Past Online Seminars WG3

Hi everyone!

Here you will find the list of online seminars already held by the Working Group 3, with the corresponding links to the recordings. Remember that in the YouTube channel of RQI - Cost Action you may find all the seminars and also the talks of conferences organised by the Cost Action:

• Nick Ormrod, Causal Relationalism in Quantum Theory
  Spoiler

  Abstract

  It has often been argued that causation plays no role in fundamental physics. In contrast, I’ll argue for a view of fundamental physics in which causal structure plays a vital role. Causalism is the view that kinematical notions emerge from causal structure. Relationalism is the view that kinematical notions are well-defined only relative to systems. Causal Relationalism combined these two views: kinematical notions are relational because they emerge from causal structure. I describe Causal Relationalism at the intuitive level before formalizing it using a notion of causal influence as the non-commutativity of unitary interactions. This involves showing that the causal structure of an arbitrary unitary interaction selects a preferred commutative algebra of observables that are “stably accessible”, and then that stable accessibility defines a unique consistent set of histories relative to any subset of a set of unitarily interacting subsystems. I show how Causal Relationalism leads to an interpretation of quantum theory that solves the measurement problem, complete with a mathematical formalism that makes its conceptual claims precise. I provide evidence that Causal Relationalism will ultimately help one tackle the problem of quantum gravity. In short, not only does causation play a role in fundamental physics — it might just hold the key to solving some of its most crucial outstanding problems.

• Philipp Höhn, Gravitational algebras, entropies and quantum reference frames
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  Abstract

  I will explain how the recent discussion about observers and crossed product algebras in perturbative quantum gravity is related to quantum reference frames and the Page-Wootters formalism and how this leads to an observer-dependence of gravitational entropies.

• Simon Milz, Resource Theory of Causal Connection and Higher Order Quantum Transformations
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  Abstract

  The capacity of distant parties to send each other signals is a fundamental requirement in many information-processing tasks. This ability is determined by the causal structure connecting the parties, and more generally, by the intermediate processes carrying signals from one laboratory to another. In this talk, I will detail how a fully fledged resource theory of such causal connection can be derived for all multi-party communication scenarios, encompassing those where the parties operate in a definite causal order, as well as those where the order is indefinite. Varying the choice of free objects -- based on different assumptions of what transformations are 'easy' to implement -- results in three distinct resource theories of causal connection with vastly different properties. I will provide an overview of the features of this hierarchy of resource theories, discuss the stratification of the space of communication scenarios by means of resource monotones, and provide a short introduction to the theory general higher order quantum transformations, the theoretical framework underlying all presented results. Finally, I will connect these results to the resource theory of causal non-separability, where causal indefiniteness -- instead of causal connection -- is considered valuable.

• Andrzej Dragan, Quantum theory of tachyons
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  Abstract

  It was recently shown that special relativity extended to superluminal frames of reference leads to the emergence of a non-deterministic quantum-mechanical picture involving quantum superpositions and complex probability amplitudes. In this talk we expand on this idea and show that the theory of tachyons should be covariantly quantized, in order to describe the Higgs mechanism of spontaneous symmetry breaking. We demonstrate how our approach links with the two-state vector formalism introduced by Aharonov et al.

• Francesco Buscemi, A new regime of non-Markovianity: non-causal revivals VS genuine backflows
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  Abstract

  In quantum dynamics, non-Markovianity is often identified through information revivals, which are typically interpreted as backflows from the environment to the system. However, as I will argue in this talk, this equivalence may be misleading because not all revivals require backflow. I will present a refined framework that distinguishes noncausal revivals, which are explainable by inert extensions that are causally separated from the system, from genuine backflows, which entail actual information exchange with the environment. Using conditional mutual information and the concept of "squashed non-Markovianity", we establish operational criteria for certifying genuine backflow based solely on the system's degrees of freedom. This distinction also resolves the long-standing "nonconvexity problem" of Markovianity and enables a convex resource theory of genuine non-Markovianity. Beyond clarifying the structure of open quantum dynamics, these results imply new applications of resource theory in quantum communication and thermodynamics.

• José de Ramón, Relativistic causality in detector models
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  Abstract

  Detector models have become a standard tool to study local aspects of quantum field theory. They provide operational probes that interact with quantum fields in a localized manner, making them ideal for understanding how the notion of particles depends on the observer, be it non-inertial, moving in curved spacetime, or coupled to a field for only a finite time. Detector models have also proven valuable in giving operational meaning to quantum-information–theoretic tasks, such as entanglement dynamics, relativistic communication, and measurement theory in regimes where relativistic effects cannot be ignored. However, detector models encounter significant challenges when relativistic causality is taken seriously. Fully relativistic behavior requires the detector itself to be a relativistic quantum field, but interacting quantum field theories are technically demanding, and their nonrelativistic limits are still not completely understood. This complicates the goal of formulating simple models that smoothly bridge standard nonrelativistic quantum information protocols with fully relativistic quantum field theory. In this talk, I will discuss the tension between detector models and relativistic causality, framing the problem through the lens of Sorkin’s ''impossible measurements''. I will also illustrate how detector models allow to study ''effective'' relativistic causality in the particular case in which detector field interactions are not localized. I will also present nonperturbative detector-field models that avoid causality violations while retaining many of the practical advantages that make detector models so useful. Time permitting, I will outline some applications of these constructions.

• Costantino Budroni, Quantum memory and quantum correlations in time
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  Abstract

  We present some recent progress on the investigation of temporal correlations, namely, sequences of outputs from repeated measurements on physical systems. We identify scenarios where quantum systems outperform classical ones and investigate the associated limits using numerical methods. We observe that the key resource for generating these correlations is the memory encoded within the internal states of the physical system. Our results contradicts the common belief that entanglement-breaking channels represent classical memory: a qudit passing through such a channel cannot be simulated by a classical system of the same size.

• Zixuan Liu, Parity erasure: a foundational principle for indefinite causal order
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  Abstract

  Processes with indefinite causal order can arise when quantum theory is locally valid and they allow accomplishing new informational tasks. Despite recent progress, the correlations allowed in such processes have not been clearly understood. Here, we propose to study the constraints on information exchange through such processes in a paradigm of locally sequential operations. In this paradigm, we identify an information-theoretic principle constraining the correlations, termed parity erasure, which follows from the local validity of causality. We show that this principle completely characterizes the local-tomography representation of higher-order processes with indefinite causal order: among all multipartite channels, the ones describing valid higher-order processes are those and only those whose input-output correlations respect parity erasure. This approach reveals a fundamental property of information exchange in scenarios with indefinite causal structure and opens a new arena for exploring their potential applications.

• Fabio Costa, Operations, causality, and memory in continuous time
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  Abstract

  Quantum combs efficiently model multi-time processes, where a system can be probed multiple times, without any assumption about underlying system-environment interactions. However, they are restricted to scenarios where any system-environment interaction `halts’ when the system is probed, an assumption not always justifiable in realistic scenarios. More foundationally, combs and general process matrices underpin the study of quantum causal structures, but they are limited to discrete models of spacetime, or instantaneous operations in spacetime. I will present a continuous-time version of combs/process matrices based on process and operation functionals, which generalize the Feynman–Vernon influence functional and yield a continuous Born rule that cleanly separates processes from operations. Operational continuous-time definitions of causality and Markovianity arise, which characterise, respectively, processes compatible with a fixed causal order and processes without memory. The framework has direct applications to continuous time sensing of dissipative systems and opens the way to extend the study of quantum causal structure to the continuum.

• Luis Pedro García-Pintos, Reshaping the quantum arrow of time
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  Abstract

  While the microscopic laws of physics are often symmetric under time reversal, most natural processes that we observe are not. The emergent asymmetry between typical and time-reversed processes is referred to as the arrow of time. In quantum physics, an arrow of time emerges when a sequence of measurements is performed on a system. I will show new quantum control tools that can yield dynamics more consistent with time flowing backward than forward. The control tools are based on the explicit construction of a Hamiltonian that can replicate the stochastic trajectories of a monitored quantum system. Such a Hamiltonian can reverse the effect of monitoring and, via a feedback process, generate trajectories consistent with a reversed arrow of time. I will also show how these tools inform the design of feedback-driven continuous measurement engines powered by the energy pumped into the system by the monitoring process.