Foundations of Quantum Physics beyond Bell: Celebrating 60 years of Bell's theorem

15 Apr 2024, 09:00 → 19 Apr 2024, 15:00 Europe/Zurich

Alberto Rolandi (University of Geneva), Flavio Del Santo (University of Geneva)

CERN, CC BY 4.0, via Wikimedia Commons

The year 2024 marks the 60th anniversary of Bell's theorem, a result that arguably had the single strongest impact on modern foundations of quantum physics, both at the conceptual and methodological level, as well as at the level of its applications in information theory and technology. We will celebrate Bell’s legacy with two joint workshops, one in Switzerland and one in Austria, which will gather physicists working on quantum foundations as well as philosophers and historians. These workshops will focus on the state-of-the-art research on quantum foundations, with the aim of going beyond Bell’s inequalities and showing the breadth and the depth that foundational research in quantum theory has reached.

Swiss Event

The first workshop will take place in Les Diablerets (Switzerland) on April 15-19, 2024. The topical focus of the workshop, although not limited to that, will be on measurements in quantum physics in a broad sense: from the realizability of joint (non-local) measurements to the “impossible measurements” in quantum field theory, from the thermodynamic cost of quantum measurements to the quantum measurement problem in Wigner’s friend scenarios. This workshop is by invitation only, but the talks and the discussion will be available on streaming. At the moment, the timetable and speakers list are available.

Organizers

• Nicolas Gisin
• Renato Renner
• Nicolas Brunner
• Flavio Del Santo
• Alberto Rolandi
• Gianmichele Blasi
• Roope Uola

  

Austrian Event 

The second Vienna Quantum Foundations conference (September 23-27, 2024) will celebrate two anniversaries at once: 60 years of Bell’s theorem, but also 20 years of IQOQI Vienna. Continuing a long tradition of quantum physics in Vienna, the conference brings together leading international researchers to discuss and shape the future of quantum foundations. While the talks will be by invitation only, there will be an opportunity to contribute a poster and an associated brief flash talk. More information will be available on the website https://vqf.iqoqi.oeaw.ac.at.

Organizers

• Časlav Brukner
• Borivoje Dakić
• Markus Müller
• Miguel Navascués
• Isabel Griesshammer

Bell schedule.pdf

participants.pdf

Monday 15 April

12:30 → 14:00 Lunch

14:15 → 14:30 Introduction

14:30 → 15:15 Talks: Session 1.1

14:30 "Impossible measurements"

https://quantum-journal.org/papers/q-2024-02-27-1267/

Speaker: Nicolas Gisin (Université de Genève)

15:15 → 16:45 Talks: Session 1.2

15:15 Measurement in quantum field theory

Presentations of quantum mechanics often include a postulate that the state
of a system undergoes an instantaneous change following a measurement.
This is clearly incompatible with special and general relativity and raises
questions concerning the description of measurement in quantum field theory (QFT).
Attempts to extend measurement postulates to QFT by hand have produced
pathologies, such as the "impossible measurements" described long ago by Sorkin
(although, arguably, a better name would have been "impossible state updates").

I will describe a framework for measurement in QFT that is causal, covariant and
consistent, based on the idea of measuring one QFT by coupling it locally to another,
and in which state update rules may be derived rather than posited.

Among other things, this framework resolves the problematic aspects of "impossible measurements/updates" within QFT.

I will also explain how the violation of Bell inequalities can be seen using these ideas.

The talk is mostly based on joint works with Rainer Verch, Henning Bostelmann, Maximilian Ruep and Ian Jubb.

See arXiv:2304.13356 for a short presentation, and Commun. Math. Phys. 378 (2020) 851-889 (https://link.springer.com/article/10.1007%2Fs00220-020-03800-6) for the original paper.

Speaker: Prof. Chris Fewster (University of York)

16:00 Impossible measurements revisited

It is by now well recognized that the naive application of the projection postulate on composite quantum systems can induce signalling between their constituent components, indicative of a break- down of causality in a relativistic spacetime context. Here we introduce a necessary and sufficient condition for an ideal measurement of an observable to be nonsignalling. As well as being particularly simple, it generalizes previous no-signalling conditions in that it allows for degeneracies and can be applied to all bounded self-adjoint operators. The condition is used to establish that arbitrary sums of local observables will not signal, in accordance with our expectations from relativistic quantum field theory. On the other hand, it is shown that the measurement of the tensor product of commuting local observables, for example bipartite operators of the form A ⊗ B, can in fact signal, contrary to the widely held belief that such measurements are always locally realizable. The implications for the notion of measurement in relativistic quantum field theory are addressed; it appears that the most straightforward application of the standard quantum formalism generically leads to violations of causality. We conclude that either the class of observables that can be measured should be restricted and/or that the naive translation of the measurement framework of quantum theory, in particular the projection postulate, to quantum field theory should be reevaluated.

Speaker: Leron Borsten (University of Hertfordshire)

16:45 → 17:15 Coffee break

17:15 → 18:45 Talks: Session 1.3

17:15 Causality in QFT measurements: the scattering paradigm and beyond

The formulation of local measurement theory for QFT has recently been an active area of research. In contrast to the asymptotic measurement framework that was enshrined in QED the new proposals aim to supply a measurement framework for measurement in local spacetime regions. In the history of QFT, in parallel to the establishment of the asymptotic scattering paradigm, there is a series of attempts to model local measurements in QFT that we will briefly review in the first part of this talk (based on [1]). Considerations of relativistic causality played an important role in these developments. Causality issues, such as impossible measurements, today have been addressed for local QFT measurements that are formulated as local scattering maps. I will focus on the analysis of causality in the detector model approach to QFT measurements, where the local scattering maps are induced by non-relativistic probes coupled to the field. Finally, I will discuss causality beyond the scattering paradigm, for histories-based approaches to QFT measurements (based on [2]).

[1] https://link.springer.com/article/10.1140/epjh/s13129-023-00064-1
[2] https://philsci-archive.pitt.edu/22887/

Speaker: Maria Eftychia Papageorgiou (IQOQI, Vienna)

18:00 TBA

Speaker: Valentino Jadrisko (Constructor University)

18:45 → 19:30 Constructor demos & drinks

19:30 → 20:30 Dinner

Tuesday 16 April

09:30 → 11:00 Talks: Session 2.1

09:30 Turing, Wigner, Bell: the high frontier for experimental metaphysics

Experimental metaphysics is the study of how empirical results can reveal facts about the fundamental nature of the world, independent of any theory. It is a field born from Bell’s 1964 theorem, and the experiments it inspired. I argue that the high frontier for the field will come from combining Bell’s 1964 paper with those of two other scientists of roughly the same era. Specifically, realising a thinking machine (à la Turing) on a quantum computer in the role of Wigner’s friend on one side of a Bell experiment would yield either a more acute contradiction between metaphysical assumptions or new physics.

A few papers are relevant, but especially this one: https://doi.org/10.22331/q-2023-09-14-1112

Speaker: Howard Wiseman (Griffith University)

10:15 Localization of events in classical and non-classical spacetimes

A natural response to Einstein's hole argument is that the spacetime points on a manifold have no a priori physical meaning. But if that is so, what are the experientialists trying to achieve when they conduct experiments towards macroscopic superpositions in space?

The link to the paper: https://arxiv.org/abs/2402.10267

Speaker: Prof. Caslav Brukner (University of Vienna)

11:00 → 11:30 Coffee break

11:30 → 12:15 Talks: Session 2.2

11:30 Wigner's friend's perception and the no-signaling principle

Speaker: Dr Veronika Baumann (University of Vienna)

12:30 → 14:00 Lunch

16:00 → 16:45 Talks: Session 3.1

16:00 Bell’s theorem and the scientists who worked on it.

Bell’s theorem history ran from a cold reception in the first years of its inception to a diversified plethora of scientists working on it, which included Nobel prize winners. I have coined the term quantum dissidents to portray the physicists who worked on it in its early decades. They believed, against the views shared by most physicists, that there was good physics to be done about hidden variables and quantum mechanics. I will present a short collective biography of the physicists who worked on this theorem throughout its sixty years of existence.

Speaker: Olival Freire (Universidade Federal da Bahia)

16:45 → 17:15 Coffee break

17:15 → 18:45 Talks: Session 3.2

17:15 Non-locality, Preferred Foliations, and Emergent Relativity

Speaker: Prof. Tim Maudlin (New York University)

18:00 John S. Bell Natural Philosopher

In his contributions to the foundations of quantum mechanics, John S. Bell turned out to be also a true natural philosopher, sensitive to the conceptual implications of physics. In my talk, I will focus on some aspects of the ideal dialogue that Bell developed with Albert Einstein and Niels Bohr, two of the main natural philosophers of the twentieth century.

Speaker: Prof. Federico Laudisa (University of Trento)

18:45 → 19:30 Drinks

19:30 → 21:00 Dinner

Wednesday 17 April

09:30 → 11:00 Talks: Session 4.1

09:30 Bell's theorem, randomness and secrecy.

Speaker: Prof. Antonio Acín (ICFO)

10:15 Experimental quantum key distribution certified by Bell’s theorem

https://www.nature.com/articles/s41586-022-04941-5

Speaker: Prof. Nicolas Sangouard (Université Paris-Saclay)

11:00 → 18:45 Glacier 3000 Excursion

18:45 → 19:30 Drinks

19:30 → 20:30 Dinner

20:30 → 21:45 Talks: Session 4.2

20:30 Quantum Network Correlations

Quantum networks are promising tools for the implementation
of long-range quantum communication. The characterization of
quantum correlations in networks and their usefulness for
information processing is therefore central for the progress
of the field, but so far only results for small basic network
structures or pure quantum states are known.

In my talk I will discuss two topics related to quantum networks.
First, I will introduce notions of network entanglement and study
their properties. Specifically, I will provide an analytical approach
to characterize correlations in large network structures with
arbitrary topologies. Second, I will present an efficient method to
characterize the network topology from few measurement data.

References:
K. Hansenne et al., arXiv:2108.02732
L. Weinbrenner et al., arXiv:2309.12907

Speaker: Prof. Otfried Gühne (University of Siegen)

21:15 The Elegant Joint Measurement is Nonlocal in the Triangle Network

Proving the nonlocality of a distribution in a general quantum network that goes beyond the Bell scenario is generally a difficult task, due to the non-convex nature of the problem. Only few examples of proven quantum nonlocality exist even in the simple triangle network. The Elegant Joint Measurement is a quantum measurement scheme that can be used to obtain an outcome distribution in the triangle network. This distribution has been conjectured to be nonlocal 6 years ago. In this work, we provide a computer-assisted proof of nonlocality for this distribution, thereby developing a toolkit to tackle general network nonlocality problems. The proof is based on the inflation technique for causal inference, but scaled to higher levels than what has generally been considered tractable. This is made possible by a number of optimization techniques, involving symmetry reductions, branch-and-bound optimization, and most importantly, the use of a Frank-Wolfe algorithm to bypass the need to call a standard linear program solver.

Speaker: Victor Gitton (ETH Zürich)

Thursday 18 April

09:30 → 11:00 Talks: Session 5.2

09:30 Quantum systems as gravitational sources: theory-independent and theory-specific predictions on the nature of gravity

Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, for instance a massive quantum system prepared in a quantum superposition of positions and sourcing a gravitational field, has attracted a lot of attention: experiments are working towards realising such a scenario in the laboratory, and measuring the gravitational field associated to a quantum source is expected to give some information about quantum aspects of gravity. However, there are still open questions concerning the precise conclusions that these experiments could draw on the nature of gravity.

In this talk, I will argue that a full answer to this question requires two different approaches: on the one hand, a theory-specific description that identifies precise effects that cannot be explained by classical gravity; on the other hand, a theory-independent description, inherited from Bell’s theorem, that tests the internal consistency of the theory and constrains from first principles which scenarios are compatible with a classical nature of gravity and which ones are not.

Speaker: Dr Flaminia Giacomini (ETH Zürich)

10:15 Could Einstein have been right after all?

One of the most surprising aspects of quantum theory is that it tells us that we live in a nonlocal universe in which random correlations seem to appear instantaneously between arbitrarily distant locations. This idea was completely abhorrent to Einstein, who dismissed it as "spooky action at a distance", yet the 2022 Nobel Prize in Physics was awarded for experimental demonstrations half a century ago of this phenomenon. It is even said that so-called loophole-free experiments confirmed nonlocality beyond any reasonable doubt. But have they really? I shall argue that no experiment whose purpose is to confirm the predictions of quantum theory can possibly be used as an argument in favour of nonlocality because any theory of physics that does not allow instantaneous signalling to occur and has reversible dynamics (such as unitary quantum theory) can be explained in a purely local and realistic universe. What if Einstein was right after all?... Once again!

This talk is based on the original doctoral work of Paul Raymond-Robichaud while under my supervision, which was published in the journal Entropy and is available open access at https://www.mdpi.com/1099-4300/21/1/87. More advanced material can be found at https://doi.org/10.1098/rspa.2000.0585, https://doi.org/10.1098/rspa.2020.0897 and https://doi.org/10.48550/arXiv.1710.01380.

Speaker: Gilles Brassard (University of Montréal)

11:00 → 11:30 Coffee break

11:30 → 12:15 Talks: Session 5.1

11:30 A problem with Many Worlds

Recently, a prominent critic of the Many Worlds (MW) interpretation summarised: 'Its main shortcoming is simply this: The interpretation is completely contentless.' In my talk, I will adopt a more generous approach and ask: What if we equip the MW interpretation, specifically its central notions of 'worlds' and 'branching,' with physical content? I will then present a no-go theorem, asserting that this endeavour is doomed: If one attempts to render these MW notions compatible with quantum theory, one will inevitably encounter contradictions.

Speaker: Renato Renner (ETH Zurich)

12:30 → 14:00 Lunch

16:00 → 16:45 Talks: Session 6.1

16:00 MEH

Speaker: Prof. Marcus Huber (TU Wien)

16:45 → 17:15 Coffee break

17:15 → 18:45 Talks: Session 6.2

17:15 Quantum measurements from the second law of thermodynamics

The measurement postulate of quantum theory stands in conflict with the laws of thermodynamics and has evoked debate regarding what actually constitutes a measurement. With the help of modern quantum statistical mechanics, we take some first steps in formalising the hypothesis that quantum measurements are driven by the natural tendency of closed systems to maximize entropy. In this paradigm, we investigate how objective measurement outcomes can emerge within a purely unitary framework, and find that: (i) the interactions used in standard measurement models fail to spontaneously feature emergent objectivity and (ii) while ideal projective measurements are impossible, we can (for a given form of Hamiltonian) approximate them exponentially well as we collect more physical systems together into an ``observer'' system. We thus lay the groundwork for self-contained models of quantum measurement, proposing improvements to our simple scheme.

Speaker: Dr Maximilian Lock (TU Wien)

18:00 TBA

Speaker: Prof. Sandu Popescu (University of Bristol)

18:45 → 19:30 Drinks

19:30 → 20:30 Dinner

Friday 19 April

09:30 → 11:00 Talks: Session 7.2

09:30 Locality and quantum particle statistics

Speaker: Prof. Borivoje Dakic (University of Vienna)

10:15 Monogamy relations for relativistically causal correlations

Speaker: Dr Mirjam Weilenmann (University of Geneva)

11:00 → 11:30 Coffee break

11:30 → 12:15 Talks: Session 7.1

11:30 Imprecise measurements in entanglement and steering tests

The standard method for detecting entanglement and steering is via measuring suitable witness operators. This requires that some prescribed measurements are performed on the local systems. I discuss what happens when the assumption of perfect measurements no longer is employed but instead the measurements are allowed to have tiny imprecisions. I argue that small specific imprecisions can give rise to large corrections to witnesses, which makes it possible to fake large amounts of entanglement from tiny misalignments. I discuss how this problem can be addressed, and how cleverly chosen witnesses allow corrections to be made "for free".

Speaker: Prof. Armin Tavakoli (Lund University)

12:30 → 14:00 Lunch