RQI Circuit

RQI Circuit Waterloo - Opening Remarks

The RQI Circuit Waterloo will start at 9:45am EST, and will be broadcast in the ISRQI YouTube Channel. The local organizers are María Rosa Preciado-Rivas, Everett Patterson, Caroline Lima, and José Polo-Gómez.

Quantum Probes of Black Holes

Black holes remain amongst the most fascinating objects in gravitational physics. Classical probes can tell us something about their exterior behaviour, but quantum probes can tell us much more. I will discuss in general terms the research I am carrying out in my group on what we can learn about black holes using 2-level quantum detectors.

Signatures of Rotating Black Holes in Quantum Superposition

" A new approach for operationally studying the effects of spacetime in quantum superpositions of semiclassical states has recently been proposed by some of the authors. This approach was applied to the case of a (2+1)-dimensional Bañados-Teitelboim-Zanelli (BTZ) black hole in a superposition of masses, where it was shown that a two-level system interacting with a quantum field residing in the spacetime exhibits resonant peaks in its response at certain values of the superposed masses. Here, we extend this analysis to a mass-superposed rotating BTZ black hole, considering the case where the two-level system co-rotates with the black hole in a superposition of trajectories. We find similar resonances in the detector response function at rational ratios of the superposed outer horizon radii, specifically in the case where the ratio of the inner and outer horizons is fixed. This suggests a connection with Bekenstein's seminal conjecture concerning the discrete horizon spectra of black holes in quantum gravity, generalized to the case of rotating black holes. Our results suggest that deeper insights into quantum-gravitational phenomena may be accessible via tools in relativistic quantum information and curved spacetime quantum field theory. "

Quantum detectors freely falling into black holes

The Unruh-DeWitt model for particle detectors has been widely used to probe quantum fields for noninertial observers or curved spacetime. However, little is known about their response as they freely fall across the event horizon of black holes despite more than four decades of studying the response of these detectors. We present the numerical results we obtained for the transition rate of particle detectors in a three-dimensional black hole spacetime, investigating their potential to serve as an ‘early warning system’ that indicates if an observer is about to cross the horizon of a black hole.

Probing Hidden Topology with Quantum Detectors

We consider the transition rate of a static Unruh-DeWitt particle detector in a variety of spacetimes built out of quotients of $\text{AdS}_3$ spacetime. In particular, we contrast the behavior of a Unruh-DeWitt detector interacting with a quantum scalar field in the $\mathbb{R}\text{P}^{2}$ geon spacetime and a spacetime constructed by \r{A}minneborg \textit{et al}. The Wightman functions of these spacetimes are obtained using the method of images. We find a number of features that distinguish the two spacetimes, which are identical outside of the black hole's event horizon, most notably, in the response functions of gapless detectors in the sharp-switching limit. This points to a way in which the interior topology of a black hole may be discerned by an external observer.

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Career Development Talk

In this talk Eduardo Martín-Martínez will talk about the career opportunities in Waterloo, and will share important information for researchers who intend to apply for positions at the University of Waterloo, Perimeter Institute and the Institute of Quantum Computing.

Duality between amplitude and derivative coupled particle detectors in the limit of large energy gaps

In this talk, we are going to present a duality between models of particle detectors, namely, between a model where the particle is coupled to the amplitude of the field and a model where the particle couples to the derivative of the field. We show that, in the limit of large energy gaps, the models can be mapped to each other in a one-to-one fashion, modulo a rescaling by the detector’s energy gap. This analysis is valid for scalar fields in arbitrary curved spacetimes and requires minimal assumptions regarding the detectors. The duality also applies to cases where more than one detector is coupled to the field, which means that many examples of entanglement harvesting with amplitude-coupled UDW detectors yield exactly the same result as derivative-coupled detectors that interact with the field in the same region of spacetime.

Particle detector models from path integrals of localized quantum fields

Using the Schwinger-Keldysh path integral, we draw a connection between particle detector models and localized quantum field theories. By integrating and then tracing out the inaccessible modes of the localized field being used as a probe, we show that, at leading order in perturbation theory, the dynamics of any finite number of modes of the probe field is exactly that of a finite number of harmonic-oscillator Unruh-DeWitt (UDW) detectors coupled to the target field. The result vindicates recent analysis by arXiv:2308.11698, and greatly extends the conclusions therein by explicitly showing that the equivalence is valid for a rather general class of input states of the probe-target field system, as well as for any arbitrary number of modes included as part of the probe; it also provides a closed-form, systematic way of obtaining the corrections to the UDW model at higher order in perturbation theory due to the existence of the additional modes that have been traced out. Finally, this perspective also allows for potential connections between particle detector models in RQI and other areas of physics where path integral methods are more commonplace, such the Wilsonian approach to the renormalization group and effective field theories.

Dynamically ObtainingLocalized Quantum Fields From a Free Field Theory

In this presentation I will discuss ongoing work on the evolution of a quantum field subject to a time-dependent, confining potential well. By making the potential gradually stronger with time, we expect to see an initially spatially delocalized field slowly become localized within a region that is determined by the minima of the potential. This can be interpreted as a concrete demonstration of how one can obtain a localized quantum field theory starting from quantum fields in free space. We then use these results to study how the time-dependent potential influences the mixedness of local modes defined from the quantum field, which is of relevance to the use of field-theory-based local probes in important protocols in Relativistic Quantum Information such as entanglement harvesting.

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Some perspectives into RQI research

Relativistic quantum information is an extremely diverse field. In fact, it has been recently proven that the duration of a broad talk about research interests in this field does not converge to a finite time. Within this limitation I will try to review some of the research lines that we currently pursue at Barrio-RQI. From the measurement problem in QFT to the thermodynamics of quantum fields theory with stops at quantum communication assisted by relativistic effects and experimental proposals in RQI.

Correlation harvesting between particles in uniform motion.

Correlation harvesting is a phenomenon where two particles can become entangled via local interactions with a quantum field. The Wightman function describing this situation for the case of uniformly accelerated particle detectors can be characterized by a master equation, where different types of motion (linear, cusped, catenary and circular) arise from tuning a single parameter in the equation. We investigate the correlation harvesting protocol for this generalized case of uniformly accelerated particles. We investigated both the case of stationary (time-translationally invariant Wightman function) and non-stationary configurations and compared the amount of harvestable classical & quantum correlations in each case. Remarkably, we found that although we cannot harvest entanglement from causally separated uniformly accelerated detectors, we can still achieve "genuine" (i.e. non-communication-assisted) entanglement harvesting in regions where the detectors are in causal contact.

Three particle detectors extract entanglement from a black hole spacetime

Understanding the correlations of the quantum vacuum provides insight into fundamental problems in cosmology, gravity, and quantum field theory, providing important clues to understanding the structure of quantum spacetime. Most research has focused on bipartite entanglement of the quantum vacuum. Here I present the first results on multipartite entanglement outside of a black hole. Specifically, I examine the tripartite entanglement harvesting protocol of three Unruh-DeWitt detectors in the (2+1)-dimensional BTZ black hole spacetime. Previous research has shown that two detectors cannot extract bipartite entanglement from a quantum field in the vicinity of an event horizon due to intense Hawking radiation. I show that it is possible to harvest tripartite entanglement in regions where bipartite entanglement cannot be extracted. Curiously, tripartite entanglement is easier to harvest than bipartite entanglement.

Experimental Entanglement Harvesting: Bridging the gap between theory and experiments

Recent advances in superconducting circuits have provided a method to tune the coupling between superconducting qubits and a waveguide, going from strong coupling to no coupling in nanoseconds [https://arxiv.org/abs/2208.05571]. These advances open the possibility to build an experimental platform to test predictions from relativistic quantum information (RQI). In particular, entanglement harvesting is an established prediction of RQI which has not been verified experimentally, but that could be tested in the aforementioned superconducting platform. To make this possible, we upgrade the UDW particle detector model to include features of the more complex experimental superconducting qubits coupled to waveguides. We will present how these new features affect the amount of entanglement harvested from the waveguide, which will be essential for the entanglement harvesting experiments that are planned for the near future.

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Entanglement structure of quantum fields through local probes

I will present a framework to study the entanglement structure of a quantum field theory inspired by the formalism of particle detectors in RQI. This framework can in principle be used to faithfully capture entanglement in a QFT between arbitrary-shaped regions of spacetime without encountering UV divergences, bypassing many of the issues typically present in other approaches. The results also establish the limits of the efficiency of entanglement harvesting, and may also be used to motivate an operational definition of entanglement between spacetime subregions in field theory.

Quantum Information in Gravity: Holographic Complexity

We investigate the holographic complexity of CFTs compactifed on a circle with a Wilson line, dual to magnetized solitons in AdS4 and AdS5. These theories have a confnement-deconfnement phase transition as a function of the Wilson line, and the complexity of formation acts as an order parameter for this transition. Through explicit calculation, we show that proposed complexity functionals based on volume and action obey a scaling relation with radius of the circle and further prove that a broad family of potential complexity functionals obeys this scaling behavior. As a result, we conjecture that the scaling law applies to the complexity of conformal feld theories on a circle in more general circumstances.

Representations and the emergence of spacetime

I will begin by reviewing the general mathematical concept of representation and I will then show that representation theory is more generally applicable than one might expect. In particular, in quantum gravity, the notion of representation can yield a new mechanism for how spacetime can change dimensions with increasing energies and how spacetime itself could emerge from Planck scale physics.

RQI Circuit Nottingham - Opening Remarks

The RQI Circuit Nottingham will start at 11:55pm GMT, and will be broadcast in the ISRQI YouTube Channel. The local organizers are Cameron Bunney, Leo Parry, and Cisco Gooding.

Quantum fields, acceleration, and now, simulation

It is known since the seventies that the acceleration of a local observer is predicted to affect the way how the observer experiences a relativistic quantum field. Probing this prediction experimentally in a genuinely relativistic spacetime is challenging, but the experimental prospects are better in condensed matter laboratory systems whose dynamics simulates a relativistic quantum field. In this talk we give an overview of research in Nottingham at this theory-versus-experiment interface.

Field back-reaction in the circular Unruh effect

The predominant approach to calculations using the Unruh-DeWitt detector is to analyse the probability of the detector transitioning away from its initial state in response to the field. In this presentation, we pose the opposite question: How does the field respond to the detector, and what kind of particles are generated through this interaction? This talk explores the emission of particles into a massless scalar field in (2+1)-dimensional Minkowski by a detector coupled to it undergoing circular motion. We examine several field observables and uncover distinct structures with intriguing physical interpretations.

Circular motion Unruh effect and other Unruh-like effects

The original Unruh effect pertains to a uniformly, linearly accelerated observer, however it is possible to associate an "Unruh-like" effect with each class of stationary worldline in Minkowski spacetime. Among these classes of stationary worldlines, circular motion is of particular interest as it would be the easiest to implement in a laboratory setting and thus offers the most promising way of measuring the Unruh effect in an analogue system. In this talk, we explore how circular motion is geometrically related to other types of stationary motion in 3+1 Minkowski space, and to what extent these relationships are reflected in the experience of an Unruh-DeWitt detector.

Quantum Fields in Analogue Spacetime

At sufficiently low temperatures, ultra-cold atoms can behave as an effective relativistic quantum field. I will describe how this quantum field can be probed with lasers, and discuss an interpretation in terms of Unruh-DeWitt detectors. Restricting attention to flat analogue spacetime, experiments have been proposed to observe the Unruh effect and to harvest vacuum entanglement. I will provide updates on these proposals, with particular focus on quantum signatures in observable quantities.

Early universe phenomena in a fluid-dynamical EFT simulator

Past research from our collaboration has shown that the study of parametric instabilities on a liquid-liquid interface and their subsequent non-linear breakdown exhibit key features of preheating: the leading theory for the thermalisation of the early Universe. The scattering of large amplitude Faraday waves in the experimental setup causes the appearance of secondary instabilities. This phenomenon closely resembles the behavior of the cosmological scenario; the oscillating inflaton field efficiently transfers its energy to matter fields, which undergo parametric resonance. Initially displaying primary unstable modes, these matter fields then decay into secondary instabilities, thereby creating more particles and leading to the thermalisation of the Universe. Our current study aims to extend this work by reproducing the characteristic aspects of different inflationary scenarios. One approach to this end is our investigation of a two-fluid interface in a strong magnetic field as a simulator for inflation dynamics. Another subject of ongoing research is the far from equilibrium dynamics that occur in thermalisation. During this process, a regime of self-similar dynamics emerges which is universal across a wide variety of systems, including both preheating and the turbulent evolution of interfacial modes in our experiment. Our fluid mechanical system thus provides an excellent test ground for studying both the general phenomena and the specifics for preheating

Career Development Talk

In this talk David Hawker and Sally Hall will talk about the career opportunities in Nottingham, and will share important information for researchers who intend to apply for positions at the University of Nottingham.

Classical and quantum field theory simulators based on normal and quantum liquids

I will provide an overview of the research conducted at the University of Nottingham's Quantum Simulators for Fundamental Physics and the Black Hole Laboratory. Our primary focus is on investigating non-equilibrium quantum field theory processes in normal and quantum liquids, both theoretically and experimentally, with direct relevance to the early universe and black holes.

Draining vortex flows of superfluid 4He as gravity simulators

Several experimental efforts in the gravity simulator programme over the last decades have succeeded in witnessing elusive phenomena closely connected to predictions of black hole physics and quantum field theory on curved spacetimes, such as Hawking radiation and Penrose superradiance, in classical and quantum fluid systems. In this regard, superfluid helium-4 (He II) provides a suitable framework to recreate unique features of quantum field theory in curved spacetime while also shedding light on the complex phenomenology of quantised vortices. Here we present an experimental set-up that has been developed in Nottingham to create, control, and characterise the most extensive draining vortex in He II with the purpose of employing it as a macroscopic, finite temperature quantum field theory simulator. The set-up comprises a fully transparent glass cryostat and a bespoke recirculation system for the superfluid that allows for the creation of an almost purely irrotational draining vortex flow, currently probed at a steady temperature of 1.95 K. We show that such vortex flow can be employed as a simulator for quantum field theories in 2+1 dimensional spacetimes, where the dynamics of a scalar field around a black hole can be retrieved by looking at the interaction between surface waves and the centrally confined vortex. Exploiting full optical access into the experiment, we were able to make use of a minimally invasive technique to reconstruct the surface of our sample with micrometric sensitivity, and resolve the wave dynamics with high resolution in space and time.

Wave-vortex interactions in a superfluid helium gravity simulator

Gravity simulators offer the prospect of emulating quantum field dynamics on curved spacetime in tabletop experiments. Our research focuses on gravity simulations of curved spacetime scenarios such as rotating black holes on superfluid helium interfaces. Our experiment offers an alternative platform to two-dimensional ultra-cold atom systems, polaritons and other quantum fluids. Specifically, our set-up, based on a large draining vortex flow in superfluid helium, covers a distinct parameter space, resulting in a controlled, finite temperature quantum field theory simulator equipped with a simultaneous readout of micrometre fluctuations of the superfluid interface in time and space. We exploit intricate interactions between interface waves and the underlying vortex flow to show that our approach results in a compact, central vortex structure with significant circulation, overcoming current limitations in other systems. We detect bound states in some wave patterns, highlighting their spatial confinement due to the irrotational flow created by the central vortex. Additionally, we identify signatures of black hole ringdown in specific vortex configurations, opening avenues to study black hole phenomena using superfluid helium.

Multiplexed digital holography for fluid surface profilometry

Digital holography (DH) has been widely used for imaging and characterization of microstructures and nanostructures in materials science and biology and also has the potential to provide high-resolution, nondestructive measurement of fluid surfaces. DH setups capture the complex wavefronts of light scattered by an object or reflected from a surface, allowing the quantitative measurements of their shape and deformation. However, their use in fluid profilometry is scarce and has not been explored in much depth to the best of our knowledge. We present an alternative use for a DH setup that can measure and monitor the surface of fluid samples. Based on DH reflectometry, our modeling shows that multiple reflections from the sample and the reference interfere and generate multiple holograms of the sample, resulting in a multiplexed image of the wavefront. The individual interferograms can be isolated in the spatial frequency domain, and the fluid surface can be digitally reconstructed from them. We further show that this setup can be used to track changes in the surface of a fluid over time, such as during the formation and propagation of waves or the evaporation of surface layers.

Optical beam in circular motion on a liquid surface: probing the fluid's phase evolution by using Off-Axis Digital Holography

The Unruh effect predicts that a uniformly accelerated observer in vacuum perceives a thermal spectrum whose temperature is proportional to its acceleration. In a recent experimental proposal for the observation of an analogue of the Unruh effect, a laser beam in circular motion is used as an accelerated local detector of surface modes (i.e. height fluctuations) on a thin film of superfluid helium-4. Before operating on this medium, we firstly test the experimental setup for the generation of the laser detector beam in circular motion and its effect on a free surface of water, and we report the corresponding results. Here, the surface modes are probed by means of a Mach-Zender interferometric setup based on off-axis digital holography, which allows to spatially and temporally resolve the phase fluctuations of a light beam. By using this technique, we can reconstruct the sub-micrometric height fluctuations of the free surface under the influence of the laser beam in circular motion.

RQI Circuit Vienna - Opening Remarks

The RQI Circuit Vienna will start at 12:25pm CET, and will be broadcast in the ISRQI YouTube Channel. The local organizer is Anne-Catherine de la Hamette.

Quantum reference frames, indefinite causal order, Wigner and his friends

Our group’s research is based on the premise that physical phenomena must ultimately be described in terms of operationally well-defined concepts. We apply operational and information-theoretic tools to a range of questions: Can causal relations be indefinite in a quantum mechanical sense? How does physics “look like” from the perspective of a quantum particle? Is the nature of reality ultimately relational? I will give an overview of the state of knowledge on these topics.

Quantum Reference Frames for Lorentz Symmetry

Since their first introduction, Quantum Reference Frame (QRF) transformations have been extensively discussed, generalising the covariance of physical laws to the quantum domain. Despite important progress, a formulation of QRF transformations for Lorentz symmetry is still lacking. The present work aims to fill this gap. We first introduce a reformulation of relativistic quantum mechanics independent of any notion of preferred temporal slicing. Based on this, we define transformations that switch between the perspectives of different relativistic QRFs. We introduce a notion of “quantum Lorentz transformations” and “superposition of Lorentz boosts”, acting on the external degrees of freedom of a quantum particle. We analyse two effects, superposition of time dilations and superposition of length contractions, that arise only if the reference frames exhibit both relativistic and quantum-mechanical features. Finally, we discuss how the effects could be observed by measuring the wave-packet extensions from relativistic QRFs.

Different perspectives in (non)-causal quantum processes

The process matrix framework was developed to allow for scenarios with indefinite or quantum causal order, a phenomenon that is expected to be relevant for quantum gravity. A popular approach towards a quantum theory of gravity is the Page-Wootters formalism, which describes time-evolution of systems via correlations between a clock system and other quantum systems encoded in history states. We combined the process matrix framework with a generalization of the Page-Wootters formalism with multiple clocks. Each of these clocks can be thought of as corresponding to an agent and conditioning on a certain clock gives the respective agent’s perspective inside an a priori general quantum process. We implemented scenarios where different definite causal orders are coherently controlled and explain why certain non-causal processes might not be compatible within this framework.

The Möbius game and other Bell tests for relativity

We derive multiparty games that, if the winning chance exceeds a certain limit, prove the incompatibility of the parties’ causal relations with any partial order. This, in turn, means that the parties exert a back-action on the causal relations; the causal relations are dynamical. The games turn out to be representable by directed graphs, for instance by an orientation of the Möbius ladder. We discuss these games as device-independent tests of spacetime’s dynamical nature in general relativity. To do so, we design a relativistic setting where, in the Minkowski spacetime, the winning chance is bound to the limits. In contrast, we find otherwise tame processes with classical control of causal order that win the games deterministically. These suggest a violation of the bounds in gravitational implementations. We obtain these games by uncovering a “pairwise central symmetry” of the correlations in question and by reducing the problem to the acyclic subgraph problem, a known NP-complete linear combinatorial optimization problem. In addition, we derive multiparty games in a scenario where the polytope dimension grows only linearly in the number of parties. Here, exceeding the limits not only proves the dynamical nature of the causal relations, but also that the correlations are incompatible with any global causal order.

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Presentation of IQOQI Vienna

In this talk you will hear about the career opportunities in the Institute of Quantum Optics and Quantum Information, IQOQI.

How spacetime symmetries and the structure of quantum theory constrain each other

In the talk, I will describe two complementary but related research directions of our group: 1) the conceptual and mathematically rigorous treatment of quantum reference frames; and 2) insights into how spacetime and quantum physics mutually constrain each other. Topic 2 utilizes quantum foundations techniques like generalized probabilistic theories and the (semi-)device-independent approach, and it turns out that these tools enable us to obtain fascinating insights into the relation between space, time and probabilities. As examples, I will present a recent paper establishing a one-to-one correspondence between the amount of entanglement in the Page-Wootters mechanism and the amount of time-translation asymmetry that the imperfect clock generates (Phys. Rev. Lett. 129, 260402, 2022), and an earlier result relating the three-dimensionality of the qubit Bloch ball to relativity of simultaneity (Proc. R. Soc. A 473, 20170596, 2017). After this talk, Stefan Ludescher will present a result on “spacetime boxes”, namely a semi-device-independent randomness generation scheme whose security relies only on spatial symmetries and not on the validity of quantum theory (arXiv:2210.14811).

Theory-independent randomness generation with spacetime symmetries

We introduce a class of semi-device-independent protocols based on the breaking of spacetime symmetries. In particular, we characterise how the response of physical systems to spatial rotations constrains the probabilities of events that may be observed: in our setup, the set of quantum correlations arises from rotational symmetry without assuming quantum physics. On a practical level, our results allow for the generation of secure random numbers without trusting the devices or assuming quantum theory. On a fundamental level, we open a theory-agnostic framework for probing the interplay between probabilities of events (as prevalent in quantum mechanics) and the properties of spacetime (as prevalent in relativity).

Table Top Quantum Gravity: Some perspectives for the future

The topic of Table Top Quantum Gravity has occupied a large space in the scientific literature the past 5 years both on theory, experiment and philosophy/epistemology. I will give a quick overview of a selection of main developments in TTQG as well as some tangential research avenues partly inspired by and entangled with this topic.

Principle based approaches to the study of the quantum nature of the gravitational field.

In this talk I will outline the difference between a 'principles' approach to physics as opposed to a 'model based' approach. I will show how one can apply the principle based approach to the current discussions around table top tests of the quantum nature of the gravitational field, by using the framework of general probabilistic theories. Finally I will argue that principle based approaches are especially relevant to interdisciplinary research areas such as this one, and outline some open research questions.

When does relativistic locality imply subsystem locality?

Relativity and quantum information use different notions of locality. In relativity, locality is tied to spacetime regions while, in QI, locality is based on the notion of subsystems. What is the relation between these notions? In this talk we will investigate this question for a simple quantum field theory model and see how relativistic causality implies subsystem locality---approximately. We will then comment on whether we can expect this result to generalise to more realistic QFTs, and how it relates to no-go theorems about low-energy quantum gravity.

Aspects of QFT measurements

The development of a consistent and practicable local measurement theory for QFT is an exciting ongoing project that is currently being pursued by many different communities, from quantum information and mathematical physics to history and philosophy of QFT. Considerations of relativistic causality have played an important role in these developments. For example, as I will discuss, general morals can be drawn from the variety of recent responses to the "impossible measurements" problem, first raised by Sorkin. Then, I will focus on detector models in QFT, which can be viewed as useful tools for modeling local QFT measurements. In this context, I will mention work that is motivated by the following questions: are the signaling relations between detectors compatible with the spacetime notions of relativistic causality? which field observables are being recorded by certain detector models? Finally, I will discuss the advantages of solvable models (that work beyond perturbation theory) in interpreting the detector's response in the weak and strong coupling regime, as well as possible frictions with relativistic causality.

Unruh effect and Hawking radiation for detectors in quantum superposition of trajectories

Unruh effect and Hawking radiation are two of the most important predictions of Quantum Field Theory in curved space. As it is well known, these thermal radiations are perceived differently by different observers in a given spacetime. Such dependence can be analyzed by considering the excitation of a quantum particle detector coupled to the radiation field and following different trajectories. We present recent works in which we extend this sort of analysis to the case in which the detectors do not follow definite trajectories, but rather quantum superpositions thereof. More concretely, we consider a multilevel particle detector coupled to a massless real scalar field, and following a superposition of accelerated trajectories in a given Rindler wedge (for the Unruh effect), or of static trajectories outside a Schwarzschild black hole (for Hawking radiation). We find that, after the interaction with the field, the state of the detector is not in general a classical mixture of the excitations expected for each of the trajectories in superposition separately, but rather some coherences survive. These coherences are a result of the non-distinguishability of the different possible states in which the radiation field is left. Their dependence on the different excited energy levels and the different superposed trajectories can be associated physically to the characteristics of the absorbed particle of the field. The results are briefly discussed in the context of Quantum Reference Frames, and future extensions are considered.

Background Independence and Quantum Causal Structure

One of the key ways in which quantum mechanics differs from general relativity is that it requires a fixed background reference frame for spacetime. In fact, this appears to be one of the main conceptual obstacles to uniting the two theories. Additionally, a combination of the two theories is expected to yield `indefinite' causal structures. In this paper, we present a background-independent formulation of the process matrix formalism---a form of quantum mechanics that allows for indefinite causal structure---while retaining operationally well- defined measurement statistics. We do this by imposing that the probabilities arising in the formalism---which we ascribe to measurement outcomes across the points of a discrete spacetime---be invariant under permutations of spacetime points. We find (a) that one still obtains nontrivial, indefinite causal structures with background independence, (b) that we lose the idea of local operations in distinct laboratories, but can recover it by encoding a reference frame into the physical states of our system, and (c) that permutation invariance imposes surprising symmetry constraints that, although formally similar to a superselection rule, cannot be interpreted as such.

Relational Quantum Relativity

Maybe instead of quantizing relativistic field theories, which we argue to be problematic both conceptually and formally, we could try to relativize the quantum mechanical framework itself? I wish to advertise a research program constituting a direction in the general landscape of the so-called Quantum Reference Frames. The broad idea is to treat reference frames as both physical systems and quantum-mechanical objects, aiming to reformulate the quantum mechanical formalism to account for relational degrees of freedom. Our new approach is well-supported mathematically and is heavily influenced by quantum measurement theory, making it more operationally oriented than other similar attempts. In this talk, I will outline the guiding principles of our research, introduce the main elements of our proposed framework, update on our current progress and mention potential future research directions.

Non-Perturbative and Numerical Treatment of Detectors and Fields in Relativistic Scenarios

The mathematical treatment of the interaction between matter and light, especially in relativistic scenarios, is challenging. Even fundamental models, such as the Unruh-DeWitt detector model, present significant obstacles when seeking to treat exactly detector responses, communication scenarios, or entanglement extraction processes. In many cases, perturbation theory allows for analytic derivations of fascinating effects. They give rise to the question of what happens beyond perturbation theory? In other cases, even perturbative calculations require advanced numerics, for example, in the neighborhood of black holes. These challenges motivated some recent works which I will outline in my talk. In particular, I will focus on [1], in which we employ star-to-chain transformations to non-perturbatively and numerically exactly treat, for example, response and radiation emission in the Unruh effect. Furthermore, I review works on entanglement extraction and signaling in Schwarzschild spacetime [2,3], which were enabled by advanced numerical Green function methods. Time permitting, I will touch upon connections to recent and ongoing works concerning the entanglement structure of Gaussian states.

RQI Circuit Stockholm - Opening Remarks

The RQI Circuit Stockholm will start at 2pm CET and will be broadcast in the ISRQI YouTube Channel. The local organizers are Germain Tobar, Evan Gale, and Vasileios Fragkos.

Localisation problem in Unruh-DeWitt detectors with quantised centre of mass

The topic of particle localisation, namely how one defines a position operator or centre of mass in relativistic quantum mechanics, has been a longstanding question in the foundations of quantum mechanics since the mid-20th century. However, despite the length and breadth of study, the localisation problem is still not well understood. I examine the implications of particle localisation in the Unruh-DeWitt model, which provides a simple model of a two-level system (aka 'particle detector') coupled to a scalar quantum field. By comparing the first- and second-quantised formulations of a detector with a quantised centre of mass, one is naturally led to two distinct notions of localisation. I considerthe consequences of these two localisation schemes in the context of spontaneous emission, finding that the two localisations lead to distinguishable physical consequences, which can in principle be tested by future experiments.

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Hawking radiation for detectors in superposition of locations outside a black hole

Hawking radiation is the proposed thermal black-body radiation of quantum nature emitted from a black hole. One common way to give an account of Hawking radiation is to consider a detector that follows a static trajectory in the vicinity of a black hole and interacts with the quantum field of the radiation. In the present work, we study the Hawking radiation perceived by a detector that follows a quantum superposition of static trajectories in Schwarzschild spacetime, instead of a unique well-defined trajectory. We analyze the quantum state of the detector after the interaction with a massless real scalar field. We find that for certain trajectories and excitation levels, there are non-vanishing coherences in the final state of the detector. We then examine the dependence of these coherences on the trajectories followed by the detector and relate them to the distinguishability of the different possible states in which the field is left after the excitation of the detector. We interpret our results in terms of the spatial distribution and propagation of particles of the quantum field.

A simple model of a black hole and its radiation

Free quantum field theories on spheres can be used to model important aspects of black holes. I describe examples which were initially inspired by holography in Anti-De Sitter space and discuss some thermodynamics. I also sketch techniques to operationally test their behaviour by scattering, or by monitoring how they radiate.

Gravimetry through nonlinear optomechanics

The large mass of optomechanical systems make them ideal for coupling to and detect weak gravitational fields. In addition, the nonlinear dynamics of the systems offer interesting sensing advantages. In my talk, I will outline the research direction of deriving the fundamental sensing limits of these systems and consider some applications, including to searches of modified gravity theories.

A quantum optics approach to testing the GR-QM interface

In this talk I will give an overview of the work in our group, where we focus on the interface between general relativity and quantum theory at low energies. We study how quantum signatures of gravity can show in table-top experiments, novel phenomena that arise from the interplay of quantum theory and gravity, and a quantum optics approach to physics beyond the Standard Model.

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Composite quantum particles at the interface with relativity and gravity

A major goal of modern physics is to understand and test the regime where quantum mechanics and general relativity both play a role. I will discuss why looking at composite particles subject to relativistic effects opens new avenues for conceptual insights into the interface between quantum theory and gravity, for new experiments, and will likely be crucial for next-generation high-precision technologies.

Statistical arrow of time and clocks in the quantum regime: a quantum measurements' perspective

In this talk, I will briefly introduce methods to characterize the irreversibility of time-continuous and weak quantum measurements from a thermodynamic viewpoint. By defining a statistical arrow of time for individual realizations of the measurement process, I will show that measurements are absolutely irreversible, similar to the free expansion of a single gas particle in a box. I will present a cold-atom realization of this idea and conclude by discussing some examples where quantum measurement added noise can be rectified to produce useful work, aid quantum ground state cooling, and fuel the ticks of an autonomous quantum clock.

Detecting single gravitons with quantum sensing

The quantisation of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyse the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental evidence of the quantisation of gravity.

Gravity is a weirdo!

After more than a 100 yrs of General Relativity, we still argue how we should quantize gravity across all possible energy scales. In this talk, I will highlight some of the unexpected features of gravitational phenomena that might be responsible for the hardness of such a task. In particular, I will argue that quantum field theory might not be the correct framework to embed a non-perturbative theory of quantum gravity; I will also put to question the notion that we can talk about spatially-local subsystems when gravity is on, and finally argue that these hints towards an emergent nature of gravitational physics.

Career Development Talk

In this talk Magdalena Zych will talk about the career opportunities in Stockholm, and will share important information for researchers who intend to apply for positions in NORDITA and at the University of Stockholm. For more information visit the website https://indico.fysik.su.se/event/8398/page/624-career-development-opportunities

RQI Circuit Bremen - Opening Remarks

The RQI Circuit Bremen will start at 12pm CET, and will be broadcast in the ISRQI YouTube Channel. The local organizers are Dennis Rätzel, Ekim Hanimeli, Roy Barzel, Emanuel Schlake, and Marian Cepok.

Career Development Talk

In this talk Claus Laemmerzahl will talk about the career opportunities in Bremen, and will share important information for researchers who intend to apply for positions in the University of Bremen.

On the foundations of quantum mechanics

Each fundamental theory of physics has to be tested at best. Quantum mechanics is defined through a set of mainly mathematical postulates which phenomenological richness in terms of physical applications has so far been explored on a modest scale only. The fact that this theory is not based on a constructive approach makes is particularly difficult to introduce in a systematic way modifications which then can be subject to experiments. In this contribution we present and evaluate various modifications of quantum mechanics.

Gravity as free fall in graded geometry

Deformations of the algebra of quantum operators lead to a description of fundamental interactions that generalizes and in a sense unifies the principles of gauge theory and the geometric description of gravity as free fall in curved spacetime. This approach is already quite well established for electromagnetism and is for example useful for the description of charged particles in a magnetic monopole background. We shall show that also gravitational interactions find such an algebraic description, but the construction requires a graded (super) geometry setting. The construction suggests a novel somewhat more algebraic interpretation of key ingredients of general relativity. A practical motivation for this line of research is to make methods and results from gauge theory available for gravitational physics. Potential applications include Aharanov-Bohm type (geometric phase) effects for gravitational fields.

Break

General Linear Electrodynamics: Causal Structure, Propagators, Quantization and quantum energy inequalities

From an axiomatic point of view, the fundamental input for a theory of electrodynamics are Maxwell’s equations dF=0 (or F=dA) and dH=J, and a constitutive law H=# F, which relates the field strength 2-form F and the excitation 2-form H. In this talk we consider general linear Electrodynamics, the theory of Electrodynamics which is defined through a local and linear constitutive law. The best known application of this theory is the effective description of Electrodynamics inside (linear) media including for example birefringence. We will analyse the classical theory of the electromagnetic potential A thoroughly before we present the quantisation of premetric electrodynamics. The resulting theory of quantum premetric electrodynamics is a well defined relativistic quantum field theory, which is not locally Lorentz invariant. As a specific application of this theory of quantum premetric electrodynamics I will present the derivation of a quantum energy inequality, that is satsified by the energy density of the electromagnetic field averaged along observer wordlines inside a uniaxial crystal. The later is geometrically described by a constitutive law that depends not only on a metric, but in addition on two vector fields describing the crystal's optical axis and rest frame.

Coupling quantum matter to gravity: a systematic post-Newtonian approach

Modern quantum-optical experiments are close to reaching a precision that will allow for the observation of post-Newtonian effects of gravity in the lab. In recent years, several interesting proposals in this direction have been made; and even if the observation of gravitational effects is not the aim of an experiment, they still have to be taken into account. For such considerations, one needs a well-defined scheme according to which the coupling of quantum matter to the classical gravitational field is determined. Such a scheme needs to be complete (i.e. account for all terms to a given post-Newtonian order), systematic, and generally applicable (i.e. without a priori restrictions on the quantum states of matter). Of course, for full ‘relativistic’ gravity, such a scheme exists in the form of quantum field theory in curved spacetimes (QFTCS). If one is interested in post-Newtonian gravity, however, one can employ an easier approach that is mathematically and conceptually less heavy than full-blown QFTCS, based on systematic post-Newtonian expansions. In this talk, I will motivate this perspective and illustrate it by a brief discussion of two such systematic derivations of post-Newtonian descriptions of quantum systems under gravity: (a) a toy-model ‘atom’ (electromagnetically bound two-particle system) in a static weak gravitational background field, and (b) a massive spin-half particle in the vicinity of a slowly accelerating observer in a general weakly-curved spacetime.

Atom interferometers in weakly curved spacetimes using Bragg diffraction and Bloch oscillations

We present a systematic approach to determine all relativistic phases up to O(c^−2) in light-pulse atom interferometers in weakly curved spacetime that are based on elastic scattering, namely Bragg diffraction and Bloch oscillations. Our analysis is derived from first principles using the parameterized post-Newtonian (PPN) formalism. In the treatment developed here, we derive algebraic expressions for relativistic phases for arbitrary interferometer (IF) geometries in an automated manner. As case studies, we consider symmetric and antisymmetric Ramsey-Borde interferometers, as well as a symmetric double diffraction interferometer with baseline lengths of 10 m and 100 m. We compare our results to previous calculations conducted for a Mach-Zehnder interferometer.

Break

Cold atom interferometry with extended free fall time – recent progress and ideas

Over the last decade, cold atom interferometry has matured into a sensitive tool that can be applied both for practical applications as well as sensitive tests of fundamental physics. In particular, several long baseline instruments as well as microgravity experiments with access to extended free fall times push the achievable sensitivity into new regimes. This holds the promise to explore the interface of gravity and quantum physics in new and exciting ways. In my talk, I will first present some of our own work on interferometry with extended free fall times in the Bremen drop tower. I then want to discuss some of the recent ideas that have been brought forward to perform test experiments for gravity using interferometry on long baselines and with extended free fall.

Gravitationally induced entanglement dynamics

We investigate the effect of gravitationally induced entanglement dynamics -- the basis of a mechanism of universal decoherence -- for photonic states in a quantum field theoretical framework. We discuss the prospects of witnessing the effect by use of quantum memories and delay lines via Hong-Ou-Mandel interference. This represents a genuine quantum test of general relativity, combining a multi-particle effect predicted by the quantum theory of light and the general relativistic effect of gravitational time dilation.

Using optomechanical systems to test gravitational theory - possibilities and limitations

More than 100 years after the first development of a relativistic theory of gravity, there is an ever-increasing amount of predicted, yet untested, phenomena and unsolved scientific puzzles revolving around gravity. There are many proposals to apply quantum sensors to test for such phenomena or experimentally resolve some of the puzzles. In this talk, I will present my perspective on three proposals based on optomechanical systems: measurement of the gravitational field of light and relativistic particle beams, obtaining bounds on Chameleon-field dark energy models, and testing for quantum properties of the gravitational field. I will give a short introduction to the models involved and discuss fundamental constraints.

RQI Circuit Sydney - Opening Remarks

The RQI Circuit Sydney will start at 10:30am AEDT December 1st (note that the time difference shifts the day by one), and will be broadcast in the ISRQI YouTube Channel. The local organizers are Daniel Terno, Nicholas Funai, and Fil Simovic.

The Hawking temperature of dynamical black holes via Conformal transformations

Assumptions of finite-time formation of a trapped region and finiteness of curvature scalars on its boundary are sufficient to constrain a general spherically symmetric metric to correspond to two classes of solutions of the Einstein equations. Depending on the dynamic behavior of the horizon, each solution can describe an expanding white hole or an evaporating black hole. In the leading order approximation, evaporating black hole (or accreting white hole) solution takes the form of the ingoing (or outgoing) Vaidya metric. This suggests a universal description of the near-horizon geometry of evaporating black (or accreting white) holes in terms of the Vaidya metric. As the Vaidya metric captures many essential features of evaporating black hole spacetimes, we demonstrate that the linear Vaidya metric can be brought into manifestly conformally static form, allowing us to determine its Hawking temperature with respect to the conformal vacuum. Since back-reaction is implicitly accounted for, we conclude that slowly evaporating black holes are indeed accurately described by quasistatic sequences of Schwarzschild metrics even when dynamical effects are present.

Implications of singularity regularization in black hole thermodynamics

Regular black holes have become a popular alternative to the singular mathematical black holes predicted by general relativity as they circumvent mathematical pathologies associated with the singularity while preserving crucial black hole features such as the trapping of light. Here, we will analyze how to generate these geometries and study their thermodynamic properties within the framework of general relativity. Our study reveals that the regularization of the singularity, through the introduction of a minimal length scale, has a plethora of implications, one of which is the absence of a Hawking-Page phase transition. We extend our study to the dynamical case, showing that supplemental terms are required in the dynamical first law of black hole thermodynamics to maintain its essence. The additional terms manifest in the dynamical first law as work terms, while the notion of internal energy is captured by the Misner-Sharp mass. Furthermore, we explicitly demonstrate that the linear coefficient of the Misner-Sharp mass expansion near the outer apparent horizon suffices for a complete thermodynamic description.

Quantum-classical hybrid dynamics

In this talk we will describe the coupling of classical and quantum systems, and the consequences of different approaches to this problem.

The Euclidean path integral and its role in black hole physics

The Euclidean path integral plays a central role in both quantum field theory and quantum gravity, and enjoys wide application in relativistic quantum information and black hole physics. It is an especially powerful tool for studying properties of quantum fields on black hole backgrounds, providing a means of studying their thermodynamic properties at the level of the partition function. In this talk, I discuss some of the key insights gained through the Euclidean path integral. I describe how it can be used to compute entanglement entropy in black hole spacetimes, and how it is applied to the gravitational field itself. I further discuss how the Euclidean path integral can be used to generalize the laws of black hole thermodynamics to settings where the Hamiltonian description is unavailable, with a particular focus on cosmological backgrounds.

Career Development Talk

In this talk Nicolas Menicucci will describe the RQI research conducted in Australia, and give career advice for future researchers in the field interested in coming to Australia for their future research.

Quantum sensors as particle detectors

Theories beyond the celebrated Standard Model of particle physics indicate that new particle and forces are manifest themself through very feeble interactions with ordinary matter. The standard method of detecting such particles and forces (pioneered by Rutherford) is based on the resolution of small distance and time scales by depositing large amount of energy and momentum of colliding particles and accurately measuring the classical quantities such as deposited energy and momentum of the particles resulted in those collisions. This is how the large particle colliders, such as the Large Hadron Collider, operate and they have obvious limitations. I will argue that rapidly developing quantum technologies that are capable of accurate measurements of quantum properties of systems, such as entanglement and interference, provide with new, and in some cases, unique opportunities to detect beyond the Standard Model Physics. I will illustrate this point with the specific example of utilising ultra-sensitive trapped electron and ion devices to search for isolated magnetic charges (magnetic monopoles) in the regime that in inaccessible by currently employed experimental techniques.

Covariant bandlimitation consequences on UDW interactions and entanglement harvesting

In QFT UV cutoffs are often used as renormalisation or calculational tools, however as we know from condensed matter physics these cutoffs can have physical origins and relevance. The generalised uncertainty principles introduce the notion of a physical minimum length scale that should appear in QFTs, with the added requirement that any cutoff should be Lorentz covariant. The covariant cutoff introduced by Kempf and Pye acts on the spectrum of the d'Alembertian operator, whose main effects are on the Feynman propagator. Past work has focussed on the consequences of this cutoff on scattering amplitudes in phi-4 theory, however in this talk we shall be considering the effects of the covariant bandlimit on detector-field interactions, of which the simplest affected example is entanglement harvesting.

Shannon wavelets and Scaled QFT

The likely presence of a fundamental minimum length scale to the universe (motivated by generalised uncertainty principles and UV divergences in quantum field theory to name a few) has led to the application of information theoretic techniques such as bandlimitation to quantum field theory. For example; an ultraviolet cut-off to quantum field theory provides a natural minimum length scale and gives an isomorphism between continuous and discrete representations of a quantum field through Shannon's sampling theorem. A QFT discretised in such a way will still possess the translational symmetries and conserved Noether charges generally associated with fundamentally continuous systems. We extend on this notion by showing that non-bandlimited quantum field theories can be decomposed into bandlimited ones using Shannon wavelets. Each scale of the wavelet decomposition gives a field theory possessing an ultraviolet cut-off and, as a result, an equivalent discrete theory. As such, one can use wavelets to decompose an N+1 dimensional continuous field theory into a 2N+1 dimensional discrete theory (where the scale of the wavelet decomposition is treated as a spatial dimension). We show that for non-interacting quantum fields (and certain engineered interacting ones) the physics of the field at one scale is entirely isolated from that of other scales, meaning that no events at one scale can have any effect on the field at any other scale. For fields that can self-interact we find that despite non-zero couplings between the scales of the field, quantities such as the Feynman propagator between scales remain zero.

Scale-limited fields and the Casimir effect

We revisit the calculation of the Casimir effect in the case of scale-limited quantum fields. We use the continuous wavelet transform (CWT) to introduce a scale degree of freedom to the field and then restrict it to simulate either an observational or fundamental resolution limit. The Casimir force is derived in this setting for a free complex massless scalar field between two infinite plates with both Dirichlet and periodic boundary conditions. We find that the force is highly dependent on the characteristics of the wavelet used in the CWT and we discuss this dependence using several wavelets as examples. This is a joint work with Simon Vedl and Gavin Brennen.

TBA

RQI Circuit Online - Opening Remarks

The RQI Circuit Online will start at 8am GMT, and will be broadcast in the ISRQI YouTube Channel.

Quantum heat engine driven by quantum fields

In this study, we investigate a quantum heat engine employing an Unruh-DeWitt (UDW) detector immersed in a quantum field. We explore a generalized framework where the detector's trajectory is arbitrary, and the field is only required to be in a quasi-free state. From this general setup, we derive both the extracted work and the condition necessary for this work to be positive. We find that the previously known condition for positive work can be recovered as a special case when the detector thermalizes with the field.

Exploring the Unruh effect via Rindler horizon mechanics

In this presentation we will examine the Unruh effect present in the high energy channeling radiation experiments of CERN-NA63 via the use of Rindler horizon thermodynamics. By directly mapping the second law of thermodynamics to the measured particle spectrum, we find the data set to be a direct broadband measurement of the Fulling-Davies-Unruh temperature. This technique also enables the measurement of physical constants directly from the dataset.

On the feasibility of detecting quantum delocalization effects on gravitational redshift in optical clocks

We derive the predicted time dilation of delocalized atomic clocks in an optical lattice setup in the presence of a gravitational field to leading order in quantum relativistic corrections. We investigate exotic quantum states of motion whose gravitational time dilation is outside of the realm of classical general relativity, finding a regime where $^{24}\mathrm{Mg}$ optical lattice clocks currently in development would comfortably be able to detect this quantum effect (if the technical challenge of generating such states can be met). We provide a detailed experimental protocol and analyse the effects of noise on our predictions. We also show that the magnitude of our predicted quantum gravitational time dilation effect remains just out of detectable reach for the current generation of $^{87}\mathrm{Sr}$ optical lattice clocks. Our calculations agree with the predicted time dilation of classical general relativity when restricting to Gaussian states.

Entangled universes in dS wedge holography

"We develop a new setting in the framework of braneworld holography to describe a pair of coupled and entangled uniformly accelerated universes. The model consists of two branes embedded into AdS space capping off the UV and IR regions, giving rise to a notion of dS wedge holography. Specializing in a three-dimensional bulk, we show that dS JT gravity can emerge as an effective braneworld theory, provided that fluctuations transverse to the brane are included. We study the holographic entanglement entropy between the branes as well as the holographic complexity within the `complexity=anything' proposal. We reproduce a Page curve with respect to an observer collecting radiation on the UV brane, as long as we take the limit where gravity decouples in that universe, thus acting as a non-gravitating bath. The Page curve emerges due to momentum-space (UV/IR) entanglement and can be understood as analogous to the `confinement-deconfinement' transition in theories with a mass gap. Moreover, the analysis of complexity shows that the hyperfast growth phenomenon is displayed within a set of proposals, while late-time linear growth can be recovered for a different set. Our framework thus provides new test grounds for understanding quantum information concepts in dS space and dS holography."

Noninertial effects in quantum systems

"The essence of many of the phenomena at the interface of quantum theory and general relativity can be studied under the much simpler setting of quantum physics in noninertial reference frames. But, as their gravitational counterparts, the noninertial effects in quantum systems are usually feeble, requiring large accelerations to be observable in traditional settings. We shall address the question of how to isolate and optimally enhance noninertial effects in quantum systems so that they can be probed with current or near-future technology. We deploy theoretical techniques from quantum optics, cavity-QED, open quantum system dynamics, and the response of correlated quantum systems to tap into the latest experimental advances in quantum measurement techniques to determine the optimal setups in which weak effects such as the ones mentioned can be detected. We shall discuss quantum effects such as the geometric phase and radiative shifts in atomic spectra in noninertial setups. The geometric phase is an observable of interest due to its sensitive and accumulative nature. At the same time, radiative shifts hold interest due to intense experimental activity surrounding atomic spectroscopy and the resultant high-precision measurements of the spectral lines. We shall see that both the geometric phase and radiative shifts lend themselves to the detection of noninertial effects in laboratory settings."

How analogue gravity resolves theoretical physics problems

Information paradox is one of the biggest problem in theoretical physics. Island prescription through holography provides an ad hoc resolution to the problem. In this talk we show that how analogue gravity retrieve the island prescription in an non-ad hoc procedure. The talk is based on arXiv:2302.08742 [hep-th].

How models of Indefinite Causality can be understood within the Causaloid Framework

Towards the studies of Indefinite Causality from a post-quantum lens, a few frameworks have been proposed and studied. The Process Matrix Framework is one such framework that generalises quantum channels to process matrices. This framework has been vastly studied due to the direct applicability of known quantum information tools. Another framework, that chronologically preceded the process matrices is the Causaloid Framework which starts from a more general starting point and can be thought of as incorporating Generalised Probability Theories as well as Indefinite Causality. The Framework has been worked on more recently to be made more accessible. Further a hierarchy within the Causaloid Framework that is distinct from Sorkin's hierarchy, that depends on composition has been introduced. A natural question arises as to how the Process Matrix Formalism and the Causaloid Framework are related and how they can be understood through the introduced hierarchy. We answer this question by showing how the bipartite Process Matrices are a special instance in the Causaloid Framework. This work helps fill a theoretical gap in the literature and by understanding Indefinite Causality from the lens of Generalised Probability Theories helps us make attempts towards an axiomatic approach to Indefinite Causality. Based on N. Sakharwade, An Operational Road towards Understanding Causal Indefiniteness within Post-Quantum Theories, 2022 and ongoing work.

Relativistic quantum communication between harmonic oscillator detectors

We propose a model of communication employing two harmonic oscillator detectors interacting through a scalar field in a background Minkowski spacetime. In this way, the scalar field plays the role of a quantum channel, namely a Bosonic Gaussian channel. The classical and quantum capacities of the communication channel are found, assuming that the detectors' spatial dimensions are negligible compared to their distance. In particular, we study the evolution in time of the classical capacity after the detectors-field interaction is switched on for various detectors' frequencies and coupling strengths with the field. As a result, we find a finite value of these parameters optimizing the communication of classical messages. Instead, a reliable communication of quantum messages turns out to be always inhibited.

Entanglement area law violation from field-curvature coupling

We investigate the entanglement entropy of a massive scalar field nonminimally coupled to spacetime curvature, assuming a static, spherically symmetric background. We discretize the field Hamiltonian by introducing a lattice of spherical shells and imposing a cutoff in the radial direction. We then study the ground state of the field and quantify deviations from area law due to nonminimal coupling, focusing in particular on Schwarzschild-de Sitter and Hayward spacetimes, also discussing de Sitter spacetime as a limiting case. We show that large positive coupling constants can significantly alter the entropy scaling with respect to the boundary area, even in case of small field mass. Our outcomes are interpreted in view of black hole entropy production and early universe scenarios.

Spontaneous Ginzburg excitations of a `superluminal’ Unruh-DeWitt detector in a dispersive and dissipative medium

"An inertial Unruh-DeWitt detector moving through a medium with a constant refractive index n perceives the modes of a surrounding field to have Lorentz-boosted energies. If the detector speed v exceeds the phase velocity c/n of the medium, those Lorentz-boosted energies grow negative. Without violating energy conservation, the detector may hence get excited by creating a photon in a negative-energy mode via the Ginzburg effect [1]. For particle detectors that are realised by fast inertial atoms, reaching velocities v > c/n is experimentally very challenging. However, the phase velocities in realistic dielectrics are typically affected by dispersion and may be notably reduced close to the medium resonances. Since absorption plays a prominent role at those resonances, the effects of dissipation also need to be considered when assessing whether `slow’ field modes at a medium resonance can indeed excite inertial particle detectors. We present a consistent treatment of quantum fluctuations in a dispersive and dissipative dielectric [2] and discuss the experimental conditions under which the Ginzburg effect may be observable [3]. "

Quantifying Entanglement in Minkowski Spacetime and Unveiling the Source of Harvested Entanglement

" It is well known that entanglement is ubiquitous in quantum field theory: even the simplest states within the simplest field theories are highly entangled. The foundation of this statement rests on two results: (1) the Reeh-Schlieder theorem, which shows that all field variables in any one region of spacetime are entangled with variables in other regions, and (2) the calculations of entanglement entropy between a region and its complement, which show that entanglement between adjoining spacetime regions is not just large but UV divergent. In this talk, I will argue that these results do not provide much information about the entanglement between individual local degrees of freedom. I will then present a way of quantifying such entanglement, involving only a finite number of degrees of freedom, finite regions of space, and quantities that are directly measurable. Contrary to conventional beliefs, our analysis reveals that finding entangled pairs of modes supported in disjoint regions of space is challenging. This leads to a crucial question: where does the entanglement harvested by a pair of Unruh-DeWitt detectors originate? Rigorous examination exposes the source as multi-mode entanglement, surpassing traditional pair-wise entanglement structures. Our findings have profound implications for the entanglement harvesting protocol, advancing the current understanding of this approach. By unraveling the role of multi-mode entanglement, we deepen our understanding of quantum correlations and their applications in quantum information theory. "

RQI Circuit Online - Opening Remarks

The RQI Circuit Waterloo will start at 2pm GMT, and will be broadcast in the ISRQI YouTube Channel.

Thermal radiation from an accelerated electron

Thermal radiation from an accelerated electron is found. The calculation is entirely from a classical point of view linking thermodynamics and electrodynamics in the canonical example of moving point charge radiation. Despite being classical, the result is shown to have an immediate connection to quantum field theory. Observational confirmation is given by data collected during the free-neutron beta decay RDK II experiment.

Experimental study of superradiance in fluids of light

Rotational superradiance is a universal phenomenon of wave amplification. In the context of black holes, it is predicted to occur at the ergosurface. Two complementary and equally valid pictures of the effect may be drawn: a classical one whereby a wave impinging on the ergosurface would be amplified, and a quantum one whereby vacuum fluctuations at the ergosurface would yield the entangled emission of pairs. This scattering process occurs because frame dragging in the ergoregion allows for negative-norm waves to propagate, enabling the mixing of field operators at the ergosurface. The generation of entanglement is particularly interesting but impossible to study in astrophysics because of the interplay with the Hawking effect at the horizon. In this talk, I show how we use the flexibility of our fluid of light of microcavity polaritons to realise a rotating geometry with an ergosurface but without a horizon. Thus we can study superradiance without the Hawking effect. On the one hand we thus theoretically study entanglement in a controlled setup and on the other hand we experimentally measure the conditions for rotational superradiance, which we stimulate with a coherent state impinging on the ergosurface. This opens the way to the expeirmental study of entanglement at the ergosurface.

Quantum Coherence of a Uniformly Accelerated Detector

We investigate the impact of uniform acceleration on the quantum coherence of an Unruh-DeWitt detector. By considering the detector in an initial superposition state of a qubit, we observe a loss of coherence induced by the acceleration. Our findings reveal that the extent of coherence loss depends on the interaction time and effective coupling parameter. Notably, longer interaction times or stronger effective coupling result in more pronounced loss of coherence.

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Generalized entropy in quantum field theory and gravity

Two fundamental notions in quantum field theory and gravity are locality and generalized entropy. The latter has informed much of our insights on quantum black holes, but seems to apply in more general situations, while the former allows us to be operational about measurement. A first attempt at reconciling the two is to assign local algebras of observables to regions of spacetime. However, the natural entropy on such objects diverges and cannot be immediately identified with generalized entropy. One may then ask what the minimal extension of the algebra is so that its von Neumann entropy is the generalized entropy of the local subregion. The answer to this question involves incorporating the local dynamics of the algebra of observables, in the form of a crossed product. We comment on the constraints imposed on locality by gravity, namely diffeomorphism invariance, and connect with holography by proving that the bulk dual of a boundary subregion is its entanglement wedge.

Realizations of UdW detectors in tabletop experiments

"Entanglement preserving communication between qubits is essential for connecting quantum processors in a multiprocessor architecture. In this talk, I will describe several proposed experimental realizations of Unruh-DeWitt detectors in solid-state condensed matter settings that may achieve such entanglement-preserving communication[1]. The best candidate experiment involves qubits placed near one-dimensional conducting edge states of a topological material. Due to the topological protection, these one-dimensional “wires” can be tunably deformed in space and time, enabling them to approach or be repelled from the qubits. I will support this proposal with experimental considerations, numerical simulations, and quantum field theory arguments. I will also discuss other experimental proposals, some of which sacrifice coherent information but could take place cheaply within six months to a year. If entanglement preserving communication is observed, these experimental demonstrations will establish the field of experimental UdW detectors that may lead to the development of solid-state multiprocessor quantum computers. [1] E.W. Aspling, J.A. Marohn, M.J. Lawler, Design Constraints for Unruh-DeWitt Quantum Computers, unpublished. See https://arxiv.org/abs/2210.12552."

Applications of Relativistic Quantum Information Channels in Conformal Field Theories

High energy physicists have been using Unruh-DeWitt detectors in quantum information sciences to elevate the theory to a relativistic one. The field of Relativistic Quantum Information (RQI) is rapidly growing, but the connection to experiments is nontrivial. One solution to this problem may be to introduce the language of RQI to that of conformal field theories with specific applications to condensed matter systems. In this talk, I outline a pedagogical approach to the construction of the Unruh-DeWitt Quantum Computer (UDWQC), a system coupling spin-qubits to topological insulators modeled as an Unruh-DeWitt interaction, with an emphasis on the quantum capacities associated with the interaction blocks of the Tomonaga-Luttinger liquid. General properties of quantum Shannon theory can be extrapolated out of the calculated channel capacities allowing a connection between the fields of topological insulating phases and quantum information. Through this pedagogical approach, we aim to provide parallels into general conformal field theories that not only provide insights into this model but many others.

Horizons Decohere Quantum Superpositions

"We show that if a massive (or charged) body is put in a quantum superposition of spatially separated states in the vicinity of a black hole or cosmological horizon, the mere presence of the horizon will eventually destroy the coherence of the superposition. This occurs because, in effect, the long-range fields sourced by the superposition register on the horizon which forces the emission of entangling “soft gravitons/photons” through the horizon. This enables the horizon to harvest “which path” information about the superposition. We provide estimates of the decoherence time for such quantum superpositions in the presence of a black hole and cosmological horizon. Additionally, we show that this decoherence is distinct from—and larger than—the decoherence resulting from the presence of thermal radiation from the horizon. Finally, we further sharpen and generalize this mechanism by recasting the gedankenexperiment in the language of (approximate) quantum error correction. This yields a complementary picture where the decoherence is due to an “eavesdropper” (Eve) inside the black hole attempting to obtain “which path” information by measuring the long-range fields of the superposed body. We compute the quantum fidelity to determine the amount of information such an interior observer can obtain, and use the information-disturbance tradeoff to give a direct relationship between the Eve’s information and the decoherence of the superposition in the exterior. In particular, we show that the decoherence of the superposition corresponds to the “optimal” measurement performable in the black hole interior. We comment on how this phenomenon can be interpreted as a low-energy probe of the so-called central dogmas of black hole and cosmological horizons. Based on arXiv:2112.10798, arXiv:2205.06279, arXiv:2301.00026, and work to appear."

Can measurements change Hilbert space?

"We may view the construction of a quantum field theory in two parts: the specification of the field algebra, especially the CCR; and the construction of a representation of that algebra on a Hilbert space. The choice of representation is an essential physical input, and is usually determined by invariance (in special-relativistic cases) or adaptations of this (the Hadamard condition, for quantum fields in curved space-time). I will show here that there are apparently physically plausible circumstances in which infinitely many commuting measurements may be made, which would carry the state out of the original Hilbert space, that is, alter the representation. In some cases causality issues would restrict the consequences of this to only gradually become apparent on any one worldline, the evidence for it accumulating at later and later events. But I will also describe a situation in which it would seem that the effects could become known at individual, finite, events. The points here relate also to the questions of whether one ought to allow nonseparable Hilbert spaces, the energetic costs of making measurements, the information knowable by particular observers or in principle extractable from the state as a whole. "

Hands on with the positive formalism

The positive formalism is a universal operational framework for probabilistic predictions in classical, quantum and generalized probabilistic theories. Its different versions subsume the standard formulation of quantum theory and the process matrix formalism among others. After a short introduction and overview I focus attention on the paradigmatic example of predicting the bounce time in a simple model of black hole to white hole bounce.

Quantum strong cosmic censorship and black hole evaporation

The information loss puzzle (or paradox) arises from semiclassical arguments that suggest that in the process of black hole formation and subsequent evaporation an initial pure state can evolve into a final mixed state. This puzzle remains one of the most important open questions in fundamental physics. In this talk, I argue that a quantum version of strong cosmic censorship indicates that the semiclassical description of black hole evaporation breaks down at the final evaporation stage. I argue further that it should be expected that semiclassical gravity predicts a future singularity instead of a post-evaporation region where information is lost. Thus, one should not expect a failure of unitarity semiclassically or for any quantum gravity theory that can `cure' spacetime singularities. Precise conjectures for quantum strong cosmic censorship are provided, explained and justified, the hope being to promote them to theorems in the future. -- Based on arXiv:2305.01617.