Causalworlds

Sep 16, 2024, 8:00 a.m. → Sep 20, 2024, 6:00 p.m. America/Toronto

PI/1-100 - Theatre (Perimeter Institute for Theoretical Physics)

Understanding causality is fundamental to science and inspires wide-ranging applications, yet there are several distinct notions of causation. Recently, there have been important developments on the role of causality in quantum physics, relativistic physics and their interplay. These have unearthed a plethora of fascinating open questions regarding the nature of causation, emergence of space-time structure and the limits of quantum information processing. At the same time, causal reasoning has become an important tool in machine learning and statistics, with applications ranging from big data to healthcare. This conference brings together experts from different areas of physics working on questions related to causality, as well as selected researchers who bridge the gap between fundamental research and current industrial applications. The aim of the conference is to provide a venue for cross-pollination of these ideas through scientific exchange between these communities. The conference will focus on the following facets of causality:

 

• Quantum and classical causal inference

• Indefinite causal order and quantum reference frames

• Causality in quantum field theory and quantum gravity

• Experiments and applications of causality

 

Sponsored in part by:

 

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Important dates

Paper submission deadline: 24 May 2024 // 31 May 2024
Paper notification: 3 July 2024

Registration deadline (with application for financial assistance): 18 July 2024
Registration deadline: 28 August 2024
Conference: 16-20 September 2024

 

Update: The submission deadline has been extended to 31st May 2024 for
papers which clearly justify their relevance for the following three
topics: 

1) classical causal inference, 

2) causality in relativistic physics (including quantum field theory and quantum gravity) and 

3) experiments in causality. 

 

As we have received a sufficiently high number of submissions on the remaining topics (particularly indefinite causality and quantum causal models), the original deadline of 24th May still holds for submissions in this category.

 

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Call for Abstracts

Prospective speakers can submit a paper for a contributed talk (in person or online) and/or a poster (in person only) via the Call for Abstracts. The Call for Abstracts is now open! Submissions for a talk will automatically be considered for a poster if not accepted for a talk. 

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Invited Speakers

Jessica Bavaresco (University of Geneva)
Cyril Branciard (CNRS, University Grenoble Alpes)
Rafael Chaves (Federal University of Rio Grande do Norte)
Giulio Chiribella (The University of Hong Kong)
Doreen Fraser (University of Waterloo)
Anne-Catherine de la Hamette (IQOQI Vienna)
Ciarán Lee (Spotify)
Tein van der Lugt (University of Oxford)
Joris M. Mooij (University of Amsterdam)
Mio Murao (University of Tokyo)
Alejandro Pozas-Kerstjens (University of Geneva)
Huw Price (Trinity College, Cambridge)
Renato Renner (ETH Zürich)
Thomas Richardson (University of Washington)
Sally Shrapnel (The University of Queensland)
Sumati Surya (Raman Research Institute)
Rainer Verch (University of Leipzig)

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Programme Committee

V Vilasini (ETH Zürich & Inria, University Grenoble Alpes) (PC Chair)
Augustin Vanrietvelde (Télécom Paris) (PC Co-chair)
Alastair Abbott (Inria, University Grenoble Alpes)
Časlav Brukner (IQOQI Vienna & University of Vienna)
Eric Cavalcanti (Griffith University)
Chris Fewster (University of York)
Lucien Hardy (Perimeter Institute)
Hlér Kristjánsson (Perimeter Institute & IQC & Université de Montréal)
Giulia Rubino (University of Bristol)
Nitica Sakharwade (Università degli Studi di Napoli Federico II)
Robert Spekkens (Perimeter Institute)
Jacopo Surace (Perimeter Institute)
Elie Wolfe (Perimeter Institute)
Lin-Qing Chen (ETH Zürich & IQOQI Vienna)
Hippolyte Dourdent (ICFO Barcelona)
Tamal Guha (University of Hong Kong)
Robin Lorenz (Quantinuum, Oxford)
Maria Papageorgiou (IQOQI Vienna)
Nicola Pinzani (Université libre de Bruxelles)
Marco-Túlio Quintino (Sorbonne Université, Paris)
Marc-Olivier Renou (Inria Paris-Saclay & CPHT, École polytechnique)
David Schmid (ICTQT, University of Gdańsk)
John Selby (ICTQT, University of Gdańsk)
Akihito Soeda (National Institute of Informatics, Tokyo)
Matthew Wilson (University College London)
 

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Scientific Organizers

Hlér Kristjánsson (Perimeter Institute & IQC & Université de Montréal) (Chair)
V Vilasini (ETH Zürich & Inria, University Grenoble Alpes) 
Robert Spekkens (Perimeter Institute)
Lucien Hardy (Perimeter Institute)
Elie Wolfe (Perimeter Institute)
Jacopo Surace (Perimeter Institute)
Marina Maciel Ansanelli (Perimeter Institute)
Yìlè Yīng (Perimeter Institute)
María Ciudad Alañón (Perimeter Institute)
Daniel Centeno Díaz (Perimeter Institute)
Khushi Gandhi (Perimeter Institute & University of Waterloo)

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Previous editions:

Causalworlds 2022: The interface between quantum and relativistic causality, foundations and practicalities
Organised at ETH Zürich in 2022. Website: https://causalworlds.ethz.ch/"

Support Materials

• 2024 Mitacs Presentation.pdf
• Causalworlds_2024_Book_of_abstracts_16/9/24.pdf
• pdf_timetable

Monday, September 16

8:30 a.m. → 9:00 a.m. Registration

9:00 a.m. → 9:20 a.m. OPENING REMARKS

9:20 a.m. → 10:20 a.m. Tutorial: Causal Inference Meets Quantum Physics

Can the effectiveness of a medical treatment be determined without the expense of a randomized controlled trial? Can the impact of a new policy be disentangled from other factors that happen to vary at the same time? Questions such as these are the purview of the field of causal inference, a general-purpose science of cause and effect, applicable in domains ranging from epidemiology to economics. Researchers in this field seek in particular to find techniques for extracting causal conclusions from statistical data. Meanwhile, one of the most significant results in the foundations of quantum theory—Bell’s theorem—can also be understood as an attempt to disentangle correlation and causation. Recently, it has been recognized that Bell’s result is an early foray into the field of causal inference and that the insights derived from almost 60 years of research on his theorem can supplement and improve upon state-of-the-art causal inference techniques. In the other direction, the conceptual framework developed by causal inference researchers provides a fruitful new perspective on what could possibly count as a satisfactory causal explanation of the quantum correlations observed in Bell experiments. Efforts to elaborate upon these connections have led to an exciting flow of techniques and insights across the disciplinary divide. This tutorial will highlight some of what is happening at the intersection of these two fields.

Speaker: Robert Spekkens (Perimeter Institute)

10:20 a.m. → 10:50 a.m. Break

10:50 a.m. → 11:30 a.m. Counterfactual and Graphical Frameworks for Causal Modeling

In the Statistics literature there are three main frameworks for causal modeling: counterfactuals (aka potential outcomes), non-parametric structural equation models (NPSEMs) and graphs (aka path diagrams or causal Bayes nets). These approaches are similar and, in certain specific respects, equivalent. However, there are important conceptual differences and each formulation has its own strengths and weaknesses. These divergences are of relevance both in theory and when the approaches are applied in practice. This talk will introduce the different frameworks, and describe, through examples, both the commonalities and dissimilarities. In particular, we will see that the “default” assumptions within these frameworks lead to different identification results when quantifying mediation and, more generally, path-specific effects.

Speaker: Thomas Richardson (University of Washington)

11:30 a.m. → 12:10 p.m. Bipartite graphical causal models: beyond causal Bayesian networks and structural causal models

Based on the immense popularity of causal Bayesian networks and structural causal models, one might expect that these representations are appropriate to describe the causal semantics of any real-world system, at least in principle. In this talk, I will argue that this is not the case, and motivate the study of more general causal modeling frameworks. In particular, I will discuss bipartite graphical causal models.

Real-world complex systems are often modelled by systems of equations with endogenous and independent exogenous random variables. Such models have a long tradition in physics and engineering. The structure of such systems of equations can be encoded by a bipartite graph, with variable and equation nodes that are adjacent if a variable appears in an equation. I will show how one can use Simon’s causal ordering algorithm and the Dulmage-Mendelsohn decomposition to derive a Markov property that states the conditional independence for (distributions of) solutions of the equations in terms of the bipartite graph. I will then show how this Markov property gives rise to a do-calculus for bipartite graphical causal models, providing these with a refined causal interpretation.

Speaker: Joris M. Mooij (University of Amsterdam)

12:10 p.m. → 1:45 p.m. Lunch + Discussion Time

1:45 p.m. → 3:00 p.m. Session A: Classical Causal Inference

1:45 p.m. Modeling Latent Selection with Structural Causal Models

Speaker: Leihao Chen (University of Amsterdam, Korteweg-de Vries Institute for Mathematics)

2:10 p.m. Zero Inflation as a Missing Data Problem: a Proxy-based Approach

Speaker: Trung Phung (Johns Hopkins Whiting School of Engineering)

2:35 p.m. Which causal scenarios might support non-classical correlations?

Speaker: Shashaank Khanna (University of York)

1:45 p.m. → 3:00 p.m. Session B: Causality in Wigner's Friend

1:45 p.m. Relating Wigner's Friend Scenarios to Nonclassical Causal Compatibility, Monogamy Relations, and Fine Tuning

Speaker: Yìlè Yīng (Perimeter Institute for Theoretical Physics)

2:10 p.m. A quantum circuit framework for extended Wigner's friend scenarios: logical and causal reasoning without objective events

Speaker: V. Vilasini (ETH Zürich and Inria University Grenoble Alpes)

2:35 p.m. Emergent classicality, relativistic causality, and quantum causal structure

Speaker: Nick Ormrod (University of Oxford)

3:00 p.m. → 3:45 p.m. Break

3:45 p.m. → 5:00 p.m. Flash Talks: Lightning Talks

3:45 p.m. Generalized tensors and partial traces over quantum networks

Speaker: Pablo Arrighi (Université Paris-Sarclay and Inria)

3:52 p.m. The Elegant Joint Measurement is nonlocal in the triangle network

Speaker: Victor Gitton (ETH Zurich)

3:59 p.m. Subsystem decompositions of quantum circuits and transformations between causal perspectives

Speaker: Julian Wechs (Université Libre de Bruxelles)

4:06 p.m. Paraconsistency of relativistic nonsignalling, and some other features of causal spectral toposes

Speaker: Ryszard Paweł Kostecki (University of Gdańsk)

4:13 p.m. Everything that can be learned about a causal structure with latent variables by observational and interventional probing schemes

Speaker: Marina Maciel Ansanelli (Perimeter Institute)

4:20 p.m. From Locality to Causality in the Heisenberg Picture

Speaker: Charles Alexandre Bédard (Università della Svizzera Italiana)

4:27 p.m. Self-testing quantum supermaps, with an application to the quantum switch

Speaker: Alastair Abbott (Inria Grenoble)

4:34 p.m. Higher-order Quantum Processes are Characterized by the Logic of their Signaling Relations

Speaker: Timothée Hoffreumon (Université Paris-Saclay)

4:41 p.m. Cluster Decomposition and Two Senses of Isolability

Speaker: Michael Miller (University of Toronto)

4:48 p.m. The geometry of spacetime from quantum measurements

Speaker: Tales Rick Perche (Perimeter Institute)

Tuesday, September 17

9:00 a.m. → 10:20 a.m. Tutorial - Indefinite quantum causality

Recent advances in quantum foundations have unveiled the idea that the causal order between quantum events may not always be fixed or even well-defined, allowing for some form of indefinite quantum causality. This tutorial will introduce the key concepts and motivations behind this rapidly developing area of research. Focusing on one of the main frameworks developed to explore indefinite quantum causality—the process matrix formalism—I will present key theoretical results, highlight the potential of indefinite causal orders as a resource for quantum information processing, and discuss experimental implementations as well as the physical interpretation of indefinite causal structures.

Speaker: Cyril Branciard (Institut Néel - CNRS)

10:20 a.m. → 10:50 a.m. Break

10:50 a.m. → 11:30 a.m. Indefinite causal order and the arrow of time

At the fundamental level, the dynamics of quantum particles and fields is time-symmetric: their dynamical equations are invariant under inversion of the time coordinate, possibly in conjunction with the change of other physical properties, such as charge and parity. At the operational level, the time-symmetry of the fundamental equations implies that certain quantum devices are bidirectional, meaning that the role of their inputs and outputs can be exchanged. Here we characterize the largest set of operations that can in principle be implemented on bidirectional devices, and show that this set includes operations in which the role of the input and output ports of the given devices becomes indefinite. An example of such an operation, called the “quantum time flip,” achieves input-output indefiniteness by adding quantum control to the direction in which a single device is used. We show that quantum operations with indefinite input-output directions can in principle achieve information-theoretic advantages over all possible operations with definite time direction, and can lead to an exetremely strong form of indefinite causal order.

Speaker: Giulio Chiribella (University of Hong Kong)

11:30 a.m. → 12:10 p.m. Can the quantum switch be deterministically simulated?

Higher-order transformations that act on a certain number of input quantum channels with an indefinite causal order, such as the quantum switch, cannot be described by standard quantum circuits that use the same number of calls of the input quantum channels. But could they be simulated, i.e., could their action on their input channels be deterministically reproduced, for all arbitrary inputs, by a quantum circuit that uses on a larger number of calls of the input channels? In this work, we prove that, when only one extra call of each input channel is available, the quantum switch cannot be simulated. We demonstrate the robustness of this result by showing that even when probabilistic and approximate simulations are considered, higher-order transformations that are close to the quantum switch can be at best simulated with a probability strictly less than one. This result stands in stark contrast with the known fact that, when the quantum switch acts exclusively on unitary channels, its action can be simulated. We also show other particular cases where a restricted simulation of the quantum switch is possible. Finally, we discuss the implications of our findings to the analysis of experiments based on the quantum switch.

Speaker: Jessica Bavaresco (University of Geneva)

12:10 p.m. → 1:45 p.m. Lunch + Discussion Time

1:45 p.m. → 3:00 p.m. Session A: Classical Causal Models

1:45 p.m. What, in the world, is a violation of causal faithfulness?

Speaker: Patrick Fraser (University of Toronto)

2:10 p.m. Classical causal models in string diagrams

Speaker: Robin Lorenz (Quantinuum)

2:35 p.m. Identifying classical Markovian causal models from generalized observation

Speaker: Isaac Friend (University of Oxford)

1:45 p.m. → 3:00 p.m. Session B: Indefinite Causal Order 1

1:45 p.m. Generalizing Bell non locality without global causal constraints

Speaker: Ravi Kunjwal (Aix-Marseille University)

2:10 p.m. Tsirelson bounds for quantum correlations with indefinite causal order

Speaker: Zixuan Liu (The University of Hong Kong)

2:35 p.m. Correlations and quantum circuits with dynamical causal order

Speaker: Raphaël Mothe (Institut Néel, CNRS)

3:00 p.m. → 3:45 p.m. Break

3:45 p.m. → 5:00 p.m. Session A: Quantum causality and compositionality

3:45 p.m. Causally faithful unitary circuit decompositions

Speaker: Tein van der Lugt (University of Oxford)

4:10 p.m. Partitions in quantum theory, and causal decompositions of 1D Quantum Cellular Automata

Speaker: Augustin Vanrietvelde (Télécom Paris)

4:35 p.m. Causality of meaning in compositional language models (an invitation to collaborate)

Speaker: Bob Coecke

3:45 p.m. → 5:00 p.m. Session B: Quantum reference frames

3:45 p.m. Quantum reference frames, measurement schemes and the type of local algebras in quantum field theory

Speaker: Daan Janssen (University of York)

4:10 p.m. Fighting non-locality with non-locality: microcausality and boundary conditions in QED

Speaker: Joshua Kirklin

4:35 p.m. Quantum Permutations as Quantum Coordinate Transformations

Speaker: Ofek Bengyat (Institute for Quantum Optics and Quantum Information - Vienna)

5:00 p.m. → 6:30 p.m. Poster Session: Poster Session 1

Wednesday, September 18

9:00 a.m. → 9:40 a.m. Quantum algorithms for classical causal learning

Given the large number of proposed quantum machine learning (QML) algorithms, it is somewhat surprising that ideas from this field have not yet been extended to causal learning. While deep learning and generative machine learning models have taken centre stage in the industrial application of automated learning on classical data, it is nonetheless well known that these techniques don't reliably capture causal concepts, leading to significant performance vulnerabilities. Increasingly, classical ML experts are taking ideas from causal inference, a field traditionally limited to small data sets of low dimensionality, and injecting modern ML elements to create new algorithms that benefit from the best of both worlds. These hybrid classical approaches provide new opportunity to search for potential quantum advantage. In this talk I explore this new research direction and propose several new quantum algorithms for classical causal inference.

Speaker: Sally Shrapnel (University of Queensland)

9:40 a.m. → 10:20 a.m. Nonlocality via Constrained Colliders

We propose that Bell correlations are explicable as a combination of (i) collider bias and (ii) a boundary constraint on the collider variable. We show that the proposal is valid for a special class of ('W-shaped') Bell experiments involving delayed-choice entanglement swapping, and argue that it can be extended to the ordinary ('V-shaped') case. The proposal requires no direct causal influence outside lightcones, and may hence offer a way to reconcile Bell nonlocality and relativity.

Speaker: Huw Price (Trinity College, Cambridge)

10:20 a.m. → 10:50 a.m. Break

10:50 a.m. → 11:30 a.m. Joint measurements on distant physical systems

It is not explicitly obvious that relativity and quantum mechanics are consistent with each other. Extensive research has shown that quantum states are consistent with relativity, in that they do not allow for faster-than-light transferring of information. In contrast, much less research has been done in quantum measurements, and in fact, naive attempts to put together relativity and quantum measurements lead to signaling between space-like separated regions. In this talk I will describe how this same problem arises in non-relativistic quantum physics, where measurements on systems kept spatially separated in general lead to signalling. By giving away the projection postulate, it is possible to alleviate this problem and measure non-local variables without signaling by exploiting pre-shared entanglement as a resource. I will describe a protocol for implementing any joint measurement in a non-signaling manner, and argue that this leads to a complete classification of all joint quantum measurements, based on the required amount of entanglement necessary to measure them.

Speaker: Alejandro Pozas-Kerstjens (Université de Genève)

11:30 a.m. → 12:30 p.m. Unconference: Unconference: Science and Society

12:30 p.m. → 2:00 p.m. Lunch + Discussion Time

2:00 p.m. → 3:00 p.m. Colloquium - Causal and counterfactual inference and what they're good for

Causal reasoning is vital for effective reasoning in many domains, from healthcare to economics. In medical diagnosis, for example, a doctor aims to explain a patient’s symptoms by determining the diseases causing them. This is because causal relations, unlike correlations, allow one to reason about the consequences of possible treatments and to answer counterfactual queries. In this talk I will present some recent work done with my collaborators about how one can learn and reason with counterfactual distributions, and why this is importantly for decision making. In all cases I will strive to motivate and contextualise the results with real word examples.

Speaker: Ciarán Gilligan-Lee (Spotify)

3:00 p.m. → 3:45 p.m. Break

3:45 p.m. → 5:00 p.m. Session A: Quantum Causal Models

3:45 p.m. QUANTUM LATENTS: DISTINGUISHING CAUSAL SCENARIOS WITH INDISTINGUISHABLE CLASSICAL ANALOGS

Speaker: Daniel Centeno Díaz

4:10 p.m. Cyclic causal modelling with a graph separation property in classical and quantum theories

Speaker: Carla Ferradini (Institute for Theoretical Physics, ETH Zurich)

4:35 p.m. A Semantics for Counterfactuals in Quantum Causal Models

Speaker: Eric Cavalcanti (Griffith University)

3:45 p.m. → 5:00 p.m. Session B: Indefinite causal order 2

3:45 p.m. Routing Quantum Control of Causal Order

Speaker: Maarten Grothus (Inria Grenoble)

4:10 p.m. Simulating the quantum switch using causally ordered circuits requires at least an exponential overhead in query complexity

Speaker: Hlér Kristjánsson (Perimeter Institute & IQC)

4:35 p.m. Knot invariants and indefinite causal order

Speaker: Samuel Fedida (University of Cambridge)

5:00 p.m. → 6:00 p.m. Poster Session: Poster Session 2

6:00 p.m. → 8:00 p.m. Banquet Dinner

Thursday, September 19

9:00 a.m. → 9:40 a.m. Quantum non-causality in spacetime may be not exclusively quantum

There are several non-causal effects that have been attributed to quantum physics. These include the analogues of "closed timelike curve effects" in quantum circuits proposed by David Deutsch (D-CTC), and the "impossible measurements" in relativistic quantum field theory discussed by Raphael Sorkin. Based on previous work, it will be pointed out in the talk that the alleged non-causality features arise not only in quantum systems, but in the very same manner in systems that are described in the framework of classical (non-quantum) statistical mechanics or classical field theory. Therefore, although the said non-causality scenarios have been portrayed as pertaining to quantum systems or quantum fields, they are in fact not based on, nor characteristic of, the quantum nature of physical systems.

Speaker: Rainer Verch (University of Leipzig)

9:40 a.m. → 10:20 a.m. Quantum discreteness and spacetime causality: what's in the mix?

The notion of causality is intimately tied to both, a transitive ordering on events, and the possibility of unrelated events. Thus, any causality structure is a partially ordered set or poset. This is the case in Lorentzian spacetime, which possesses a single time direction. In causal set quantum gravity, this spacetime causality structure is "first quantised" by discretising it. However, as with any dynamical quantum theory of spacetime, background notions of causality are insufficient. I will discuss how ordering and discreteness, as manifested in the sequential growth paradigm, provide a broad framework for quantum dynamical notions of causality.

Speaker: Sumati Surya (Raman Research Institute)

10:20 a.m. → 10:50 a.m. Break

10:50 a.m. → 11:30 a.m. Relativistic causality principles in QFT

In QFT, one aspect of relativistic causality is the principle of microcausality, which requires that observables associated with spacelike separated regions commute. But this principle is not by itself sufficient to rule out superluminal signalling, as examples of ‘impossible’ measurements demonstrate. Representations of the dynamics that respect relativity also play a necessary role in upholding relativistic causality in QFT. This talk will focus on the important role that principles of relativistic dynamics play in representations of local measurement in QFT.

Speaker: Doreen Fraser (University of Waterloo)

11:30 a.m. → 12:10 p.m. Enhancing Non-Classicality Detection with Interventions

Generalizations of Bell's theorem, particularly within quantum networks, are now being analyzed through the causal inference lens. However, the use of interventions, a central concept in causality theory, remains unexplored. As will be discussed, if we are not limited to observational data and can intervene in our experimental setup, we can witness quantum violations of classical causal bounds even when no Bell-like violation is possible. Through interventions, the quantum behavior of a system, that would seem classical otherwise, can be demonstrated. We will then present a photonic experiment implementing those ideas and consider applications of this framework for measurement-based quantum computation, quantification of causality in quantum gates and quantum network protocols.

Speaker: Rafael Chaves (International Institute of Physics (IIP-UFRN))

12:10 p.m. → 1:30 p.m. Lunch

1:30 p.m. → 3:00 p.m. Unconference: Interdisciplinary Topics on Causality (1)

3:00 p.m. → 3:45 p.m. Break

3:45 p.m. → 5:00 p.m. Session A: Network nonlocality 1

3:45 p.m. Two convergent NPA-like hierarchies for the quantum bilocal scenario

Speaker: Xiangling Xu (Inria Saclay Île-de-France)

4:10 p.m. Escaping the Shadow of Bell’s Theorem in Network Nonlocality

Speaker: Maria Ciudad Alañón

4:35 p.m. Observational-Interventional Bell Inequalities

Speaker: Davide Poderini (International Institute of Physics, UFRN, Natal)

3:45 p.m. → 5:00 p.m. Session B: Causality in Spacetime Physics

3:45 p.m. Towards Relational Quantum Field Theory

Speaker: Jan Głowacki (University of Oxford)

4:10 p.m. Making Sense of Relativistic Causality Conditions

Speaker: Wayne Myrvold (University of Western Ontario)

4:35 p.m. Relativistic Concepts in Point-Free Spaces

Speaker: Nesta van der Schaaf (The University of Edinburgh)

5:15 p.m. → 6:15 p.m. Business Meeting - Future of Causalworlds (Optional)

Friday, September 20

9:00 a.m. → 9:40 a.m. Higher-order quantum computation and causality

Supermaps are higher-order transformations that take maps as input. We explore quantum algorithms that implement supermaps of unitary operations using multiple calls to a black-box unitary operation. We investigate how the causal structure and spacetime symmetry of these unitary black-boxes affect their performance in implementing higher-order quantum operations. We analyze several tasks, inversion, complex conjugation, and transposition of black-box unitaries.

Speaker: Mio Murao (The University of Tokyo)

9:40 a.m. → 10:20 a.m. Causally faithful circuits for relativistic realisability, or: What can you do in a spacetime?

Multipartite quantum channels realisable in a spacetime obey the no-superluminal-signalling constraints imposed by relativistic causality. But what about the converse: Can every channel that exhibits no superluminal signalling also be realised through relativistically valid dynamics? To our knowledge, only special cases of this question have been studied. For bipartite channels, the answer has been found to be negative in general (Beckman et al., 2001), though we will argue that counterexamples must necessarily involve a form of fine-tuning. Another special case of the question has been extensively explored under the name of nonlocal quantum computation in the context of position-based cryptography. We will pose and motivate the question in generality, conjecture a positive answer for all but the fine-tuned channels, and present results towards proving it, drawing on insights from nonlocal quantum computation and the new field of causally faithful circuit decompositions of unitary transformations (see also Tuesday). Beyond their relevance to spacetime realisability, the circuit decompositions involved in addressing the question also find applications in quantum causal modelling.

Speaker: Tein van der Lugt (University of Oxford)

10:20 a.m. → 10:50 a.m. Break

10:50 a.m. → 11:30 a.m. Indefinite causal order and quantum reference frames

Recent research on quantum reference frames (QRFs) has shown that whether a system is in a superposed state of locations, momenta, and other properties can depend on the quantum reference frame relative to which it is being described. Whether an event is localized in spacetime or not can change under QRF transformations, in that case so-called quantum-controlled diffeomorphisms. This raises a critical question: can quantum reference frame transformations render indefinite causal order definite? In this talk, I propose a relativistic definition of causal order based on worldline coincidences and proper time differences, establishing it as an operationally meaningful observable in both general relativity and quantum mechanics. Using this definition, we can analyse the indefiniteness of causal order in the optical and gravitational quantum switch on equal footing. This analysis suggests an operational rather than a spacetime-based understanding of events. I will compare these findings to other recent results and conclude with broader implications for events in non-classical contexts

Speaker: Anne-Catherine de la Hamette (University of Vienna / IQOQI Vienna)

11:30 a.m. → 12:10 p.m. Fundamental limits for realising quantum processes in spacetime

Causality is a core concept in both General Relativity (GR) and Quantum Information Theory (QIT), yet it manifests differently in each domain. In GR, causal cones appear as a defining property of spacetime. Conversely, in QIT, causality relates to the abstract flow of information in quantum processes, independent of spacetime. This raises a crucial question: under what conditions can an abstract quantum process be realised within spacetime? The question is especially intriguing for quantum processes with indefinite causal structure, like the Quantum Switch, which resist classical causal descriptions. In this talk, I will present no-go theorems that reveal fundamental limitations on the realisability of such processes in spacetime and, thus, more generally, on the interplay between GR and QIT. This is based on joint work with V. Vilasini (Physical Review Letters, 133 080201, 2024).

Speaker: Renato Renner (ETH Zurich)

12:10 p.m. → 1:30 p.m. Lunch

1:30 p.m. → 3:00 p.m. Unconference: Interdisciplinary Topics on Causality (2)

3:00 p.m. → 3:45 p.m. Break

3:45 p.m. → 4:35 p.m. Session A: Network Nonlocality 2

3:45 p.m. Quantum non-classicality in the simplest causal network

Speaker: Pedro Lauand (University of Campinas)

4:10 p.m. Limitations of causal models of the triangle network, in the symmetric subspace and beyond

Speaker: Tamás Kriváchy (ICFO, The Institute of Photonic Sciences (Barcelona))

3:45 p.m. → 4:35 p.m. Session B: Interface of Information-theoretic and Relativistic Causality

3:45 p.m. A decompositional framework for process theories in spacetime

Speaker: Matthias Salzger (ICTQT, University of Gdansk)

4:10 p.m. Characterizing Signalling: Connections between Causal Inference and Space-time Geometry

Speaker: Maarten Grothus (Inria Grenoble)

4:35 p.m. → 5:00 p.m. CLOSING REMARKS