# Schedule for: 21w5104 - Quantum Foundations, Gravity, and Causal Order (Online)

Beginning on Sunday, May 30 and ending Friday June 4, 2021

All times in Banff, Alberta time, MDT (UTC-6).

Monday, May 31 | |
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07:00 - 07:15 |
Introduction and Welcome by BIRS Staff ↓ A brief introduction to BIRS with important logistical information, technology instruction, and opportunity for participants to ask questions. (Online) |

07:15 - 07:45 |
Giulio Chiribella: Quantum operations with indefinite direction of time: the quantum time flip ↓ The standard operational framework of quantum theory is time-asymmetric. This asymmetry reflects the capabilities of ordinary agents, who are able to deterministically pre-select the states of quantum systems, but not to deterministically post-select the outcomes of quantum measurements. However, the fundamental dynamics of quantum particles is time-symmetric, and is compatible with a broader class of operations where pre-selections and post-selections are combined in general ways that do not presuppose a definite direction of time. In this talk I introduce a framework for quantum operations with indefinite time direction, providing an example, called the quantum time flip, where an unknown, time-symmetric process is accessed in a coherent superposition of two alternative time directions. In certain information-theoretic tasks, a hypothetical agent with access to the quantum flip can in principle outperform all agents who operate in a definite time direction.
Related paper: G. Chiribella an Z. Liu, The quantum time flip, https://arxiv.org/abs/2012.03859 (Online) |

07:45 - 08:15 |
Aleks Kissinger: 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 capture the environment 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. (Online) |

08:15 - 08:25 | Break on Gather.town (Online) |

08:25 - 08:55 |
Gavin Morley: Levitating nanodiamond experiments: towards a test of quantum gravity ↓ We are building an experiment in which a nitrogen-vacancy-centre electron spin would be used to put a levitated nanodiamond into a spatial quantum superposition [1-3]. This would be able to test theories of spontaneous wavefunction collapse and is the first step of a much more ambitious experiment to test if gravitational effects can be in a quantum superposition [4, 5]. This talk will describe our current experimental design [6-11], and our latest experimental progress: the first steps on a long journey.
[1] A. T. M. A. Rahman, A. C. Frangeskou, M. S. Kim, S. Bose, G. W. Morley & P. F. Barker, Sci. Rep. 6, 21633 (2016).
[2] A. T. M. A. Rahman, A. C. Frangeskou, P. F. Barker & G. W. Morley, RSI 89, 023109 (2018).
[3] A. C. Frangeskou, A. T. M. A. Rahman, L. Gines, S. Mandal, O. A. Williams, P. F. Barker & G. W. Morley, NJP 20, 043016 (2018).
[4] S. Bose, A. Mazumdar, G. W. Morley, H. Ulbricht, M. Toroš, M. Paternostro, A. A. Geraci, P. F. Barker, M. S. Kim & G. Milburn, PRL 119, 240401 (2017).
[5] C. Marletto & V. Vedral, PRL 119, 240402 (2017).
[6] S. Bose & G. W. Morley, arXiv:1810.07045 (2018).
[7] M. Scala, M. S. Kim, G. W. Morley, P. F. Barker & S. Bose, PRL 111, 180403 (2013).
[8] C. Wan, M. Scala, G. W. Morley, A. T. M. A. Rahman, H. Ulbricht, J. Bateman, P. F. Barker, S. Bose & M. S. Kim, PRL 117, 143003 (2016).
[9] J. S. Pedernales, G. W. Morley & M. B. Plenio, PRL 125, 023602 (2020).
[10] R. J. Marshman, A. Mazumdar, G. W. Morley, P. F. Barker, S. Hoekstra & S. Bose, NJP 22, 083012 (2020).
[11] Z.-q. Yin, T. Li, X. Zhang & L. M. Duan, PRA 88, 033614 (2013). (Online) |

08:55 - 09:25 |
Philip Walther: Shining light on the interface of gravity and quantum physics: precision measurements using photonic quantum interferometry ↓ Quantum mechanics and general relativity are two fundamentally different theories and have both been tested independently with very high precision. However, even after a century of research, the interplay of those two very different theories has never been tested experimentally. Within this talk I will present the experimental research aiming to explore this interface between quantum mechanics and general relativity by performing high-precision experiments at the level of single quanta of light, the photons. Such quantum systems allow one to examine the influence of gravity on interference effects. For this purpose, a high-precision interferometer whose paths are subject to different gravitational potentials will be used. (Online) |

09:10 - 09:20 | Break on Gather.town (Online) |

09:35 - 09:40 |
Group Photo ↓ Please turn on your cameras for the "group photo" -- a screenshot in Zoom's Gallery view. (Online) |

09:40 - 10:15 | Discussion (Online) |

16:00 - 16:30 |
Fabio Costa: Background independent quantum causal structures ↓ The process matrix formalism formalises non-classical causal structures, which are expected to emerge in a theory that combines quantum indeterminacy with the dynamical causal structure of general relativity. The framework relies on the notion of local laboratories—an abstraction of spacetime events or regions—where local operations and measurements can be performed. In a theory of quantum gravity, we expect that it should not be possible to label laboratories a priori: this requires a background reference frame (although possibly non-classical), while general relativity is background independent. I will present a background independent formulation of the process matrix formalism that incorporates a discrete version of background independence: invariance under permutations. Although the formalism abandons an absolute notion of “local operation”, such a notion re-emerges as relative to a physical reference frame. Permutation symmetry also implies some type of superselection rule, although, for quantum processes, this cannot be interpreted as implying the existence of a conserved charge with a definite value. Finally, I will comment on the possibility to extend the approach to include quantum coordinate transformations. (Online) |

16:30 - 17:00 |
Laura Henderson: Entanglement Harvesting with a Temporal Superposition ↓ Quantum field theory is completely characterized by the field correlations between spacetime points. In turn, some of these can be accessed by locally coupling to the field simple quantum systems, a.k.a. particle detectors. In this work, we consider what happens when a quantum- controlled superposition of detectors at different space-time points is used to probe the correlations of the field. We show that, due to quantum interference effects, two detectors can gain information on field correlations which would not be otherwise accessible. This has relevant consequences for information theoretic quantities, like entanglement and mutual information harvested from the field. In particular, the quantum control allows for extraction of entanglement in scenarios where this is otherwise provably impossible. (Online) |

17:00 - 17:10 | Break on Gather.town (Online) |

17:10 - 17:40 |
Andrew White: Communicating via ignorance ↓ Noise is ubiquitous: communication protocols aim to optimise the amount of information that can be sent through a channel with a given amount of noise. Classically, in the limit of a completely noisy channel no information can be transmitted. Quantum physics offers strategies to transmit information in this scenario: via superposition of path, or via superposition of causal order. Previous work showed respectively that: placing two completely noisy channels in a superposition of paths—that is in different arms of an interferometer—allows some information to be communicated, up to 0.16 bits; placing two completely noisy channels in a superposition of order—so the order of the channels is indefinite—allows up to 0.049 bits can be communicated. Here we analyse, and experimentally demonstrate, communication through various combinations of noisy and unitary channels in an indefinite causal order and provide an example where perfect transmission is possible, but only in the case of order superposition [1].
We use a quantum switch [2] to achieve indefinite causal order: the order is controlled by the light polarisation and the quantum channels act on the transverse spatial mode of the light. We investigate two limiting cases. When both channels are fully-depolarising, the ideal limit is communication of 0.049 bits; experimentally we achieve 0.0341$±0.0015 bits. When one channel is fully-depolarising, and the other is a known unitary, the ideal limit is communication of 1 bit, a capacity not possible with simple path superposition. We experimentally achieve 0.636$±0.017 bits. Our results offer intriguing possibilities in applications ranging from communication through to imaging in turbulent media.
[1]. K. Goswami, Y. Cao, G. A. Paz-Silva, J. Romero, and A. G. White,
Increasing communication capacity via superposition of order
Physical Review Research 2, 033292 (2020).
doi:10.1103/PhysRevResearch.2.033292
[2]. K. Goswami, C. Giarmatzi, M. Kewming, F. Costa, C. Branciard, J. Romero, and A. G. White
Indefinite Causal Order in a Quantum Switch,
Physical Review Letters 121, 090503 (2018).
doi:10.1103/PhysRevLett.121.090503 (Online) |

17:30 - 19:30 | Placeholder (Online) |

17:40 - 17:45 |
Group Photo ↓ Please turn on your cameras for the "group photo" -- a screenshot in Zoom's Gallery view. (Online) |

17:45 - 18:10 |
Flaminia Giacomini: Discussion session ↓ Discussion with the speakers of the previous session (Online) |

Tuesday, June 1 | |
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07:00 - 07:30 |
Markus Müller: Quantum reference frame transformations as symmetries and the paradox of the third particle ↓ In a quantum world, reference frames are ultimately quantum systems too -- but what does it mean to "jump into the perspective of a quantum particle"? In this work, we show that quantum reference frame (QRF) transformations appear naturally as symmetries of simple physical systems. This allows us to rederive and generalize known QRF transformations within an alternative, operationally transparent framework, and to shed new light on their structure and interpretation. We give an explicit description of the observables that are measurable by agents constrained by such quantum symmetries, and apply our results to a puzzle known as the `paradox of the third particle'. We argue that it can be reduced to the question of how to relationally embed fewer into more particles, and give a thorough physical and algebraic analysis of this question. This leads us to a generalization of the partial trace (`relational trace') which arguably resolves the paradox, and it uncovers important structures of constraint quantization within a simple quantum information setting, such as relational observables which are key in this resolution. While we restrict our attention to finite Abelian groups for transparency and mathematical rigor, the intuitive physical appeal of our results makes us expect that they remain valid in more general situations. (Online) |

07:30 - 08:00 |
Marios Christodoulou: Quantum Superpositions of Graphs ↓ Graphs are a discrete topological canvas that in many applications can completely replace a continuous manifold. Nevertheless, invariance of dynamics defined on graphs under changes of reference frames is typically attempted by embedding the graph in an ambient manifold. This seems superfluous as node names already label points similarly to a choice of coordinates in continuous space. As node names are fiducial, graph renamings can be seen as a change of coordinates on the graph. Thus, graph renamings correspond to a natively discrete analogue of diffeomorphisms. In quantum theory, node names become even more important. We first provide a robust notion of quantum superpositions of graphs and argue with a simple example that in quantum theory in order to avoid instantaneous signalling it is necessary to use node names to define the ‘localisation’ of a node, rather than values of a physical field. We propose renaming invariance as a symmetry principle of similar weight to diffeomorphism invariance and show how to impose it at the level of quantum superpositions of graphs. (Online) |

08:00 - 08:10 | Break on Gather.town (Online) |

08:10 - 08:40 |
Esteban Castro Ruiz: Relative subsystems and quantum reference frame transformations ↓ Transformations between reference frames play a crucial role in our understanding of physical processes. In practice, reference frames are realised by physical systems, which are standardly treated as classical. However, assuming that every physical system is ultimately quantum, it is interesting to ask how a theory of transformations wrt quantum reference frames would look like, and what implications it would have for our description of spacetime. Recently, there has been a lot of effort towards developing a quantum generalisation of reference frame transformations, unveiling novel phenomena that are absent in the classical treatment of reference frames. Here, we develop a first-principles framework for quantum reference frame transformations which clarifies important conceptual issues of previous treatments. Based on the algebra of relative observables between a system and a reference frame, our operational perspective leads naturally to a mixed-state approach (incoherent twirling), in contrast to current pure-state approaches (coherent twirling). Within our framework, the full invariant quantum subsystem contains not only the algebra of relative observables between the system and the reference frame but also an “extra particle,” related to the invariant degrees of freedom of the reference frame itself. Importantly, this extra particle contains information about the “quantumness” of the reference frame and is essential to the unitarity of quantum reference frame transformations. Our approach is general, in the sense that it can be applied to a vast set of symmetry groups and to any type of system. We illustrate the physical meaning of the concepts developed by analysing quantum reference frame transformations with respect to the (centrally extended) Galilei group. (Online) |

08:40 - 09:10 |
Ognyan Oreshkov: Quantum processes on time-delocalised systems ↓ I will discuss the notion of time-delocalised quantum systems and its relevance to the question of realisability of processes with indefinite causal order in standard quantum theory and quantum gravity. I will explain how given a description of an experiment in the form of a (generally cyclic) circuit, the experiment can be described with respect to an arbitrary alternative choice of (sub)systems. This provides a very general notion of transformation between different equivalent descriptions of an experiment. I will review how the quantum SWITCH can be seen as realisable on time-delocalised systems in standard quantum mechanics and will show that all unitarily extendible tripartite processes admit such realisations. Notably, this includes processes violating causal inequalities, among which some purely classical ones. I will discuss the meaning of causal inequality violations with such time-delocalised variables. Finally, I will speculate on the link between time-delocalised systems and quantum reference frames. (Online) |

09:10 - 09:20 | Break on Gather.town (Online) |

09:20 - 10:00 |
Lucien Hardy: Discussion session ↓ Discussion session with the speakers (Online) |

16:00 - 16:30 |
Alexander Smith: Relational dynamics and quantum time dilation ↓ The lesson of general relativity is background independence, which results in a Hamiltonian constraint. This presents a challenge for quantum gravity because the quantization of this constraint demands that physical states of geometry and matter are frozen, leading to the problem of time. We must then explain how the conventional notion of time evolution emerges, which motivates the need for a relational description of quantum dynamics. Using quantum clocks and covariant time observables, I will introduce a formulation of relational quantum dynamics that allows for a probabilistic notion of relativistic time dilation. This framework will then be used to describe a quantum time dilation effect that occurs when a clock moves in a superposition of different relativistic momenta. I will argue that this time dilation effect may be observable with present-day technology and offers a new test of relativistic quantum mechanics. Implications for causal structure will be discussed. (Online) |

16:30 - 17:00 |
Eduardo Martin-Martinez: A tale of two detector models: Causal structure and measurements in quantum fields ↓ We will discuss how relativistic causality and covariance play a role in the measurement problem in quantum fields and even in our ability to put information into a quantum field. We will compare the Fewster-Verch formalism with the Unruh-DeWitt-like (UDW) particle detector models. We will discuss the limitations of both measurement frameworks as well as the typical abuses that are often made with Unruh-DeWitt-like detector models and how they matter in general relativistic scenarios. Finally we will discuss the claim in [arXiv:2103.13400] that weakly coupled detectors cannot harvest entanglement and show what is the effect of that claim, if any, on the feasibility of entanglement harvesting with particle detectors. (Online) |

17:00 - 17:10 | Break on Gather.town (Online) |

17:10 - 17:40 |
Magdalena Zych: Clocks and detectors for characterising quantum causal structures ↓ Understanding and characterising causal structures which posses provably non-classical features is of direct relevance for the research on quantum theory with indefinite causal order as well as for quantum gravity. Due to diffeomorphism invariance of general relativity (GR), causal structure of spacetime in GR requires an operational description — in terms of physical systems and their ability to exchange information. In quantum theory such systems can be taken as composite quantum particles, modelling ideal clocks as well as particle detectors. I will discuss how such particles allow insights into quantum causal structures by allowing us to construct quantum spacetimes and quantify non-classical features of their causal structures. Surprisingly, this approach also shows that operational means may fail to distinguish a genuine superposition of different (non-diffeomorphic) spacetimes from a single classical spacetime. This opens a question: under what conditions non-classical spacetimes can be operationally distinguished from the classical ones? (Online) |

17:40 - 18:10 | Robert Mann: Discussion session (Online) |

Wednesday, June 2 | |
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07:00 - 07:30 |
Sougato Bose: Quantum Nature of Gravity in the Lab: Assumptions, Implementation and Applications on the Way ↓ There is no empirical evidence yet as to “whether” gravity has a quantum mechanical origin. Motivated by this, I will present a feasible idea for testing the quantum origin of the Newtonian interaction based on the simple fact that two objects cannot be entangled without a quantum mediator. I will show that despite its weakness, gravity can detectably entangle two adjacent micron sized test masses held in quantum superpositions even when they are placed far apart enough to keep Casimir-Polder forces at bay. A prescription for witnessing this entanglement through spin correlations is also provided. Further, I clarify the assumptions underpinning the above proposal such as our reasonable definition of “classicality”, as well as relativistic causality. We note a few ways to address principal practical challenges: Decoherence, Screening EM forces and Inertial noise reduction. I will also describe how unprecedented compact sensors for classical gravity (including meter scale sensors for low frequency gravitational waves) will arise on the way to the above grand goal. (Online) |

07:30 - 08:00 |
Anupam Mazumdar: Quantum test of gravity by colliding Schrödinger's kittens ↓ Quantum gravity is yet to be tested in a laboratory. I will provide criteria for testing the quantum fluctuation of a graviton in a laboratory by colliding two non-relativistically Schrödinger’s kittens and study how the final states would be entangled in this process. Realising such an experiment will be filled with various challenges, from developing new technologies to logistics to financial costs. I will motivate the community, despite all these challenges, why doing such an experiment is necessary to understand both the foundations of quantum mechanics and gravity. (Online) |

08:00 - 08:10 | Break on Gather.town (Online) |

08:10 - 08:40 |
Alessio Belenchia: Quantum Superposition of Massive Objects and the Quantization of Gravity ↓ When a massive quantum body is put into a spatial superposition, it is of interest to consider the quantum aspects of the gravitational field sourced by the body. In this talk, I will discuss a Gedankenexperiment where Alice and Bob control masses in quantum superposition. I will show that the analysis of this experiment does not lead to any inconsistency provided the 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 and that table-top experiments testing entanglement of systems interacting via gravity do probe the quantum nature of gravity, even if no ``gravitons'' are emitted during the experiment. (Online) |

08:40 - 09:10 |
Ivette Fuentes: Quantum Frequency Interferometry: with applications ranging from gravitational wave detection to dark matter searches ↓ We introduce a quantum interferometric scheme that uses states that are sharp in frequency and delocalized in position. The states are frequency modes of a quantum field that is trapped at all times in a finite volume potential, such as a small box potential. This allows for significant miniaturization of interferometric devices. Since the modes are in contact at all times, it is possible to estimate physical parameters of global multi-mode channels. As an example, we introduce a three-mode scheme and calculate precision bounds in the estimation of parameters of two-mode Gaussian channels. This scheme can be implemented in several systems, including superconducting circuits, cavity-QED and cold atoms. We consider a concrete implementation using the ground state and two phononic modes of a trapped Bose-Einstein condensate. We apply this to show that frequency interferometry can improve the sensitivity of phononic gravitational waves detectors by several orders of magnitude, even in the case that squeezing is much smaller than assumed previously and that the system suffers from short phononic lifetimes. Other applications range from magnetometry, gravimetry and gradiometry to dark matter/energy searches. (Online) |

09:10 - 09:20 | Break on Gather.town (Online) |

09:20 - 10:00 | Flaminia Giacomini: Discussion session (Online) |

10:00 - 10:15 | MITACS presentation (Online) |

16:00 - 16:30 |
Dan Carney: Tests and implications of gravitational entanglement ↓ I'll briefly overview the basic notion that one can look for entanglement generation via gravity in experiments. In particular I will present a recent proposal (joint with H. Muller and J. M. Taylor) using an atom interferometer coupled to a high-Q mechanical mass. The central idea relies on a new method of entanglement verification in time-dependent systems. Time permitting, I'll also make some comments about the interpretation of these entanglement experiments more broadly, particularly the relation to gravitons. (Online) |

16:30 - 17:00 |
Achim Kempf: Spacetime and causality when the notion of distance is replaced by the notion of correlation. ↓ Quantum field fluctuations are the more strongly correlated the smaller their spacetime distance. As a consequence, the very notion of distance can be replaced by the notion of correlation strength. This suggests a picture in which all degrees of freedom are described by the same abstract structure, namely (multi-point) correlators, a picture which is essentially information theoretic. At low energies, these abstract correlators possess a mathematical representation as the correlation functions of matter fields that live on a curved spacetime with a definite causal structure. As one approaches the Planck scale, the abstract correlators may no longer possess such a representation, thereby superseding the notions of matter and of spacetime, but the abstract correlators can still be examined information theoretically (Online) |

17:00 - 17:10 | Break on Gather.town (Online) |

17:10 - 17:40 |
Thomas Galley: Quantum Relativity of Subsystems ↓ One of the most basic notions in physics is the partitioning of a system into subsystems, and the study of correlations among its parts. In this talk, we will explore these notions in the context of quantum reference frame (QRF) covariance, in which this partitioning is subject to a symmetry constraint. We will see that different reference frame perspectives induce different sets of subsystem observable algebras, which leads to a gauge-invariant, frame-dependent notion of subsystems and entanglement. We will then see that subalgebras which commute before imposing the symmetry constraint can translate into non-commuting algebras in a given QRF perspective after symmetry imposition. Such a QRF perspective does not inherit the distinction between subsystems in terms of the corresponding tensor factorizability of the kinematical Hilbert space and observable algebra. Since the condition for this to occur is contingent on the choice of QRF, the notion of subsystem locality is frame-dependent. Finally I will comment on the relevance of these results for existing programs in quantum gravity, as well as relate them to existing results on the relativity of quantum systems within approaches in quantum information. This talk is based on joint work with Philipp A. Hoehn, Maximilian P. E. Lock, Shadi Ali Ahmad and Alexander R. H. Smith which can be found at https://arxiv.org/abs/2103.01232 (Online) |

17:40 - 18:10 | Lucien Hardy: Discussion session (Online) |

Thursday, June 3 | |
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07:00 - 07:30 |
Sumati Surya: Growing a Quantum Discrete Universe, Causally ↓ In this talk I will discuss the criteria for constructing a covariant quantum dynamics for causal sets using the Rideout-Sorkin sequential growth paradigm. In the histories-formulation, the growth process determines the complex weight or quantum measure, with the covariant observables or be-ables given by the covariant measurable sets. The existence of covariant observables in turn puts constraints on the allowed growth dynamics. I will present some recent results with Stav Zalel on a simplified “abelian” model which demonstrates the existence of classes of covariant quantum dynamics. I will end the talk with a broad discussion of this observer independent formulation of quantum gravity. (Online) |

07:30 - 08:00 |
Fay Dowker: Recovering General Relativity from a Planck scale discrete theory of quantum gravity ↓ I will present an argument that if a theory of quantum gravity is physically discrete at the Planck scale and the theory recovers General Relativity as an approximation, then, at the current stage of our knowledge, causal sets must arise within the theory, even if they are not its basis. (Online) |

08:00 - 08:10 | Break on Gather.town (Online) |

08:10 - 08:40 |
Caslav Brukner: Quantum reference frames and the weak and Einstein equivalence principles ↓ In physics, a reference frame is an abstract coordinate system that specifies observations within that frame. Recently, it has been proposed to consider reference frames as physical objects that ultimately obey quantum mechanical laws. The transformations between such quantum reference frames can be seen as an extension of the transformations between classical reference frames. In my talk, I will show how the notion of quantum reference frames can help us formulate and test the weak and Einstein equivalence principles when a "free-falling laboratory" is in a quantum state (which can be spatiotemporally extended, for example), or when it falls into a superpositon of gravitational fields. This gives rise to the quantum Einstein equivalence principle, which could pave a fruitful way to the conceptual foundation of a future theory of quantum gravity. (Online) |

08:40 - 09:10 |
Jorma Louko: Thermality of circular motion ↓ An observer in uniform linear acceleration responds to the Minkowski vacuum thermally, in the Unruh temperature $T_U =\frac{proper\, acceleration}{2 \pi}$. An observer in uniform circular motion experiences a similar Unruh-type temperature $T_c$, with better prospects of detection in analogue spacetime laboratory experiments, but $T_c$ depends not just on the proper acceleration but also on the orbital radius and on the excitation energy. We establish a range of analytic and numerical results for $T_c$ for a massless scalar field in $3+1$ and $2+1$ spacetime dimensions, the latter being motivated by proposed condensed matter experiments. In particular, we find that the circular motion analogue Unruh temperature grows arbitrarily large in the near-sonic limit, encouragingly for the experimental prospects, but less quickly in effective spacetime dimension $2+1$ than in $3+1$. [Based on Biermann et al, Phys. Rev. D 102, 085006 (2020)] (Online) |

09:10 - 09:20 | Break on Gather.town (Online) |

09:20 - 10:00 | Robert Mann: Discussion session (Online) |

16:00 - 16:30 |
Daniel Terno: Why astrophysical black holes do not have horizons, and what it means if they do ↓ Do astrophysical black holes observed by LIGO and EHT actually trap light? We argue that properties of the near-horizon regions, such as an atmosphere of exotic matter violating the null energy condition or a firewall produced by an accreting black hole, combined with our current understanding of physics make this unlikely. If, however, a distant observer does detect an apparent horizons this will indicate the onset of radically new physics. (Online) |

16:30 - 17:00 |
Valentina Baccetti: Vacuum entanglement harvesting with delocalized matter ↓ Entanglement harvesting has been studied extensively using the Unruh-deWitt (UdW) detector. In this detector the matter systems are modelled as two-level quantum detector systems while the classical centre of mass degrees of freedom are described using a smearing profile function. As has been extensively shown, entanglement harvesting depends very sensitively on the detector details. In this talk we consider two quantum delocalized detectors in their respective
ground states [1], and we ask how their ability to become entangled with each other is affected by their mass and their initial centre of mass delocalization. For comparison we consider entanglement harvesting from two UdW detectors with classical centre of mass and Gaussian smearing profile. We will show that the process of entanglement harvesting is affected by the coherent delocalization of matter and, in particular, that delocalized detectors harvest less entanglement than detectors whose centre of mass degrees of freedom are assumed to behave classically. We will also identify the limit in which the results for entanglement harvesting for coherently delocalized detectors reduce to the results for detectors with classical external degrees of freedom [2].
[1] N. Stritzelberger and A. Kempf, “Coherent delocalization in the light-matter interaction”, Phys. Rev. D, 101, 036007, 2020.
[2] Nadine Stritzelberger, Laura J. Henderson, Valentina Baccetti, Nicolas C. Menicucci, and Achim Kempf, “Entanglement harvesting with coherently delocalized matter”, Phys. Rev. D 103, 016007, 2021. (Online) |

17:00 - 17:10 | Break on Gather.town (Online) |

17:10 - 17:40 |
Hui Wang: Coherently amplifying photon production from vacuum with a dense cloud of accelerating photodetectors ↓ An accelerating photodetector is predicted to see photons in the electromagnetic vacuum. However, the extreme accelerations required have prevented the direct experimental verification of this quantum vacuum effect. In this work, we consider many accelerating photodetectors that are contained within an electromagnetic cavity. We show that the resulting photon production from the cavity vacuum can be collectively enhanced such as to be measurable. The combined cavity-photodetectors system maps onto a parametrically driven Dicke-type model; when the detector number exceeds a
certain critical value, the vacuum photon production undergoes a phase transition from a normal phase to an enhanced superradiant-like, inverted lasing phase. Such a model may be realized as a mechanical membrane with a dense concentration of optically active defects undergoing gigahertz flexural motion within a superconducting microwave cavity. We provide estimates suggesting that recent related experimental devices are close to demonstrating this inverted, vacuum photon lasing
phase. (Online) |

17:40 - 18:10 | Flaminia Giacomini: Discussion session (Online) |

Friday, June 4 | |
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07:00 - 07:30 |
Markus Aspelmeyer: Quantum tests of (quantum) gravity ↓ No experiment today provides evidence that gravity requires a quantum description. Two type of table-top searches have been suggested to provide answers: experiments that test low-energy consequences of quantum theories of gravity, and experiments that directly probe the phenomenology of superposition states of gravitational source masses (in the spirit of a quantum-Cavendish experiment). The latter requires to bridge the gap between two different realms: precision measurements of gravity with microscopic source masses (currently 10^21 atoms) and quantum state preparations of massive solid state objects (currently 10^9 atoms). I will review the current status in the lab and the challenges to be overcome for future experiments. (Online) |

07:30 - 08:00 |
Fabio Sciarrino: Experimental tests on quantum causality ↓ The fields of quantum non-locality, in physics, and causal discovery, in machine learning, both face the problem of deciding whether observed data are compatible with a presumed causal relationship between the variables. Bell’s theorem shows that quantum mechanical correlations can violate the causal constraints imposed on any classical explanation of experiments performed by space-like separated parties, the phenomenon of non-locality. Recently, it has been realized that many of the concepts and tools from the field of causal inference, such as Bayesian networks, are useful not only to reinterpret known results but most importantly to provide generalizations of Bell’s theorem. We will report several experiments aimed at developing a deep understanding of the departure between classical and quantum causality by starting from elementary but fundamental causal structures. We will then briefly discuss applications to quantum information processinng [1-8].
[1] G. Carvacho, F. Andreoli, L. Santodonato, M. Bentivegna, R. Chaves, F. Sciarrino. “Experimental violation of local causality in a quantum network”, Nature Communications 8, 14775 (2017).
[2] F. Andreoli, G. Carvacho, L. Santodonato, R. Chaves, F. Sciarrino, “Maximal qubit violation of n-locality inequalities in a star-shaped quantum network”, New J. Phys. 19, 113020 (2017).
[3] F. Andreoli, G. Carvacho, L. Santodonato, M. Bentivegna, R. Chaves, F. Sciarrino, “Experimental bilocality violation without shared reference frames”, Phys. Rev. A 95, 062315 (2017).
[4] R. Chaves, G. Carvacho, I. Agresti, V. Di Giulio, L. Aolita, S. Giacomini, F. Sciarrino, “Quantum violation of an instrumental test”, Nature Physics (2017). doi:10.1038/s41567-017-0008-5.
[5] E. Polino, I. Agresti, D. Poderini, G. Carvacho, G. Milani, G. Barreto Lemos, R. Chaves, F Sciarrino, “Device independent certification of a quantum delayed choice experiment”, Phys. Rev. A 100, 022111 (2019)
[6] I. Agresti, D. Poderini, L. Guerini, M. Mancusi, G. Carvacho, L. Aolita, D. Cavalcanti, R. Chaves, F. Sciarrino, “Experimental device-independent certified randomness generation with an instrumental causal structure”, Communications Physics 3, 110 (2020)
[7] D. Poderini, I. Agresti, G. Marchese, E. Polino, T. Giordani, A. Suprano, M. Valeri, G. Milani, N. Spagnolo, G. Carvacho, R. Chaves and F. Sciarrino, “Experimental violation of n-locality in a star quantum network”, Nature Communications 11, 2467 (2020).
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08:00 - 08:10 | Break on Gather.town (Online) |

08:10 - 08:40 |
Silke Weinfurtner: Quantum simulators for fundamental physics ↓ The dynamics of the early universe and black holes are fundamental reflections of the interplay between general relativity and quantum fields. The essential physical processes occur in situations that are difficult to observe and impossible to experiment with: when gravitational interactions are strong, quantum effects are important, and theoretical predictions for these regimes are based on major extrapolations of laboratory-tested physics.
We will discuss the possibility to study these processes in experiments by employing analogue classical/quantum simulators. Their high degree of tunability, in terms of dynamics, effective geometry, and field theoretical description, allows one to emulate a wide range of elusive physical phenomena in a controlled laboratory setting. We will discuss recent developments in this area of research. (Online) |

08:40 - 09:10 |
Christopher Wilson: Analog Quantum Simulation of Strongly-Coupled Field Theories with a Parametric Cavity ↓ TBA (Online) |

09:10 - 09:20 | Break on Gather.town (Online) |

09:20 - 10:00 | Lucien Hardy: Discussion session (Online) |

16:30 - 17:00 |
Philippe Allard Guerin: A no-go theorem for the persistent reality of Wigner's friend's perception ↓ The notorious Wigner's friend thought experiment has in recent years received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In this work we formulate a no-go theorem for the persistent reality of Wigner's friend's perception, which allows us to conclude that the perceptions that the friend has of her own measurement outcomes at different times cannot "share the same reality", if seemingly natural quantum mechanical assumptions are met. We show that there is no joint probability distribution for the friend's perceived measurement outcomes at two different times, that depends linearly on the initial state of the measured system and whose marginals reproduce the predictions of unitary quantum theory. This theorem entails that one must either (1) propose a nonlinear modification of the Born rule for two-time predictions, (2) sometimes prohibit the use of present information to predict the future --thereby reducing the predictive power of quantum theory-- or (3) deny that unitary quantum mechanics makes valid single-time predictions for all observers. We briefly discuss which of the theorem's assumptions are more likely to be dropped within various popular interpretations of quantum mechanics. (Online) |

17:00 - 17:10 | Break on Gather.town (Online) |

17:10 - 17:40 |
Timothy Ralph: Weak value Bohmian Trajectories of Relativistic particles ↓ TBA (Online) |

17:40 - 18:10 | Robert Mann: Discussion session (Online) |