Quantum Information and Computation Initiative (QICi) of The University of Hong Kong. |

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## Upcoming Virtual Seminar

**24th of June 2021,**

**2pm GMT**

Bob Wald, University of Chicago

**Quantum Superposition of Massive Bodies**

We analyse a gedankenexperiment previously considered by Mari et al. that involves quantum superpositions of charged and/or massive bodies ("particles'') under the control of the observers, Alice and Bob. In the electromagnetic case, we show that the quantization of electromagnetic radiation (which causes decoherence of Alice's particle) and vacuum fluctuations of the electromagnetic field (which limits Bob's ability to localize his particle to better than a charge-radius) both are essential for avoiding apparent paradoxes with causality and complementarity. We then analyze the gravitational version of this gedankenexperiment. We show that the analysis of the gravitational case is in complete parallel with the electromagnetic case provided that gravitational radiation is quantized and that vacuum fluctuations limit the localization of a particle to no better than a Planck length. This provides support for the view that (linearized) gravity should have a quantum field description.

**Join**QISS Virtual Seminar by following the link hku.zoom.us/j/728065144 or using the Zoom Meeting ID 728 065 144 (password is QISS). Alternatively, watch the seminar

**live**on youtube.com/c/QISSproject. All seminars are archived and available to watch at a later time on our youtube channel.

(The `Zoom client for meetings' can be downloaded here: zoom.us/download)

Seminars are

**open**to participation.

To follow

**seminar announcements**join our mailing list.

**The idea**

Seminar topics range from fundamental theory to experiments and the philosophy of science, in particular on questions at the intersection of Quantum Gravity, Quantum Information and Quantum Foundations. The

**goal is to stimulate discussion**across disciplines, rather than presenting incremental technical results pertinent only for specialists in a sub-field.

For this reason, the talk duration is relatively short (<30 mins) and speakers are encouraged to make it pedagogical. One or two commentators will first engage the speaker(s) for ~10mins to get the discussion going. After this stage, questions and comments are open to all participants for one hour.

**QISS Virtual Seminar organising committee**

*Pierre Martin-Dussaud*

Lucas Hackl

Marios Christodoulou

Lucas Hackl

Marios Christodoulou

**Winter-Spring 2021 Program**

(to be updated)

(to be updated)

**7th of January, 2PM GMT**

**Richard Healey**

**University of Arizona**

*`Are facts relative in a quantum world?'*Recent arguments purport to show that if quantum theory is universally applicable then there is no objective fact about the outcome of a quantum measurement in certain extended Wigner’s friend

*Gedankenexperimenten*. This calls for an examination of the notions of fact and objectivity. If quantum theory is universally applicable then the facts about the physical world include a fact about each quantum measurement outcome. I will argue that these and other physical facts lack an ideal kind of objectivity but their more modest objectivity is all that science needs.

**28th of January 2021,**

**3pm GMT**

**Lee Smolin**

**Perimeter Institute**

*The quantum universe as a collection of partial views of itself*I describe a recent proposal for a simultaneous completion of quantum mechanics and general relativity, called the causal theory of views (CTV). Among its postulates are that time, in the sense of causal relations amongst events, is fundamental, and that space is emergent-along with everything that depends on space, such as distances, derivatives, fields, locality, non-locality etc. Also assumed real and fundamental are energy and momentum. Each event than has a view of the rest of the universe, which is made by the energy and momentum transferred to it by its causal precedents. To define dynamics we must introduce a measure of distance on the space of views. The idea is that differences of views substitutes for spacial distances and derivatives. The potential energy is then postulated to be a measure of the total diversity of views in the universe, called the variety. The kinetic energy is then related to the variety’s rate of change under causal evolution. The dynamics is defined by a sum over causal histories, from which space and spacetime emerge at the semiclassical approximation. N body nonrelativistic quantum mechanics is also derived, due to the variety reducing to Bohm’s quantum potential. Further steps are sketched.Based on papers: arXiv:1712.04799. with Marina Cortes: arXiv:1307.6167, arXiv:1407.0032, arXiv:1703.09696 , arXiv:1902.05082,

arXiv:1104.2822, arXiv:1506.02938, arXiv:1205.3707

**18th of February 2021, 5pm GMT**

**Stephen Wolfram**

*wolframphysics.org*

**A Surprisingly Promising Approach to a Fundamental Theory of Physics**

See:See:

**15th of April 2021,**

**2pm GMT**

**Aleks Kissinger, Oxford University**

**Extending the Logic of Influence and Causation**I will talk about some recent developments in the framework of "black box causal reasoning". In this minimal setting, we assume access to some abstract process and attempt to describe, quantify, or prove properties about the causal relationships between its inputs and outputs. This works both for first-order processes, which can capture e.g. a device shared by multiple agents, or higher-order processes, which captures the universe in which those agents live. This higher-order picture leads naturally to a particular categorical structure that has long been studied in theoretical computer science called a *-autonomous category. Whereas first order processes (e.g. quantum gates) only have two natural notions of composition (in series and in parallel), higher-order processes have an extremely rich and multi-faceted notion of composition guided by the "internal logic" of a *-autonomous category. In this talk, I will highlight some aspects of this logic, show how they can be used for causal reasoning, and discuss some recent extensions and open problems.

**13th of May 2021,**

**2pm GMT**

Marcus Aspelmeyer, Oxford U

**3rd of June 2021,**

**2pm GMT**

Ognyan Oreshkov, Université Libre de Brussels

**24th of June 2021,**

**2pm GMT**

Bob Wald, University of Chicago

**Autumn 2020 Program**

3rd of September, 3PM GMT (4PM London, UK)

**Carlo Rovelli**,

**CPT/**

**Aix-Marseille U and Perimeter Institute,**

*`Why can we influence the future?'*

Seminar Poster

Watch archived seminar on youtubeSeminar Poster

Watch archived seminar on youtube

Abstract: The two main components of the QISS community --quantum information and quantum gravity-- have opposite views on the arrow of time. This is generally taken as foundational in the dominant instrumentalist approach of the fist; while is often considered to be only a contingent aspect of the macroworld in the second. I show how to reconcile the two perspectives. This requires two steps: a careful analysis of the arrow of implication; and an understanding of the physical source of the time orientation of the agent. A number of papers have recently addressed these issues offering a compelling solution to the apparent disagreement.

****October 1st, 2020, 2:30PM GMT (3PM UK)

**Alejandro Perez,**

**CPT/**

**Aix-Marseille U,**

*`Two birds with one stone: discreteness, the cosmologic*

**al constant problem, and the Hawking information puzzle'**

*Seminar Poster*

Watch archived seminar on youtubeWatch archived seminar on youtube

8th of October, 2PM GMT (3PM London, UK)

**Nick Huggett, U of Illinois, Chicago, with Tushar Menon and Fedele Lizzi,**

**`Missing the Point in Noncommutative Geometry'**

*Seminar Poster*

Watch archived seminar on youtubeWatch archived seminar on youtube

Abstract: Noncommutative geometries generalize standard smooth geometries, parametrizing the noncommutativity of dimensions with a fundamental quantity with the dimensions of area. The question arises then of whether the concept of a region smaller than the scale makes sense in such a theory. We argue that it does not, in two interrelated ways. In the context of Connes’ spectral triple approach, we show that arbitrarily small regions are not definable in the formal sense. While in the scalar field Moyal-Weyl approach, we show that they cannot be given an operational definition. We conclude that points do not exist, and that continuous spacetime is an appearance, in such geometries.

29th of October, 2PM GMT (2PM London, UK)

**Anna Pearson with Natalia Ares, Oxford U, `Testing gravitational decoherence through the heating of a mechanical resonator'**

*Seminar Poster*

Watch archived seminar on youtubeWatch archived seminar on youtube

Abstract: The theory of classical channel gravity models gravitational interactions as classical measurement channels. These channels are a source of decoherence even if the results of the measurements are never recorded in a lab and thus the gravitational interaction can be thought of as having the same effect as an observer. This leads to two potentially observable effects – decoherence in the position basis and a density dependent heating effect. We have set up an experiment to test for the latter, using a cavity optomechanical setup at cryogenic temperatures to measure the mode heating of a silicon nitride membrane.

19th of November, 3PM GMT (3PM London, UK)

**Lucien Hardy, Perimeter Institute, `**Time Symmetry in Operational Theories'

*Seminar Poster*

Watch archived seminar on youtubeWatch archived seminar on youtube

The standard framework for probabilistic operational theories is time asymmetric. The fact that future choices cannot affect the probability of earlier outcomes is mathematized by the statement that the deterministic effect is unique. However, deterministic preparations are not unique and, correspondingly, earlier choices can influence later probabilities of outcomes. This time asymmetry is rather strange because abstract probability theory knows nothing of time. Furthermore, the Schoedinger equation is time symmetric and, additionally, measurement situations can be treated by very simple models (without invoking the Second Law at all). In this talk I will outline how it is possible to give a time symmetric treatment of operational probabilistic theories with particular application to Quantum Theory. In so doing, we will see that the usual formulation of operational quantum theory is, in some sense, missing half of the picture.

17th of December, 3:30PM GMT (4:30PM Vienna)

**Caslav Brukner, University of Vienna,**

**`**

*Quantum superposition of processes with opposing thermodynamic arrows of time.'*Fundamental laws of physics are generally time-symmetric. The directionality of time is then often explained with the thermodynamic arrow of time: the entropy of an isolated system increases during a process, and it is constant only if the process is reversible. In this talk, I will consider a quantum superposition between two processes with opposing thermodynamic arrows of time. How is a definite arrow of time established for such a superposition? I will show that a quantum measurement of entropy change (for values larger than the thermal fluctuations) can be accountable for this. In particular, while the individual result of the measurement is random, once the value of the entropy variation has been observed, the system continues its evolution according to a definite arrow of time. Furthermore, for entropy variations lower than (or of the order of) the thermal fluctuations, interference effects can cause entropy changes describing more or less (ir)reversible processes than either of the two constituents, or any classical mixture therefrom.

## Archived Seminars

Robert Oeckl

UNAM, Morelia

July 2, 2020

The invention of quantum theory in the 1920s represented a paradigm shift in our approach to describing the natural world. The focus on the object as a primitive shifted to the observation as a primitive. At the time, the first applications of interest came with a classical description in the language of Hamiltonian evolution, canonical variables and states. Staying close to this particular language lead to the development of the quantum formalism of Hilbert spaces, operators, Schrödinger equation and Born rule. Somewhat unfortunately, this standard formulation has come to dominate our understanding of what quantum theory is. While it was successfully employed in describing the micro-structure of matter and its relevant interactions, describing the dynamics of spacetime itself is outside of its scope. With the present talk I want to promote the idea that quantum theory is much more general than this standard formulation. I aim to clarify the essence of the paradigm shift that lies at the heart of the transition from classical to quantum theory. On this basis I then review the derivation from first principles of a more fundamental formulation of quantum theory, the positive formalism, and the recovery of the standard formulation as a special case.

UNAM, Morelia

July 2, 2020

*`What is Quantum Theory?'***Watch archived seminar on youtube**

**Abstract**The invention of quantum theory in the 1920s represented a paradigm shift in our approach to describing the natural world. The focus on the object as a primitive shifted to the observation as a primitive. At the time, the first applications of interest came with a classical description in the language of Hamiltonian evolution, canonical variables and states. Staying close to this particular language lead to the development of the quantum formalism of Hilbert spaces, operators, Schrödinger equation and Born rule. Somewhat unfortunately, this standard formulation has come to dominate our understanding of what quantum theory is. While it was successfully employed in describing the micro-structure of matter and its relevant interactions, describing the dynamics of spacetime itself is outside of its scope. With the present talk I want to promote the idea that quantum theory is much more general than this standard formulation. I aim to clarify the essence of the paradigm shift that lies at the heart of the transition from classical to quantum theory. On this basis I then review the derivation from first principles of a more fundamental formulation of quantum theory, the positive formalism, and the recovery of the standard formulation as a special case.

Chiara Marletto

University of Oxford

July 2, 2020

After briefly discussing constructor theory (a recently proposed generalisation of the quantum theory of computation), I will present the general argument that provides a robust theoretical underpinning for the recently proposed experiments to witness quantum effects in gravity.

University of Oxford

July 2, 2020

*`Witnessing non-classicality beyond quantum theory'***Watch archived seminar on youtube****Abstract**After briefly discussing constructor theory (a recently proposed generalisation of the quantum theory of computation), I will present the general argument that provides a robust theoretical underpinning for the recently proposed experiments to witness quantum effects in gravity.

Sougato Bose

University College London

May 14, 2020

*`Table-top Testing of the Quantum Nature of Gravity: Assumptions, Implications and Practicalities of a Proposal'*

**Watch archived seminar on youtube**

**Abstract**

A lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Motivated by this, I will present a feasible idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. A prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, is also provided and can be measured through simple spin correlations. Further, I clarify the assumptions underpinning the above proposal such as our reasonable definition of "classicality", as well as the crucial aspect of the locality of physical interactions. The role of off-shell processes is also highlighted to clarify what the mediators actually are according to the standard theory of quantum gravity. How the experiment sits within relativistic quantum field theory is clarified. Lastly, a list of practical challenges are noted.

Eugenio Bianchi

Penn State University

April 23, 2020

The quantum field vacuum is highly entangled, even in causally disconnected regions. In contrast, the state of a quantum geometry of space can be unentangled, resulting in an uncorrelated network of elementary quanta of space. In this talk I discuss how the architecture of spacetime emerges from entanglement between these elementary quanta. I will focus on loop quantum gravity, causal structures and the primordial universe.

Penn State University

April 23, 2020

*`Entanglement and the Architecture of Spacetime'***Watch archived seminar on youtube****Abstract**The quantum field vacuum is highly entangled, even in causally disconnected regions. In contrast, the state of a quantum geometry of space can be unentangled, resulting in an uncorrelated network of elementary quanta of space. In this talk I discuss how the architecture of spacetime emerges from entanglement between these elementary quanta. I will focus on loop quantum gravity, causal structures and the primordial universe.

Jeremy Butterfield and Henrique Gomes

University of Cambridge

April 2, 2020

Various programmes and results in the philosophy/foundations of spacetime theories illustrate points about reduction and functionalism in general philosophy of science. I will focus on some programmes and results about how the physics of matter contributes to determining, or even determines, or even explains, chrono-geometry. I will say something about most of the following examples: the Helmholtz-Lie theorem on free mobility implying constant curvature; and in the philosophical literature, Robb (1914), and Mundy (1983). I also hope to mention from the physics literature: Barbour and Bertotti (1982), Hojman, Kuchar and Teitelboim (1976); Dull, Schuller et al. (2012, 2018); and Gomes & Shyam (2016: 1608.08236 = J. Math. Phys. 57, 112503).

University of Cambridge

April 2, 2020

**Watch archived seminar on youtube**

`On Reduction and Functionalism about Space and Time.'**Title**`On Reduction and Functionalism about Space and Time.'

**Abstract**Various programmes and results in the philosophy/foundations of spacetime theories illustrate points about reduction and functionalism in general philosophy of science. I will focus on some programmes and results about how the physics of matter contributes to determining, or even determines, or even explains, chrono-geometry. I will say something about most of the following examples: the Helmholtz-Lie theorem on free mobility implying constant curvature; and in the philosophical literature, Robb (1914), and Mundy (1983). I also hope to mention from the physics literature: Barbour and Bertotti (1982), Hojman, Kuchar and Teitelboim (1976); Dull, Schuller et al. (2012, 2018); and Gomes & Shyam (2016: 1608.08236 = J. Math. Phys. 57, 112503).