RQI-N Online 2022 (aka RQI-North-12)

Welcome and Opening Remarks

A Brief History of Relativistic Quantum Information

Relativistic Quantum Information, or RQI, has emerged over the past 20 years as an active area of research concerned with the relationship between relativistic effects and quantum information tasks. I will give an overview of the pre-history of RQI, its emergence as a distinct sub-discipline, its subsequent development, and its present-day status.

Black hole disinformation problem, version 2

My title follows the provocative title of Hayward from 2005, where the necessary conditions for a consistent formulation of the paradox were formulated. Since then we learned a number of things: on the one hand, intrinsic non-unitarity is shown not necessarily to be in conflict with observations, and on the other hand the concept of physical black holes allows to quantify the costs of formulation of the information loss problem/paradox. I describe how formulation of the information loss problem imposes constrains on solutions of the Einstein equations, and what are their consequences both for the black hole physics and the alleged paradox.

Entangling three detectors in a black hole spacetime

Although significant work has been done on exploring the extraction of bipartite entanglement from a quantum field by a pair of Unruh-DeWitt detectors, a process known as entanglement harvesting, there have been relatively few studies on the extraction of multipartite entanglement. It has been previously shown that in flat space three detectors can harvest entanglement in regions of the parameter space where bipartite harvesting is not possible. We apply the method of tripartite entanglement harvesting to the case of three detectors in the BTZ spacetime, and find that three detectors with fixed proper separations can harvest tripartite entanglement much closer to the horizon then bipartite entanglement. Additionally, we find there are detector configurations that appear to completely eliminate this "tripartite entanglement shadow". We also explore the case where the three detectors are in an equilateral triangle with the black hole located in the centre and find that for this configuration tripartite entanglement harvesting is only guaranteed for a small range of black hole masses and AdS lengths.

Entanglement Harvesting From the Gravitational Field

In this presentation, we extend the entanglement harvesting protocol to the case where two detectors couple to a quantum gravitational field in the linearized regime. We present the effects on the entanglement harvested by the detectors using different detector models and compare our results to the case of entanglement harvesting from the electromagnetic and real scalar fields.

A General Characterization of Particle Detectors in Curved Spacetimes

In this talk we will discuss a recently developed formalism which allows one to embed a localized non-relativistic quantum theory in a background curved spacetime while undergoing a timelike trajectory. This formalism allows for a general formulation of smeared particle detectors in curved spacetimes, stipulating a well defined regime of validity for these models. We will also discuss modifications to the dynamics of the detectors, which should be taken into account when probing quantum fields in curved spacetimes. Reference: Phys. Rev. D 106, 025018 (arXiv:2206.01225)

Expanding Edges of Quantum Hall Systems in a Cosmology Language - Hawking radiation from de Sitter horizon in edge modes

Expanding edge experiments are promising to open new physics windows of quantum Hall systems.In a static edge, the edge excitation, which is described by free fields decoupled with the bulk dynamics, is gapless, and the dynamics preserve conformal symmetry. When the edge expands, such properties need not be preserved. We formulate a quantum field theory in 1+1 dimensional curved spacetimes to analyze the edge dynamics. We propose methods to address the following questions using edge waveforms from the expanding region: Does the conformal symmetry survive? Is the nonlinear interaction of the edge excitations induced by edge expansion? Do the edge excitations interact with the bulk excitations? We additionally show that the expanding edges can be regarded as expanding universe simulators of two-dimensional dilaton-gravity models, including the Jackiw-Teitelboim gravity model. As an application, we point out that our theoretical setup might simulate emission of analog Hawking radiation with the Gibbons-Hawking temperature from the future de Sitter horizon formed in the expanding edge region.

Entanglement is better teleported than transmitted

We consider the quantum communication scenario where Alice is entangled with an ancilla and intends to communicate with Bob, through an intermediary system such as a quantum field, so as to make Bob entangled with the ancilla. We find that if Alice and Bob directly couple to the intermediary system, such as a field, then they can generate negativity between Bob and the ancilla only at orders that are higher than second perturbative order. Here, we propose a protocol based on teleportation combined with entanglement harvesting or generation through the intermediary system, such as a field. We show that this protocol can transfer negativity already to second perturbative order. An analytic formula for the transferred negativity indicates that our protocol is optimal to transfer negativity in a maximally entangled state by using identical detectors in entanglement harvesting or generation.

Quantum time dilation in a gravitational field

According to relativity, the reading of an ideal clock is interpreted as the elapsed proper time along its single classical trajectory. In contrast, quantum theory allows the association of many simultaneous paths with a single quantum clock. Naturally, we can ask how the superposition principle affects the gravitational time dilation observed by a simple clock---a decaying two-level atom. Placing such an atom in a superposition of positions enables us to analyze a quantum contribution to a classical time dilation, manifested in the process of spontaneous emission. In particular, we show that the emission rate of an atom prepared in a coherent superposition of separated wave packets in a gravitational field is different from the transition rate of the atom in a classical mixture of these packets, which gives rise to a nonclassical-gravitational time dilation effect.

A Little Drama Across the Horizon

We analyse numerically the transitions in an Unruh-DeWitt detector, coupled linearly to a massless scalar field, in radial infall in (3+1)-dimensional Schwarzschild spacetime. In the Hartle-Hawking and Unruh states, the transition probability attains a small local extremum near the horizon-crossing and is then moderately enhanced on approaching the singularity. In the Boulware state, the transition probability drops on approaching the horizon. The unexpected near-horizon extremum arises numerically from angular momentum superpositions, with a deeper physical explanation to be found.

Probing BTZ Black Hole via Fisher Information

Relativistic quantum metrology is a framework that not only accounts for both relativistic and quantum effects when performing measurements and estimations, but further improves upon classical estimation protocols by exploiting quantum relativistic properties of a given system. Here I present results of the first investigation of Fisher information associated with a black hole. I briefly review recent work in relativistic quantum metrology that examined Fisher information for estimating thermal parameters in (3+1)-dimensional de Sitter and Anti-de Sitter (AdS) spacetimes. Treating Unruh-DeWitt detectors coupled to a massless scalar field as probes in an open quantum systems framework, I extend these recent results to (2+1)-dimensional AdS and black hole spacetimes. While the results for AdS are analogous to those in one higher dimension, I note new non-linear results arising from the BTZ mass.

Newton's cradle spectra

We present broadly applicable nonperturbative results on the behavior of eigenvalues and eigenvectors under the addition of self adjoint operators and under the multiplication of unitary operators, in finite dimensional Hilbert spaces. To this end, we decompose these operations into elementary 1 parameter processes in which the eigenvalues move similarly to the spheres in Newton's cradle. As special cases, we recover level repulsion and Cauchy interlacing. We discuss two examples of applications. Applied to adiabatic quantum computing, we obtain new tools to relate algorithmic complexity to computational slowdown through gap narrowing. Applied to information theory, we obtain a generalization of Shannon sampling theory, the theory that establishes the equivalence of continuous and discrete representations of information. The new generalization of Shannon sampling applies to signals of varying information density and finite length.

Thermal and Bounded Unruh-DeWitt Detectors in Circular Motion

Given the difficulty in experimentally measuring the Unruh effect for a linearly accelerated observer, it has become relevant to probe other Unruh-like regimes which are more experimentally viable. One such approach looks into the experience of an observer in uniform circular motion. Recently, Biermann et al. (2020) have investigated this circular acceleration version of the Unruh effect by analysing the excitations and de-excitations of a two-level detector in several limiting regimes. In this talk, I will extend the analysis to regimes accessible in analogue spacetime laboratory experiments. In particular, I will consider a non-zero background temperature and a field confined in a cavity.

Entanglement Harvesting of Inertially Moving UDW Detectors

We investigate the effects of relative motion on entanglement harvesting by considering a pair of Unruh-Dewitt detectors moving at arbitrary, but independent and constant velocities, both linearly interacting with a massless scalar field vacuum in four dimensional Minkowski spacetime. Working within the weak coupling approximation, we find that the Negativity is a (non-elementary) function of the relative velocity of the detectors, as well as their energy gaps and minimal separation. We find parameter regions where Negativity increases with velocity up to a maximum and then decreases, reaching zero at some sublight velocity. At any given relative velocity, the harvested entanglement is inversely correlated with the detector energy gap (at sufficiently high values) and the distance of closest approach of two detectors.

Operational models of ``temperature superpositions’’

The interplay between quantum theory, thermodynamics, and general relativity emerges as a promising arena for exploration and insights into foundational questions — thermalisation process often come into play in scenarios of importance for the interplay of quantum and gravitational theories, and quantum thermodynamics is of interest in its own right. In this talk I will discuss new insights into the notion of temperature and thermalisation obtained from an operational approach, where the notion of temperature is associated with a quantum system serving as a thermometer. I will also discuss how these results Unruh DeWitt detectors with quantised trajectories.

Measurement-based, Lorentz covariant Bohmian trajectories of multiple photons

Bohmian mechanics is a nonlocal hidden-variable interpretation of quantum theory which predicts that particles follow deterministic trajectories in spacetime. Historically, the study of Bohmian trajectories has mainly been restricted to nonrelativistic regimes due to the widely held belief that the theory is incompatible with special relativity. Contrary to this belief, my collaborators and I recently developed a new approach for constructing the relativistic Bohmian-type velocity field of single particles via weak measurements of the particle's momentum and energy [1]. In this talk, I present an extension of our weak-value based relativistic Bohmian theory to include multiparticle interactions. I show that the Bohmian-type velocity fields of two photons constructed via weak measurements is entirely equivalent to that obtained from a manifestly Lorentz-covariant multitime, multiparticle Klein-Gordon theory. I specifically apply our formalism to a two-photon position-symmetrised state and calculate the resulting trajectories in the bunched interference pattern. In contrast with prior expectations, our results demonstrate that there is a consistent way of understanding multiparticle interactions in Bohmian mechanics alongside the tenets of special relativity. I discuss the future outlook of our research regarding the reformulation of our model to account for scenarios involving particle creation and annihilation processes. [1] Joshua Foo, Estelle Asmodelle, Austin P. Lund, and Timothy C. Ralph, Nature Communications 13, 4002 (2022).

Entanglement in quantum field theory via wavelet representations

Quantum field theory (QFT) describes nature using continuous fields, but physical properties of QFT are usually revealed in terms of measurements of observables at a finite resolution. We describe a multiscale representation of a free scalar bosonic and Ising model fermionic QFTs using wavelets. Making use of the orthogonality and self similarity of the wavelet basis functions, we demonstrate some well known relations such as scale dependent subsystem entanglement entropy and renormalization of correlations in the ground state. We also find some new applications of the wavelet transform as a compressed representation of ground states of QFTs which can be used to illustrate quantum phase transitions via fidelity overlap and holographic entanglement of purification.

Vaidya to Rindler transformation and the Hawking radiation

Assumptions of finite time formation of trapped region and finiteness of curvature scalars on their boundary are enough to constrain a general spherically symmetric metric to correspond to two classes of solutions of the Einstein equations. Each solution can describe an expanding white hole or an evaporating black hole. In the leading order approximation, evaporating black hole solution takes the form of the Vaidya metric, and we intend to study the nature of the quantum field in the Vaidya background here. Schwarzschild spacetime behaving Rindler-like near the horizon allows us to use the periodicity time trick to derive the Hawking temperature. Rindler coordinate transformation from the Vaidya metric to apply the periodicity time trick is an entirely nontrivial task because of the time dependence of the metric. We obtain the Hawking temperature for the Vaidya metric using the periodicity time trick here to show that this trick would be useful not only for stationary spacetimes but also for dynamical spacetimes. However, there are limitations to the finite time periodicity trick as we could not know the hidden assumptions behind the extracted temperature from this method, including the choice of the particular vacuum state. To identify the limitations and to uncover the hidden assumptions behind this trick, we rederive the Hawking temperature explicitly using the field-theoretic calculation on the Vaidya background. This is still a work in progress.

Infrared acceleration radiation

We present an exactly soluble electron trajectory that permits an analysis of the soft (deep infrared) radiation emitted, the existence of which has been experimentally observed during beta decay via lowest order inner bremsstrahlung. Our treatment also predicts the time evolution and temperature of the emission, and possibly the spectrum, by analogy with the closely related phenomenon of the dynamic Casimir effect. *In collaboration with Paul C.W. Davies (ASU).

Surface gravity and the information loss problem

The prediction that black holes evaporate through the emission of Hawking radiation is widely regarded as one of the most impressive achievements of quantum field theory in curved spacetimes. Extensions of this result to various dynamical scenarios require generalisations of surface gravity. While different generalisations do in general not agree exactly, they are believed to be suitably close, particularly in the quasi-static limit. Nonetheless, we find that the two principal generalisations of surface gravity to dynamic spherically symmetric spacetimes are irreconcilable, and neither of them can describe the emission of nearly-thermal radiation. If semiclassical gravity is valid, this implies that it is impossible to simultaneously realise all of the necessary elements (event horizon, evaporation, thermal character of the radiation) that would be required for a self-consistent formulation of the information loss paradox. It also raises the question of which (if any) definition of surface gravity (or some closely related quantity) is related to the Hawking temperature of an evolving black hole. Note: this talk is based on https://doi.org/10.1103/PhysRevD.105.124032

The effect of dynamical collapse models within scalar cosmological perturbations theory

In this talk, I will provide a brief review on the basic concepts of cosmological inflation and the theory of cosmological perturbations. Moreover, I will explain the main concepts of dynamical collapse models regarding them as phenomenological modifications to the standard Schrödinger evolution. I will discuss the corrections to the power spectrum of the comoving curvature perturbation during the inflationary era which arise when considering dynamical collapse models. Finally, I will point out some aspects which need to addressed when considering these models within a cosmological context.

Gauge invariant observables in perturbative algebraic QFT

In this talk I will review the ideas and concepts around the notion of observables in perturbative quantum field theory and effective quantum gravity, using the algebraic approach that stems from the ideas of Haag and Kastler (local quantum physics), but goes beyond that, introducing some non-local effects. I will discuss both local and non-local observables and how they can be defined on a large class of curved spacetimes.

Non-classicality of scalar field dark matter

The axion-like particle (ALP) is a class of dark matter candidates where single particle states are highly occupied. The usual approach to model the non-relativistic ALP dynamics relevant to cosmic large scale structure formation is to treat it as a single classical field, in contrast to cold dark matter (CDM) which is treated as a collection of classical particles. Astrophysical “smoking gun” signatures of ALPs that set it apart from CDM are due to this difference. Literature on possible effects beyond the classical field description of ALPs is sparse and controversial, with some arguing that quantum effects are essential for an accurate description of large scale structure, while others conclude that quantum effects will arise only at times exceeding the age of the universe. These discrepancies are to some extent due to the application of different benchmarks for ``quantumness,'' or ``non-classicality.'' Here we introduce a quantum benchmark that is novel for ALPs, based on gravitationally induced self-squeezing. Self-squeezing, or Kerr-squeezing, is a process well known in the context of quantum optics and Bose-Einstein condensates. We show using a simple yet conservative model based on the validity of the classical Schrödinger-Poisson equation (a type of Gross-Pitaevskii equation) that for a typical QCD axion, squeezing can become as large as r=36 on very short time scales of order few thousand years, with the onset of squeezing r=1 reached within 0.1 milliseconds. According to this benchmark any viable cosmological ALP, such as the QCD axion or ultralight ``fuzzy dark matter’', is non-classical. This indicates that a deeper understanding of the quantum-classical transition of the ALP is required. All our conclusions were based on enforcing the validity of the Gross-Pitaevskii equation, so that exploring the observability of squeezing in future work is tantamount to scrutinizing the validity of the classical field description in ALP cosmology. Preprint: https://arxiv.org/abs/2105.13451

Sewing spacetime with Lorentzian Threads

nspired by the universality of computation, we advocate for a principle of spacetime complexity, where gravity arises as a consequence of spacetime optimizing the computational cost of its own quantum dynamics. This principle is explicitly realized in the context of the Anti-de Sitter/Conformal Field Theory correspondence, where complexity is naturally understood in terms of state preparation via Euclidean path integrals, and Einstein's equations emerge from the laws of quantum complexity. We visualize spacetime complexity using Lorentzian threads which, conceptually, represent the operations needed to prepare a quantum state in a tensor network discretizing spacetime. Thus, spacetime itself evolves via optimized computation.

Noether charge formalism for Weyl-transverse gravity

The gravitational Noether charge (Iyer-Wald) formalism has been shown to provide a systematic way to calculate conserved quantities, such as canonical energy or Wald entropy. Its original version applies to local, fully diffeomorphism invariant theories of gravity. In my talk, I introduce an extension of the Noether charge formalism to local theories of gravity invariant under transverse diffeomorphisms and Weyl transformations. Among these theories, Weyl-transverse gravity is of particular interest, having the same classical solutions as general relativity. However, the difference in symmetry group leads to a radiatively stable value of the cosmological constant. Given these attractive features of Weyl-transverse gravity, I discuss the application of our formalism to the first law of black hole mechanics and the Wald entropy formula in this theory. Especially, I focus on the contributions coming from the cosmological constant and from possible violations of local energy conservation, which are in principle allowed in Weyl-transverse gravity.

Can One Hear the Shape of a Quantum Spacetime?

Quantum gravity is a difficult problem in part because Quantum Field Theory uses the mathematics of analysis while General Relativity is in terms of differential geometry. One area of mathematics that can incorporate both the analytical and the geometrical is Spectral Geometry. This field tries to answer the question: Can one hear the shape of a drum? More specifically, it studies the geometric information encoded in the spectra of operators on (traditionally Riemannian) manifolds. In quantum gravity, we are interested in applying ideas from Spectral Geometry to spacetimes, which are conventionally described by Lorentzian manifolds. I will discuss our work in this direction, within the context of causal set theory, a theory of quantum gravity in which spacetime is fundamentally discrete and described by a partial order of events (a causal set). Causal sets provide a powerful framework in which to study spectra associated with spacetimes. This is because operators on causal sets are finite matrices with a finite number of eigenvalues, and the discretization is frame-independent. I will give an overview of the information contained in the spectra of causal set operators. In particular, I will present evidence that these spectra can help us determine the shape of a quantum spacetime, specifically its features such as dimension and ”manifoldlike-ness”.

Predictions for Quantum Gravitational Signatures from Inflation

I report on our recent computation of the corrections to the primordial power spectrum that should arise in realistic inflationary scenarios if there exists a generic covariant ultraviolet (UV) cutoff, as commonly motivated by considerations of quantum gravity. The UV cutoff can also be viewed as a covariant form of bandlimitation. The corrections to the primordial power spectrum consist of small superimposed oscillations whose frequency, phase, and amplitude are functions of the comoving wave number. Given cosmological parameters that characterize the slow roll during inflation, the frequency predicted for these oscillations depends only on the value of the UV cutoff. The specificity of this prediction can be used to increase experimental sensitivity through the filtering for template signatures. This will allow experiments to put new bounds on where a natural UV cutoff can be located between the Planck scale and the Hubble scale during inflation. It may bring imprints of Planck-scale physics in the cosmic microwave background and in structure formation within the range of observations.

Observable artefacts and preserved symmetries of bandlimited quantum field theories

It has been known for decades that quantum field theory exhibits mathematical pathologies at very small scales. This is why an ultraviolet (high-energy) cutoff is assumed to exist, although its details are often left unspecified. The standard approach to this problem is to employ renormalisation, which seeks to model low-energy behaviour for field theories whose high-energy details are unspecified. In this talk, I will describe a different approach inspired by information theory: Bandlimited quantum field theory is an approach to quantum fields that allows them to simultaneously be described as both continuous and discrete. This eliminates the need for renormalisation and leads to new detectable artefacts of the bandlimit. I will discuss two effects of these features. The first is a collection of detectable signatures of the bandlimit that appear when coupling such a field to local quantum systems. The second is a way to map between discrete theories and continuous theories in a way that preserves continuous translational symmetry – and corresponding conserved linear momentum – at the quantum level. A number of interesting physical and conceptual features arise from this perspective.

Quantised mass-energy effects in a particle detector

The Unruh-DeWitt (UDW) detector is a simple but powerful model of a quantum particle interacting with an external environment, e.g. a field. The particle's internal state can change in response to the field---e.g., internal energy can increase at the expense of decreasing field energy. Research with UDW models has mostly used a classical description of the detector's external degrees of freedom (DOFs), i.e. assigning it a classical trajectory, and treating only the internal state as a quantum DOF. On the other hand, formal field-theoretic versions describe the detector itself as a quantum field. Neither model captures the natural scenario of a low-energy quantum particle, e.g. an atom, interacting with a quantum field, e.g. light. Hence much recent interest has arisen in more realistic quantum descriptions of the detector's centre of mass, where it has been described either as moving in superposition along classical trajectories, or dynamically evolving under a non-relativistic Hamiltonian. Yet results in atomic physics show mass-energy equivalence plays a crucial role in energy and momentum conservation for atom-light interactions. Neither of the above UDW models can capture this effect, as absorption or emission of field quanta must also change the detector's rest mass by an equivalent energy. Here we address this problem and incorporate quantisation of the detector's mass-energy into the UDW model. We show that changes in internal energy due to emission/absorption persist even at low energies. Specifically, corrections to transition rates due to mass changes cannot be ignored unless the entirety of the center of mass dynamics is also ignored. Our results imply that one cannot model a massive particle interacting with a relativistic quantum field consistently without at the least including relativistic mass-energy equivalence in the particle's dynamics.

Unruh-DeWitt Detectors with Relativistic Centre of Mass

Models of quantum particles interacting with an external field, which may for example describe the physics of atom-light interactions, are frequently referred to as particle detector models. One of the most widely used is a model where a quantum particle is linearly coupled to a relativistic quantum field, which is known as the Unruh-DeWitt (UDW) model. In this model, it is traditionally assumed that the detector (i.e. the atom or particle) follows a classical worldline, and only its internal degrees of freedom are quantised. However, a fully quantum treatment should also include a quantised description of the detector’s external centre-of-mass degrees of freedom. This was previously proposed in Ref. [1], and studied for uniformly accelerating detectors in Ref. [2]. However, these past analyses have so far only considered detectors in the non-relativistic regime, which is insufficient to fully model the dynamics and is known to lead to spurious effects and unphysical friction forces acting on the detector [3, 4]. To resolve these conflicts, we extend the non-relativistic model to include the full relativistic dispersion relation of the UDW detector’s quantised centre of mass, and consequently derive an analytical description of the detector’s dynamics in the regime of a “first-quantised” relativistic quantum mechanics. Moreover, we compare these results to a fully relativistic “second-quantised” model, where the particle detector is instead modelled by a quantised scalar field. Our extension, beyond simply a demonstration of self-consistency across various regimes, explores more deeply the transition of relativistic quantum field theory to non-relativistic quantum mechanics, and in particular studies the nature of mass-energy and localisation of particle states between these two theories. [1] N. Stritzelberger and A. Kempf, Phys. Rev. D 101, 036007 (2020). [2] V. Sudhir, N. Stritzelberger, and A. Kempf, Phys. Rev. D 103, 105023 (2021). [3] M. Wilkens, Phys. Rev. A 47, 671 (1993); M. Wilkens, Phys. Rev. A 49, 570 (1994). [4] M. Sonnleitner, N. Trautmann, and S. M. Barnett, Phys. Rev. Lett. 118, 053601 (2017); M. Sonnleitner and S. M. Barnett, Phys. Rev. A 98, 042106 (2018).

Experimental observation of curved light-cones in a quantum field simulator

We investigate non-equilibrium dynamics in an experimental system of two parallel one-dimensional quasi-condensates described by quantum field theory in inhomogeneous background metric. By measuring local phononic fields, we observe the propagation of correlations along sharp light-cone fronts of curved shape, which under certain conditions get reflected at the system’s boundaries and return at the recurrence time. By extracting the space-dependent variation of the front velocity from the data, we find agreement with theoretical modeling, suggesting that the inhomogeneous atomic density controls this space dependence. The presented advances open the pathway toward a wide range of quantum simulations of field dynamics influenced by curved spacetime metrics. Presenting on behalf of collaboration with: M. Tajik, N. Sebe, P. Schüttelkopf, F. Cataldini, J. Sabino, F. Møller, S.-C. Ji, S. Erne, G. Guarnieri, S. Sotiriadis, J. Eisert, and J. Schmiedmayer

Vacuum entanglement from cold atoms

By coupling a pair of laser fields to a Bose-Einstein condensate, the entanglement harvesting protocol can be implemented for spacelike-separated analogue vacuum field detectors. The relativistic analogy is found to be robust in experimentally-accessible parameter regimes, justifying the neglect of dispersive corrections to the effective vacuum field. Entanglement harvested from the effective vacuum field can be deduced from correlations in a pair of photodetectors.

Primary thermalisation mechanism of Early Universe observed from Faraday-wave scattering on liquid-liquid interfaces

Parametric instabilities can be responsible for dramatic events, from the collapse of bridges and rolling of ships at sea to the thermalisation of our universe following cosmic inflation, 13.8 billion years ago. In a leading theory for the thermalisation of the Early Universe, known as preheating, broad parametric resonance efficiently transfers the energy of the inflaton field to other fields and particles, thus producing the hot plasma required for the Big Bang theory to proceed. However, direct observations of the non-linear dynamics of preheating in the early universe are not feasible. Here, we conduct a controlled experiment to simulate the key aspects of inflationary preheating in a parametrically driven interface between two fluids. We study the scattering of large amplitude Faraday waves and observe a broadening of primary resonance bands and the subsequent appearance of secondary instabilities and their estimated growth rates, as predicted in preheating. Adapting the statistical machinery from field theories, namely two-point functions and the factorisation properties of higher order correlators, we show that the interfacial evolution is accurately captured by leading terms in an effective perturbative description. Our results demonstrate the robustness of preheating dynamics in a strongly interacting and damped system.

The demonstration of switchable detector-field coupling implemented using superconducting systems

We present our recent work on the demonstration of switchable coupling between a detector and a quantum field [1]. The detector is implemented as a superconducting flux quantum bit, with a transition frequency in the GHz range, and the field is the electromagnetic field in a superconducting waveguide. The coupling is realized based on a magnetic field controlled superconducting device. The normalized coupling strength, defined as the ratio of the qubit emission rate to the qubit frequency, ranges from effectively zero (with an experimental bound of 6 x 10-5) to the ultrastrong coupling regime (2 x 10-2). We performed a detailed characterization of the system using microwave scattering experiments, which allows for the extraction of the transition frequency, coupling to the field, and decoherence, relative to the system control parameters. We discuss the prospects for using this system for future experimental investigations of relativistic quantum information. 1. N. Janzen, X. Dai, S. Ren, J. Shi, A. Lupascu, arXiv:2208.05571 (2022).

Harvesting entanglement from fermionic fields with linearly coupled detectors

Entanglement harvesting protocols have been studied in a variety of setups. For the case of non-Hermitian fields, until recently only quadratic couplings had been investigated. In this talk, I will discuss entanglement harvesting with particle detectors that couple linearly to non-Hermitian fields. In particular, I will discuss particle detectors coupled to a complex scalar quantum field, as well as to a spin 1/2 fermionic field. I will conclude by showing that the complex scalar model can be a good approximation for the fermionic model in the protocol of entanglement harvesting when the mass of the field is sufficiently large compared to the inverse interaction time.

Probing entanglement area laws with particle detectors

Studying the entanglement structure of a quantum field theory provides rich insight into its information-theoretic properties. However, the tools presently available to evaluate entanglement in quantum fields can only be feasibly applied in a small subset of physical systems, such as vacuum ground states or systems with holographic descriptions. Thus, we still lack a method of computing a true entanglement measure which is both practical and broadly applicable. The solution may come in the form of particle detector models, a class of systems which serve as probes of the quantum field, and provide such benefits as being inherently regularized and more directly connected to experimental protocols. As a first proof-of-concept, we are working to reproduce the area laws characteristic of local quantum field theories using detector models. In this talk, I will share initial results and future directions of this exploration.

Particle detector and mirror in GUP modified vacuum.

We consider quantized scalar field with GUP (Generalized uncertainty principle) modified dispersion relation. GUP will modify the Klein-Gordon equation. We find the solution of modified Klein-Gordon equation in presence of a plane reflecting boundary in (1+1) dimension. Now we consider the excitation of a two level atom (Unruh DeWitt detector) by a single GUP modified mode in following two scenarios : (a) The atom is accelerating in present of a static mirror and (b) The atom is static in presence of an accelerating mirror. We calculate the spontaneous excitation probability of the atom in the two cases. We show that in case (b), the GUP modifiers the spatial oscillation of the probability. This destroys the symmetry, that exists in absence of GUP, between the excitation probability in the case (a) and (b). We discuss the significance of our result in context of the test of weak equivalence principle in quantum electrodynamical systems. Journal ref: Riddhi Chatterjee, Sunandan Gangopadhyay, and A. S. Majumdar, Phys. Rev. D 104, 124001 (2021).

Detection of evanescent particles

We show that an Unruh-DeWitt detector is sensible to the presence of evanescent particles. These particles come from the quantization of classical evanescent modes of a scalar field in a neighborhood of a timelike hypersurface. Crucially, this quantization is achieved via the recently proposed α-Kähler quantization. The probability amplitudes of the detection are computed for different trajectories of the detector.

Measurement proposal for acceleration temperature in BEC vacuum, with laser energy interferometer.

Black Hole radiation has arguably been seen in experiments in BECs. A much more difficult task is to see acceleration temperatures. Again BECs seem the natural venue for this. Arising out of discussions at a meeting in Nottingham almost 4 years ago, a group of us have come up with a proposal to measure the acceleration temperature in a BEC sonic vacuum. To accomplish this will require a new type of detector (essentially a microphone which converts sound waves or phonons into electomagnetic waves or photons. borrowing key ideas from the quantum nature of LIGO detectors, we suggest the use of a novel interferometer in which the two arms live in frequency rather than physical space. The quantum mechanics of this system is also fascinating, in which damping and amplification can be used to minimize the effect of measurement on the quantum system.

Black hole entropy in causal set quantum gravity

Quantum, thermal and gravitational physics are all implicated in the values of the temperature and entropy of a black hole. The value of the entropy -- the number of Planck-sized plaquettes tiling the horizon -- speaks of discreteness and is one of the strongest motivations for the causal set approach to the problem of quantum gravity. I will talk about progress towards a statistical mechanical account of black hole entropy -- and causal horizon entropy more generally -- within causal set theory.

Disjoint Region Entanglement Entropy in Causal Set Theory

Understanding entanglement entropy in a covariant manner is considered a potential pathway toward a theory of quantum gravity, and entanglement entropy in itself has a broad range of connections to other areas of physics. We investigate the entanglement entropy of a 1+1D scalar field in disjoint intervals within the causal set framework. This involves using a truncation scheme for the spacetime commutator and correlator, the Pauli-Jordan and Wightman functions. We extend existing entanglement entropy calculations to disjoint regions via a new truncation scheme for disjoint causal diamonds, which follows from the single diamond truncation scheme. We then show the results from setups including two and three disjoint causal diamonds, as well as a single causal diamond that shares a boundary with a larger global causal diamond. In all these cases, our results agree with the expected area laws calculated via the continuum theory. The ease of our calculations indicate our methods to be a useful tool for numerically studying such systems. We end with a discussion of some of the strengths and future applications of the spacetime formulation we use in our entanglement entropy computations, both in causal set theory and in the continuum.

Harvesting mutual information from BTZ black hole spacetime

We investigate the correlation harvesting protocol for mutual information between two Unruh-DeWitt detectors in a static BTZ black hole spacetime. Here, the effects coming from communication and change in proper separation of the detectors are set to be negligible so that only a black hole affects the extracted mutual information. We find that, unlike the entanglement harvesting scenario, harvested mutual information is zero only when a detector reaches an event horizon, and that although the Hawking effect and gravitational redshift both affect the extraction of mutual information, it is extreme Hawking radiation that inhibits the detectors from harvesting.

When entanglement harvesting is not really harvesting

We revisit the entanglement harvesting protocol when two detectors are in causal contact. We study the role of field-mediated communication in generating entanglement between the two detectors interacting with a quantum field. We provide a quantitative estimator of the relative contribution of communication versus genuine entanglement harvesting. For massless scalar fields in flat spacetime, we show that when two detectors can communicate via the field, the detectors do not really harvest entanglement from the field, and instead they get entangled only via the field-mediated communication channel. In other words, in these scenarios the entanglement harvesting protocol is truly "harvesting entanglement" from the field only when the detectors are not able to communicate. In contrast, for massive scalar fields both communication and genuine harvesting contribute equally to the bipartite entanglement when the detectors are causally connected. These results emphasize the importance of taking into account the causal relationships between two parties involved in this relativistic quantum information protocol before we can declare that it is truly entanglement harvesting.

Entanglement Harvesting in 3+1D Schwarzschild Spacetime

Title: Entanglement Harvesting in 3+1D Schwarzschild Spacetime joint work with: João G. A. Caribé, Robert H. Jonsson, Marc Casals, Achim Kempf, Eduardo Martín-Martínez Abstract: Exploring the impact of spacetime curvature on vacuum entanglement is at the core of the study of entanglement harvesting from quantum fields, in particular, around black holes. In this work we are able to address entanglement harvesting in 3+1D Schwarzschild spacetime, employing advanced methods to calculate the field's Wightman function in the Boulware, the Hartle-Hawking and the Unruh vacuum state. We demonstrate a lensing effect around the caustics of the black hole spacetime. Here, even for causally connected detectors, pre-existing vacuum entanglement can dominate the harvesting. This is in contrast to flat spacetime where pre-existing entanglement dominates only outside of the causal cone, as was recently shown by Tjoa and Martín-Martínez.

Asymptotic measurement schemes for all observables of a quantum field theory

Measurement schemes have been implemented in a covariant and general fashion in quantum field theory [1] providing a fundamental description of the measurement chain from which state update rules may be derived. The main idea is that the system QFT is coupled to a probe QFT in a compact coupling region and that measurements of probe observables may be interpreted as measurements of local system observables. Among other things, the framework is free of the "impossible measurements" originally discussed by Sorkin nearly 30 years ago [2]. However there remains the question of how comprehensive this approach is, and whether all local observables can be measured using one of these measurement schemes. In this talk I present work with Jubb and Ruep [3] which answers this question for the real Klein-Gordon field. In particular: (a) if one drops the requirement of compact coupling regions from [1], measurement schemes exist for all local observables; (b) if one maintains the assumption of compact coupling regions, we show that every local observable can be obtained as the limit of a sequence of observables for which measurement schemes exist, which we describe as an asymptotic measurement scheme. [1] CJ Fewster & R Verch, Commun. Math. Phys. 378 (2020) 851-889 arXiv:1810.06512. [2] H Bostelmann, CJ Fewster & MH Ruep, Phys. Rev. D 103 (2021) 025017 arXiv:2003.04660. [3] CJ Fewster, I Jubb & MH Ruep. arXiv:2203.09529.

How to make measurement possible in QFT

Arguments by Sorkin (1993) and Borsten, Jubb, and Kells (2021) establish that a natural extension of quantum measurement theory from non-relativistic quantum mechanics to relativistic quantum theory leads to the unacceptable consequence that expectation values in one region depend on which non-selective measurement is performed in a spacelike separated region. We analyze two recent proposals for measurement theories for QFT that adopt different approaches to responding to Sorkin's no-go result: a measurement theory based on detector models proposed in Polo-Gómez, Garay, and Martín-Martínez (2022) and a measurement framework for algebraic QFT proposed in Fewster and Verch (2020). Our comparison of these two approaches aims to clarify both their differences and their similarities. There are two similarities that are of particular interest for the foundations of QFT. First, the traditional operational interpretation of a local algebra of observables A(O) as representing possible operations carried out in region O is abandoned in both. Second, the respective state update rules cannot be interpreted as representing a physical change of state of the system that occurs in any region of spacetime. This is an example of how the formulation of a measurement theory for QFT that addresses Sorkin's no-go result can change the form taken by the Measurement Problem, which was originally framed using NRQM and its measurement theory. This is joint work with Maria Papageorgiou.

Effective notions of causality in detector models

Detector models in QFT model measurements of quantum fields as interactions with quantum devices. Detector models based on the light-matter interaction often require the detectors to be delocalized in space and time. For example, the localization of an atom in flat spacetime will be given by the wavefunction of its initial state, which is defined everywhere unless the potentials involve infinite potential wells. Physically, however, the atoms are ‘’mostly’’ localized in a region of spacetime and its wavefunction decays exponentially away from this region. When several of such delocalized detectors are involved, it becomes necessary to discuss signalling relations between them in an effective way. I will argue that the localization of general detectors, even in curved spacetime, can be made effective from the point of view of the signalling relations between detectors. The quantitative figure of merit that I will discuss is the Quantum Fisher information, which we will use to encode signalling relations and to define effective causal relations between detectors. Possible applications to theories with faster than light signalling, such as bandlimited or analogue fields, will also be briefly discussed.

Measuring quantum fields with particle detectors and machine learning

We demonstrate how we can use machine learning techniques to bypass the technical difficulties of designing an experiment and translating its outcomes into concrete claims about concrete features of quantum fields. In practice, all measurements of quantum fields are carried out through local probes. Despite measuring only a small portion of the field, these local measurements have the capacity to reveal many of the field's global features. This is because, when in equilibrium with their environments, quantum fields store global information locally, albeit in a scrambled way. We show that neural networks can be trained to unscramble this information from data generated from a very simple one-size-fits-all local measurement protocol. To illustrate this general claim we will consider the case-study of how a particle detector can learn about the field's boundary conditions before signals can propagate from the boundary to the detector. The exact same simple fixed local measurement protocol and machine-learning ansatz can be used successfully for a wide variety of features of the QFT, supporting the claim that the framework proposed here can be applied to any kind of local measurement on a quantum field to reveal nearly any of the field's global properties in a one-size-fits-all manner.

Spacetime curvature from ultra rapid measurements of quantum fields

In this talk we will show a formulation of the notion of curvature of spacetime from quantum fields via measurements with particle detectors. This is done by rewriting the curvature of spacetime in terms of the excitation probability of particle detectors ultra-rapidly coupled to a scalar quantum field on an arbitrary background. Using a short distance expansion for the Wightman function, we express the excitation probability of a detector as the transition probability in Minkowski spacetime plus correction terms written as a function of the curvature tensors and the detector size. Comparing the excitation probability in curved spacetimes with its flat analog, we can express the components of the Ricci and Riemann curvature tensors as a function of physically measurable excitation probabilities of different shaped detectors.

Uncovering the Thermodynamics of Quantum Fields

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 and heat is actually an open problem with multiple incompatible answers. Even more complications appear when looking from the perspective of quantum field theory. Many of the notions arising from quantum thermodynamics, such as the popular two-point measurement scheme of work, become ill-defined. In this talk we will present tools from quantum thermodynamics that could be adapted to quantum fields, including work and heat distributions, fluctuation theorems and coarse-grained or observational entropy. Moreover, we will present the complications that arise for quantum fields and how to overcome them, using KMS thermal states and probes. Finally, we will go over some current advances on the thermodynamics of quantum fields: work distributions, the first law of thermodynamics and heat engines fuelled by the Unruh effect.

Quantum Field Theory based Quantum Information: Measurements and Correlations

We contend that a relativistic quantum information theory requires a formulation of relativistic quantum measurements in terms of quantum fields that is (i) local and causal and (ii) applicable to current and proposed experiments in the relativistic regime. We present the Quantum Temporal Probabilities (QTP) formalism, which identifies detection probabilities as linear functionals of QFT correlation functions. We show the relation of QTP to the Schwinger-Keldysh formulation of QFT, and we present some applications, emphasizing the convergence with notions from non-equilibrium QFT.

Experimental Observation of Acceleration-Induced Thermality

The incorporation of classical general relativity into the framework of quantum field theory yielded a rather surprising result -- thermodynamic particle production. In short, for fundamental deformations in the structure of spacetime, quantum mechanics necessitates the creation of thermalized particles from the vacuum. One such phenomenon, known as the Unruh effect, causes empty space to effervesce a thermal bath of particles when viewed by an observer undergoing uniformly accelerated motion. These highly accelerated systems will also have an associated Rindler horizon which produces this Unruh radiation at the celebrated Fulling-Davies-Unruh temperature. For accelerated charges, the emission and absorption of this Unruh radiation will not only affect the associated Rindler horizon in accordance with the Bekenstein-Hawking area-entropy law, but will also imprint the FDU temperature on any photons emitted and subsequently detected in the laboratory. A recent series of high energy channeling experiments carried out by the NA63 collaboration at CERN have finally brought about the first observations and insights into the nature of the Unruh effect. In this presentation, I will discuss the various aspects of acceleration-induced thermality measured by these experiments at NA63.

Falling through masses in superposition: quantum reference frames for indefinite metrics

The current theories of quantum physics and general relativity alone do not allow us to study situations where the gravitational source is quantum. In my talk, I will propose a strategy to determine the dynamics of probe quantum systems in the presence of mass configurations in superposition, and thus an indefinite spacetime metric, using quantum reference frame (QRF) transformations. In particular, I will establish the formalisms that allow us to move from a QRF where the metric is indefinite to a QRF where the metric is definite. Assuming the covariance of the dynamical laws under the QRF transformation, this will transform the problem of the dynamics of probe quantum systems in indefinite metrics into a physically equivalent problem of the dynamics in a definite metric.

Quantum vacuum excitation of a quasi-normal mode in an analog model of black hole spacetime

Vacuum quantum fluctuations near horizons are known to yield correlated emission by the Hawking effect. We use a driven-dissipative quantum fluid of microcavity polaritons as an analogue model of a quantum field theory on a black-hole spacetime and numerically calculate correlated emission. We show that, in addition to the Hawking effect at the sonic horizon, quantum fluctuations may result in a sizeable stationary excitation of a quasi-normal mode of the field theory. Observable signatures of the excitation of the quasi-normal mode are found in the spatial density fluctuations as well as in the spectrum of Hawking emission. This suggests a general and intrinsic fluctuation-driven mechanism leading to the quantum excitation of quasi-normal modes on black hole spacetimes.

On inference of quantization from gravitationally induced entanglement

Observable signatures of the quantum nature of gravity at low energies have recently emerged as a promising new research field. One prominent avenue is to test for gravitationally induced entanglement between two mesoscopic masses prepared in spatial superposition. In this talk, we will briefly analyse such proposals and what one can infer from them about the quantum nature of gravity.

Gravitationally induced entanglement dynamics of photon pairs and quantum memories

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

The positive formalism - or - how to do quantum theory in the absence of classical temporal order

I will provide a lightning overview of the positive formalism with an emphasis on how it allows us to do quantum theory without a fixed temporal order between processes. This is necessary not only for quantum gravity, but also in certain situations of quantum field theory.

Quantum superposition of massive objects and the nature of gravity in quantum theory and beyond

Recently, table-top experiments involving massive quantum systems have been proposed to test the interface between quantum theory and gravity. I will first analyse a Gedankenexperiment in which entanglement is generated via gravitational interaction between two quantum systems A and B. I will argue that, within a field-theoretic description of the gravitational field and in a Newtonian regime, it is necessary to include in the description some quantum features of gravity (vacuum fluctuations and emission of quantised radiation) to avoid faster than light signalling. I will then present a no-go theorem, which holds independently of the specific description of gravity and for general non-classical systems A and B, stating that the following statements are incompatible: i) gravity is able to generate entanglement; ii) gravity locally mediates the interaction between the non-classical systems; iii) gravity is classical. Finally, I will comment on the implications of the violation of each condition.

Uniqueness of the Fock quantization of charged scalar and fermionic fields under Schwinger effect

In quantum field theory, breaking of time translational invariance caused by an intense external field, such as an electromagnetic field, produces particle creation. This leads to an ambiguity in the definition of the Fock vacuum. In cosmological backgrounds this ambiguity has been reduced by imposing that the quantization preserves the symmetries of the system and that the dynamics is unitarily implemented. In this talk, we apply these requirements to the quantization of a massive charged field coupled to a classical time-dependent homogeneous electric field, considering both scalar and fermionic fields. For both cases, we characterize the quantizations fulfilling the criteria above and we show that they form a unique equivalence class of unitarily related quantizations, which provide a well-defined number of created particles at all finite times.

Vacuum ambiguities in the Schwinger effect and the quantum kinetic approach

The Schwinger effect is a non-perturbative particle creation phenomenon due to the application of a strong electric field. When this external agent is time-dependent, there appear ambiguities when choosing the canonical vacuum of the quantum theory. We analyze how the time evolution of the number of created particles depends on these ambiguities. Moreover, after providing a generalization of the standard quantum Vlasov equation for general vacua, we focus on the family of quantizations that allow for unitary dynamics. As a result, we propose a new criterion to further reduce the family of physically acceptable vacua.

Black Holes Decohere Quantum Superpositions

We show that if a massive body is put in a quantum superposition of spatially separated states, the mere presence of a black hole in the vicinity of the body will eventually destroy the coherence of the superposition. This occurs because, in effect, the gravitational field of the body radiates soft gravitons into the black hole, allowing the black hole to acquire "which path" information about the superposition. A similar effect occurs for quantum superpositions of electrically charged bodies. We provide estimates of the decoherence time for such quantum superpositions. We believe that the fact that a black hole will eventually decohere any quantum superposition may be of fundamental significance for our understanding of the nature of black holes in a quantum theory of gravity.

Avoiding the black hole information paradox with semiclassical dynamics

In this talk I will present the possible outcomes of the evolution of black holes in semiclassical gravity. Among them, I will focus particularly on the ones in which the formation of a singularity is avoided, without passing through regions of Planckian curvature (which could break the validity of the semiclassical approximation). I will show how such configurations can be related to the ubiquitous instability of inner horizons, when the source of perturbation is the energy-positivity-violating expectation value of a stress energy tensor operator.

Influence of cosmological expansion in local experiments

Whether the cosmological expansion can influence the local dynamics, below the galaxy clusters scale, has been the subject of intense investigations since the ’40s. In this talk, I will consider the effect of the global cosmological expansion on local experiments in an idealized setting by employing McVittie and Kottler spacetimes, embedding a spherical object in an FLRW spacetime, as approximate, idealized models of a local gravitational environment. In particular, I will discuss the influence of the cosmological expansion on the frequency shift of an optical resonator and estimate its effect on the exchange of light signals between local observers, clarifying some of the statements present in the literature. References [1] F. Spengler, A. Belenchia, D. Rätzel, and D. Braun, Influence of cosmological expansion in local experiments, Class. Quantum Grav. 39 055005 (2022).

Black semiclassical stars

The gravitational collapse of massive stars serves to manifest the most severe deviations of general relativity with respect to Newtonian gravity: the formation of horizons and spacetime singularities. Both features have proven to be catalysts of deep physical developments, especially when combined with the principles of quantum mechanics. Nonetheless, it is difficult to combine all these developments into a unified theoretical model, while maintaining reasonable prospects for the independent experimental corroboration of its different parts. I will present evidence that semiclassical gravitational collapse can give place to self-consistent ultra-compact stars, indistinguishable up to current observations from black holes but very different from the structural point of view. In particular, the matter content plays a fundamental role. Hopefully, near-future gravitational-wave observations will be able to discern among these different models.

What can gravity mediated entanglement really tell us about quantum gravity?

If gravity can entangle systems of masses, does that mean that there’s something quantum about gravity? if so, what is it? if not, why not? We will review some of the assumptions made in the interpretation of proposed tabletop experiments for quantum gravity and what we can learn from them.