicine, for example, by guiding the development of efficient catalysis, innovative materials, and targeted drugs. In this talk, I will describe our work on imaging time-resolved molecular dynamics using two distinct and complementary techniques.

In the first part of my talk, I will discuss the use of coincident Coulomb explosion imaging for the direct visualization of roaming reactions. These reactions represent unconventional pathways that allow fragments to remain weakly bonded, leading to the formation of unexpected final products. Typically, the neutral character of the roaming fragment and its indeterminate trajectory make direct experimental identification challenging. However, I will demonstrate that by leveraging the power of coincidence imaging, we can reconstruct the momentum vector of the neutral roamer and thus identify an unambiguous signature for roaming.

In the second part of my talk, I will discuss the imaging of UV-induced ring-opening and dissociation dynamics using ultrafast electron diffraction. I will demonstrate that by harnessing the superior temporal and structural resolution of this technique, we can explore the competition among different molecular pathways as well as their wavelength-dependent behavior.

Prof. Jed Pixley, Rutgers University

Novel Strongly Correlated Phases in Stacked TMD Bilayers

Two-dimensional transition metal dichalcogenides (TMDs) have emerged as an exciting platform to stack and twist bilayers to engineer strongly correlated quantum phases. Here we present a theory to describe the recent realization of a heavy fermion state in stacked MoTe2/WSe2 bilayers. An extension of this theory that allows for the formation of unconventional superconductivity through repulsive nearest neighbor interactions will be used to show how to realize the p-wave BEC to BCS transition.

Dr. François Légaré, Institut national de la recherche scientifique, Energy Materials Télécommunications center

Ultrafast IR/mid-IR laser technologies and their applications at ALLS

The Advanced Laser Light Source (ALLS) is a unique user facility located at INRS-EMT (Varennes, Canada) counting on 40M CDN$ of investment since 2002. Since 2019, this facility has jointed the LaserNetUS network and is now funded as a national research infrastructure by the Canada Foundation for Innovation – Major Science Initiatives. These fundings ease access to the facility for academic and government users. In the first part of my talk, I will give an overview of the facility’s capabilities including the most powerful laser in Canada with 750 TW. In the second part, I will discuss novel approaches developed by my team for the generation of ultrashort pulses in the IR and mid-IR spectral range. This includes multidimensional solitary states in hollow core fibers [1,2] as well as using the frequency domain optical parametric amplification for the generation of tunable CEP stable mid-IR laser pulses [3,4]. Pulse characterization in the mid-IR spectral range will be presented [5]. Finally, I will present recent results on the generation of high-dose MeV electrons from tight focussing in air [6].

References

[1] R. Safaei, G. Fan, O. Kwon, K. Légaré, P. Lassonde, B. E. Schmidt, H. Ibrahim, and F. Légaré (2020), High-energy multidimensional solitary states in hollow core fiber, Nature Phot. 14, 733-739.

[2] L. Arias, A. Longa, G. Jargot, A. Pomerleau, P. Lassonde, G. Fan, R. Safaei, P. Corkum, F. Boschini, H. Ibrahim, and F. Légaré, Few-cycle Yb laser source at 20 kHz using multidimensional solitary states in hollow-core fibers, Opt. Lett. 47, 3612-3615 (2022).

[3] A. Leblanc, G. Dalla-Barba, P. Lassonde, A. Laramée, B. Schmidt, E. Cormier, H. Ibrahim, and F. Légaré (2020), High-field mid-infrared pulses derived from frequency domain optical parametric amplification, Opt. Lett. 45, 2267-2270.

[4] G. Dalla-Barba, G. Jargot, P. Lassonde, S. Tóth, E. Haddad, F. Boschini, J. Delagnes, A. Leblanc, H. Ibrahim, E. Cormier, and F. Légaré, Mid-infrared frequency domain optical parametric amplifier, Opt. Express 31, 14954-14964 (2023).

[5] A. Leblanc, P. Lassonde, S. Petit, J.-C. Delagnes, E. Haddad, G. Ernotte, M. R. Bionta, V. Gruson, B. E. Schmidt, H. Ibrahim, E. Cormier, and F. Légaré (2019), Phase-matching-free pulse retrieval based on transient absorption in solids, Opt. Express 27, 28998.

[6] S. Vallières, J. Powell, T. Connell, M. Evans, M. Lytova, F. Fillion-Gourdeau, S. Fourmaux, S. Payeur, P. Lassonde, S. MacLean, and F. Légaré, High Dose-Rate MeV Electron Beam from a Tightly-Focused Femtosecond IR Laser in Ambient Air (2024), Laser Photonics Rev. 18, 2300078.

François Légaré is a chemical physicist who specializes in developing novel approaches for ultrafast science and technologies, as well as biomedical imaging with nonlinear optics (Ph.D. in chemistry, 2004 – co-supervised by Profs. André D. Bandrauk and Paul B. Corkum). Full professor (2013 - …) at the Energy Materials Telecommunications center of the Institut national de la recherche scientifique (INRS-EMT), he was the director of the Advanced Laser Light Source (ALLS) until 2023. Since 2022, he is the director of the INRS-EMT center and CEO of ALLS. Under his scientific leadership, INRS has received in 2017 a grant of 13.9M CDN$ from the Canada Foundation for Innovation and the Quebec government, with 11.9M CDN$ to upscale the ALLS facility with high average power Ytterbium laser systems and advanced instrumentation for time-resolved material characterization. He is a Fellow and senior member of OPTICA and Fellow of the American Physical Society. He is a member of The College of New Scholars, Artists and Scientists of the Royal Society of Canada (2017). He was awarded the Herzberg medal from the Canadian Association of Physics in 2015 and the Rutherford Memorial Medal in physics of the Royal Society of Canada in 2016. He has contributed to about 200 articles in peer reviewed journals including prestigious ones such as Nature, Science, Nature Photonics, Nature Physics, Nature Communications, and Physical Review Letters. According to Google Scholar, his h-index is 59 with more than 13,000 citations.

Dr. Thomas Weinacht, Stony Brook University

Long Lived Electronic Coherences in Molecules

I will discuss experiments and calculations that demonstrate long lived electronic coherences in molecules using a combination of measurements with shaped octave spanning ultrafast laser pulses, 3D velocity map imaging and calculations of the light matter interaction. Our pump-probe measurements prepare and interrogate entangled nuclear-electronic wave packets whose electronic phase remains well defined despite vibrational motion along many degrees of freedom. The experiments and calculations illustrate how coherences between excited electronic states survive even when coherence with the ground state is lost, and may have important implications for light harvesting, electronic transport and attosecond science.

Dr. Yossathorn Tawabutr, University of Jyväskylä

Fully Consistent NLO Calculation of Forward Single-Inclusive Hadron Production in Proton-Nucleus Collisions

We study the single-inclusive particle production from proton-nucleus collisions in the dilute-dense framework of the color glass condensate (CGC) at next-to-leading order (NLO) accuracy. In this regime, the cross section factorizes into hard impact factors and dipole-target scattering amplitude describing the eikonal interaction of the partons in the target color field. For the first time, we combine the NLO impact factors with the dipole amplitude evolved consistently using the NLO Balitsky-Kovchegov (BK) equation with the initial conditions fitted to HERA structure function data.

The resulting neutral pion cross section with all parton channels included are qualitatively consistent with the recent LHCb measurement. In particular, the NLO evolution coupled to the leading order impact factor is shown to produce a large Cronin peak that is not visible in the data, demonstrating the importance of consistently including NLO corrections to all the ingredients. Furthermore, the transverse momentum spectrum is found to be sensitive to the resummation scheme and the running coupling prescription in the BK evolution. This demonstrates how additional constraints for the initial condition of the BK evolution can be obtained from global analyses including both the HERA and LHC data. In light of the upcoming upgrades to the LHC, the dependence of our results on rapidity will also be discussed.

Prof. Lance Dixon, Stanford University

The DNA of Particle Scattering

Scattering amplitudes are the arena where quantum field theory meets particle experiments, for example at the Large Hadron Collider where the copious scattering of quarks and gluons in quantum chromodynamics (QCD) produces Higgs bosons and many backgrounds to searches for new physics. Particle scattering in QCD and other gauge theories is far simpler than standard perturbative approaches would suggest. Modern approaches based on unitarity and bootstrapping dramatically simplify many computations previously done with Feynman diagrams. Even so, the final results are often highly intricate, multivariate mathematical functions, which are difficult to describe, let alone compute. In many cases, the functions have a “genetic code” underlying them, called the symbol, which reveals much of their structure. The symbol is a linear combination of words, sequences of letters analogous to sequences of DNA base pairs. Understanding the alphabet, and then reading the code, exposes the physics and mathematics underlying the scattering process, including new symmetries. For example, the two scattering amplitudes that are known to the highest orders in perturbation theory (8 loops) are related to each other by a mysterious antipodal duality, which involves reading the code backwards as well as forwards. A third scattering amplitude, which contains both of these as limits, has an antipodal self-duality which “explains” the other duality. However, we still don’t know `who ordered’ antipodal (self-)duality, or what it really means.

Dr. Jon Curtis, UCLA

Multimode cavity control of ferroelectric fluctuations

Electromagnetic cavities and metamaterials have been used to great effect in the field of AMO physics and electrical engineering. By shaping the spatial, spectral, or polarization characteristics of the electromagnetic environment, the coherent interaction between light and matter can be focused and amplified, leading to phenomena such as lasing, the Purcell effect, the Casimir effect, and superradiance. In this talk I will show how these ideas may be extended and applied to solid state quantum materials. In particular, I will consider polarization fluctuations in a quantum paraelectric insulator, and consider their coupling to a Fabry-Perot type optical cavity. By using the full multimode continuum description of the system, I will show how the ferroelectric fluctuations respond in a local, spatially resolved manner. The presence of the cavity indeed is shown to renormalize the soft-mode frequency, with effects primarily confined to the surface, and thus for thin films this effect can be pronounced. The temperature dependence shows this effect only onsets at low temperatures, indicating its origin from quantum electrodynamics effects – in close analogy with the Casimir effect.

Prof. Alicia Kollár, University of Maryland

Lattices in Circuit QED

The field of circuit QED has emerged as a rich platform for both quantum computation and quantum simulation. These systems exhibit a high degree of both spatial and temporal control which can be used to create synthetic lattice systems. Spatial lattices can be formed using periodic arrays of resonators. Combined with strong qubitphoton interactions, these systems can be used to study dynamical phase transitions, many-body phenomena, and spin models in driven-dissipative systems. I will show that lattices of coplanar waveguide (CPW) resonators permit the creation of unique devices which host photons in curved spaces, gapped flat bands, and novel forms of qubit-qubit interaction [1,2]. I will show that graph theory is the natural language for describing these microwave photonic systems and present preliminary data on the development of a new generation of CPW lattice devices with unconventional band structures. Periodic modulation in superconducting-qubit systems also provides a route to Floquet systems with topological band structures. I will present preliminary experimental steps toward the realization of a topological energy pump which can “boost” smaller non-clalssical states of light into larger ones [3].

[1] A. J. Koll´ar et al., Nature 45, 571 (2019).

[2] A. J. Koll´ar et al., Comm. Math. Phys. 376, 1909 (2020).

[3] D. Long et al., Phys. Rev. Lett. 128, 183602 (2022).

Dr. Edwin Pedrozo, MIT

Quantum Sensors and the Search for New Physics

The continuously improving performance of quantum sensors is enabling the exploration of fundamental physics with unprecedented precision. Notable examples of these systems include optical atomic clocks and atom interferometers, which are among the most precise devices ever invented by humankind. As a result, they are increasingly utilized in the search for new physics. The application of Atomic, Molecular, and Optical (AMO) Physics techniques to such inquiries in the realm of nuclear physics has been gaining attention in the current decade. The level of control and precision achievable in AMO tabletop experiments, especially with ultracold atoms, enhances the measurement capabilities in complex experimental systems that pursue tests of fundamental physics and symmetries, the search for the electron electric dipole moment (eEDM), and physics beyond the standard model. In this talk, I will explain how incorporating entanglement into these systems can further improve their measurement capabilities. Additionally, I will discuss several proposals that employ laser-cooled atoms and molecules in the search for physics beyond the Standard Model.

Prof. Steve Lamoreaux, Yale University

Title: TBA

Prof. Tigran Sedrakyan, University of Massachusetts Amherst

Moat-band physics and emergent excitonic topological order in correlated electron-hole bilayers”

The role of the particle-particle interaction becomes increasingly important if the spectral band structure of a free system has increasing degeneracy. Ultimately, it will be the role of interactions to choose the state of the system. Examples include the systems with the lowest band having a degenerate minimum along a closed contour in the reciprocal space – the Moat. Any weak perturbation can set a new energy scale describing the state with qualitatively different properties in such a limit of infinite degeneracy. In this talk, I will discuss the general principles behind the universal properties of correlated bosons on moat bands, which host topological order with long-range quantum entanglement. In particular, I will discuss moat-band phenomena in shallowly inverted InAs/GaSb quantum wells, where we observe an unconventional time-reversal-symmetry breaking excitonic ground state under imbalanced electron and hole densities. I will show that the strong frustration of the system leads to a moat band for excitons, resulting in a time-reversal-symmetry breaking excitonic topological order, which explains all our experimental observations.

Daniel Norman, Department of Physics, University of Connecticut

The Complex Analytic Properties of Bandwidth Limited Signals and their Application to Conformal Cosmology and Signal Processing

Conformal gravity is an alternative theory of gravity derived from the conformally invariant Weyl squared action as opposed to the standard Einstein-Hilbert action. The general equations of conformal gravity were applied to the cosmological scale to create a theory of Conformal Cosmology where the geometry of space-time is described by first order fluctuations on a conformal-to-flat background of constant negative curvature. The differential equations of this cosmological model have solutions in terms of Legendre functions with complex degree and order. In order to calculate the multipole expansion of Cosmic Background Radiation (CMB) anisotropy within this model, it is necessary to integrate the Legendre function solutions with respect to their complex degree. An analytic method for solving this integration problem was developed which makes use of the fact that the Legendre functions are Bandwidth Limit Signals (BLS’s) which are functions with a finite domain in frequency space. A general analysis of the properties of BLS’s in the complex plane was done which has yielded new theorems and expansion formulas applicable to all BLS’s as well as to other related families of complex functions. These results have both specific applications to Conformal Cosmology as well as broader applications to the field of signal processing. The methods of complex integration developed in this work, initially for the purpose of computing the CMB anisotropy in Conformal Cosmology, have been used to provide a novel solution to the infamous Borweinn integral as well as a novel proof of the Nyquist-Shannon sampling theorem.

Professor Chii-Dong Lin, Kansas State University

Post-Nobel Award on attosecond Science – Challenges and opportunities in the field going forward

It is an exciting month for the attosecond and strong-field physics communities after the announcement of the three Nobel Laureates earlier. How will this field evolve going forward? While it is very attractive to talk about the shortest light pulses to the general public, and even to the physical science community, the field still faces great challenges but also opportunities. I will “talk” about the challenges and will share with you some of the recent progress toward developing theories that can be compared to experiments.

Prof. Cyrus Dreyer, Stony Brook University and Flatiron Institute

Nonadiabatic lattice dynamics in metals and magnets

In electronic structure theory, lattice vibrations are usually treated under the Born-Oppenheimer approximation, where electronic degrees of freedom are assumed to be fast compared to nuclear dynamics. However, going beyond this adiabatic approximation is necessary in many situations for an accurate description of phonons, and their effects on materials properties. I will discuss two such cases. The first case involves Born effective charges, which are crucial to understanding, e.g., ferroelectric polarization, phonon dispersions in ionic insulators, electron-phonon scattering, dielectric screening, electromechanical coupling, and optical spectra in the IR/THz regime. Via density-functional perturbation theory (DFPT) calculations, I will show that going beyond the adiabatic approximation extends the definition of Born effective charges from insulators to conducting systems and relates them to a seemingly unrelated fundamental property of metals: the Drude weight. The second case I will discuss is the coupling of magnetism and phonons in materials. Specifically, I will demonstrate a DFT-based methodology for including the velocity-dependence of interatomic forces, which explicitly accounts for time-reversal symmetry breaking in the nuclear equations of motion. I will show that in some magnetic materials, such as CrI3, the assumption of adiabatic separation between electron and nuclear dynamics breaks down completely due to the role of (slow) spin dynamics in the coupling between phonons and the magnetic order.

Dr. Romain Vasseur, University of Massachusetts  Amherst

Learning global charges from local measurements

Monitored random quantum circuits (MRCs) exhibit a measurement-induced phase transition between area-law and volume-law entanglement scaling. In this talk, I will review the physics of such entanglement transitions, and discuss the current status of this field as well as recent experimental realizations. I will argue that MRCs with a conserved charge additionally exhibit two distinct volume-law entangled phases that cannot be characterized by equilibrium notions of symmetry-breaking or topological order, but rather by the non-equilibrium dynamics and steady-state distribution of charge fluctuations. These include a charge-fuzzy phase in which charge information is rapidly scrambled leading to slowly decaying spatial fluctuations of charge in the steady state, and a charge-sharp phase in which measurements collapse quantum fluctuations of charge without destroying the volume-law entanglement of neutral degrees of freedom. I will present some statistical mechanics description of such charge-sharpening transitions, and relate them to the efficiency of classical decoders to “learn” the global charge of quantum systems from local measurements.

Dr. Sandra Beauvarlet, UConn and PULSE, SLAC National Laboratory

Attosecond X-ray pulse pair generation at SLAC- LCLS X-ray Free Electron Laser and application to probe ultrafast electron dynamics in aminophenol

 

I will present in this AMO seminar general notions about how Free Electron Laser (FEL) work and will present some of the characteristics and more recent developments at the SLAC X-ray FEL light source. I will discuss the key advances to reach the sub-fs timescale enabling the production of isolated attosecond X-ray pulses but also the generation of attosecond X-ray pulse pairs with controllable delays. These pulses open the way to pump-probe measurements of ultrafast dynamics with attosecond temporal resolution and angstrom spatial resolution due to the X-ray nature of the light produced. Going from a more technical development perspective to an experimentalist vision, I will report on two examples of attosecond X-ray pump - attosecond X-ray probe measurements conducted on gas phase Aminophenol molecules (C ₆ H ₇ NO). The first one relies on carbon K-shell ionization and the effect of post-collision interaction (PCI). The second one relies on X-ray absorption spectroscopy to probe charge migration across the molecules on a sub-10 fs timescale.

Dr. Abhishek Bannerjee, Harvard University

 

Dr. Heide Ibrahim, INRS, Canada

 

Filming ultrafast chemical reactions in real-time – from coherent motion to roaming dynamics

 

Upon photoexcitation, molecules can undergo different types of transformations such as simple vibrations or rotations, but also migrations, or dissociations. Due to the required combination of ultrafast time-scales and ultra-small length-scales involved, specific tools are required to follow such reactions in real-time. Here, we are using time-resolved Coulomb explosion imaging (CEI) as the ultrafast tool to image such dynamics. With CEI, we can not only image coherent molecular dynamics such as proton migration in acetylene; it also works for various dissociation pathways, even if they occur differently from one molecule to another.

In the formaldehyde molecule, we can see fragments following the direct, conventional dissociation path, as well as fragments deviating from this minimum energy path. So-called roaming fragments or “roamers” explore the potential energy landscape in a statistical manner and were directly captured in real-time, despite the signal’s statistical character. This is possible due to the single-molecule sensitivity of CEI. In addition to the first direct observation of roaming fragments undergoing dynamics, we could show that the onset of roaming occurs actually several orders of magnitude earlier than previously expected. We thus show that CEI provides the means to extract new, unexpected pathways, which would otherwise remain hidden underneath a strong background.

 

Provakar Datta, Department of Physics, University of Connecticut

Probing the Neutron’s Internal Structure via High-Q2 Electromagnetic Form Factor Measurements

The electromagnetic form factors (EMFFs) are among the most basic observables sensitive to the nucleon’s internal structure. Knowing their values with high precision in a wide range of squared four-momentum transfer (Q2) is essential for the advancement of QCD. The high Q ² precision data of the nucleon EMFFs are scarce due to the challenges associated with such measurements. However, the Super BigBite Spectrometer (SBS) collaboration is currently running multiple experiments in Jefferson Lab’s experimental Hall A to precisely measure the proton and neutron EMFFs with unprecedented Q ² reach, which will vastly improve the situation. In this talk, I will give an overview of the SBS high Q ² program with a focus on the SBS-GMn experiment. SBSGMn, the very first SBS experiment, was completed during Oct. 2021 - Feb. 2022 running period to measure the neutron magnetic form factor (GnM) up to Q ² = 13.6 (GeV/c) ² using “ratio” method. I will briefly discuss the underlying theory, measurement technique, associated technical challenges, and present our progress of physics analysis including preliminary data/MC comparisons. I will also show realistic projections of the final uncertainties on GnM, emphasizing the high-Q ² data points.

Prof. Chris Laumann, Boston University

Condensed-matter systems provide alternative “vacua” exhibiting emergent low-energy properties dramatically different from those of the standard model. A case in point is the emergent quantum electrodynamics (QED) in the family of magnetic materials known as quantum spin ice. The emergent QED possesses many features familiar from our universe, such as charges, anti-charges and photons, but also
many unfamiliar one, such as magnetic monopoles. Thus these magnetic insulators provide a laboratory for exploring effective QED in regimes quite inaccessible to traditional Maxwell electromagnetism.

In this talk, I will review the beautiful picture of how QED emerges in these frustrated magnets when quantum fluctuations cause the spins to fractionalize into spinons – and what this has to do with ice and pyrochlore materials. We will then turn to several results regarding its fine structure. We will see that the fine structure constant – the dimensionless coupling which controls the interactions between
emergent light and charges – generically takes values ~0.1 in quantum spin ice, much larger than the \(\alpha\) ~ 1/137 of our universe [1]. This leads to a variety of predictions regarding the coherent dynamics of the spinons which we expect can be probed by neutron scattering [2]. Finally, we will consider how true electric fields couple into this magnetic insulator – and may permit the indirect observation of
the emergent magnetic monopole and a curious inverted Lorentz force [3].

[1] Pace, Morampudi, Moessner, Laumann. Phys. Rev. Lett. 127, 117205 (2021).
[2] Morampudi, Wilczek, Laumann. Phys. Rev. Lett. 124, 097204 (2020).
[3] Laumann, Moessner. arXiv:2302.06635.

Shaswat Tiwari, NCSU 

Entanglement measures in scattering and the S-matrix Bootstrap

Quantum information theory has been instrumental in offering new
insights into old physics problems. In this talk, I shall use it to
study 2-2 scattering in quantum field theories[1], defining the
Entanglement entropy and the relative entropy in this setting. I will
evaluate these measures for theories like φ4, chiral perturbation
theory ( χP T ), etc. Using known bounds on the differential elastic
cross section, I will argue a general bound on relative entropy at
high energy. I will also find definite sign properties of relative
entropy that shall help constrain the S-matrix Bootstrap. Reviving the
old 1960s effort to study Quantum field theory using S-matrices, the
S-matrix bootstrap is a recent attempt to study S-matrices
numerically. I will briefly describe the bootstrap methods and how
relative entropy becomes helpful in this study. These constraints help
isolate S-matrices, which have leading Regge trajectory compatible
with experiments[2].


[1] A. Bose, P. Haldar, A. Sinha, P. Sinha, and S. S. Tiwari,
“Relative entropy in scattering and the
S-matrix bootstrap,” SciPost Phys. 9, 081 (2020),arXiv:2006.12213 [hep-th]

[2]A. Bose, A. Sinha, and S. S. Tiwari, “Selection rules for the
S-Matrix bootstrap,” SciPost Phys. 10, 122 (2021),arXiv:2011.07944
[hep-th].

Dr. Oleg Gorobtsov, Cornell University

In quantum materials, coexistent degrees of freedom such as spin, charge, and lattice respond in concert to various external stimuli including light and electrical current. Understanding the interactions between these degrees of freedom and stimuli promises advances in technology from next-generation electronics to energy storage. From coexistent emergent phases and far-from-equilibrium states to topological defects and nanomaterial morphology, nanostructural heterogeneities in space and time connected to crystal lattice commonly underpin the material behavior. Breakthrough coherent X-ray scattering techniques at novel X-ray sources capture nanoscale changes in lattice in-situ to connect them to macroscopic properties. Precise control over lattice dynamics by photoexcitations on sub-picosecond timescales sheds light on interaction between lattice dynamics and electronic order in chromium, an archetypal density wave material. In Mott insulators, nanoscale measurements reveal novel hierarchy and transition behavior of heterogeneities during metal-insulator transitions. Unique capabilities of coherent X-ray science offer us glimpses into the physics of phase transition in quantum materials and future design possibilities.

Prof. George Gibson, Department of Physics, University of Connecticut

The influence of molecular structure on strong-field interactions and the problem of coupling three electrons

1D 1-electron models explain a surprising amount of phenomena in the strong-field interaction with atoms and molecules, such as ionization rates, enhanced ionization at a critical internuclear separation, and high-harmonic generation. 1D 2-electron models predict multielectron phenomena, such as enhanced multiphoton susceptibilities leading to strong multiphoton excitation. In this talk, we examine 1D 3-electron systems to address the possibility of direct inner-orbital ionization. Indeed, an old question of excitation through inner-orbital ionization, versus a two-step process, in which ionization is followed by excitation can finally be answered. Based on symmetry arguments, we show unambiguously that direct inner-orbital ionization is possible.

Prof. P. Mannheim, Department of Physics, University of Connecticut

Normalization of the vacuum and the ultraviolet completion of Einstein gravity

Second-order-derivative plus fourth-order-derivative gravity is the ultraviolet completion of second-order-derivative quantum Einstein gravity. While it achieves renormalizability through states of negative Dirac norm, the unitarity violation that this would entail can be postponed to Planck energies. As we show in this paper the theory has a different problem, one that occurs at all energy scales, namely that the Dirac norm of the vacuum of the theory is not finite. To establish this we present a procedure for determining the norm of the vacuum in any quantum field theory. With the Dirac norm of the vacuum of the second-order-derivative plus fourth-order-derivative theory not being finite, the Feynman rules that are used to establish renormalizability are not valid, as is the assumption that the theory can be used as an effective theory at energies well below the Planck scale. This lack of finiteness is also manifested in the fact that the Minkowski path integral for the theory is divergent. Because the vacuum Dirac norm is not finite, the Hamiltonian of the theory is not Hermitian. However, it turns out to be PT-symmetric. And when one continues the theory into the complex plane and uses the PT-symmetry inner product, viz. the overlap of the left-eigenstate of the Hamiltonian with its right-eigenstate, one then finds that for the vacuum this norm is both finite and positive, the Feynman rules now are valid, the Minkowski path integral now is well behaved, and the theory now can serve as a low energy effective theory. Consequently, the theory can now be offered as a fully consistent, unitary, and renormalizable theory of quantum gravity.

Graduate student Phi-Hung Tran, Department of Physics, University of Connecticut

An accurate semiclassical method for constructing photoelectron momentum distribution for atoms and molecules in intense lasers

We propose a version of the semiclassical method based on the Herman-Kluk propagator combined with the strong-field approximation for atoms and molecules in intense lasers. Here we illustrate the application of the method for calculation of photoelectron momentum distribution (PMD) for different atoms (H, Ar, Ne, and Xe) in intense few-cycle laser pulses. We show that the constructed PMDs agree very well with the exact numerical solutions of the time-dependent Schrödinger equation. We further show that our method provides a clear physical interpretation for the validity of the factorization of the recollision process. In contrast to the common belief that there are two paths, so-called long and short trajectories, we found that there are typically multiple trajectories that lead to a given final momentum in the high-energy region. Accurate phases of those trajectories are needed to obtain correct interference patterns in the PMD. Our method can be used to extend current capabilities of laser-induced electron diffraction and other ultrafast imaging and strong-field spectroscopic techniques.

 

Prof. Michael Lublinsky, Department of Physics, Ben-Gurion University

Magnetic monopole in chiral plasma

Chiral anomaly induces qualitatively new transport phenomena in finite temperature plasma of massless fermions. In the first part of the talk, we will explore the effect of a magnetic monopole inserted into chiral plasma. Next, we will present various novel anomaly induced phenomena discovered in a holographically defined model.

Fatma Aslan, University of Connecticut Basics of factorization in a scalar Yukawa field theory The factorization theorems of QCD apply equally well to most simple quantum field theories that require renormalization but where direct calculations are much more straightforward. Working with these simpler theories is convenient for stress testing the limits of the factorization program and for examining general properties of the parton density functions or other correlation functions that might be necessary for a factorized description of a process. With this view in mind, we review the steps of factorization in a real scalar Yukawa field theory for both deep inelastic scattering and semi-inclusive deep inelastic scattering cross sections. In the case of semi-inclusive deep inelastic scattering, we illustrate how to separate the small transverse momentum region, where transverse momentum-dependent parton density functions are needed, from a purely collinear large transverse momentum region, and we examine the influence of subleading power corrections. We also review the steps for formulating transverse momentum-dependent factorization in transverse coordinate space, and we study the effect of transforming to the well-known scheme. Within the Yukawa theory, we investigate the consequences of switching to a generalized parton model approach and compare it with a fully factorized approach. Our results highlight the need to address similar or analogous issues in QCD.