Talks

Although many-body localization (MBL) is most widely studied for systems with random potentials, various interesting phenomena occur naturally in quasiperiodic potentials. In particular, there is the lack of low-disorder rare regions which may destabilize MBL, the existence of a regime which is non-ergodic but with extended states, and a regime where all single-particle orbitals are critical. In this talk, I will give a theoretical picture of these phenomena (and some generalizations) based on the structure of the quasiperiodic potentials. First, I will show that regularly spaced deep wells in the potential create a separation of particle and information spreading timescales, leading to the non-ergodic extended behavior. Secondly, I will demonstrate that local mirror symmetries in the potential give rise to a hierarchy of resonances in the presence of interaction, leading to many-body critical states. Finally, I will discuss how such resonances may lead to rare-region effects in quasiperiodic systems, even though there is no low-disorder region in such systems.

Non-local games are a powerful tool to distinguish between correlations possible in classical and quantum worlds. Kalai et al. (STOC’23) proposed a compiler that converts multipartite non-local games into interactive protocols with a single prover, relying on cryptographic tools to remove the assumption of physical separation of the players. While quantum completeness and classical soundness of the construction have been established for all multipartite games, quantum soundness is known only in the special case of bipartite games. In this paper, we prove that the Kalai et al.’s compiler indeed achieves quantum soundness for all multipartite compiled non-local games, by showing that any correlations that can be generated in the asymptotic case correspond to quantum commuting strategies. Our proof uses techniques from the theory of operator algebras, and relies on a characterisation of sequential operationally no-signalling strategies as quantum commuting operator strategies in the multipartite case, thereby generalising several previous results. On the way, we construct universal C*- algebras of sequential PVMs and prove a new chain rule for Radon-Nikodym derivatives of completely positive maps on C*-algebras which may be of independent interest.

The past decade has seen rapid advancement in electron excitation-based direct detection experiments, which target light, MeV-GeV mass dark matter. Recently, the DAMIC-M collaboration began to probe the benchmark light dark matter models, renewing questions about the broader range of dark matter models these experiments can detect, and the necessity of new experiments to search beyond. In this talk we'll discuss a novel mechanism to cosmologically produce light, keV-GeV mass, dark matter coupled to electrons via a magnetic dipole moment. We consider a minimally extended dark sector in which only heavier states are charged under the kinetically mixed dark photon. These states are produced via freeze-in and subsequently thermalize the rest of the dark sector, efficiently populating the lighter, neutral, magnetic dipole-coupled dark matter. Finally, we'll discuss how magnons, collective excitations of spin waves, can be used to search for keV-MeV mass magnetic dipole-coupled dark matter, outside the reach of any semiconductor based direct detection technology.

Lambda Cold Dark Matter (LCDM) is the most successful theory for the formation of structure in the Universe. Although its predictions have been verified on large scales, they are still contested on the scale of dwarf galaxies, whose dynamical properties are often cited as evidence for the need to revise some of LCDM’s basic tenets. In this context, I will discuss the recent discovery of the faintest galaxies known to date, and how their properties may be used to place constraints on the clustering of dark matter on the smallest galactic and sub-galactic scales, as well as on the viability of some of the proposed alternatives to LCDM.