Hi all,
Next Tuesday, we have two talks:
(1) At 11:15, Lorenzo Sannino will talk about “A spacetime generalisation of mutual information”. The talk will take place in HIT E41.1.
(2) At 14:00, Bruna Sahdo will talk about “Adding and removing subsystems in QRFs”. The talk will take place in HIT J51.
See below for the abstracts.
Best,
Ladina
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Title:
A spacetime generalization of mutual information
Abstract:
In this talk, we investigate an ongoing work on a generalization of the mutual information to timelike separated quantum systems, where no natural factorization of the Hilbert space is established. In this setting, defining a meaningful joint state of the two subsystems becomes non-trivial, as their degrees of freedom cannot be simply combined through a tensor product. To address this issue, we construct a minimal subspace that contains the information of both systems as a subsystem of the panorama state space, which we introduce as an abstract, information-theoretic Hilbert space encoding all quantum information of the world model considered. To construct this partition, we analyze how the algebra describing a local subsystem transforms through time. Under entangling dynamics, operations that are local at one instant generally become non-local at different times, yet they continue to represent the same physical subsystem, now described virtually by a non-local algebra. In this view, time evolution reshapes the tensor-product structure of the global Hilbert space rather than altering the quantum state itself. This algebraic perspective allows us to describe multiple subsystems, defined locally at different times, as virtual systems embedded within the same panorama state space, where they can be combined to define an abstract joint subsystem useful for our generalization of mutual information. Finally, we illustrate this framework through examples and discuss a concrete application to quantum reservoir processing.
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Title:
Adding and removing subsystems in QRFs
Abstract:
The tensor product rule for composing subsystems is central to information-theoretic formulations of Quantum Theory. Adopting a relational view, we study how to describe the adding and removing of subsystems in a quantum reference frame (QRF). In a translation-invariant setup following E. Castro-Ruiz, O. Oreshkov (2025), we show that textbook compositional rules only hold in a QRF if it is compatible with being in a 'classical' external state. This issue is closely related to the so-called paradox of the third particle. However, we also show that any QRF can be unitarily mapped to a ‘classicalized’ version including the external observer, where the tensor product rule applies unambiguously. We can use this to derive composition rules suitable for descriptions relative to QRFs. The map can be understood as a coordinate change. It generates an analogy between QRFs and non-inertial frames through the equivalence principle: an accelerated frame can have its description modified so that it appears inertial. In a similar way, the description of a QRF in a coherent state can be re-expressed to mimic a classical perspective. Although there is an operational subtlety in measuring absolute acceleration, this does not undermine the importance of the inertial/non-inertial distinction in classical physics. The analogy suggests that it might be comparably subtle to infer a QRF’s ‘quantumness’ from within, but that the classical/quantum frame distinction present in this formalism remains fundamental. The procedure also recovers transformations from inequivalent QRF approaches—such as Giacomini et al. (2019) and Krumm et al. (2021)—without projections, hinting at a conceptual framework where different formalisms are consistent at the same time.