# RQI-Online 2020/21

Date :
Jan. 27, 2021
Location :
Jan. 15, 2021, 11:59 p.m.
Online version of the yearly meeting of the Relativistic Quantum Information Society. The conference will happen on a weekly basis, every Wednesday at two sessions: the Waterloo and Australian sessions. The Waterloo sessions will happen at 9am EST (2pm UTC) on the following Wednesdays: 27 Jan, 3 Feb, 10 Feb, 17 Feb, 24 Feb, 3 Mar, 10 Mar, 17 Mar, 24 Mar, 31 Mar, 7 Apr, 14 Apr, 21 Apr, 28 Apr, ... The Australian sessions will happen at 7pm EST (10am AEST) on the following Wednesdays: 27 Jan, 3 Feb, 17 Feb, 24 Feb. (28 Jan, 4 Feb, 18 Feb, 25 Feb in AEST) The recordings of the sessions above can be found in the following playlist: https://www.youtube.com/playlist?list=PLeoh1MW56PeIWwyjeGRT6sDKfw3wCTvn5

## Programme

Jan 27,2021

### Does Decoherence Make Observations Classical?

Page, Don (University of Alberta)

The fact that we rarely directly observe much quantum uncertainty is often attributed to decoherence. However, decoherence does not reduce the quantum uncertainty in the full quantum state. Whether or not it reduces the quantum uncertainties in observations depends on the yet-unknown rules for getting observations (and their measures or probabilities') from the quantum state. Some schematic possibilities for what these rules might be in simple toy models are discussed.

### On the Heisenberg limit

Schuetzhold, Ralf (Helmholtz-Zentrum Dresden Rossendorf, TU Dresden)

For extremely weak signals (such as vacuum birefringence or gravitational waves), the Heisenberg limit is important for determining the achievable accuracy. Here, bounds on the Heisenberg limit in terms of the available energy of the probing system are discussed.

### Vacuum entanglement in the presence of gravitational waves

A remarkable fact about the vacuum state of a quantum field theory is that it is entangled across spacelike separated regions. This entanglement is strong enough to violate a Bell inequality and is ultimately responsible for some of the most important predictions of quantum field theory on curved space, such as the Unruh and Hawking effects. In this talk, I will introduce the entanglement harvesting protocol as an operational way to probe vacuum entanglement. This protocol relies on two atoms, modeled by Unruh-DeWitt detectors, that are initially un-entangled. These atoms then interact locally with the field and become entangled. Because the atoms do not interact with one another, any entanglement between the atoms is a result of entanglement that is 'harvested' from the field, and thus quantifying this entanglement serves as a proxy for how entangled the field is across the regions in which the atoms interacted. Using this protocol, I will show that while the local statistics of each atom are unaffected by the presence of a gravitational wave, the entanglement between them depends sensitively on both the amplitude and frequency of the gravitational wave. This suggests that the entanglement signature left by a gravitational wave may be useful in characterizing its properties.

### Effects of Horizons in Mirror Spacetimes on Entanglement Harvesting

Cong, Wan (University of Waterloo)

We study the effects of horizons on the entanglement harvested between two Unruh-DeWitt detectors through the use of moving mirrors in 1+1D with and without strict horizons. The horizon in this case is a null line through the spacetime beyond which wavepackets originating from left past null infinity fails to reach future left null infinity. Our study reveals the sensitivity of the entanglement harvested to the global dynamics of the trajectories.

Jan 28,2021

### Holographic teleportation in higher dimensions

Bak, Sang-Eon (Gwangju Institute of Science and Technology)

We study the higher-dimensional traversable wormholes in the context of Rindler-AdS/CFT. From the holographic principle's perspective, information transfer through traversable wormholes can be viewed as quantum teleportation. We show that traversable wormholes can be made by violating the average null energy condition (ANEC) in the higher-dimensional bulk via non-local boundary interaction. We find an analytic formula for the ANEC violation that generalizes Gao-Jafferis-Wall result to higher-dimensional cases, and we read off the ANEC by computing the two-point correlation function with the backreaction of shock waves. We discuss that the bound on information transferred through wormholes could depend on the dimensionality of the spacetime. Also, we argue that traversability is closely related to the scrambling effect that can be diagnosed by the butterfly velocity. [arXiv:2011.13807] I would like to appreciate it if I can get an opportunity to talk, whether it is the Waterloo session or the Australian session. However, I prefer the Waterloo session time.

### Spherically symmetric black holes in semiclassical and modified gravity

Murk, Sebastian (Macquarie University, Sydney)

Assuming only the existence of an apparent horizon and its regularity, we derive universal properties of the near-horizon geometry of spherically symmetric black holes. General relativity admits only two distinct classes of physical black holes, and both appear at different stages of the black hole formation. If semiclassical gravity is valid, then accretion after horizon formation inevitably leads to a firewall that violates the quantum energy inequalities. Consequently, physical black holes can only evaporate once a horizon has formed. Comparison of the required energy and time scales with the known semiclassical results suggests that the observed astrophysical black hole candidates are horizonless ultra-compact objects, and the presence of a horizon is associated with currently unknown physics. This has interesting implications for the information loss paradox. We describe how these results extend to modified theories of gravity and derive constraints that any self-consistent modified gravity theory must satisfy to be compatible with their existence. Note: these results are summarised in arXiv:2010.03784 and arXiv:2012.11209

MacLaurin, Colin (University of Queensland)

One toy model of Hawking radiation is a scalar field in Schwarzschild spacetime. I interpret the field's Noether current as the flow of probability for a single particle. Because this current is conserved, it allows a generally covariant treatment of Hawking radiation, which is internally consistent for arbitrary observers or foliations of spacetime. [Australian part of the conference preferred]

### Radiation from an inertial horizon

Good, Michael (Nazarbayev University)

Radiation is investigated from asymptotic zero acceleration motion where a horizon is formed and subsequently detected by an outside observer. A perfectly reflecting moving mirror (black hole analog) is used to model this system and compute the energy and spectrum. The trajectory is asymptotically inertial (zero proper acceleration)-ensuring negative energy flux (NEF), yet approaches light-speed with a null ray horizon at a finite advanced time. We compute the spectrum and energy analytically. The results signal the potential existence of a geometry that, like a black hole, has an event horizon that radiates energetic particles to observers at infinity, however at late-times, the horizon of this system is shining with negative energy Hawking radiation.

Feb 03,2021

### Charged scalar field with quantum unitary dynamics and Schwinger effect

We analyze the canonical quantization of a scalar field in the presence of an external electromagnetic classical field in a flat background. Given the Bogoliubov transformation that codifies the field dynamics, we show that the criterion of demanding its implementation as a unitary operator in Fock space serves to select a unique equivalence class of unitarily equivalent Fock representations. Each of these quantizations admit a particle number density defined at all times in the evolution, and with the correct asymptotic behavior when the electric field vanishes. Although we perform the field quantization in a specific gauge, we also show the equivalence between the procedures taken in different gauges.

### Casimir effect in conformally flat spacetimes

Dębski, Kacper (University of Warsaw)

We discuss several approaches to determine the Casimir force in inertial frames of reference in different dimensions. On an example of a simple model involving mirrors in Rindler spacetime we show that Casimir’s and Lifschitz’s methods are inequivalent and only latter can be generalized to other spacetime geometries. For conformally coupled fields we derive the Casimir force in conformally flat spacetimes utilizing an anomaly and provide explicit examples in the Friedmann–Lemaître–Robertson–Walker (k=0) models.

### Quantum simulating relativistic quantum field theories

Schmiedmayer, Joerg (Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien)

Jörg Schmiedmayer Vienna Center for Quantum Science and Technology (VCQ), Atominstitut, TU-Wien Ultra-cold atoms and quantum degenerate matter are an ideal system to quantum simulate relativistic quantum fields. In this talk I will give an overview of the basic concepts to build such quantum simulators where the zero-temperature condensed phase corresponds to the ‘vacuum’ on top of which one can experiment with the excitations that can be massless (linear dispersion relation in the Luttinger Liquid) [1] or have a mass as in the Sine-Gordon quantum field [2,3]. Besides giving theoretical arguments illustrating how far these simulators can go [4] I will concentrate on the experimental demonstrations of relativistic quantum fields and their physics. This will include among others the light cone like propagation of information [5], the creation of quantum noise when cutting a quantum field [6], squeezed modes [7] and the verification of a sin Gordon quantum simulator [3]. Work performed in collaboration with the groups of E. Demler (Harvard), Th. Gasenzer und J. Berges (Heidelberg) and J. Eisert (Berlin). Supported by the Wittgenstein Prize, the FWF SFB FoQuS, DFG-FWF: SFB ISOQUANT: and the EU: ERC-AdG QuantumRelax [1] T. Giamarchi and O. U. Press, Quantum Physics in One Dimension, International Series of Monographs on Physics (Clarendon Press, Oxford, UK, 2004). [2] V. Gritsev et al. Phys. Rev. B 75, 174511 (2007) . [3] T. Schweigler et al. Nature 545, 323 (2017). [4] T. Kitagawa et al. Phys. Rev. Lett. 104, 255302 (2010); K. Agarwal, et al. Phys. Rev. Lett. 113, 190401 (2014),;K. Agarwal, et al. Phys. Rev. B 95, 195157 (2017); M. Michel et al. Phys. Rev. A 99, 053615 (2019). [5] T. Langen et al., Nature Physics, 9, 640 (2013). [6] M. Gring et al., Science, 337, 1318 (2012). [7] T. Langen et al., Science 348 207-211 (2015).

### Extracting quantum field theory descriptions from experimental data

Erne, Sebastian (Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien)

Analogue quantum simulators present an ideal platform to study fundamental questions of quantum field theory. Therein, only a small subset of the microscopic details of the underlying quantum device affect the simulation outcome for physical quantities of interest, based on the renormalization program of QFT. This fact raises the important question of how to determine from experimental data the relevant information content of simulated QFTs. In this talk I will present results [1] on how the irreducible building blocks of the quantum field theoretical descriptions of a quantum many-body system can be extracted directly from experimental data. Introducing the general framework based on equal-time one-particle irreducible (1PI) correlation functions I will thereafter concentrate on the sine-Gordon model, quantum simulated experimentally through two coupled superfluid quantum wires. [1] T. V. Zache, T. Schweigler, S. Erne, J. Schmiedmayer, J. Berges, Phys. Rev. X 10, 011020 (2020)

Feb 04,2021

### Effect of environment on the interferometry of clocks

Verma, Harshit (University of Queensland)

Mach-Zehnder interferometry involving spatial superposition of a massive particle with its internal degrees of freedom (DOF) modelled as clocks had been previously proposed as an experiment encapsulating genuine general relativistic effects on quantum systems. We have analysed a realistic model of the clocks in which they are subject to the effects of environment leading to noise during their transit through the arms of the interferometer. By modelling the environment as quantum DOF interacting with the clock, we have shown that interferometric visibility is affected by the noise. We have shown that the visibility can increase or decrease depending on the type of noise and also the time scale and transition probabilities in the noise models. The quantification of the aforementioned effect on the visibility paves the way for a better estimate on the expected outcome of an experiment wherein the clocks cannot be effectively shielded from the noise.

### Entanglement and decoherence due to gravity

Miki, Daisuke (Department of Physics, Kyushu University)

The unification of quantum mechanics and general relativity is one of the most fundamental problems. Many theories have been proposed, but there are almost no experimental studies on quantum gravity. However, by the development of quantum technology, some models to test the quantum property of gravity have been proposed recently. We show the dynamics of the gravity-induced entanglement and decoherence in a multi-particle system. We consider the N-particle system on a straight line and each particle is initially in a superposition of two spatially localized states. We analyze the entanglement between a specific pair of particles for the multi-particle system. The entanglement in a specific pair may be generated at earlier times, but the entanglement tends to disappear through other particles due to gravity. We discuss the characteristic time of the decoherence. I have a preference to talk on the Australian sessions.

### Gravity-induced entanglement in optomechanical systems

Matsumura, Akira (Department of Physics, Kyushu University)

We investigate the phenomenon of gravity-induced entanglement in optomechanical systems. Assuming photon number conservation and the Newtonian potential expanded up to the quadratic order of the oscillator positions, we exactly solve the dynamics of the optomehcanical systems. Then, we find that the phase difference due to the Newtonian gravity leads to the large entanglement of photons in separated cavities. We clarify the generating mechanism of large gravity-induced entanglements in optomechanical systems in an exact manner. We also determine the characteristic time to generate the maximal entanglement of photons. Finally, by comparing the characteristic time with the decoherence time due to photon leakage, we evaluate the range of the dissipation rate required for testing the gravity-induced entanglement. Australian sessions

Feb 10,2021

### Local Measurement of quantum fields in curved spacetimes

Fewster, Christopher (University of York)

A standard account of the measurement chain in quantum mechanics involves a probe (itself a quantum system) coupled temporarily to the system of interest. Once the coupling is removed, the probe is measured and the results are interpreted as the measurement of a system observable. Measurement schemes of this type have been studied extensively in Quantum Measurement Theory, but they are rarely discussed in the context of quantum fields and still less on curved spacetimes. In this talk I will give a brief over view of how such measurement schemes can be implemented for quantum fields on curved spacetime within the general setting of algebraic QFT. In particular, I will show how measurements of probe observables correspond to measurements of "induced" local system observables, describe the role of state update rules, and how the framework can be used to shed light on an old problem due to Sorkin concerning "impossible measurements" in which measurement apparently conflicts with causality. Finally, I will discuss new results indicating that the system observables that can be induced by measurement schemes form a large subspace of the algebra of all local observables. The talk is based on work with Rainer Verch [Leipzig], (Comm. Math. Phys. 378, 851–889(2020), arXiv:1810.06512; see also arXiv:1904.06944 for a summary), with Henning Bostelmann and Maximilian Ruep [York] ( Phys. Rev. D 103 (2021) 025017; arXiv:2003.04660) and work in progress with Ian Jubb [Dublin] and Maximilian Ruep.

### Relativistic causality in particle detector models: Faster-than-light signalling and Impossible measurements''

de Ramon, Jose (University of Waterloo)

We consider violations of causality in Unruh DeWitt-type detector models in relativistic quantum information. We proceed by first studying the relation between faster-than-light signaling and the causal factorization of the dynamics for multiple detector-field interactions. We show that non-relativistic detector models predict superluminal propagation of the field's initial conditions. We draw parallels between this characteristic of detector models, stemming from their non-relativistic dynamics, and the so-called Sorkin's impossible measurements in QFT. Based on these features, we discuss the validity of measurements in QFT modeled with non-relativistic detectors.

### Weakly coupled local particle detectors cannot harvest entanglement

Ruep, Maximilian Heinz (University of York)

Entanglement harvesting is the process in which two particle detectors, initially in an uncorrelated product state, (e.g. two Unruh detectors in their respective Gaussian pure ground states) are coupled to a linear real scalar quantum field in two spacelike separated regions and end up in an entangled final state. In this talk I will present an attempt to avoid unphysical singular or non-local coupling and introduce the notion of local particle detectors, given by a local Bosonic mode of a scalar probe field (a proxy for a truly local measurement device based on the measurement theory of Fewster and Verch [CMP. 378, 851–889 (2020)]). I will show that local particle detectors cannot harvest entanglement below a critical coupling strength when the corresponding probe fields are initially prepared in quasi-free Reeh-Schlieder states. This is a consequence of the fact that such states restrict to truly mixed states on any local Bosonic mode. I would like to apply for a contributed talk in the Waterloo session.

### Entanglement and other gravity-induced effects from quantum matter: a first-principles analysis

Anastopoulos, Charis (University of Patras)

We analyze the weak-field limit of Quantum Geometrodynamics. This provides an effective quantum theory for describing the interactions of matter through weak gravity, that is fully consistent with both quantum field theory and General Relativity. This theory fully accounts for recently proposed experiments on the generation of (Newtonian) gravitational forces from quantum distributions of matter, and phenomena like gravity induced entanglement, gravitational cat states and gravity-induced Rabi oscillations. Our main results include: (i) the demonstration that these phenomena do not involve true gravitational degrees of freedom, (ii) the definition of operators for the spacetime metric, that allows us to analyze fundamental issues like light-cone fluctuations, (iii) the conflict between the symmetry of general covariance and the usual quantization procedure persists even at the weak gravity limit, and could have experimental consequences, and (iv) gravity is fully described by constraints that define instantaneous laws even in the Newtonian regime. This fact requires a significant upgrade of usual quantum notions of locality (and hence of entanglement) in order to deal with gravitational effects at a fundamental level.

Feb 17,2021

### A new type of particle detector

Unruh, William (The University of British Columbia)

In 1976 I pointed out that a two level system coupled to a quantum field could act like a particle detector, and, if accelerated through the vacuum, would react as though surrounded by a bath of particles. In designing a test of this prediction in a BEC analog system, we came up with a new type of detector, in which a laser set up to act locally with the quantum field, could transform particles in that field into photons in the laser beam. Using this idea, we have proposed an experiment to see the phonons in the phonon state of the BEC, and to detect the phonons expected due to circular acceleration (Bell Leinaas) through the phonon-vacuum of the BEC. By setting up an interferometer in frequency, rather than physical, space for the laser one should be able to detect the thermal bath due to the accelerated trajectory of interaction region between the laser and the BEC.

### Analogue Unruh Detectors

Gooding, Cisco (University of Nottingham)

It has been over four decades since the discovery that accelerated observers perceive the vacuum as a thermal state. This result - known as the Unruh effect - is both central to our understanding of relativistic quantum fields and notoriously difficult to verify experimentally. In this talk I describe how one can significantly enhance the observability of the circular Unruh effect with an analogue system, using either a 2D Bose-Einstein condensate or a thin-film superfluid. I will then discuss open challenges involved with extracting quantum signatures from measurements made on the analogue detectors.

### Rotational superradiance in Bose-Einstein condensates

Patrick, Sam (University of Nottingham)

Rotational superradiance is a process in which waves are amplified through scattering with a rotating system. This phenomenon is well understood when the waves are non-dispersive, which is the case for example when waves scatter with a black hole. But what happens in systems where the waves are dispersive? In this talk, I will describe a set of techniques which can be used to study superradiance in dispersive media, focussing on the example of a draining vortex in a Bose-Einstein condensate.

### Decay of quantum sensitivity due to three-body loss in Bose-Einstein condensates

Rätzel, Dennis (Humboldt Universität zu Berlin)

In view of the coherent properties of a large number of atoms, Bose-Einstein Condensates (BECs) have a high potential for sensing applications. Several proposals have been put forward to use collective excitations such as phonons in BECs for quantum enhanced sensing in quantum metrology. However, the associated highly non-classical states tend to be very vulnerable to decoherence. In this article, we investigate the effect of decoherence due to the omnipresent process of three-body loss in BECs. We find strong restrictions for a wide range of parameters and we discuss possibilities to limit these restrictions. Rätzel D. and Schützhold R., arXiv:2101.05312

Feb 18,2021

### Adiabatic vacua from linear complex structures

Hackl, Lucas (University of Melbourne)

Adiabatic vacua play an important role as initial quantum states of quantum fields on a dynamical spacetime. As Gaussian states, their quantum information properties are well understood, so they can be naturally used as resource relativistic quantum information. Adiabaticity requires that the vacuum evolves slowly under the background dynamics and the standard method requires a WKB approximation to find the vacuum order by order. In this talk, I present an alternative approach that utilizes the concept of a linear complex structure to label field theory vacua and allows one to find the adiabatic vacua from a simple recursion relation. I will do this explicitly for FLRW spacetimes and comment on its relation to the adiabatically renormalized energy-momentum tensor.

### Gaussian states don’t propagate semi-classically in spacetime

Wood, Carolyn (The University of Queensland)

As experimental quantum technologies increase in complexity and precision, the loss of spatial coherence is a problem which will only increase. The same complexity and precision, however, is bringing us closer to exciting tests of fundamental physics. Composite particles such as atoms and molecules are promising tools for these future experiments. However, in all theoretical studies of such particles, modelled as freely-propagating Gaussian wave packets, they delocalise into separate internal mass-energy components, and their spatial coherence is destroyed. Besides calling into question the suitability of composite particles as experimental tools, this behaviour is contrary to even our most naive understanding of atoms and molecules as cohesive entities in the ‘real’ world, where we expect them to have well-localised spacetime trajectories. In this work, we identify the optimal way to prepare composite particles to fully avoid the delocalization and related loss of spatial coherence. We find the correct theoretical approach required to discuss limitations on the space-time trajectories of composite quantum particles --- it requires a new uncertainty principle which includes mass as an operator. We show that the quantum states which minimise the inequality propagate coherently in spacetime, and transform covariantly under boosts. This result highlights the fundamental differences between phase and configuration space for composite particles, reconciling theory with our naïve view, while the new minimum uncertainty states will find applications in upcoming precision experimental tests. Preference to talk at Australian sessions, but Waterloo times are also workable.

### Building block approach to asymptotic symmetries in general relativity

Yamaguchi, Koji (Department of Physics, Tohoku University)

Asymptotic symmetries of black hole spacetimes have received much attention as a possible origin of the Bekenstein-Hawking entropy in black hole thermodynamics. In general, it takes heavy efforts to find appropriate asymptotic conditions on a metric and a Lie algebra generating the transformation of symmetries with which the corresponding charges are integrable. We here propose a powerful approach to construct building blocks of asymptotic symmetries of a given spacetime metric. Our algorithmic approach makes it easier to explore asymptotic symmetries in any spacetime than in conventional approaches. As an explicit application, we analyze the asymptotic symmetries on Rindler horizon. We find a new class of symmetries related with dilatation transformations perpendicular to the horizon, which we term superdilatations. This talk will be based on arXiv:2012.14050 [gr-qc]. ******** I want to give a talk at the Australian sessions. Since I have a PhD defense on 4 Feb, I want to give a talk on 17 Feb or 24 Feb.

### Quantum speed limit time in relativistic frame

Ali Khan, Niaz (Department of Physics, HITEC university)

We investigate the roles of the relativistic effect on the speed of evolution of a quantum system in a purely amplitude damping channels. We find that the relativistic effect speed-up the quantum evolution to a uniform evolution speed of an open quantum system for the damping parameter $p_{\tau}\lesssim p_{\tau_{c0}}.$ Moreover, we point out a non-monotonic behavior of the quantum speed limit time (QSLT) with acceleration in the damping limit $p_{\tau_{c0}}\lesssim p_{\tau}\lesssim p_{\tau_{c1}},$ where the relativistic effect first speed-up and then slow down the quantum evolution of the damped system. As a consequence, the QSLT decays and revives with increasing acceleration. For the damping limit $p_{\tau_{c1}}\lesssim p_{\tau}$, we notice a monotonic increasing behavior of QSLT, leads to a slow down the quantum evolution of the damped system. In addition, we examine the roles of the relativistic effects on the speed limit time for the system coupled with the phase damping channels.

Feb 24,2021

### Positivity and compose-ability in a path integral framework for quantum theory

Dowker, Fay (Imperial College London)

The path integral provides the basis for a framework for the foundations of quantum theory and it is a work in progress to establish the axioms of this framework. I will examine the positivity axiom.’’ Boes and Navascues (BN) proved that the set of all *strongly positive* systems—which I will explain---is maximal among sets of systems that are closed under composition. That means no other system can be added to the strongly positive set without spoiling closure under composition. I will show that the strongly positive set is the unique set satisfying this condition---maximal among sets of systems that are closed under composition—which is a good argument to adopt the strong positivity axiom for path integral based quantum theories, including quantum gravity.

### Causality violations in cavity quantum fields: the rotating wave approximation

Funai, Nicholas (University of Waterloo)

The rotating wave approximation is often used to simplify light-matter interactions from optomechanical systems to quantum optics; making the results analytic or at least numerically manageable. Here I will be discussing the rotating wave approximation, the causality violations it introduces, as well as when and where it is reasonable to use. I shall also be introducing a mathematical trick I developed that simplifies infinite sums, permitting the numerical evaluation of non RWA calculations on 3+1 D cavities; a growing necessity given continuing improvements in experimental techniques.

### What makes a detector click: interaction with Fock wavepackets

Tjoa, Erickson (University of Waterloo)

We highlight fundamental differences in the models of light-matter interaction between the behaviour of Fock state detection in free space versus in a very large optical cavity. To do so, we study the phenomenon of resonance of detectors with Fock wavepackets as a function of their degree of monochromaticity, the number of spatial dimensions, the linear or quadratic nature of the light-matter coupling, and the presence (or not) of cavity walls in space. In doing so we show that intuition coming from quantum optics in cavities does not straightforwardly carry to the free-space case. For example, in (3+1)-dimensions the detector response to a Fock wavepacket will go to zero as the wavepacket is made more and more monochromatic and in coincidence with the detector’s resonant frequency. This is so even though the energy of the free-space wavepacket goes to the expected finite value of ħΩ in the monochromatic limit. This is in contrast to the behaviour of the light-matter interaction in a cavity (even a large one) where the probability of absorbing a Fock state is maximized if the quantum is more monochromatic at the detector’s resonance frequency. We trace this crucial difference to the fact that monochromatic Fock states are not normalizable in the continuum and physical Fock states need to be constructed out of normalizable wavepackets.

### Assisted harvesting of quantum superpositions from a coherent scalar field

Kollas, Nikolaos (University of Patras)

Recently it has been demonstrated that it is possible to harvest quantum resources other than entanglement from a quantum field. By coupling a massless scalar field in a coherent state to an Unruh-Dewitt detector we study the amount of quantum coherence that can be harvested and its dependence on various parameters such as the dimension of the underlying spacetime, the size and the frequency of the detector, the mean energy per field quanta and the mean interaction time between the two. For an inertial extended detector it is shown that this amount is maximised for a sudden interaction. Nonetheless by changing the phase of the coherent amplitude distribution of the field it is possible to attain a maximum value for discrete interaction times. We observe that when the mean energy of a single quantum of the field is comparable to that of the detector it is possible to extract more coherence if it is static, while for larger or smaller energies this situation, which affects its rate of decoherence, is reversed.

Feb 25,2021

### Super(posed) Unruh-deWitt detectors

Foo, Joshua (The University of Queensland)

We extend the well-known UdW detector framework to include detectors traveling in arbitrary superpositions of trajectories. Such detectors can gain information about the quantum field that is otherwise inaccessible to a single system. In particular, the state of the detector includes non-local field correlations between the different trajectories that produces interference effects in the detector observables. We apply our model to relativistic (superpositions of Rindler trajectories, Phys. Rev. D 102, 085013) and curved spacetime (de Sitter universe, arXiv:2005.03914) settings, showing that these interference effects are traced back to the causal relations between the superposed trajectories. The interference also allows such detectors to discern the global properties of the background spacetime when it would be otherwise impossible for detector on classical trajectories. Promisingly, our model can be applied to numerous foundational questions, such as a phenomenological description of spacetime in a superposition of curvatures (see arXiv:2012.10025) and the amplification of entanglement harvesting due to spatiotemporal delocalisation (see arXiv:2101.01912).

### Vacuum entanglement harvesting with delocalized matter

Baccetti, Valentina (Royal Melbourne Institute of Technology)

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.

### Unruh De-Witt-detector model to the electro-optic sampling of the electromagnetic vacuum

Onoe, Sho (The University of Queensland, University of Queensland)

In this work, a new theoretical framework to describe the novel experimental advances in electro-optic detection of broadband quantum states, specifically the quantum vacuum, is devised. By making use of fundamental concepts from quantum field theory in curved space-time, the nonlinear interaction behind the electro-optic effect can be reformulated in terms of an Unruh-De-Witt detector coupled to a scalar bosonic field during a very short time interval. When the duration of this coupling achieves a regime in which its functional form has an envelope of similar extension to its oscillations (i.e. a subcycle regime), virtual field particles inhabiting the field vacuum are transferred to the detector in the form of real excitations. We demonstrate that this behavior can be translated to the scenario of electro-optic sampling of the quantum vacuum, in which the detected field mode (the probe) works as an Unruh-De-Witt detector, with part of its interaction-generated photons arising from virtual particles inhabiting the electro-magnetic vacuum. We discuss the consequences and working regimes of such process with the aid of the characterization of the main features of the quantum light involved in the detection.

### Quantum simulations of holographic duality

Brennen, Gavin (Macquarie University, Sydney)

We propose a way to demonstrate observable features of holographic duality based on the concept of lifted tensor networks. Specifically, we describe how to lift the MERA (multiscale entanglement renormalisation ansatz) tensor network representation of the ground state of a 1D quantum critical system, effectively described by a CFT (conformal field theory), to a 2D bulk quantum state. Such a construction can be realized using a logarithmic, in the size of the boundary, depth quantum circuit. The key innovation is the introduction of a basis independent lifting procedure for the MERA which introduces physical degrees of freedom on the network bonds leading to a tighter bulk/boundary correspondence. We show how the resulting 2D bulk state exhibits holographic screens, i.e. horizon like regions, and also derive a formula analogous to the quantum-corrected Ryu-Takayanagi formula of AdS/CFT whereas prior work only showed the classical correspondence. The latter quantity is verified numerically for a family of unitary minimal model CFTs. Our basis independent lifting procedure provides a prescription to map between operators in the bulk and along the boundary, similar to that observed in the holographic correspondence. We also prove that in order for a global on-site symmetry on the boundary theory to be present at any renormalization scale, the tensor network representation of it must consist of locally symmetry tensors. This leads to bulk gauging of the symmetry in the lifted network, a feature that is also found in gauge/gravity duality.

Mar 03,2021

### Quantum Time Dilation

Smith, Alexander (Saint Anselm College and Dartmouth College)

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 theory of quantum dynamics. Using covariant time observables, I will demonstrate the equivalence of two previously thought to be distinct approaches to relational quantum dynamics: the evolving constants of motion program and the Page-Wootters formalism. This analysis allows for the computation of the probability that a quantum clock reads a particular time conditioned on another quantum clock reading a different time. From this conditional probability distribution, I will demonstrate a novel quantum time dilation effect that occurs between two clocks when one moves in a superposition of relativistic momenta. Using the lifetime of a hydrogen-like atom as a clock, I will argue that this quantum time dilation effect may be observable in present-day spectroscopy experiments, thus offering a new test of relativistic quantum mechanics.

### Einstein's Equivalence principle in the presence of superpositions of spacetimes

Giacomini, Flaminia (Perimeter Institute for Theoretical Physics)

The Principle of Equivalence, stating that all laws of physics should take their special-relativistic form in any and every local inertial frame, lies at the core of general relativity. Because of its fundamental status, this principle could be a very powerful guide in formulating physical laws at regimes where both gravitational and quantum effects are relevant. However, it is unclear whether the Principle of Equivalence will continue to hold when quantum systems, which can be in a superposition or entangled with other physical systems, are taken as reference frames. Here, we tackle this question by introducing a relational formalism to describe quantum systems in a superposition of curved spacetimes. We build a unitary transformation to the quantum reference frame associated to a quantum system in curved spacetime, and in a superposition of curved spacetimes. In both cases, in such quantum reference frame the metric looks locally flat, so that one cannot distinguish, with a local measurement, whether the spacetime is flat or curved, and whether it is in a classical configuration or not. This transformation identifies a Quantum Local Inertial Frame (QLIF), which thus extends the Principle of Equivalence to quantum reference frames in a superposition of gravitational fields.

### Violations of the Quantum Superposition Principle in the Entropic Dynamics of Quantum Fields Coupled to Gravity

Ipek, Selman (University at Albany-SUNY)

When dealing with the concept of information in physics, we are often inclined to think of the ways in which we can use physics to control, manipulate, and access information. The implication is that the laws of physics are primary and that the notion of information takes a secondary role. In Entropic Dynamics (ED) we explore a different but related question, one which goes back to the work of E.T. Jaynes in statistical mechanics, namely, that of whether the laws of physics are themselves an application of the rules for processing information. While such a viewpoint may appear controversial, it has already proven quite successful in deriving many aspects of the quantum formalism, including the quantum theory of fields on a fixed curved space-time, as well as shedding light on conceptual issues in gravity. Here we discuss the ED approach to quantum scalar fields interacting with a dynamical background. Our approach rests on a few key ingredients: (1) Rather than modelling the dynamics of the fields, ED models the dynamics of their probabilities. (2) In accordance with the standard entropic methods of inference, the dynamics is dictated by information encoded in constraints. (3) The choice of the physically relevant constraints is dictated by principles of symmetry and invariance. The first of such principles imposes the preservation of a symplectic structure which leads to a Hamiltonian formalism with its attendant Poisson brackets and action principle. The second symmetry principle is foliation invariance, which, following earlier work by Hojman, Kuchar, and Teitelboim, is implemented as a requirement of path independence. The result is a hybrid ED model that approaches quantum field theory in one limit and classical general relativity in another, but is not fully described by either. A particularly significant prediction of this ED model is that the coupling of quantum fields to gravity implies violations of the quantum superposition principle. We discuss this loss of linearity and its possible implications for studies of quantum information processing.

### Hierarchy of Theories with Indefinite Causal Structures: A Second Look at the Causaloid Framework

Sakharwade, Nitica (Perimeter Institute for Theoretical Physics)

The Causaloid framework is useful to study Theories with Indefinite Causality; since Quantum Gravity is expected to marry the radical aspects of General Relativity (dynamic causality) and Quantum Theory (probabilistic-ness). To operationally study physical theories one finds the minimum set of quantities required to perform any calculation through physical compression. In this framework, there are three levels of compression: 1) Tomographic Compression, 2) Compositional Compression and 3) Meta Compression. We present a diagrammatic representation of the Causaloid framework to facilitate exposition and study Meta compression. We show that there is a hierarchy of theories with respect to Meta compression and characterise its general form. Next, we populate the hierarchy. The theory of circuits forms the simplest case, which we express diagrammatically through Duotensors, following which we construct Triotensors using hyper3wires (hyperedges connecting three operations) for the next rung in the hierarchy. Finally, we discuss the implications for the field of Indefinite Causality. [1] Journal of Physics A: Mathematical and Theoretical, 40(12), 3081

Mar 10,2021

### The anti-Hawking Effect

Mann, Robert (University of Waterloo)

One of the more surprising early predictions of quantum field theory in curved spacetime is that accelerating detectors respond as though they were in a thermal bath of particles, with  a temperature proportional to their acceleration. Counter-intuitively, it has been shown recently that sometimes particle detectors can click less often, and even cooling down as their acceleration increases, in contrast to the heating one would expect.  I describe recent work demonstrating the existence of black hole analogues of these effects, which we call  anti-Hawking phenomena.  I shall discuss the implications of this work for future investigations in black hole physics and RQI.

### The Unruh effect in slow motion

Grimmer, Daniel (University of Waterloo)

We show under what conditions an accelerated detector (e.g., an atom/ion/molecule) thermalizes while interacting with the vacuum state of a quantum field in a setup where the detector's acceleration alternates sign across multiple optical cavities. We show (non-perturbatively) in what regimes the probe forgets' that it is traversing cavities and thermalizes to a temperature proportional to its acceleration. Then we analyze in detail how this thermalization relates to the renowned Unruh effect. Finally, we use these results to propose an experimental testbed for the direct detection of the Unruh effect at relatively low probe speeds and accelerations, potentially orders of magnitude below previous proposals.

### Stimulated Unruh Effect, Catalyzed Unruh Effect and Acceleration-Induced Transparency

Šoda, Barbara (University of Waterloo)

New acceleration-induced quantum effects are presented. The first observation is that stimulation can be applied not only to rotating but also to counter-rotating wave effects. This leads to new phenomena which may be called the stimulated and the catalyzed Unruh effects. The second observation is that acceleration is impacting not only counter-rotating wave terms but also rotating wave terms. This leads to the new phenomenon of acceleration-induced transparency. By combining stimulation with acceleration-induced transparency, the observability of the Unruh effect is potentially greatly improved.

### Unruh-like effects

Juárez-Aubry, Benito A. (Universidad Nacional Autónoma de México)

Detectors along stationary worldlines in Minkowski spacetimes record detailed-balance temperatures, which can generally depend on the detector's frequency difference. For example, in the case of a (i) linearly uniformly accelerated trajectory this is the (frequency-independent) Unruh temperature. In this talk, I will present a general study of the temperatures along (ii) cusped, (iii) circular, (iv) catenary and (v) helix uniformly accelerated motions, using a combination of numerical and analytic techniques. In each case, the temperatures will be functions of up to three motion parameters, the worldline's curvature, torsion and hypertorsion. I will discuss in which asymptotic regimes of this parameter space the temperatures of the different motions asymptote to each other, and the physical meaning of these limiting behaviours.

Mar 17,2021

### Locality and causality in quantum field theory

Rejner, Kasia (University of York)

In this talk I will give an overview of various notions of locality used in quantum field theory (QFT) and their relation to causality. I will also discuss how locality could be violated and what are the consequences of this for QFT description.

### The First Law of Quantum Field Thermodynamics

Thermodynamics has arguably been one of the most powerful branches of physics, and the most universal in its breadth of applications. From steam engines to black holes, the laws of thermodynamics seem to rule it all. However, even the most basic concepts in thermodynamics quickly become problematic in quantum theory. Defining simple notions such as work is actually an open problem with multiple incompatible answers. It gets worse: the most accepted definition of work in quantum thermodynamics, the two-point measurement scheme (TPM), generally violates the first law. When looked at from the perspective of quantum field theory, the situation gets even worse since the TPM scheme becomes ill-defined. We will analyse alternative definitions of work distributions for quantum field theory that are a) well defined b) physically understandable c) amenable to computations in a non-perturbative way and d) fulfil the first law of thermodynamics on average and in variance as well as some of the most important non-equilibrium theorems such as Crooks theorem and the Jarzinsky equality. We will show how, for KMS (thermal) states, we can provide the exact statistics of work and energy increase for unitary operations on the quantum field which are localized in space and time.

### Probing thermal fluctuations through scalar test particles

Leite Ferreira Junior, Alexsandre (Universidade Federal do Espírito Santo)

The fundamental vacuum state, related to Minkowski empty space, produces divergent fluctuations of the quantum fields that have to be subtracted in order to bring reality to the description of the physical system. This is a methodology well confirmed in laboratory with impressive accuracy. Nonetheless, when we subtract such empty space contribution, we open the possibility to have negative vacuum expectation values of classically positive-defined quantities. This is what has been addressed in the literature as subvacuum phenomenon. Here it is investigated how a scalar charged particle is affected by the vacuum fluctuations of massive scalar field in D + 1 spacetime when the background evolves from empty space to a thermal bath, and also when a perfectly reflecting boundary is included. It is shown that when the particle is brought into a thermal bath it gains an amount energy by means of positive dispersions of its velocity components. The magnitude of this effect is dependent on the temperature and also on the field mass. As an outcome no subvacuum effect occurs. However, when a reflecting wall is inserted into this system, the dispersions can be positive or negative, showing that subvacuum effect happens even in a finite temperature environment. Another relevant aspect is that the magnitude of the effects here discussed are fundamentally dependent on the switching interval of time the system takes to evolve between two states. I would prefer to talk on the Waterloo sessions.

### Sabotaging correlation harvesting between qubits coupled to a quantum field

Sahu, Abishek ()

We study the effect of third parties in the acquisition of correlations from quantum systems that communicate through a quantum field (both through harvesting and information exchange). In particular, we consider a scenario with an arbitrary number of two-level systems that couple to a quantum field locally in time. In this setup we study non-perturbatively the impact of the presence of intermediate detectors (interlopers) in the ability for two detector operators (Alice and Bob) to acquire quantum and classical correlations through their interaction with the field. We demonstrate that even a single interloper can completely sabotage all correlation harvesting between Alice and Bob by acting on the causal past of one of them. Furthermore, the interloper is able to interact with the field so that the interaction with the field floods' a target detector with entropy such that it becomes maximally entangled with the field. This prevents the target detector from acquiring any correlations with any other quantum system, while Alice and Bob cannot defend against this attack. This can be used as an attack against shared resources in relativistic quantum information, or to shield a bipartite system from acquiring correlations through its interactions with quantum field fluctuations.

Mar 24,2021

### From Analogue to Emergent Gravity: three small lessons

Liberati, Stefano (SISSA)

“The analogue gravity framework uses condensed matter systems to simulate phenomena characteristics of quantum field theory in curved spacetimes (e.g. cosmological particle production or black hole evaporation). In this seminar I will review the state of this field and explore its extension towards the simulation of the emergent gravity paradigm. In doing so I will discuss three lessons that we can draw from this framework about long standing puzzles in black hole thermodynamics and cosmology.”

### Entanglement measure for the characterisation of Hawking emission in analogues to gravity

Jacquet, Maxime (Paris Sorbonne University)

Quantum fluctuations on curved spacetimes cause the emission of pairs of particles from the quantum vacuum, as in the Hawking effect from black holes. We use an optical analogue to gravity [1,2,3] to investigate the influence of the curvature on quantum emission. We explain how, due to dispersion, the spacetime curvature varies with frequency and the radiation is not described by a single temperature [4]. We analytically calculate the particle flux, correlations and entanglement (as measured with the logarithmic negativity) at all frequencies [5]. Thus we show that horizons increase the flux with a characteristic spectral shape over an otherwise monotonous background of spontaneous emission. There, the photon number correlations transition from multi- to two-mode, with close to maximal entanglement. This exemplifies how the spacetime curvature rules the kinematics and the quantum state that is generated in optical and non-optical analogue systems. References [1] Philbin et al. Science 2008 [2] Finazzi et al. PRA 2013 [3] Jacquet et al. PRA 2015 [4] Jacquet et al. PRA 2020 [5] Jacquet et al. SciPost Phys 2020

### « Transplanckian Physics » in analogue condensed matter systems (dissipation and dispersion): Hawking radiation at zero frequency and dispersive wormhole travel with water waves.

Rousseaux, Germain (CNRS)

We report on what is to our knowledge the first scattering experiment of surface waves in a decelerating trans-critical flow induced by internal dissipation, which in the Analogue Gravity context is described by an effective spacetime with a white-hole horizon. The existence of a sub-luminal dispersive branch is at the origin of 2 additional solutions of the dispersion relation at zero frequency supplementary to the trivial flat free surface for zero wavenumber. Both non-zero wave-numbers combine to generate a stationary undulation downstream from the white hole horizon where the long wavelength wave velocity matches exactly the flow speed. We show that the triptych of solutions induces a 3 by 3 checker-board pattern in the momentum correlation space for the downstream region featuring the undulation. The additional super-luminal branch controls a reverse interstellar travel in an analogue wormhole between both analogue black and white hole horizons that we probe experimentally with water waves in an open channel flow. We can tune the appearance of both Hawking radiation and its corresponding Cherenkov-like undulation by controlling the creation of negative norm modes because of the existence of a dispersive Landau threshold (minimum of the phase velocity as a function of the wavenumber/momentum). I would like to attend to the Waterloo session.

### Gravity through a feedback mechanism

Gaona Reyes, José Luis (University of Trieste)

I will review some of the recent work in which consistent hybrid quantum-classical dynamics models have been developed by adding fluctuations to the classical variables, through a continuous measurement and feedback mechanism. I will focus on two models, due to Kafri, Taylor and Milburn (KTM) and Tilloy and Diósi (TD). I will discuss possible ways to generalize the KTM model. I will compare the decoherence effects predicted in both models and discuss about the construction of full-gravitational interaction models through a feedback mechanism. I will argue that the TD model is the only viable framework within the simplest conditions for implementing the feedback.

Mar 31,2021

### Quantum gravity when the notion of distance is replaced by the notion of correlation

Kempf, Achim (University of Waterloo)

Quantum field fluctuations are the more strongly correlated the shorter their spacetime distance. Therefore, the notion of spacetime distance can be replaced by the notion of correlation strength. Instead of the usual picture, in which the degrees of freedom of spacetime and of matter are considered qualitatively different, this suggests a new picture in which all fundamental degrees of freedom are described by the same abstract structure, namely multi-point correlators, a picture which is essentially information-theoretic. In the quantum gravity regime, these abstract correlators may not possess representations as correlation functions of matter fields that live on a curved spacetime. As one approaches the low energy regime, the notions of spacetime and of matter emerge when the abstract correlators become approximately representable as correlation functions of quantum fields on a curved spacetime. I discuss a simple solvable model for the quantum gravity regime.

### Bandlimited Entanglement Harvesting

Henderson, Laura (University of Waterloo)

There are many reasons to believe that there is a fundamental minimum length scale below which distances cannot be reliably resolved. One method of constructing a quantum field with a finite minimum length scale is to use bandlimited quantum field theory, where the spacetime is mathematically both continuous and discrete. This is a modification to the field, which has been shown to have many consequences at the level of the field. We consider an operational approach and use a pair of particle detectors (two-level qubits) as a local probe of the field, which are coupled to the vacuum of the bandlimited massless scalar field in a time dependent way through a switching function. We show that mathematically, the bandlimit modifies the spacial profile of the detectors so that they are only quasi-local. We explore two different types of switching functions, Gaussian and Dirac delta. We find that with Gaussian switching, the bandlimit exponentially suppress the de-excitation of the detectors when the energy gap between the two levels is larger than the bandlimit. If the detectors are prepared in ground state, in certain regions of the parameter space they are able to extract more entanglement from the field than if there was no bandlimit. When the detectors couple with Dirac-delta switching, we show that a particle detector is most sensitive to the bandlimit when it couples to a small but finite region of spacetime. We find that the effects of a bandlimit are detectable using local probes.

### Late-Time Inferences for Unruh-DeWitt Detectors

Kaplanek, Gregory (McMaster University)

This talk is based on the articles arXiv:1912.12951, 1912.12955 & 2007.05984 and focuses on making late-time inferences on the state of a single (point-like) Unruh-DeWitt (UDW) detector coupled to a real scalar field, as the UDW detector either (i) accelerates through flat Minkowski space or (ii) hovers near the event horizon of a Schwarzschild black hole. The calculation is carried out using the Nakajima-Zwanzig master equation as a perturbation in the dimensionless qubit-field coupling. When the field is prepared in appropriately-chosen vacua the late-time state for the UDW detector is thermal. The calculation uses Open EFT techniques to restrict the calculation to the Markovian regime, where the qubit evolution is described by a Lindblad equation. By systematically assessing the approximations required in this regime, we find the region of parameter space in which Markovianity applies, where in particular we find that the renormalized qubit gap must be small compared to the temperature.

### Bandlimited UDW detector dynamics on 2+1 flat and spherical spacetimes

Shalabi, Ahmed (University of Waterloo)

The potential breakdown of the notion of a metric at high energy scales could imply the existence of a fundamental minimal length scale below which distances cannot be resolved. One approach to realizing this minimum length scale is construct a quantum field theory with a bandlimit on the field. We report on an investigation of the effects of imposing a field bandlimit on a curved and compact spacetime. To achieve this operationally, we couple two Gaussian-smeared UDW detectors to a scalar field on a S^2 x R spherical spacetime through Dirac-delta switching. Delta switching allows for a non-perturbative analysis that includes higher order effects due to the bandlimit. The bandlimit is implemented through a cut-off of the allowable angular momentum modes of the field. We find that the detectors are less sensitive to the bandlimit on the spherical spacetime, and observe features similar to flat spacetime. These include the response of the detectors depending on their geometry and that smaller detectors couple in a stronger manner to the field. We also explore two squeezed detector setups in both flat and spherical spacetimes, and find notable difference between the two cases. Due to the compact nature of spherical space, the lack of dissipation of any perturbation to the field results in locally excited signals of the field travelling from pole to pole in the spacetime. Quite strikingly, squeezing increases the response of the detectors in flat space but decreases the response on the spherical spacetime. Moreover, we find that squeezing on a sphere introduces extra anisotropies that could be exploited to amplify or weaken the response of the second detector.

Apr 07,2021

### Quantum simulators for fundamental physics

Weinfurtner, Silke (University of Nottingham)

Analogue gravity summarises an effort to mimic physical processes that occur in the interplay between general relativity and field theory in a controlled laboratory environment. The aim is to provide insights in phenomena that would otherwise elude observation: when gravitational interactions are strong, when quantum effects are important, and/or on length scales that stretch far beyond the observable Universe. The most promising analogue gravity systems up to date are fluids, superfluids, superconducting circuits, ultra-cold atoms, and optical systems. While deepening our understanding of the laboratory systems at hand, the long-term vision of analogue gravity studies is to advance fundamental physics through interdisciplinary research, by establishing and nurturing a new culture of collaboration between the various communities involved. I will discuss recent efforts to explore the quantum origin of the Universe, accelerated observer radiation, and rotating black hole physics in the laboratory.

### Entanglement Harvesting with Modified Dispersion Rela-tions

Sachs, Allison (University of Waterloo)

Does modifying the dispersion relation of a quantum field significantly alter the entanglement harvesting protocol? In this work, we ask this question in 1+1, 2+1, and 3+1 dimensions. We consider two classes of dispersion: hyperbolic tangent dispersion and a dispersion relation consisting of a quartic correction to the usual dispersion relation. We present results and compare them to the standard entanglement harvesting protocol from a field with linear dispersion. In many cases, we find that entanglement harvesting is robust under modifications to dispersion.

### Quantum Temporal Superposition: the case of QFT

Belenchia, Alessio (Queen's University Belfast)

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 particle detectors. In this talk, we show that a quantum-controlled superposition of detectors at different space-time points can gain information on field correlations which would not be otherwise accessible. In particular, the quantum control allows for the extraction of entanglement in scenarios where this is otherwise provably impossible.

### Broken Covariance in Particle Detector Models

Rick Perche, Tales (University of Waterloo, Perimeter Institute for Theoretical Physics)

In this talk we show that it is not possible to formulate a fully covariant theory for a smeared particle detector model that consists of a single degree of freedom coupled to a quantum field locally around a timelike trajectory. Namely, different notions of time evolution yield different predictions for these theories due to the fact that the interaction Hamiltonian densities for these models violate microcausality. We then quantify the break of covariance for smeared particle detectors and show that if the detector starts in a statistical mixture of eigenstates of its free Hamiltonian, the difference in the predictions associated to two notions of time evolution is of third order in the coupling constant between the field and detector. The talk is based on the work https://arxiv.org/abs/2006.12514.

Apr 14,2021

### Communication through quantum fields near a black hole

Jonsson, Robert (Max Planck Institute of Quantum Optics)

When Alice falls into a black hole, how much information can she send to Bob before she passes the horizon? How is communication via massless fields near a black hole affected by the fact that signals, due to spacetime curvature, can propagate both on direct and black-hole orbiting null geodesics, as well as on time-like paths? An analysis of these questions requires the calculation of the field’s Green function, which is technically challenging. We here use tools previously designed for the study of wave propagation in curved spacetimes, and apply them to the relativistic quantum information communication setup. We model the observers as particle detectors, and analyze the information carrying capacity of direct and black hole-orbiting null geodesics as well as of the timelike contributions in 3+1 dimensional Schwarzschild spacetime. Interestingly, and perhaps counter-intuitively, we find that the gravitational time-dilation reduces and limits the information receivers close to the horizon can extract from the direct null contributions of infalling signals. In this regime, in particular, the non-direct-null and timelike contributions, which do not possess an analog on Minkowski spacetime, can dominate over the direct null contributions. Reference: R. H. Jonsson, D. Q. Aruquipa, M. Casals, A. Kempf, and E. Martín-Martínez, “Communication through quantum fields near a black hole,” Phys. Rev. D, vol. 101, no. 12, p. 125005, Jun. 2020, doi: 10.1103/PhysRevD.101.125005.

### Entanglement Amplification from Rotating Black Holes

Robbins, Matthew (University of Waterloo)

The quantum vacuum has long been known to be characterized by field correlations between spacetime points. These correlations can be swapped with a pair of particle detectors, modelled as simple two-level quantum systems (Unruh-DeWitt detectors) via a process known as entanglement harvesting. We study this phenomenon in the presence of a rotating BTZ black hole, and find that rotation can significantly amplify the harvested vacuum entanglement. Concurrence between co-rotating detectors is amplified by as much as an order of magnitude at intermediate distances from the black hole relative to that at large distances. The effect is most pronounced for near-extremal small mass black holes, and allows for harvesting at large spacelike detector separations. We also find that the entanglement shadow -- a region near the black hole from which entanglement cannot be extracted -- is diminished in size as the black hole's angular momentum increases. This talk will be based on arXiv:2010.14517 [hep-th].

### Unruh-DeWitt Detector Differentiation of Black Holes and Exotic Compact Objects

Zhang, Chen (University of Toronto)

We study the response of a static Unruh-DeWitt detector outside an exotic compact object (ECO) with a variety of (partially) reﬂective boundary conditions in 3+1 dimensions. The horizonless ECO, whose boundary is extremely close to the would-be event horizon, acts as a black hole mimicker. We ﬁnd that the response rate is notably distinct from the black hole case, even when the ECO boundary is perfectly absorbing. For a (partially) reﬂective ECO boundary, we ﬁnd resonance structures in the response rate that depend on the diﬀerent locations of the ECO boundary and those of the detector. We provide a detailed analysis in connection with the ECO’s vacuum mode structure and transfer function.

### Harvesting entanglement with detectors freely falling into a black hole

Gallock-Yoshimura, Kensuke (University of Waterloo)

We study the entanglement harvesting protocol in a (1+1)-dimensional Schwarzschild black hole spacetime for Unruh and Hartle-Hawking states, where at least one of two Unruh-DeWitt (UDW) detectors is freely falling from infinity. We show that (i) the amount of extracted entanglement by static and free-falling detectors is always less than that of two static detectors due to their relative velocity and acceleration, and (ii) the previously known entanglement death-zone’ near the horizon is no longer present when two free-falling detectors are considered.

Apr 21,2021

### How thermal is circular acceleration?

Louko, Jorma (University of Nottingham)

An observer in uniform linear acceleration responds to the Minkowski vacuum thermally, in the Unruh temperature T_U = (proper acceleration)/(2 pi). An observer in uniform circular motion experiences a similar Unruh-type temperature T_c, operationally measurable via the detailed balance condition between excitation and de-excitation probabilities, but T_c depends not just on the proper acceleration but also on the orbital radius and on the excitation energy. We establish analytic and numerical results for T_c for a massless scalar field in 3+1 and 2+1 spacetime dimensions. In the ultrarelativistic limit, the (3+1)-dimensional T_c is of the order of T_U uniformly in the energy, as previously found by Unruh, but the (2+1)-dimensional T_c is significantly lower at low energies. The results adapt to an analogue spacetime nonrelativistic field theory via a straightforward gamma-factor scaling of the excitation and de-excitation energies. In particular, 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)]

### Quantum Detection of Inertial Frame Dragging

Kubiznak, David (Perimeter Institute for Theoretical Physics)

A relativistic theory of gravity like general relativity produces phenomena differing fundamentally from Newton's theory. An example, analogous to electromagnetic induction, is the dragging of inertial frames by mass-energy currents. These effects have recently been confirmed by classical observations. Here we show, for the first time, that they can be observed by a quantum detector. We study the response function of Unruh De-Witt detectors placed in a slowly rotating shell. We show that the response function picks up the presence of rotation even though the spacetime inside the shell is flat and the detector is locally inertial. Moreover, it can do so when the detector is switched on for a finite time interval within which a light signal cannot travel to the shell and back to convey the presence of rotation.

### Quantum fields and strong cosmic censorship

Klein, Christiane (Leipzig University)

The strong cosmic censorship conjecture (sCC), first stated by Penrose, proposes that at inner horizons of black holes, the spacetime should become inextendible because local observables, such as the stress-energy tensor, of classical or quantum fields should diverge. For the Reissner-Nordström-de Sitter spacetime, numerical computations indicate that there is a physical parameter space where sCC is violated classically. But Hollands et al. have show that in the vicinity of the inner horizon the effects of a real scalar quantum field can dominate those of the classical one and restore sCC. Their key finding is a quadratic divergence of the quantum stress-energy tensor on the horizon in an arbitrary Hadamard state. The results presented show that the prefactor of this divergence is non-zero in a wide range of parameters, and that it can take either sign. This means quantum effects restore sCC under quite general conditions, while the direction of the divergence depends on the specific setup. The talk is based on Phys.Rev.D 102 (2020) 8, 085004, which is joined work with S. Hollands and J. Zahn. I would prefer to talk in the Waterloo-session.

### Entanglement in Einstein-Cartan cosmology

Belfiglio, Alessio (University of Camerino)

We study the entanglement arising for Dirac and Klein-Gordon fields in an expanding space-time characterized by the presence of torsion. Torsion is introduced according to the Einstein-Cartan theory, while the space-time is described by means of an asymptotically flat Robertson-Walker metric. In this framework, torsion is represented by means of two external functions, fulfilling precise constraints, able to match the basic demands of the cosmological principle, i.e., homogeneity and isotropy of the universe. We notice that for the Dirac field the presence of torsion can increase the amount of entanglement, and this is true in particular for small values of the particle momentum. For the Klein-Gordon field we show that for peculiar forms of the unknown functions describing torsion, the model can be solved exactly. This can be done using the same scale factor as in the Dirac case and torsion can affect significantly the amount and the mode dependence of entanglement.

Apr 28,2021

### Quantum Information Processing in Space-Time

I review recent progress in the theory of summoning and related quantum information processing tasks in relativistic space-times. I also describe applications, including the implementation of novel forms of unforgeable money/tokens with quantum-enabled security.

### A detector-based measurement theory for QFT

Polo-Gómez, José (University of Waterloo)

We propose a measurement theory for QFT based on particle detectors (localized non-relativistic systems that couple to quantum fields, like the Unruh-DeWitt model). Concretely we give a relativistic equivalent to the quantum mechanical Lüders' udpate rule (a measurement that updates the state of the measured system by a projector) to update the field state after a measurement is carried out. Namely, we analyze the induced POVM on the field when we carry out an idealized measurement on a detector after it coupled to the field. We discuss how this measurement theory proposal 1) can directly connect with experimental results and 2) does not have the "impossible measurements" problem pointed out by Rafael Sorkin in the 90s that discards the use of idealized measurements in QFT.

### Perspectives of measuring gravitational effects of laser light and particle beams

Spengler, Felix (Tübingen University)

We present a study of the possibilities of creation and detection of oscillating gravitational fields from lab-scale high energy, relativistic sources. The sources considered are high energy laser beams in an optical cavity and the ultra-relativistic proton bunches circulating in the beam of the Large Hadron Collider (LHC) at CERN. These sources allow for signal frequencies much higher and far narrower in bandwidth than what most celestial sources produce. In addition, by modulating the beams, one can adjust the source frequency over a very broad range, from Hz to GHz. The gravitational field of these sources and responses of a variety of detectors are analysed. We find that if a slow (k Hz) modulation of a commercially available 500 kilo-Watt continuous-wave laser or the LHC beam can be achieved, a recently experimentally demonstrated monolithic pendulum should be able to measure the gravitational force from these beams. This opens new perspectives of studying general relativistic effects and possibly quantum-gravitational effects with ultra-relativistic, well-controlled terrestrial sources.

### Quantum time dilation in atomic spectra

Grochowski, Piotr (Center for Theoretical Physics, Polish Academy of Sciences)

Quantum time dilation occurs when a clock moves in a superposition of relativistic momentum wave packets. The lifetime of an excited hydrogen-like atom can be used as a clock, which we use to demonstrate how quantum time dilation manifests in a spontaneous emission process. The resulting emission rate differs when compared to the emission rate of an atom prepared in a mixture of momentum wave packets at order $v^2/c^2$. This effect is accompanied by a quantum correction to the Doppler shift due to the coherence between momentum wave packets. This quantum Doppler shift affects the spectral line shape at order $v/c$. However, its effect on the decay rate is suppressed when compared to the effect of quantum time dilation. We argue that spectroscopic experiments offer a technologically feasible platform to explore the effects of quantum time dilation.

### Quantum principle of relativity

Dragan, Andrzej (University of Warsaw)

We show that the local and deterministic mode of description is not only in conflict with the quantum theory, but also with relativity. We argue that elementary relativistic properties of spacetime lead to the emergence of a non-deterministic quantum-mechanical picture involving quantum superpositions and complex probability amplitudes.

May 05,2021

### A particle detector model for neutrino oscillations

Torres, Bruno (University of Waterloo)

We analyze the problem of neutrino oscillations via a fermionic particle detector model inspired by the physics of the Fermi theory of weak interactions. The model naturally leads to a description of emission and absorption of neutrinos in terms of localized two-level systems, in close parallel to the Unruh-DeWitt model commonly used in relativistic quantum information. The formalism is shown to reproduce the standard results for neutrino oscillation probabilities without mention to “flavor states” for the neutrino field. This illustrates how particle detector models provide a powerful theoretical tool to tackle measurements in quantum field theory and emphasizes that the notion of flavor states, although sometimes useful, does not play any crucial role in neutrino phenomenology. Talk based on Phys. Rev. D 102, 093003 (2020), in collaboration with T. Rick Perche, André Landulfo, and George Matsas.

### Quantum communication through a partially reflecting moving mirror

Lapponi, Alessio (University of Camerino)

Motivated by the fact that a null-shell of a collapsing black hole can be described in terms of a perfectly reflecting accelerating mirror, I here investigate the consequences of semitransparent mirrors. In particular, I take into account partially reflecting mirrors and I provide for them analytic Bogoliubov coefficients in case the mirror has an impulsive acceleration. I thus associate a bosonic Gaussian channel to a semitransparent moving mirror, studying how the particle creation of the mirror itself influences the transmission of a signal through it. An amplification of a fraction of transmitted signal occurs if compared with the static case, alongside a noisy contribution proportional to the created particles. I investigate the magnitudes of both the amplification and noise, showing the former to be smaller, i.e. demonstrating that the classical and quantum capacities of this system both increase.

### Quantum radiation in dielectric media with dispersion and dissipation

Lang, Sascha (Helmholtz-Zentrum Dresden Rossendorf)

Quantum field theory in curved space-time predicts some fascinating mechanisms of particle creation that have close analogies to other types of quantum radiation occurring in, e.g., optical fibres or waveguides. Recent studies of such analogies in dielectric media usually take into account dispersion but neglect dissipation. We extend the established Hopfield model for dispersive but non-dissipative dielectrics by explicitly adding an environment field. The associated microscopic Lagrangian allows for an ab initio treatment of quantum radiation. As an application, we study the number and dynamics of photons created by switching on and off dissipation. The particle yield in our set-up may be larger than for a changing (real) refractive index, which constitutes a famous analogue of cosmological particle creation. References: [1] S. Lang, R. Schützhold, & W. G. Unruh, Quantum radiation in dielectric media with dispersion and dissipation, Phys. Rev. D 102, 125020 (2020) [2] S.Lang & R. Schützhold, Analog of cosmological particle creation in electromagnetic waveguides, Phys. Rev. D 100,065003 (2019)

### Probing quantum states of the electromagnetic field locally in space-time

Virally, Stephane (Polytechnique Montréal)

One of the most useful probes of quantum states of the electromagnetic field is homonyme detection [Collet1987]. This generally useful tool is intrinsically non-local in space-time, which leads to the impossibility of ensuring a full tomography of quantum states of light for an arbitrarily accelerated observer [Onoe2019]. In contrast, local field measurements in space-time provide information that is not subjected to these constraints. The most prominent technique for local space-time measurement is electro-optic sampling (EOS), which has recently demonstrated its ability to locally probe the quantum vacuum [Riek2015]. EOS mixes an ultrashort, ultra-focused, near infrared (NIR) pulse with a terahertz (THz) signal in a nonlinear crystal. This provides a diffraction-limited local space-time probe volume with dimensions much shorter than the characteristic wavelength and period of the probed field. We will discuss the current limitations of this technique and means to go beyond these limitations. In particular, we will discuss quadratures in a region of space-time, their non-causal characteristics [Virally2019], and means to measure them with an EOS-like probe [Sulzer2020]. We will also show that the second-quantized photon distribution of the probe plays a major role in the current limitations, and we will introduce a version of EOS with a nonclassical probe that can bypass these limitations [Cusson2020]. This quantum-enhanced EOS technique promises to provide full state tomography of quantum fields in a finite volume of space-time. [Collet1987] M. Collet, R. Loudon and C. Gardiner, Journal of Modern Optics 34, 881 (1987) [Onoe2019] S. Onoe and T.C. Ralph, Physical Review D 99, 116001 (2019) [Riek2015] C. Riek, D.V. Seletskiy, A.S. Moskalenko, J.F. Schmidt, P. Krause, S.Eckart, S. Eggert, G. Burkard and A. Leitenstorfer, Science 350, 420 (2015) [Virally2019] S. Virally and B. Reulet, Physical Review A 100, 023833 (2019) [Sulzer2020] P. Sulzer, K. Oguchi, J. Huster, M. Kizmann, T.L.M. Guedes, A. Liehl, C. Beckh, A.S. Moskalenko, G. Burkard, D.V. Seletskiy and A. Leitenstorfer, Physical Review A 101, 033821 (2020) [Cusson2020] P. Cusson, S. Virally and D.V. Seletskiy, The 22nd International Conference on Ultrafast Phenomena, 3447437 (2020)

### A tale of particles and measurements

Martin-Martinez, Eduardo (University of Waterloo)

We will review apparently harmless notions like 'particles' or 'measuremnts' in quantum field theory and then again 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 and dicuss whether the UDW model is phyiscal or not and in what ways. We will discuss the limitations of both measurement frameworks as well as some of the typical abuses that are often made with Unruh-DeWitt-like detector models and how they matter in general relativistic scenarios.