Talks

In this talk, we analyze detector operators that measure energy to the power ∆-2 at null infinity in Einstein gravity. These operators transform as conformal primaries on the celestial sphere and provide a natural basis for describing energy-flux observables in scattering processes. Using the collinear factorization of scattering amplitudes, we derive the universal leading structure of the operator product expansion (OPE). A key consequence of our analysis is the precise identification of the ∆=2 detector, the number operator. Exploiting the fact that soft charges generate symmetries of the S-matrix, we demonstrate that the number of particles is entirely determined by the product of two supertranslation currents. This work establishes a direct link between detector observables and the soft sector of celestial holography. Analogous conclusions hold for Yang–Mills theory. 

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.