Talks

The dynamics of closed quantum systems undergoing unitary processes has been well studied, leading to notions of measures for the expressive power of parameterized quantum circuits, relative to the unique, maximally expressive, average behaviour of ensembles of unitaries. Such unitary expressivity measures have further been linked to concentration phenomena known as barren plateaus. However, existing quantum hardware are not isolated from their noisy environment, and such non-unitary dynamics must therefore be described by more general trace-preserving operations. To account for hardware noise, we propose several, non-unique measures of expressivity for quantum channels and study their properties, highlighting how average non-unitary channels differ from average unitary channels. In the limit of large composite system and environments, average noisy quantum channels are shown to be maximally globally depolarizing. Furthermore, we rigorously prove that highly-expressive parameterized quantum channels will suffer from barren plateaus, thus generalizing explanations of noise-induced phenomena. We complement our analytical findings with numerical experiments that showcase that, in certain situations, noise can increase the expressivity of parametrized quantum circuits.

Studying noncommuting conserved quantities (or `charges’) has produced a conceptual puzzle. Recent results suggest that noncommuting charges hinder thermalization in some ways yet promote it in others. To help resolve this puzzle, we demonstrate how noncommuting charges can promote thermalization by reducing the number of local observables that thermalize according to the Eigenstate Thermalization Hypothesis. We first establish a correspondence between charges and sufficient conditions for local observables not to thermalize. These conditions are known as `dynamical symmetries.' We demonstrate that a corresponding charge exists for each pair of dynamical symmetries a Hamiltonian has. We prove that the reciprocal relationship holds for many charges and Hamiltonians. Using this correspondence, we demonstrate that introducing a new charge into a system can either increase or decrease the number of non-thermalizing local observables. If the new charge commutes with the existing ones, the system’s non-thermalizing observables are preserved, and new ones may emerge. Conversely, if it does not commute, the existing non-thermalizing observables will thermalize. We illustrate our results using various models, showing that noncommuting charges eliminate the non-thermalizing dynamics arising from dynamical symmetries.

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Strongly coupled confining theories are well-motivated in many BSM frameworks. The early universe cosmological history of these theories provides possibilities for observable signals. These theories undergo confinement deconfinement phase transition in the early universe, which can result in gravitational wave signals, observable in upcoming experiments. Using AdS/CFT, these theories have been studied in the Randall-Sundrum framework, and various quantitative aspects of the phase transition have been calculated. In the models that have been considered, the rate of transition from the deconfined phase to the confined phase is very small and leads to a period of supercooling. This enhances the gravitational wave signal, but presents a tension between a low confinement scale and fitting to the standard picture of BBN. In this talk, I will briefly review the calculations leading to these conclusions, and argue that some of the issues are specific to the simplified models that have been studied. I will present two modifications that are expected on general grounds, motivated by including strong IR effects systematically. Such effects change the results significantly. In particular, new qualitative features appear which have been missed in previous investigations. I will briefly comment on the phenomenological implications and open questions. The talk will be based on 2309.10090 and 2401.09633.

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