High Energy Theory Seminar

In quantum field theory, it is not always possible to excite one state into another using only local operators. If state 2 cannot be excited out of state 1, one says that state 2 lives in a different "sector" than state 1. In upcoming work with Jonathan Sorce and Federico Capeccia, we establish general algebraic criteria for excitability; then, in the concrete setting of free field theory, we find explicit necessary-and-sufficient conditions for excitability of Gaussian states. When excitability is mutual --- i.e., state 2 can be excited from state 1 and vice versa --- our results reproduce the "quasiequivalence theorems" of Powers, Störmer, van Daele, Araki, and Yamagami.

In this talk, I will argue that excitability informs foundational questions in quantum field theory,  and I will outline how we formulate and prove the "excitability theorem." I will emphasize the key role played by the information-theoretic tool of canonical purification (à la arXiv:2512.17014), which enables us to reduce excitability of general states to the simpler question of pure-state excitability.

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