Dr. Eric Koch, Harvard Smithsonian Center for Astrophysics
Revealing the multi-phase neutral interstellar medium’s role in the star formation lifecycle: a sharpened view of nearby galaxies from LGLBS and PHANGS-JWST
The neutral interstellar medium (ISM) fuels the star formation lifecycle, yet we still lack vital constraints on the formation and destruction of molecular clouds because of challenges in observing the cold neutral ISM phases with high resolution and sensitivity. With dedicated surveys, the combination of VLA, ALMA, and JWST can now make significant advances in the coming years. In this talk, I will present multiple observational approaches that are making progress in this area: detailed 21-cm HI VLA mapping across the Local Group from the Local Group L-band Survey (lglbs.org), resolved molecular cloud studies with ALMA and JWST in M33 and PAH imaging from PHANGS-JWST as a highly sensitive resolved view of the total neutral gas tracer (phangs.org). These surveys bridge Galactic with extragalactic star formation studies and provide new constraints to guide the next generation of numerical simulations.
Bi-polaron superconductivity in the low density limit
It has been assumed for decades that high values of Tc from the electron-phonon coupling are impossible. At weak-to-intermediate coupling strength this result follows from the Migdal-Eliashberg theory, while at strong coupling, when bipolarons form, the transition temperatures are low because of the exponential effective mass enhancement. However, the latter conclusion was based on numerical solutions of the Holstein model. I will discuss a different model with coupling based on the displacement modulated hopping of electrons and argue that much larger values of the bipolaron Tc can be achieved in this setup. Non-locality of the problem gives rise to small-size, yet relatively light bipolarons, which can be studied by an exact sign-problem-free quantum Monte Carlo approach even in the presence of strong Hubbard and Coulomb potentials. We find that Tc in this model generically and significantly exceeds typical upper bounds based on Migdal-Eliashberg theory or superfluidity of Holstein bipolarons, and, thus, offers a route towards the design of high-Tc superconductors via functional material engineering. Finally, there are indications for even better prospects in systems with non-linear electron-phonon coupling.
Cortex Fusion Systems, Inc. uses shaped ultrafast laser pulses to catalyze fusion reactions in molecules. Our work comprises (1) designing transiently confining effective one-electron potentials in field-dressed molecules, (2) performing quantum chemistry calculations to validate the enhancement of nuclear tunneling by laser-modified electron screening dynamics, and (3) testing pulse shapes in the laser lab by coupling ultrafast spectroscopy techniques with nuclear radiation detection and spectrometry. In this regard, “quantum-controlled fusion” is a coherent, under-the-barrier process that does not require plasma ignition. Our goal is to repurpose the modern suite of commercial femtosecond laser amplifiers and pulse-shaping techniques to achieve compact and scalable fusion generators using quantum control.
Monica Vidaurri, Stanford University and NASA Goddard
Ethics and Aliens: the need for an ethical approach to space science
The progress of space science and exploration has seemingly inevitably fallen to private companies, to the concern of private citizens and scientists, who are directly impacted by private actions in space. Additionally, academia has reached a critical limit in terms of unchecked features that promote elitism and exclusionism, including increasingly competitive admissions to programs and fellowships, scarcity in jobs, prevalent sexual harassment, and others. As individuals, it is difficult to imagine what a truly ethical framework for our work looks like, let alone how we alone can influence laws and policy to change the actions of individuals with seemingly unlimited wealth and resources. This talk will introduce 3 main facets of what ethics means with respect to space science and exploration, including introducing space science as a historically oppressive institution, and how we can begin to move past this as individuals, labs, departments, and institutions. The norms we allow and ignore ultimately shape these broader laws, policies, and workplace culture. As a result, our science cannot be detached from the social and political framework it exists in, and the custom of early and regular collaboration with ethicists and planetary protection specialists (and other social scientists) is critical for not only mission safety, but mission and science integrity, as well as the well-being of those contributing to the mission and who gets to be included in such work. Creating a safe, responsible, and ethical space for peaceful purposes cannot wait for the international space community to create these practices de jure, but must be started at the individual level and regarded as custom for integration into international law, de facto, and require an uncomfortable self-assessment in the true goals of space science, as well as the ways that the academic structure has failed certain groups of students. By creating a new framework that prioritizes ethics, only then can we responsibly go into the unknown.
Unveiling the Physics of Galaxy Formation and its Large-Scale Effects at Cosmic Dawn
Cosmic Dawn, loosely defined here to be the first billion years of cosmic time, is an ever-intriguing era that witnessed the formation of the first generations of galaxies. Toward the end of it there was also the last major phase transition of our Universe, the epoch of reionization (EoR), which is believed to be driven by the hydrogen-ionizing background emerged from the early galaxies formed. In this talk, I will explain how Cosmic Dawn becomes a real exciting epoch for unveiling the physics of galaxy formation thanks to the James Webb Space Telescope (JWST), as well as several forthcoming facilities such as SPHEREx, Roman Space Telescope, Square Kilometer Array, and LiteBIRD focusing on the large-scale effects. I will discuss the theoretical landscape galaxy formation at Cosmic Dawn informed by new JWST observations, with a particular focus on the phenomenon of bursty star formation. I will introduce methods and ideas to shed light on different aspects of early galaxy formation, including the star formation history, stellar feedback, outflows, and the ionizing output, using both individual galaxies and their effects on the large-scale structure and cosmic background radiations. With a few case studies, I will demonstrate how to harness the power of the aforementioned facilities and their synergies for these purposes.
We discuss the anharmonic oscillator in quantum mechanics using exact WKB methods in a ‘t Hooft-like double scaling limit where classical behavior is expected to dominate. We compute the tunneling action in this double scaling limit, and compare it to the transition amplitude from the vacuum to a highly excited state. Our results, exact in the semiclassical limit, show that the two expressions coincide, apart from an irreducible and surprising instanton contribution. The semiclassical limit of the anharmonic oscillator betrays its quantum origin as a rule showing that the quantum theory is intrinsically gapped from classical behavior. Besides an example of a resurgent connection between perturbative and nonperturbative physics, this may provide a way to study transition amplitudes from tunnelling actions, and vice versa.
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)
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.
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.
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.
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.
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.
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.
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.
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.
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).
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. 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.
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.
The search for Majoranas in the context of topological quantum computing has led to remarkable progress in quantum materials and measurement technologies over a short period of just over a decade. Although topological Majoranas remain to be definitively observed, Majorana-like quantum states derived from part-semiconductor/part-superconductor parents already exist and feature sophisticated control and measurement capabilities. While this research is heavily motivated and driven by its potential applications, most notably by Microsoft, I will focus on how these new quantum platforms can help answer fundamental physics questions. I will discuss how the development of the planar Josephson junction platform for topological superconductivity allowed novel physics discoveries such as the Josephson diode effect [1], Andreev rectification [2], and phase-controlled Josephson vortices [3] - all by designing experiments that are impossible in conventional Josephson junctions. Surprisingly (or not), all these effects have topological counterparts, producing distinctive signatures in the topological regime. Careful study of these signatures in over 10 devices provide support for intermediate-disorder topological superconductivity [4]. Although not mature enough for a topological quantum processor, these relatively disordered devices may help solve open physics problems from the detection of Cooper pair entanglement to non-abelian quantum statistics on the road towards the dream of pristine topological superconductivity.