Conference

"Wigner's Friend is a variant of the Schrödinger's cat experiment in which the cat is replaced by a Friend, the difference being that the Friend is unambiguously a conscious agent who experiences definite measurement outcomes. It has received renewed attention in recent years due to the development of several Extended Wigner's Friend (EWF) arguments, aimed at ruling out certain kinds of Copenhagenish interpretations. In this talk, I will discuss the origins of Wigner's Friend in von Neumann and Wigner's discussions of what we now call the Orthodox interpretation of quantum mechanics. I will then discuss how the argument was taken up by Everett and his followers, generalized by Deutsch, and then more recently brought into the Copenhagenish context. Along the way, I emphasize how each variant of the argument has different implications for the Orthodox, Everett and Copenhagenish interpretations, concluding that all three interpretations require a theory of quantum mechanical agents in order to be complete. I outline some first steps towards such a theory."

I will review conceptual advances which paved the way for the emergence of the mature form of quantum theory (quantum mechanics) in 1925-27, focusing on the contributions of Albert Einstein [1,2]. I argue that Einstein’s 1905 paper on light quanta was motivated by his firm belief that equipartition of energy was inescapable within classical statistical mechanics. Moreover, his rejection of the ether in his work on Special Relativity freed physicists to accept the possibility of wavelike phenomena not supported by a medium. Einstein in his 1907 paper on the specific heat of solids, became the first important physicist to embrace clearly the quantization of energy, strengthening his conclusion that a radical revolution in all of physics (not just electromagnetism) was upon us. In 1909 Einstein was able to derive the first rigorous result in quantum statistical mechanics, his energy/momentum fluctuation formula, which strongly supported the necessity of wave/particle duality in the new physics. Bohr in 1913 was able to explain the hydrogen spectrum in terms of quantization of electron orbits, but at the expense of accepting the possibility of accelerating charges which do not radiate, a result that Einstein found puzzling. In 1916-7 Einstein introduced fundamental randomness into quantum theory via the hypothesis of spontaneous emission. Shortly thereafter he generalized the Bohr-Sommerfeld quantization rules by putting them in a topological form but noticed that they didn’t seem to work for what we now call chaotic systems [3]. When Heisenberg eventually replaced the semiclassical quantization rules with a discrete generalization in matrix mechanics in 1925 he unwittingly escaped this limitation, something which does not appear to have been appreciated in the historical literature. Independently of these developments, De Broglie and Bose inspired Einstein in 1925 to develop a quantum statistical mechanics of indistinguishable atoms with wave-particle duality. This work directly motivated Schrodinger to investigate a wave equation describing electrons and eventually to discover his equation for the complex wave function of the electron at the beginning of 1926, which resolved the puzzle of non-radiating charges in hydrogen

As is well known to historians, in the early days of quantum theory James Franck frequently reported fresh experimental results to Bohr and others, on the basis of which major theoretical advances were made. These data – and indeed, the design and execution of the ground-breaking experimental work whence they came – were not, contrary to the usual assumptions, due to Franck himself. This work was done by Hertha Sponer; she was the one running the spectroscopy labs in Göttingen in these years, as well as teaching the main physics seminars in these areas. It was her experimental prowess that enabled significant insights into this new theory, and her expert instruction that guided and inspired a new generation of quantum physicists in Göttingen and beyond. Yet Sponer’s name has been nearly completely erased from this history. If she is mentioned at all it is usually as Franck’s “student” or “assistant” (of which she was neither – she was his academic equal) or as Franck’s second wife (as she was, but which entirely disregards her international standing as a scientist in her own right). Extant accounts of Sponer’s life and work are few and exclusively concern her post-WWII years as a professor of physics at Duke. But Sponer was no longer working at the cutting-edge of quantum theory in these decades, and so her role in that field’s development is left largely ignored. This talk reintroduces Sponer to the early history of quantum physics in her (arguably) rightful role, maven of quantum spectroscopy. I shall present two cases where I believe she earns this title: her early understanding and experimental confirmation of electron waves, and together with Franck first using quantum tunneling to interpret hitherto unexplained molecular phenomena.

My research investigates the role of Bose-Einstein statistics in the genesis of quantum optics. It explores key developments following the emergence of quantum statistics in the late 1920s, which eventually led to the classification of elementary entities as bosons and fermions in 1945. Additionally, it examines whether the concept of an evolving statistical style of reasoning in physics provides a useful analytical framework for understanding the debate on the Hanbury Brown and Twiss (HBT) effect, the early investigations of photon correlations in the late 1950s, and the emergence of photon statistics in the early 1960s.

Einstein, Podolsky and Rosen’s “Can quantum mechanical description of reality be considered complete?”(1935) and Schrödinger’s “Die gegenwärtige Situation in derQuantenmechanik” (1936) are commonly accepted as the seminal papers for the modern study of quantum mechanical entanglement. However, not much has been known about the prehistory of these papers. By a study of Schrödinger’s correspondence and extensive research notes we show that Schrödinger was aware of what is essentially the EPR argument already in November 1931, after a talk by Einstein on the photon box thought experiment. We also note the importance of thei nput from Leo Szilard in Schrödinger’s notes, which credit Szilard both with the simplification of the photon box experiment to that of a collision between a photon and a mirror and with proposing a quantum mechanical state that is essentially the state used in the EPR argument.

Critical fluctuations coupled to Fermi surfaces can destroy conventional Fermi liquids, giving rise to diverse non-Fermi liquids that may remain metallic or become superconducting at zero temperature. I will present a classification of non-Fermi liquids with globally convex hot Fermi surfaces based on the concept of projective fixed points, which account for the flow of the Fermi momentum under renormalization. This framework organizes non-Fermi liquids into seven super-universality classes, each characterized by distinct superconducting fluctuations. Stable non-Fermi liquids exhibit universal but non-scale-invariant pairing interactions, while unstable ones display universal constraints on pairing symmetry, superconducting transition temperature, and its scaling with the Fermi momentum. I will discuss physical examples and a toy model that captures the essential universal properties of these classes.

I will introduce factorization algebras as an approach to understanding quantum field theory. I will outline some of the benefits of this approach, as well as how it connects with other topics in mathematical physics.

This study presents the modelling of the gravitational wave (GW) bias parameter by bridging between simulated GW sources and galaxies in low redshift galaxy surveys 2MPZ, WISExSCOS (WISC) and SDSS DR7. We study this connection by creating a mock host catalog for GW events and populating galaxy surveys with binary black holes (BBHs) for different scenarios of the GW host-galaxy probability as a function of the galaxy stellar mass, SFR and metallicity. For 2MPZ, WISExSCOS (WISC) galaxy surveys with only stellar mass information, we consider a phenomenological broken power law model for the host-galaxy probability function, with a potential turnover where the star formation efficiency begins to drop. We vary the parameters of the GW host-galaxy probability function and find that generically the GW bias increases as the turnover point increases. The change in the GW bias parameter shows a maximum change of about 30% for different scenarios explored in this work in comparison to the galaxy bias. For the SDSS DR7 survey with stellar mass, SFR and metallicity information, we use a joint host-galaxy probability function defined over stellar mass, star formation rate (SFR), and metallicity. This probability is modelled as the product of three broken power-law distributions, each with a turnover point motivated by astrophysical processes governing the relation between current-day galaxy properties and BBH mergers, such as galaxy quenching and BBH delay time. In this case, our results show that GW bias is most sensitive to host-galaxy probability dependence on stellar mass, with increases of up to ∼O(10)% relative to galaxy bias as the stellar mass pivot scale rises. We also find a notable relationship between GW bias and SFR: when the host-galaxy probability favors low-SFR galaxies, the GW bias significantly increases. In contrast, we observe no strong correlation between GW bias and metallicity. These findings suggest that the spatial clustering of GW sources is primarily driven by the stellar mass and SFR of their host galaxies and shows how GW bias measurements can inform models of the host-galaxy probability function.

Neural quantum states have emerged as a powerful framework for representing classical probability distributions derived from quantum many-body systems. However, fundamental questions remain about their computational learnability, particularly for physically relevant quantum states. We identify and analyze a fundamental obstacle to the efficient learning of locally indistinguishable quantum states using neural network architectures. We introduce a restricted statistical query (SQ) learning model that captures the essential features of noisy gradient descent training in autoregressive models such as recurrent neural networks networks. Within this framework, we prove that locally indistinguishable states cannot be learned in polynomial time, establishing an inherent computational barrier. Conversely, we show that distributions with finite Markov length remain efficiently learnable in the restricted SQ model. We demonstrate the practical implications through numerical experiments on paradigmatic examples of strong-to-weak symmetry breaking, including ferromagnetic ground states and syndrome distributions of quantum error-correcting codes beyond their decoding thresholds. Our findings suggest that the hardness of neural quantum state learning could serve as a novel computational probe for identifying mixed-state phase transitions and decoding thresholds in both numerical simulations and experiments.

I will talk about effective description of quantum dynamics for systems interacting with thermal environment. I will focus on a prototypical example of Brownian Motion and show how one can leverage symmetries to build a general effective path integral description with quantum/thermal corrections using methods of Non-Equilibrium QFT.