Scientific Series

The measurement of the electron magnetic moment (g-2)_e has reached the spectacular precision of sub-parts-per-trillion, and is currently the highest-precision measurement of a property of a fundamental particle. The next iteration of the measurement is expected to improve the precision by almost an order of magnitude, at which point the leading systematic uncertainty will be the “cavity shift”: the modification of the electron energy levels due to the conducting walls of the cavity in which the electron is trapped. This quantity cannot be directly measured at the required precision, and must be calculated. I will review the setup of the (g-2)_e experiment and present a new calculation of the cavity shift using the full machinery of quantum mechanics and quantum electrodynamics, which improves upon previous classical calculations with a consistent renormalization scheme while allowing for individual quality factors for each cavity mode.

I discuss work in progress on the no-boundary state of the universe in quantum cosmology, emphasizing the key role played by the inclusion of an observer in obtaining a reasonable no-boundary state. For concreteness we focus on a 2d toy model, namely dS JT gravity. This model has avatars of important issues with the wavefunction of the universe in inflationary models. The issues are that the wavefunction predicts a small universe (whereas our universe is rather large), and that it is not normalizable. I show that the non-normalizability is resolved by promoting dS JT gravity to sine dilaton gravity, a type of UV completion of dS JT gravity of which I discuss the cosmological interpretation. I then argue that the issue of predicting a small universe (in this toy model) is resolved by including an observer in the theory. With the observer, the leading contribution is a bra-ket wormhole. The observer's no boundary state is a (maximally mixed) identity matrix, and thus prefers neither small nor large universes.

In this talk, I will discuss the complexity of a fermionic analogue of Quantum k-SAT. In this Fermionic k-SAT problem, one is given the task to decide whether there is a fermionic state in the null-space of a collection of fermionic, parity-conserving, projectors on n fermionic modes, where each fermionic projector involves at most k fermionic modes. We prove that this problem can be solved efficiently classically for k = 2. In addition, we show that deciding whether there exists a satisfying assignment with a given fixed particle number parity can also be done efficiently classically for Fermionic 2-SAT: this problem is a quantum-fermionic extension of asking whether a classical 2-SAT problem has a solution with a given Hamming weight parity. We also prove that deciding whether there exists a satisfying assignment for particle-number-conserving Fermionic 2-SAT for some given particle number is NP-complete. Complementary to this, we show that Fermionic 9-SAT is QMA_1-hard. 

In 1914, Ramanujan wrote down 17 intriguing formulas for 1/pi, which motivated the modern-day machinery of computing trillions of digits of pi. Most of these formulas lay unproven until the 1980s. The Canadian Borwein brothers wrote a comprehensive treatise proving these formulas in the 1980s. We can now ask, “What is the physics behind Ramanujan’s pi”? I will argue that the physics connection can be found via logarithmic conformal field theories, for instance, those studied in the fractional quantum hall effect, polymers, and percolation. The CFT connection gives rise to an infinite number of new formulas for pi. In contrast, the myriad Ramanujan formulas provoke us into looking into faster converging basis for conformal correlators, which is, in turn, provided by the stringy dispersion relation mentioned above.

The cosmic neutrino background is like the cosmic microwave background, but less photon-y and more neutrino-ey. The CNB is also less talked about than the CMB, mostly because it's nearly impossible to detect directly. But if it could be detected, it would be interesting in several ways that are discussed.

Local subsystems play a fundamental role in theoretical physics, but they are usually defined in terms of background spacetime structures, in particular a Lorentzian metric. In gravity there are no such structures and the metric becomes dynamical. I will argue that nevertheless subsystems can be constructed in relation to other degrees of freedom, subject to the requirement that those variables reside within the subsystem itself. In operational terms an observer located within the system should be able to determine its edge by taking measurements within that region only. Within the context of classical field theory, I will demonstrate that this guarantees that the observables describing the system constitute a closed Poisson algebra. I'll discuss some examples, explain how surface charges generating subsystem symmetries, including the Bekenstein-Hawking area entropy, are incorporated, and make some remarks about how this can be extended to a fully quantum treatment within the framework of deformation quantization.

It is usually assumed that 4D instantons can only arise in non-Abelian theories. One can, however, explicitly construct instantons for QED in the background of a Dirac monopole. This is the low-energy effective field theory for fermions interacting with a 't Hooft-Polyakov monopole. This theory posesses both a topological instanton number and 't Hooft zero modes. I will show how such instantons provide the underlying mechanism for the Callan-Rubakov process: monopole-catalyzed baryon decay with a cross section that saturates the unitarity bound. 

Many aspects of classical AdS/CFT can be understood from the point of view of group theory. In this talk, I will argue that applying the same philosophy can provide us with some powerful insights regarding the construction of its flat counterpart, dubbed flat holography. The basic setup will be a massless gauge field of arbitrary (integer) spin propagating on Minkowski spacetime of any dimensions. An asymptotic analysis will reveal the data composing the solution space, as well as the asymptotic symmetries acting on the latter. I will show how part of this solution space can be understood in terms of unitary irreducible representations of the Lorentz group, seen as the group of conformal transformations of the celestial sphere. This interpretation allows us to capture some universal aspects of field theories in Minkowski space-time within the framework of celestial holography, while we will see that the other parts of the solution space have a more natural Carrollian interpretation.

The quantum marginal problem concerns the compatibility of given reduced states. In contrast, the entanglement transitivity problem takes compatible entangled marginals as input and ask if one can infer therefrom the entanglement of some other marginals. When this is possible, the input marginals are said to exhibit entanglement transitivity. Previous studies [Npj Quantum Inf 8, 98 (2022)] have demonstrated that certain families of states show entanglement transitivity. In this talk, we will show that when specific dimension constraints are satisfied, entanglement transitivity is possible and even generic among the marginals of pure state. To this end, we use the fact that given these constraints, the marginals of generic pure states (1) uniquely determine the global state and (2) are entangled. For the latter, our results generalize that of Aubrun et al. [Comm. Pure. Appl. Math. 67, 129 (2013)], which allows us to conclude further that sufficiently large parts of a generic multipartite pure state are entangled for any bipartition.

After an introduction to the notion of Leonard pairs, I explain their different uses. Then, I provide the definition of the associated Heun operator and how it allows us to simplify the computation of the quantum entanglement entropy. Finally, I show, in the simplest example, how the Bethe ansatz can be used to diagonalize the Heun operator.

Quantum biology explores how phenomena like superposition, tunneling, and entanglement influence biological systems, with growing experimental evidence supporting their role in processes such as photosynthesis, enzymatic reactions, DNA mutations, and animal navigation. These discoveries have profound implications for medicine, renewable energy, and our understanding of life. Quantum neurobiology applies this perspective to the brain, exploring how brain cells function beyond traditional models of electrical activity and chemical neurotransmission. Moving beyond classical models of chemical and electrical neuronal signaling, quantum approaches could revolutionize diagnostics and treatments for neurological disorders while offering deeper insights into cognition, behavior, and consciousness. Here will be presented an overview of quantum biology, examining the current state of quantum neurobiology, both theory and experiment, and exploring its potential applications in diagnosing and treating neuroinflammatory conditions such as Alzheimer’s and Parkinson’s disease.