RQI Circuit 2026

RQI Circuit Stockholm - Opening Remarks

The RQI Circuit Stockholm will start at 10am CEST on May 29th. The local organizers are Jerzy Paczos and Germain Tobar.

Operations, causality, and memory in continuous time

Quantum combs efficiently model multi-time processes, where a system can be probed multiple times, without any assumption about underlying system-environment interactions. However, they are restricted to scenarios where any system-environment interaction ‘halts’ when the system is probed, an assumption not always justifiable in realistic scenarios. More foundationally, combs and general process matrices underpin the study of quantum causal structures, but they are limited to discrete models of spacetime, or instantaneous operations in spacetime. I will present a continuous-time version of combs/process matrices based on process and operation functionals, which generalize the Feynman–Vernon influence functional and yield a continuous Born rule that cleanly separates pocesses from operations. Operational continuous-time definitions of causality and Markovianity arise, which characterise, respectively, processes compatible with a fixed causal order and processes without memory. The framework has direct applications to continuous time sensing of dissipative systems and opens the way to extend the study of quantum causal structure to the continuum.

What is an independent system?

I will review different notions of independence for quantum systems in the context of von Neumann algebras, both for quantum mechanics and quantum field theory. Then I will show how quantum gravity challenges these notions and point towards possible resolutions.

Theoretical, Operational, and Experimental Progress on Vacuum Entanglement

Although it is know that the vacuum of a massless scalar field contains entanglement between any pair of disjoint spacetime regions, this entanglement has not yet been quantified or experimentally measured. In this talk I will discuss recent progress on 1) the theoretical quantification of vacuum entanglement through local modes in QFT, 2) optimizations of the operational approach to vacuum entanglement extraction through the protocol of entanglement harvesting, and 3) a recent experimental proposal of entanglement harvesting, extracting entanglement from the ground state of a Bose-Einstein condensate using localized polarons that couple to it in effectively causally disconnected regions.

Optimization of Entanglement Harvesting with arbitrary temporal profiles

Experimental verification of vacuum entanglement in QFT is hindered by prohibitively small numbers. In this talk we optimize the protocol of entanglement harvesting for two spacelike separated probes that couple to a field in arbitrary temporal regions. Using analytical expressions for smeared propagators, we find optimal compactly supported switching functions that significantly improve the prospects for experimental realization.

Non-perturbatively extracting the two-point function of relativistic quantum fields

In this talk we present a non-perturbative method for extracting the two-point function of a quantum field between arbitrary spacetime regions through measurements of local probes. To do so we build a lattice of interactions in spacetime and pinpoint the precise relationship between the field’s smeared two-point function and the probe observables. We also consider the limit when the interaction regions are spacetime Dirac delta functions and quantify the uncertainty acquired in the two-point function measurement based on the interaction region sizes.

Lunch Break

Career development talk

Measuring gravitational acceleration with large-mass quantum resonators

Mechanical resonators in the quantum regime, including moving-end mirrors and levitated nano- and micro-particles are extremely well suited for measuring weak gravitational effects. The main reason is their large mass, which lies several orders of magnitude above the single-atom scale. In this talk, I'll discuss efforts to model these systems theoretically and to compute the fundamental sensing advantage for measuring gravitational acceleration. I will also speak broadly about the plans and activities in our newly established research group for studying quantum dynamics and sensing.

Partner modes and the localization of entanglement in Gaussian quantum field theory

We introduce a framework to identify where the total correlations with a chosen degree of freedom reside within the rest of the system for pure Gaussian states in quantum field theory. We first show that every correlated mode possesses a unique single–degree-of-freedom partner, distributed all over space, that fully captures its correlations (consisting of entanglement) and we provide an explicit construction of this purification partner from the complex structure of the system’s state. Then, we introduce a locally symplectic-invariant quantifier of correlations between two different arbitrary modes, denoted by Dsym. This quantity admits a simple geometric interpretation as an overlap between each mode and the purification partner of the other. We formulate a simple necessary and sufficient criterion for two-mode entanglement in Gaussian states in terms of Dsym, placing on firm quantitative footing the intuition that entanglement with a given localized mode ‘lives’ on the spatial support of its partner mode.

Bipartite entanglement harvesting with multiple detectors

In this talk, I will present our study on bipartite entanglement harvesting from the quantum vacuum of a massless scalar field between two subsystems, each composed of an arbitrary number of Unruh–DeWitt detectors. Using perturbation theory, we show that the leading-order negativity is fully determined by a submatrix of the reduced density matrix whose dimension grows linearly with the number of detectors. Within this framework, we analyze how detectors’ spatial arrangement influences harvesting, focusing on three- and four-detector configurations, as well as a linear chain of detectors. The results clarify how to arrange multiple detectors to optimize harvesting and show that increasing their number broadens the ranges of energy gaps and separations over which entanglement can be extracted from the field.

Bounding quantum measurement disturbance by weighted state exclusion statistics

Among the defining features of quantum theory is the unavoidable change experienced by a physical system when information is extracted through a measurement. This phenomenon, referred to as quantum measurement disturbance, limits the sets of observables whose statistics can be inferred from a single sample and is now recognized as a key security ingredient in quantum communication protocols. In this work, we focus on quantifying measurement disturbance using the average fidelity with respect to an arbitrary ensemble of states. Building on this, we introduce the statistical disturbance bound, which can be inferred solely from measurement statistics without requiring access to the post-measurement states. In addition to providing its mathematical definition and a semidefinite programming formulation for its computation, we show how the statistical disturbance bound can be determined experimentally via a so-called weighted quantum state exclusion task. To demonstrate that our framework offers a substantial advantage over existing information–disturbance trade-off relations, we analyze the statistical disturbance bound for measurements affected by detector inefficiency or amplitude-damping noise.

Coffee Break

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Unruh–DeWitt Detectors in and for Analogue Space-Times

Unruh–DeWitt detectors have been widely applied to topics of curved space-time quantum field theory. They have been very important for and successful in creating operational ways to study effects predicted by this field of physics. Similarly, the effects predicted by curved space-time quantum field theory have been hugely influential in driving and creating the field of analogue space-times. Comparatively, the study of Unruh–DeWitt detectors in the context of analogues is still in its infancy, despite the many facets of relativistic quantum information and its adjacent fields it will illuminate. Here, I will present my ongoing work of this in the context of two particular analogue models: (1) An electromagnetic analogue in a uniaxial crystal, and (2) a quantum fluid of light. While the former demonstrates the non-trivial interplay of dispersion relations and detector responses, the latter will give an example of the dimension-dependent detector response as predicted by Takagi and others.

Quantum metasurfaces as probes of vacuum particle content

The quantum vacuum of the electromagnetic field is inherently entangled across distinct spatial sub-regions resulting in entangled particle content across these sub-regions. However accessing this particle content in a controlled laboratory experiment has remained out of experimental reach. Here we propose to overcome this challenge with a quantum mirror made from a two-dimensional sub-wavelength array of atoms that divides a photonic cavity. The array response to light is tunable between transmissive and reflective states by a control atom that is excited to a Rydberg state. We find that vacuum photon content from non-perturbative changes of the boundary conditions and therefore distinct spatial sub regions of the vacuum causes subtle frequency shifts that are accessible to sub-wavelength atom array platforms. This novel approach for probing vacuum particle content stems from the unique ability to create coherent dynamics of superpositions of transmissive and reflective states providing a quantum enhanced platform for observing vacuum particle creation from highly non-perturbative boundary condition changes of the electromagnetic field vacuum.

Gravitational wave imprints on spontaneous emission

Gravitational-wave studies and detection efforts have traditionally focused on the effects of gravitational waves on test masses and on geodesic deviation. In contrast, we show that gravitational waves can nontrivially influence spontaneous emission from point-like atoms, inducing directionality in the emission pattern and generating sidebands in the spectrum. We examine how much information about the gravitational-wave amplitude is encoded in the quantum state of the combined atom–field system, analyzing both the classical Fisher information associated with photon-number measurements and the quantum Fisher information. Our results suggest that the requirements for gravitational wave detection via this mechanism are not prohibitive. arXiv.2506.13872

Simulating gravitational-wave effects in strongly coupled atom–cavity systems

One of the proposed platforms in which both quantum and general relativistic effects can become observable is an atom interacting with the electromagnetic field in a gravitational-wave background. The periodic modulation of field modes induced by variations of the spacetime metric modifies the atomic emission spectrum. Notably, the influence of a passing gravitational wave can be simulated through modulated boundary conditions, such as moving cavity mirrors. We analyze the impact of this modulation on atom–field interactions in the strong atom–cavity coupling regime, where Rabi oscillations occur. We show analytically that the modulation is resonantly enhanced, leading to measurable imprints in the atomic transition probability. This establishes a realistic and experimentally accessible testbed for probing general relativistic effects in quantum optical systems.

RQI Circuit Vienna - Opening Remarks

The RQI Circuit Vienna will start at 11:25 CEST on June 5th. The local organizers are Emil Broukal, Bruna Sahdo, Lin-Qing Chen, Manuel Mekonnen, and Jan Mandrysch.

Quantum Reference Frames in Quantum Foundations and Gravity

Quantum reference frames (QRFs) provide a new way of understanding how physical descriptions depend on the systems relative to which they are formulated. In this talk, I will present the broader research program on QRFs and outline current and future directions at the interface of quantum foundations, quantum information, and gravity. In particular, I will discuss how observed relative frequencies may fluctuate in the presence of nonideal QRFs, how QRFs may help recover tensor-product structures in quantum gravity guided by the Einstein equivalence principle, and how they may regularize otherwise singular quantities such as entropies, in connection with recent results on gravitational algebras.

Time delocalization and causality across temporal quantum reference frames

In relational quantum dynamics, evolution emerges via the correlations between some system of interest and a clock system, which plays the role of a temporal reference frame. Their combined state satisfies a Wheeler-de Witt-like constraint equation, and therefore does not evolve, leading to a “block universe” picture. In this talk I want to present the interplay of two aspects, namely temporal localization and causal relations, when comparing emergent dynamics with respect to different choices of clock. I will explain the extent to which two clocks can agree on the temporal localization of events. Then, focussing on the operational notion of causality, we require a clearly defined notion of interventions, i.e. quantum operations, and consider two different approaches to modeling these operations within relational dynamics. The first considers their application via the choice of solutions to the constraint equation, i.e. the choice of which “history” is considered. The second approach incorporates the operations into the constraint equation itself and thereby into its solutions, giving a dynamical picture of the interventions. From the perspective of a single clock, both approaches allow for a notion of operational causality in relational dynamics. However, for multiple clocks, only the second approach gives a consistent picture regarding causal relations, while necessarily manifest- ing some degree of temporal delocalization between frames. Moreover, this second approach, when considering certain cases of temporal delocalization, naturally describes scenarios with indefinite causal order, a well-known quantum feature of operational causality.

Linking geometry and quantum theory via quantum reference frames

The geometric structures of general relativity are naturally captured in the language of principal bundles, while the operational content of quantum theory is encoded in POVMs on Hilbert spaces. We argue that the link between the two is natural and compelling: requiring that sample spaces of quantum observables carry the structure of principal bundles, covariant under gauge group actions, yields quantum reference frames — quantum systems with a built-in geometric notion of localisation and orientation. Physical observables are then relational: invariant under joint transformations of frame and system. This architecture underlies a programme for Relational Quantum Field Theory which we will outline and motivate from this structural standpoint.

Quantum coordinate transformations via matter fields for quantum spacetime

A central challenge in quantum gravity is to formulate a theory without presupposing a background spacetime. Such a description requires relational observables and a genuinely quantum generalisation of diffeomorphisms. Here we associate coordinate systems with sets of four dynamical scalar fields, whose stress-energy tensors contribute to the constraints of linearised quantum gravity. Such quantum coordinate fields (QCFs) extend the notion of quantum reference frames and enable a relational description of other physical systems. By generalising the perspective-neutral construction of quantum reference frames, we show that relational, gauge invariant observables admit a description in each QCF perspective, and derive consistent transformations between perspectives. The resulting unitary maps implement passive or active, local quantum coordinate changes, yielding quantum linearized diffeomorphisms beyond superpositions of classical ones. We further discuss the operational aspects of relational observables, the conceptual distinction between QCFs and quantum reference frames, and identify the difference between QCFs transformations and the superposition of classical diffeomorphisms.

Quantum permutations as genuinely quantum coordinate transformations

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Adding and removing subsystems in quantum reference frames

The tensor product rule for appending new subsystems has an important role in information-theoretic formulations of Quantum Theory. Adopting a relational view, we study the adding of subsystems in a quantum reference frame (QRF) for finite discrete translations following E. Castro-Ruiz, O. Oreshkov (2025). We show that the quantum mechanics textbook composition rule only holds in a QRF perspective if the reference system is compatible with a `classical' external state. In general, a frame perspective can be inconsistent with the existence of extra systems in certain relative states. This issue is closely related to the so-called paradox of the third particle. There is, however, a way to recover the tensor product rule for any QRF by an invariance-preserving unitary map on the setup, as long as if it is redefined to include the external observer. We show how this procedure induces a modified adding rule for subsystems with a physical interpretation. We make an analogy between the map applied to a quantum frame, which recovers the standard adding properties, and changes of coordinates in classical physics which, via the equivalence principle, make non-inertial frames appear inertial. The procedure also recovers transformations from inequivalent QRF approaches without projections, hinting at a conceptual framework where different formalisms are consistent at the same time.

Lunch Break

Observables in Table-Top Quantum Gravity

Quantum superpositions of near-classical spacetimes likely represent the regime of quantum gravity most accessible to empirical investigation within our lifetimes. Yet, the theoretical implications of the proposed experimental protocols remain difficult to articulate with precision. I argue that so-called “tabletop quantum gravity” experiments will reveal the quantum indefiniteness of the geometrical invariants that characterize spacetime. This claim foregrounds a central lesson of general relativity: in the absence of a fixed spacetime background, physical observables cannot be defined locally.

When does entanglement through gravity imply gravitons?

Detection of entanglement through the Newtonian potential has been claimed to support the existence of gravitons, by extrapolating to a thought experiment which demonstrates that complementarity and causality would be in conflict unless quantum fluctuations exist. We critically assess this consistency argument using scalar field models. We show that whether complementarity or no-signalling is violated when quantum fluctuations are neglected, depends on how this approximation is taken, while in both cases entanglement is generated locally in spacetime. We clarify that the correct reading of the paradox requires making a clear distinction between two notions of causality violation: Newtonian action-at-a-distance and the quantum mechanical no-signalling at space-like separation; the latter is pertinent while the former is not. We conclude that the thought experiment (a) does not add to the epistemological relevance of entanglement through Newtonian potentials (b) lends support for the existence of gravitons, if retardation effects are detected in entanglement through gravity.

Local Operations and Field Mediated Entanglement without a Local Tensor Product Structure

Quantum information has become a powerful tool for probing the structure of quantum field theories, yet its application to gauge theories remains subtle. On the one hand, quantum information theory assumes subsystem locality, i.e.~the factorization of the total Hilbert space into subsystems. On the other hand, gauge constraints prevent the total Hilbert space to decompose into a spacetime-local tensor product structure. Because the Hilbert space structure of gauge theories does not accommodate the subsystem decomposition used in quantum information theory, standard information-theoretic results, such as the Local Operations and Classical Communication (LOCC) theorem, cannot be used straightforwardly in the context of gauge theories. In this work, we bridge this gap in the case of a two-dimensional lattice gauge model that captures key features of electromagnetism. In particular, we construct gauge-invariant local algebras and derive a physically meaningful decomposition of the Hilbert space, providing an operationally consistent notion of locality in the absence of a local tensor-product structure. We apply this framework to field-mediated entanglement protocols relevant to proposed tests of the quantum nature of gravity. We show that the discretized version of electromagnetism satisfies an analogue of the LOCC theorem: entanglement cannot be generated without genuine quantum field interactions, even in the absence of a spacetime-local tensor product factorization of the Hilbert space. This may point towards an operational way to define a subsystem structure for gauge theories.

Coffee Break

Career Development Talk

Experimental perspective on gravitationally induced entanglement

Testing the quantum nature of gravity is the goal of a broad experimental and theoretical community. I will review and compare different experimental setups and measurement strategies, and argue that gravitationally induced entanglement remains the most practical metric to achieve this goal.

How to measure quantum fields?

While we all know how to model measurements in ordinary quantum physics, measurement theories covering quantum fields are actively researched. One reason for this is that it becomes much harder to characterize measurements and operations which are, at least in principle, possible, complying with foundational principles such as relativistic covariance, locality and causality. In my talk, I will present to you a concrete scheme which allows to measure the elementary scalar field. The talk is based on joint work with Miguel Navascués, ( link.springer.com/article/10.1007/s11005-025-02001-3 ).

An inequality for relativistic local quantum measurements

We investigate the trade-off between vacuum insensitivity and sensitivity to excitations in finite-size detectors, taking measurement locality as a fundamental constraint. We derive an upper bound on the detectability of vacuum excitation, given a small but nonzero probability of false positives in the vacuum state. The result is independent of the specific details of the measurement or the underlying physical mechanisms of the detector and relies only on the assumption of locality. Experimental confirmation or violation of the inequality would provide a test of the axioms of algebraic quantum field theory, offer new insights into the measurement problem in relativistic quantum physics, and establish a fundamental technological limit in local particle detection.

On the Planckian time of thermalization.

We present results on fundamental operational limits descending from the standard rules of quantum mechanics applied on thermal states. Namely, we assess the following question: what is the minimum time required by the most general machine to thermalize a given system? Via the Boltzmann constant k_B and the Planck constant ℏ, each temperature T naturally defines a Planckian timescale as τ_Pl = ℏ/(k_B T) . Due to its appearance in several quantum many-body dissipative models, τ_Pl has been conjectured to intrinsically constrain the thermal relaxation of any system. Going beyond model-dependent arguments that have been discussed in the literature, we provide a general operational proof of this bound, by considering thermalization as a task assigned to the most general quantum machine M that receives as input a quantum system S with local Hamiltonian H_S, and satisfies two conditions: i) (Validity of Quantum Mechanics) The evolution of the compound S + M described by unitary Hamiltonian evolution. After time t the machine M is traced out to return the output state of S. ii) (Nontrivial Thermalization) The output is approximately the thermal ensemble, i.e. it should be close to the Gibbs state for the given T and H_S. This requirement has to be satisfied for a nontrivial set of Hamiltonians (at least 2, or more in general some continuous ball). Finally, we comment on possible insights our results might bring to the understanding of the fast-scrambling conjecture for black holes.

Harvesting Contextuality from the Vacuum

Quantum contextuality is the notion that certain measurement scenarios do not admit a global description of their statistics and has been implicated as the source of quantum advantage in a number of quantum information protocols. It has been shown that contextuality generalizes the concepts of non-local entanglement and magic, and is an equivalent notion of non-classicality to Wigner negativity. It is known that the vacuum of quantum fields possesses contextuality, so now the question is: can we harvest this contextuality and make use of this valuable resource? I will answer this question in the affirmative, introducing the protocol of contextuality harvesting. I will show that Unruh-DeWitt models are capable of harvesting quantum contextuality from the vacuum of a massless scalar quantum field. In particular, that gapless systems can be made to harvest contextuality given a suitable choice of measurements. I also investigate an Unruh-DeWitt qubit-qutrit system, where I show that certain tradeoffs exist between the harvested contextuality of the qutrit and the harvested entanglement between the systems, and that there are regimes where the two resources can both be present.

RQI Circuit Postdoc Meeting - Opening Remarks

The RQI Circuit at the Postdoc meeting will start at 9:10am CEST on June 12th. The local organizer is José Polo-Gómez.

Electrifying the vacuum: entanglement under time-dependent external fields

How do electric fields dynamically tune classical and quantum correlations in spacetime? We show that an external time-dependent electric field significantly modifies local mode correlations depending on system parameters. Crucially, when the electric field reaches the critical threshold to trigger the Schwinger effect, correlations undergo a phase transition.

Detecting quantum fields: from pointlike to macroscopic detectors

Since the 1990s, we know that the way we model measurements in standard quantum mechanics cannot be extended to quantum field theory, because projective measurements conflict with relativity. Particle detectors (i.e., non-relativistic quantum mechanical systems that interact with quantum fields) are a well-understood tool that allows us to formulate a general measurement scheme for quantum fields, but they are usually highly-idealized models, e.g., single qubits or harmonic oscillators. In this talk, we will introduce a model of macroscopic detector which aims to better represent the kind of detectors used in realistic experiments. We will study its dynamics and its causal behaviour, concluding with an analysis of how the measurement of some relevant observables scales with the number of constituents of the detector, enhancing some phenomena and hindering others.

Coffee Break

Localising quantum information in black hole evaporation

We revisit Hawking's black hole radiation derivation, including the quantum state of the initial matter forming the black hole. We investigate how non-vacuum initial quantum states, at the past of a black hole geometry, influence the black hole radiation observed at future null infinity. We further classify which of the initial state excitations are distinguishable from one another through measurements on the black hole radiation state. We discuss the cases of a collapsing black hole and a collapsing black hole that is evaporating. We model the black hole collapsing matter as excitations of the scalar quantum field. We use Algebraic Quantum Field Theory to provide a clear physical interpretation of the results, in terms of localised operations. In a concrete example of a black hole made of one large collapsing excitation of mass M, and compare it to a same-mass black hole formed due to the collapse of two smaller excitations, of mass M/2 each. We find using our formalism that the two cases yield different non-thermal radiation states and can, in principle, be distinguished.Our results provide a mechanism for partial information recovery in collapsing black holes, and total information recovery in fully evaporated black hole geometries that do not form singularities.

Classical-quantum gravity as quantum gravity in disguise

Classical-quantum hybrid theories have recently been proposed as frameworks in which quantum matter can backreact on a classical gravitational field while preserving complete positivity. I will discuss recent work showing that such dynamics can be embedded into a fully quantum theory on an enlarged Hilbert space: the apparently classical sector arises from an operational restriction to a commutative pointer algebra rather than from canonical quantization of phase space. This shifts the focus from the formal consistency of the hybrid evolution to the operational content that remains once the classical sector is viewed as a restricted part of an underlying quantum description. As a complementary illustration, I will consider a toy model in which a qubit interacts with a classical particle and the resulting closed, rotationally invariant dynamics violates conservation of total angular momentum. This example demonstrates a structural tension between complete positivity, nontrivial backreaction, and conservation laws, and motivates the question of whether analogous constraints appear in fully relativistic classical-quantum gravity models.

Quantum effects on local observers in an exactly solvable model of quantum gravity

Recent developments in quantum gravity have emphasized the role of an "observer" in making sense of quasi-local quantum-gravitational effects. In this talk, we will describe an explicit model of a gravitationally dressed observer in Jackiw-Teitelboim (JT) gravity. This is a dilaton gravity model in two spacetime dimensions that is fully solvable both classically and quantummechanically, thereby allowing us to study quantum gravity effects far beyond the semiclassical regime. We will show how properly accounting for gravitational dressing in JT gravity affects the experience of a local probe crossing the horizon of a black hole, and also study nonperturbative effects on the thermal atmosphere outside the black hole in this model.

RQI Circuit Waterloo - Opening Remarks

The RQI Circuit Waterloo will start at 9am EDT on June 19th. The local organizers are Ahmed Shalabi, Boris Ragula, Ireneo James Membrere, and Matheus Hrabowec Zambiaco.

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Relativistic Quantum Information Meets Holography

his talk lies at the intersection of three major pillars of modern physics: quantum theory, information theory, and gravity. In particular, we focus on relativistic quantum information (RQI) and holography. In RQI, one studies quantum information processing in relativistic settings. In holography, one studies how bulk gravitational physics is encoded in information on a lower-dimensional boundary. As a concrete example, we investigate a magic-harvesting protocol using a qutrit Unruh–DeWitt detector. We analyze how the protocol behaves both in the AdS bulk and on the CFT boundary, aiming to provide a viable route toward probing the holographic principle through quantum information protocols.

Entanglement via classical mediators and the Wheeler Feynman absorber theory

In this presentation I will discuss how quantum sources can become entangled without a quantum field mediating their interaction, but, instead, via 'quantum-controlled' fields, and how such fields relate to the Wheeler-Feynman absorber theory of electromagnetism. 'Quantum-controlled' fields have no local quantum degrees of freedom and emerge from the classical propagation of quantum sources' quantum degrees of freedom. They induce direct and relativistic couplings between quantum systems, allowing for quantum effects such as entanglement. As such, they can be applied to explain gravity-induced entanglement and the relevant experiments that aim to witness the quantum nature of the gravitational field. Interactions mediated by 'quantum-controlled' fields exhibit a striking similarity to the Wheeler-Feynman absorber theory, mathematically, conceptually and even in terms of causality violations. I will present this analogy along with the original conceptual motivation of Feynman.

Coffee Break

Probing Vacuum Energy Fluctuations with Gravitational Observables

According to Einstein’s equations, the stress-energy of matter fields determines how spacetime curves. In quantum field theory, however, the stress-energy tensor and its fluctuations become singular operators, raising the question of whether and how the short-distance divergences can influence low-energy gravitational observables. In this talk, I will present a construction of the power spectral density of interferometric gravitational time-delay fluctuations, and discuss its potential as a probe of vacuum energy fluctuations.

Entanglement harvesting in conformal field theory

In this talk, I will present recent work on entanglement harvesting in general d-dimensional conformal field theories using pointlike Unruh-DeWitt detectors coupled to scalar primary operators. This work extends standard harvesting protocols beyond free fields to interacting conformal theories and arbitrary spatial dimensions. We found that increasing the operator scaling dimension suppresses both negativity and mutual information, reflecting the faster decay of correlations. For holographic CFTs, we also showed that bulk effective field theory enables a separation between field-harvested and communication-mediated entanglement.

Lunch Break

Career Development Talk

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A Hot Take on What Infalling Observers See in a BTZ Black Hole

Recent work by Shallue and Carroll (2025) introduced an operational definition of effective temperature that accounts for the short time an infalling detector remains near a black hole horizon. In that framework, the effective temperature is defined as the temperature of the thermal bath for which an inertial detector in Minkowski spacetime, with the same energy gap and switching duration, exhibits the same response as the infalling detector. Applied to a detector interacting with the Hartle-Hawking vacuum of a Schwarzschild black hole, this definition yields the Hawking temperature far from the horizon and twice the Hawking temperature at the horizon. In this work, we study a freely falling detector interacting with the Hartle-Hawking vacuum of a Bañados-Teitelboim-Zanelli (BTZ) black hole and define an analogous effective temperature. The motivation is that, whereas Schwarzschild spacetime is locally Minkowski, BTZ spacetime is locally three-dimensional anti-de Sitter (AdS3) spacetime. We therefore adopt a notion of effective temperature based on the response of an inertial detector in a thermal bath in AdS3. Building on previous results, we numerically compute both the response of a detector freely falling in BTZ and the response of an inertial detector in thermal AdS3, allowing us to determine the effective temperature as a function of the detector's proper time or radial position during infall.

On sufficient conditions for holographic scattering

Holography implies scattering in the bulk can be mediated by entanglement on the boundary. The connected wedge theorem (CWT) of May, Penington, and Sorce is a concrete example where bulk scattering implies correlation between certain boundary regions. However the converse does not hold. We investigate a recent proposal of Leutheusser and Liu for a generalization of the CWT with converse. We prove the forward direction: having pairs of CFT “input” (and likewise “output”) regions in a phase with connected entanglement wedge implies that a particular bulk subregion (the intersection of “input” and “output” entanglement wedges) is non-empty. We then establish a modified version of the proposal which has a converse, and identify counter-examples to the stronger conjecture.

Coffee Break

Alice and Rob Revisited: How quantum reference frames can provide new insights into entanglement degradation

One of the canonical results from the field of Relativistic Quantum Information (RQI) is the degradation of entanglement between a pair of relatively accelerated observers, typically described from a 'global' perspective. But how might this degradation be manifest from the perspective of the 'quantum observers' that comprise this set-up? In this presentation, we make use of the Perspectival Quantum Reference Frame (QRF) formalism to revisit the problem of entanglement degradation between an inertial observer Alice and a non-inertial observer Rob. We find many interesting connections between our new perspectival approach and the typical 'global' approach. We derive explicit relationships for these, noting that they sometimes require quantum resources beyond entanglement. In particular, we find that the sum of perspectival entanglement and coherence, previously studied by Cepollaro *et al.* can be exactly related to the entanglement of the 'global' system. Based on work done with Sijia Wang and Robert B. Mann: https://arxiv.org/abs/2603.23601

Contextuality from quantum field vacua using qutrit Unruh-DeWitt detectors

This talk discusses how contextuality can be harvested from the vacuum of a quantum field using localized Unruh-DeWitt detectors. Contextuality is widely regarded as a key nonclassical resource for quantum advantage. Although the detectors are initially noncontextual with respect to Heisenberg-Weyl measurements, their interaction with the field can generate contextuality, quantified by the contextual fraction. The talk will also highlight the accompanying emergence of Wigner-function negativity, consistent with known equivalences between these two signatures of nonclassicality. Together, these results establish contextuality harvesting as a new phenomenon in relativistic quantum information and show that the quantum vacuum can directly supply resources associated with quantum advantage.

Quantum Energy Teleportation as a Quantum Thermodynamic Tool

Thermodynamics is one of the cornerstones of modern physics, concerning itself with the behaviours and flows of heat, energy, work, and limitations on permitted physical processes. Its principles are crucial not only for practical applications, ranging from engines to electronics, but also to further our theoretical understanding of physical laws governing the universe. With increasing advancements in our technology and theories at the smallest scales, classical thermodynamics is being revisited and extended to regimes where quantum effects begin to play a significant role. This is the basis of the emergent field of quantum thermodynamics, enabling the study of energy, entropy, and work at their (currently) most fundamental level. With this comes a growing need for reliable quantum states, as pure quantum states are a major component of many quantum technologies, including quantum computing [1] and secure communication [2]. Recently, there have been remarkable successes in using relativistic quantum informational protocols in the study of quantum thermodynamics. Quantum energy teleportation (QET) is one such protocol [3], enabling energy transfer between subsystems of a multipartite quantum system without the energy needing to physically propagate between these. QET achieves this by leveraging quantum correlations within the system. This project investigates the relation between the state purification and energy costs due to the QET protocol, as a means of verifying the efficiency of QET as a tool to produce purer quantum states. Building on prior work demonstrating the QET protocol’s ability to harness quantum correlations for purification [4], this project investigates how the purification is related to the extracted energy of the protocol. This research is primarily conducted through computational simulations implemented in Julia, which are used to compute both the optimal achievable purity and the maximum extractable energy from a system of interacting qubits in a quantum thermal state. Since the QET protocol relies in part on local unitary operations, numerical optimisation is used to identify which operations maximise purification and energy extraction, similar to the approach in [4]. In particular, since our implementation of QET relies on the introduction of an ancillary system [5], the optimal unitary operators are represented and optimised as 8 by 8 matrices. A particular focus is put on examining the relation between operators optimizing purity and operators optimizing energy extraction. This could provide insights on whether certain implementations of QET are more energy efficient for generating pure states, rendering them more promising for experimental realisation and industrial application. [1] Michael A. Nielsen, and Isaac L. Chuang, “Quantum Computation and Quantum Information (2nd ed.),” Cambridge: Cambridge University Press. ISBN 978-1-107-00217-3. OCLC 844974180 (2010). [2] Nicolas Gisin and Rob Thew, “Quantum Communication,” Nature Photonics, vol 1, No. 3, pp165-171 (2007). [3] Masahiro Hotta, “Quantum energy teleportation: An introductory review,” (2011), arXiv:1101.3954 [quant-ph]. [4] Nayeli A. Rodríguez-Briones, Eduardo Martín-Martínez, Achim Kempf, and Raymond Laflamme, “Correlation-enhanced algorithmic cooling,” Phys. Rev. Lett. 119 (2017). [5] Boris Ragula and Eduardo Martín-Martínez. 2025. A review of applications of quantum energy teleportation: from experimental tests to thermodynamics and spacetime engineering. Canadian Journal of Physics. 103(11): 1094-1118.

Causality of particle detector models in perturbation theory

We continue some of the work of [1] by proving that particle detector models respect relativistic causality at arbitrary perturbative order. Concretely, we show that the signaling contributions between two detectors can be written in terms of field commutators between their interaction regions, and therefore vanish for spacelike separation. Furthermore, we introduce a general structure of the perturbative terms and use it to compute explicit expressions for the third and fourth orders, including numerical results illustrating their dependence on detector smearing. These results have important implications for setups utilizing particle detector models, such as those seen in quantum field theory, quantum optics, and relativistic quantum information. [1] E. Mart´ın-Mart´ınez, Causality issues of particle detector models in QFT and quantum optics, Phys. Rev. D 92, 104019 (2015).

Closing Remarks

RQI Circuit Manchester - Opening Remarks

The RQI Circuit Manchester will start at 11am BST on June 26th. The local organizers are Maciej Jarema and Ilaria Dimina. For more information visit https://gravitylaboratory.com/event/1/.

(Quantum) Physics in Rotating Frames

The way quantum phenomena are affected by the frame of reference in which they occur is deeply connected to one of the most fundamental mysteries - the unification of quantum mechanics and general relativity. Some of the theorised effects are very well known, others less so, and direct experimental tests have been relatively overlooked. I will present experimental work in several different areas: photon entanglement in rotating frames, the amplification of certain field modes by rotating absorbers that lead to 'black-hole bomb' instabilities, and a potential test of the Unruh effect - the transformation of the vacuum to an accelerated observer.

Analogue gravity experiments in superfluid helium

A new experiment is in development for visualizing the dynamics of the surface of thin films of superfluid 4He. This experiment takes place in a dry, 300 mK 3He refrigerator, and uses a Mach-Zehnder interferometer to realise a combination of offaxis digital holography and heterodyne interferometry as measurement probes of the fluid surface. We have so far categorised the eigenmodes of surface excitations in a cylindrical basin [1] and further hope to also observe surface deformations in the presence of quantized vortex lines terminating at the fluid surface. The thin film serves as an analogue (2+1) dimensional spacetime, with small surface fluctuations playing the role of a scalar quantum field [2]. In the zero-temperature limit, the surface modes become quantized, arising from excitations of small numbers of phonons and are referred to as third sound modes. Using this analogue, the experiment will serve as an environment in which predictions of quantum field theory in a flat (2+1) dimensional spacetime can be tested. Ultimately, our goal is to create an analogue Unruh-DeWitt detector, capable of measuring the changing power spectral density of a scalar field from the perspective of an accelerating observer. The ability to control the geometry of superfluid thin films using temperature control, or applied electric and magnetic fields, also allows the creation of non-trivial effective spacetime geometries, in which analogue experiments can explore predictions of cosmological models. We have also proposed that such a platform would allow for the direct measurement of information theoretic quantities such as area laws of mutual information between coupled effective fields [3]. [1] ArXiv Preprint: 2509.10235 [2] New Journal of Physics 26.6 (2024): 065001. [3] ArXiv Preprint: 2508.07247 *This work was supported by the UK National Quantum Technologies Programme

The Analogue Unruh Effect in Thin-Film Superfluid Helium-4

Thin films of superfluid helium are excellent analogue platforms to test for predictions in quantum field theory in curved spacetime and relativistic thermodynamics, since they act as effective (2+1)-dimensional spacetimes. Their surface height fluctuations, also known as third sound, obey a Klein-Gordon equation with propagation speed analogous to the speed of light. We are currently developing an experiment to test for the Unruh effect in these systems. Our aim is to measure a thermal-like spectrum, analogous to that predicted by Unruh, by introducing a laser beam in circular motion through the film acting as a localised, accelerating detector. I will discuss the details of the theoretical model, and its experimental realisability.

Career Development Talk

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Back-action of an Unruh-DeWitt detector in causal covariant perturbation theory

Transitions in a local quantum system in a relativistic spacetime are accompanied by a back-action on the ambient quantum field to which the local system couples. We address this back-action for a pointlike Unruh-DeWitt detector coupled to a scalar field, in scenarios where field observable measurements are not conditioned on a measurement of the detector's final state. We present a second-order perturbative formalism that maintains spacetime covariance and relativistic causality throughout. As an application, we evaluate the renormalised stress-energy tensor of a massless scalar field for the back-action of a uniformly linearly accelerated detector in 3+1 Minkowski spacetime. The energy flux into and out of the detector accounts exactly for the energy gained and lost by the detector in its transitions due to the Unruh effect. For a detector prepared initially in its ground state, we find two regions of negative energy density, one near the acceleration horizon, the other in the far future. We anticipate the formalism to be adaptable to the back-action that occurs in analogue spacetime simulations of relativistic acceleration. (Based on 2512.16217 by A. S. Wilkinson, L. J. A. Parry, J. Louko and W. G. Unruh)

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Early Universe Cosmology In The Lab

Understanding how energy transfers across scales in far-from-equilibrium systems is central to many areas of physics, including the dynamics of the early Universe. Laboratory analogue experiments provide a way to explore such processes under controlled conditions. In this talk, I present a periodically driven two-fluid system in which surface waves are generated and interact nonlinearly, redistributing energy between different modes and leading to cascade formation. The dynamics can be described by a weakly nonlinear Lagrangian framework and are closely analogous to the parametric excitation of fields that occurs during cosmological preheating after inflation. By experimentally tracking the evolution of individual wave modes, we study how these cascades develop, providing insight into nonlinear dynamics in analogue models of early-Universe physics.

Measuring and manipulating information in QFT simulators

The encoding of information-theoretic measures, such as entropy and mutual information, across space and time, characterises the structure, history, and complexity of correlations in a system. Accessing information distributions would provide a powerful lens for investigating area laws, thermalization, fundamental communication limits, and non-linear, far-from-equilibrium dynamics. Analogue QFT simulators can provide the necessary observables to assess to these phenomena by emulating field degrees of freedom with high precision and dynamical control. However, a significant hurdle remains: while analogue simulators offer the platform, extracting information requires access to their quantum state. This remains largely out of experimental reach beyond small systems and/or simple states. In this talk, I will present a tomography scheme for assessing the Gaussian information content of thin-film superfluid helium experiments that simulate (2+1)- dimensional QFTs. I will also present experimental measurements of information area laws in strongly interacting ultracold gases that simulate the sine-Gordon scalar field. Finally, I outline our progress toward monitoring information flow and its active manipulation.

Trapped surfaces and geodesics in non-axisymmetric analogue black holes

Many phenomena associated with black holes are typically studied under symmetry assumptions, most notably in the axisymmetric Kerr spacetime, where the underlying structure allows for significant analytical simplifications. In contrast, comparatively little is known about how the breaking of such symmetries impacts these phenomena. Analogue gravity systems provide a useful platform to explore these questions in controlled settings. In this talk, I will present the construction and analysis of a non-axisymmetric analogue black hole, focusing on the identification of the effective horizon and its geometric properties. I will then discuss the behaviour of null geodesics, highlighting how propagation and trapping are modified in the absence of symmetry. Finally, I will comment on how these results provide the necessary foundation for the study of superradiant phenomena in such systems, along with some preliminary considerations on the challenges introduced by non-axisymmetry.

RQI Circuit Dublin - Opening Remarks

The RQI Circuit Dublin will start at 10:30am BST on July 3rd. The local organizer is Joshua Jones.

Career Development Talk

Entropy, von Neumann Algebras and de Sitter Space

This short talk will attempt to show how the three subjects of the title are intertwined.

Coffee Break

Semiclassical Spacetime Thermodynamics

I outline the recovery of semiclassical gravitational dynamics from entanglement equilibrium of local causal horizons. Due to backreaction of quantum fields, gravitational entanglement entropy acquires corrections encoded in the conformal anomaly. Though entropy is no longer simply proportional to area, the universal nature of the anomaly still ensures a robust entropy prescription that can be obtained without invoking the gravitational Lagrangian. Then, gravitational dynamics follows from equilibrium conditions in a self-contained and internally consistent way. I further discuss the potential of applying the formalisms of von Neumann algebras and quantum reference frames in this context. While the talk focuses on the most tractable 2D case, I also comment on the physically most relevant 4D case.

Classical and Quantum (In)stability of Cauchy Horizons in Self-similar Collapse

A thick shell of radiation can undergo gravitational collapse to a naked singularity, providing a challenge to the cosmic censorship hypothesis. To probe the stability of the resulting Cauchy horizon, we consider classical and quantum field perturbations of the spacetime, focussing on the self-similar case.

Lunch Break

Why Causal Sets?

he breakdown of the continuum hypothesis at the Planck scale, manifested by UV divergences in QFT and singularities in General Relativity, suggests that spacetime may be fundamentally discrete. This talk introduces Causal Set Theory, an approach to quantum gravity where the smooth Lorentzian manifold is replaced by a locally finite partial order. We begin by motivating discreteness not merely as a mathematical regulator, but as a physical necessity motivated by arguments from black hole entropy and information theory. We contrast Causal Sets with regular lattice discretizations, demonstrating how "sprinkling" points via a Poisson process preserves Lorentz invariance. We then explore the core tenet of the theory—"Order + Number ~ Geometry"—showing how familiar geometric concepts like geodesics, dimension, and the Einstein-Hilbert action (via the Benincasa-Dowker formalism) can be recovered purely from the causal relations and counting of vertex elements. Finally, we discuss the statistical behavior of these sets in the large-$N$ limit, specifically the dominance of non-manifold-like Kleitman-Rothschild orders, and the work being done to distinguish, at a graph theoretic level, the differences between these KR orders and manifold-like causal sets.

Causal Set Propagators in de Sitter Spacetime

This talk will give an introduction to the formulation of quantum field theory on causal sets in curved spacetimes such as de Sitter. In particular we will discuss the Sorkin-Johnston (SJ) vacuum state both for causal sets and the continuum, before highlighting how causal sets can be used to determine the relation between the SJ vacuum and the one parameter family of vacua in de Sitter spacetimes.

Entanglement Entropy in Spacetime, and a Signature of Discreteness

Entanglement entropy is one of the key candidates for the Bekenstein-Hawking entropy of black holes, although it is well known to be divergent without a regularisation scheme. Such a scheme likely originates in nature from quantum gravity. If one has a covariant theory of quantum gravity, one requires a means of performing the calculation of entanglement entropy in spacetime, so as to actually apply the scheme faithfully. I will give such a spacetime formulation of entanglement entropy for quasifree fields, and show a potential regularisation originating from a theory of quantum gravity, causal set theory. It will be seen that upon calculation, the entanglement entropy carries within it a signature of the causal set discreteness.

The Causal Propagator and its Spectral Density

In scalar quantum field theory, the causal propagator, which is proportional to the spacetime commutator of the field, plays an essential role. I will discuss some ways the causal propagator and its spectrum have been used to formulate quantum field theory in a more explicitly covariant manner in the continuum as well as in causal set theory. I will then state a conjecture for its asymptotic spectral density in a free theory, along with examples that lend evidence to the conjecture. This result also has important implications for Lorentzian spectral geometry, as it is akin to Weyl’s asymptotic law in Riemannian spectral geometry.

General Discussions and Additional Questions

Coffee Break

Counting States in Gauge Orthononal Invariant Matrix Models and Possible Connections with Black Hole Entropy in Anti de Sitter Spacetimes

Gauge gravity indicates that anti de Sitter spacetimes with black holes emerge from gauged matrix models. I will endeavour to connect these ideas by focusing on the microcanonical description of the relevant states in the simplest matrix models where there are indications of this connection.

RQI Circuit York - Opening Remarks

The RQI Circuit York will start at 10:20am BST on July 10th. The local organizers are Patricia Ribes Metidieri, Filippo Nava, and Christiane Klein. More information can be found at the website https://sites.google.com/york.ac.uk/rqicircuityork2026.

Polarisation measurements in quantum field theory

In this talk I will illustrate the "FV" theory of measurement in QFT, focussing on a specific model in which the polarisation state of a QFT can be measured. Measurements can be made for two polarisation channels and at multiple locations in spacetime. In particular, this model allows one to compute probabilities for joint measurements at spacelike separated locations, allowing discussion of Bell inequalities and the interpretation of state updates following measurement. No previous acquaintance with the measurement framework is required. The talk is based on the preprint Coupled Proca theories: Green-hyperbolicity, quantization and applications to polarization measurement, CJ Fewster and CKM Klein, arXiv:2511.11348, and further work in progress. The measurement framework was introduced in Quantum fields and local measurements, CJ Fewster and R Verch, Commun. Math. Phys. 378 (2020) 851-889 arXiv:1810.06512.

Quantum field theory models for testing Bell inequalities

Bell inequalities are an important tool to distinguish between quantum mechanics and local hidden-variable theories. Since the first work by Bell, different experimental designs have been put forward to demonstrate the violation of Bell inequalities in way that is, under certain assumptions, impossible for hidden-variable theories. In this talk, I will describe how such an experimental design can be modelled within the "FV" measurement framework and under which circumstances the same violation of the corresponding Bell inequality as for the quantum-mechanical version can be observed. The talk is based on the preprint Coupled Proca theories: Green-hyperbolicity, quantization and applications to polarization measurement, CJ Fewster and CKM Klein, arXiv:2511.11348, and further work in progress.

Short Break

Tsirelson's problem: a possible discrepancy between the predictions of relativistic and non-relativistic quantum theory in Bell non-locality

Bell non-locality is a fundamentally quantum phenomenon, arising when we measure two quantum systems in a space-like separated manner. The non-relativistic description of such an experiment prescribes the tensor product of two Hilbert spaces corresponding to the two systems. The relativistic description, however, prescribes a single Hilbert space with the measurements of the two systems commuting. Tsirelson thought that these two descriptions lead to equivalent experimental predictions in Bell non-locality. However, a recent breakthrough result showed that this is not the case: the relativistic description is strictly more general in principle [1]. The proof of this result is not constructive, leaving open the question: is the relativistic description more general in practice? That is, are the predictions showing this discrepancy actually physical? If so, how could we test which predictions are the correct ones? A possible resolution to these questions may arise from algebraic quantum field theory or group algebra representations. [1] Ji, Natarajan, Vidick, Wright and Yuen, Communications of the ACM, 64(11), 131 - 138 (2021)

Modular theory and relative entropy: overview and future perspectives

The interest in local von Neumann algebras in relativistic quantum field theory arises naturally, as they correspond to the algebras of local observables represented on the vacuum Fock space. In particular, Haag duality ensures that these algebras are maximal in the sense that they contain all local observables. Within this framework, restricting the vacuum state to a local von Neumann algebra allows one to use modular theory and, in turn, introduce the notion of Araki–Uhlmann relative entropy. This provides a natural definition of relative entropy in QFT, where the absence of a trace—stemming from the structure of local von Neumann algebras—precludes the use of standard quantum mechanical tools. In this talk, I will review these concepts in the setting of a free real scalar quantum field theory. Time permitting, I will also present results on relative entropy in fermionic QFT and discuss how Haag duality is modified when the vacuum state is replaced by a KMS state. This work is based on collaborations with Albert Much, Leonardo Sangaletti, and Rainer Verch.

Lunch Break

Career Development Talk

QRFs and semi-local observables in QFT

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Measuring Relational Observables

We discuss limitations of local measurement schemes to describe measurements in quantum field theory in probing relational observables, which naturally arise from requirements such as diffeomorphism invariance. To overcome these limitations, we introduce notions of relative measurements performed in relation to operational quantum reference frames, and discuss their associated observable quantities and state updates.

Gluing in Electromagnetism: a differential cohomology approach

In this talk, I will present an ongoing work with D. Janssen and K. Rejzner on the gluing problem in quantum electromagnetism. Put simply, the question is: can we reconstruct the global physics on a spacetime knowing the physics in its subregions? For gauge theories and spacetimes with non-trivial topologies and boundaries, the problem is theoretically challenging; from an experimental point of view, phenomena such as the Aharonov-Bohm effect motivates an in depth analysis. Using a relative differential cohomology approach, we construct a space of field configurations for electromagnetism that is compatible with gluing, and we discuss their dynamics. We derive an algebra of observables, roughly corresponding to smeared Wilson lines; in this setting, we prove that given a (nice enough) decomposition of spacetime, the algebra of observables on the full spacetime can be reconstructed from the algebras on its subregions. Eventually, we discuss quantisation and its superselection sectors, and their link with relativistic quantum information experiments.

Coffee Break

Semiclassical gravity: Why and how?

In this talk, I present semiclassical gravity and explain its impact on unresolved RQI questions. I argue that, despite its complexity, it is worth understanding this theory in full conceptual and mathematical detail. How to make progress in this task is the second point I touch on.

Wigner's friend black hole adventure: an argument for complementarity?

At the heart of both Wigner's friend paradoxes and black hole puzzles lies the question of unitarity. In Wigner’s friend setups, sealed-lab measurements are modeled unitarily, probing the measurement problem. In black hole physics, the unitarity problem concerns information preservation in evaporation. In this talk, I discuss a refined version of the Frauchiger--Renner paradox and extend a recent analogy between Wigner's friend and black hole paradoxes exposed by Hausmann and Renner [arXiv:2504.03835v1], by constructing new paradoxes that merge black hole physics with extensions of Wigner’s friend scenarios into a unified argument. I shortly discuss implications of these works and highlight subtleties in black hole puzzles.

The matter-gravity entanglement hypothesis

recall my 1998 matter-gravity entanglement hypothesis according to which the physical entropy of a quantum gravitational closed system is to be identified with its matter-gravity entanglement entropy and I explain how this is motivated by the so-called thermal atmosphere puzzle. Under the further identification of 'information' with negative entropy, one aspect of the information loss puzzle then becomes a special case (the case where the closed system consists initially of a collapsing ball of matter in an otherwise empty universe) of the second law puzzle: Why does the entropy of a closed system increase? Our proposed answer is that if (low-energy) quantum gravity is a conventional (unitary) quantum theory, then one would expect that, if it is initially low, the matter-gravity entanglement entropy of the quantum state of a closed system would increase for all time, due to interactions between matter and gravity. I also recall my more recent arguments (https://arxiv.org/abs/2206.07445) that, if this is on the right track, then one would expect the final total state of black hole evaporation for an initially large black hole formed by collapse to (be pure and) consist mainly of photons entangled with gravitons.

Closing Remarks

RQI Circuit Hangzhou - Opening Remarks

The RQI Circuit Hangzhou will start at 10am CST on July 17th. The local organizers are Prof. Hongguang Liu, email, Dr. Ioannis Soranidis, and Mr. Cong Zhao.

The Finite Black Hole Interior in Quantum Gravity

Quantum gravity faces deep tensions between the smooth geometry of classical spacetime and the discrete, finite nature of the quantum Hilbert space. One striking manifestation of this tension is the infinite size of a black hole’s interior. For an AdS black hole, the volume of its interior (the Einstein-Rosen bridge) increases almost linearly at late times. Motivated by the complexity = anything proposal, we introduce the spectral representation of infinite generating functions for both codimension-one and codimension-zero gravitational observables that probe the black hole interior. Their time evolution exhibits a characteristic slope-ramp-plateau structure, analogous to the spectral form factor in chaotic quantum systems. Upon incorporating quantum corrections from Euclidean wormholes, we find that holographic complexity measures obey a universal time evolution: they grow linearly for a long period and then saturate at a plateau at late times. We further show that this universal behaviour is governed by a specific pole structure and by spectral level repulsion, the hallmark of quantum chaos.

Chaotic dynamics and black holes in BMN theory

The study of chaotic phenomena in the vicinity of black holes has become popular in recent years, mainly because they turn out to be related to the paradox of information loss. Sekino and Susskind have conjectured that black holes are the fastest information scramblers in nature and the dynamics of their microscopic degrees of freedom is manifestly chaotic and nonlocal. Strong evidence supports the claim that quantized BMN matrix theory provides a valid description of the chaotic and non-local dynamics of the black hole degrees of freedom. In this talk we will explore the classical limit of the BMN matrix theory with bosonic membranes and present a series of results we have obtained recently.

Semiclassical realization of emergent gravity via stress tensor deformation

We present a semiclassical framework in which stress-tensor deformations of a quantum field theory (QFT) reorganize into a gravitational action evaluated at a metric saddle. The deformed partition function can be written as a gravitational path integral evaluated at the saddle, establishing a direct link between stress-tensor flows and gravitational dynamics. Two complementary routes arise: (i) from gravitational actions such as Einstein and Palatini, which map to stress-tensor deformations of a seed QFT; and (ii) from deformed QFTs such as generalized Nambu-Goto and TTbar-like deformed models, which reconstruct the corresponding gravitational actions. Finally, in a free, massive scalar theory, we show that the one-loop effective action of the nonlocal deformation contains a local curvature term; its coefficient defines an induced Newton constant at a chosen renormalization scale, thereby demonstrating a bidirectional link between stress-tensor flows and classical gravity. The talk will based on: 2508.15461 and 2603.08481

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Career Development Talk

In this talk, Hongguang Liu will present career opportunities at Westlake University and provide important information for researchers who wish to apply for positions at the Institute for Theoretical Sciences and Westlake University.

Lunch Break

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From non-vacuum to vacuum regular black holes: thermodynamics and singularity resolution

Regular black holes provide a useful framework for exploring how quantum-gravitational effects may resolve the singularities predicted by general relativity. In this talk, I will discuss recent progress on singularity-free black holes in several complementary settings. I will begin with quasitopological gravity, where an infinite tower of higher-curvature terms can generically produce regular black-hole solutions in higher dimensions. I will focus on the thermodynamic properties of these solutions, whose equation of state exhibits features reminiscent of fluids with finite molecular volume and gives rise to a rich phase structure. I will then contrast these results with regular black holes supported by nonlinear electrodynamics, emphasizing the differences between pure-gravity and matter-supported constructions. Finally, I will present recent results on regular black holes in polymerized gravity, highlighting the mechanism of singularity resolution and I will conclude with some open questions.

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RQI Circuit Hannover - Opening Remarks

The RQI Circuit Stockholm will start at 8am CEST on July 24th. The local organizer is Martin Plávala.

Embezzling Entanglement From Relativistic Quantum Fields

Embezzlement of entanglement refers to the counterintuitive possibility of extracting entangled quantum states from a reference state of an auxiliary system via local quantum operations while hardly perturbing the latter. I will discuss a deep connection between the operational task of embezzling entanglement and the mathematical classification of von Neumann algebras. Our result implies that relativistic quantum fields are universal embezzlers: any entangled state of any dimension can be embezzled from them with arbitrary precision. This provides an operational characterization of the infinite amount of entanglement present in the vacuum state of relativistic quantum field theories.

Frame Dependence of Spekkens’ Contextuality for Relativistic Spin Systems

In this talk I will discuss the notion of relativistic contextuality. In particular, I will show how the operational definition of contextuality introduced by Spekkens is, in general, frame-dependent. Indeed, while an observer in an inertial frame describes a contextual ontological model with respect to spin states and all possible spin measurements, an observer in a boosted frame can describe a non-contextual model with respect to the transformed spin states and all transformed spin measurements.

Quantum Photonic Networks With Semiconductor Quantum Dots

Quantum dots are emerging as reliable, on-demand sources of single and entangled photons, compatible with wafer-scale fabrication and precisely tunable to interface with diverse quantum systems. These emitters can be integrated into compact photonic structures, generating high-quality photon streams that couple efficiently into optical fibers, enabling secure quantum key distribution across metropolitan distances. Together, these deterministic light sources, robust encoding methods, and hybrid photonic designs pave the way for scalable quantum photonic networks.

Lunch Break

Real-time Lattice Approximations of Quantum Fields

Computer simulations are an indispensable tool to explore quantum field theories (QFTs) beyond the perturbative regime. It is common to base such numerical investigations on lattice discretizations of space (and time). Assuming that a given QFT is under sufficient control in a fixed discretization, we will discuss the problem of controlling the continuum limit. This is facilitated by treating QFTs arising in the scaling limit of near-critical models of (quantum) statistical mechanics as effective field theories. Following Wilson and Kadanoff, a QFT is understood as a coherent sequence of lattice models connected by the renormalization group, where each lattice model supplies the predictions appropriate for a certain energy scale (lattice spacing) together with a required precision – intuitively, the inverse lattice spacing should be at least double that of the features to be simulated to have reasonable error bounds following the Nyquist-Shannon theorem. This picture is mathematically implemented using operator-algebraic renormalization, a rigorous version of the Wilson-Kadanoff renormalization group in the Heisenberg picture. As a guiding example, we discuss the transverse-field Ising chain.

The Emergence of Spacetime From Quantum Mechanics

A fundamental question in the study of holographic dualities is how bulk gravity emerges from the quantum information structure of a holographic boundary theory. While the standard bulk/boundary dictionary allows one to relate local bulk operators to nonlocal boundary operators, a more microscopic understanding of the mechanism behind this relation remains hidden. One recent approach towards deepening our understanding of this relationship is concerned with the question of what properties of the boundary theory are sensitive to the bulk causal structure. This has led to the formulation of rigorous criteria by which one can judge, given any quantum-mechanical system, whether its dynamics are consistent with the dynamics of a semiclassical higher-dimensional bulk dual. This approach is based on an understanding of the bulk-to-boundary map as an encoding of quantum information in the form of subalgebra/subregion duality, where each subsystem of the boundary theory represents a particular piece of the emergent bulk and the algebraic structure of operators within it. This talk will introduce the basic methods and results that arose from these studies and the open problems that they raise.

Quantum Properties in High-energy Collider Experiments

Quantum entanglement has recently been observed in high-energy particle physics processes, exploring quantum information in relativistic regimes, but also posing conceptual and technical challenges in the interpretation of the results. In this talk, I will discuss the prospects of using concepts from quantum information theory to access quantum properties in collider experiments. Unlike in conventional quantum optics settings, the systems are relativistic and the particle momenta and measurements are not under active experimental control. Adapting methods from quantum information theory allows for enhanced ways to analyze quantum properties but also to test the soundness of the predictions from quantum field theory and the standard model in high energy experiments. As a first proof of concept we illustrate the application of such methods to top pair production in the LHC using Monte Carlo simulations.

Career Development Talk

Coffee Break

Existing Experiments Suffice to Indirectly Verify the Quantum Essence of Gravity

The gravity-mediated entanglement experiments employ concepts from quantum information to argue that if entanglement due to gravitational interaction is observed, then gravity cannot be described by a classical system. However, the proposed experiments remain beyond our current technological capability, with optimistic projections placing the experiment outside of the short term future. Here we argue that current matter-wave interferometers are sufficient to indirectly prove that gravitational interaction creates entanglement between two systems. Specifically, we prove that if we experimentally verify the Schrödinger equation for a single delocalized system interacting gravitationally with an external mass, then, under one of two reasonable assumptions, the time evolution of two delocalized systems will lead to gravity-mediated entanglement.

Atom Interferometers in Non-trivial Gravity: Where Quantum Metrology Meets Einstein

Atom interferometers (AIFs) are highly accurate instruments used to measure inertial forces such as rotations and accelerations. They leverage the principles of quantum mechanics by coherently splitting the wave function of ultracold atoms (Rb, Yb, Cs, K,…) into a superposition of two momentum states, thereby creating a spatial superposition through the use of light pulses. By recombining the wave packets and measuring the resulting phase shift between the two momentum states, AIFs can accurately determine local gravitational accelerations or the imprinted photon recoil, among other phenomena. In Hannover, we have the "Very-Long-Baseline Atom Interferometer" (VLBAI), a 10-meter facility that represents the latest advancement in large-scale AIF experiments. It is the third of its kind globally, following similar setups in Stanford and Wuhan. AIFs not only allow measurement of the fine-structure constant with cutting-edge precision but also hold potential for detecting gravitational waves in the eagerly anticipated mid-frequency range and probing dark matter, especially if baseline lengths are extended to 100 - 1000 meters. To achieve these objectives, it is crucial to develop realistic and precise theoretical models to distinguish genuine signals from background noise. This presentation provides a comprehensive introduction to this field of research and explores key aspects that will play a pivotal role in the future.

Towards Measuring Tailored Gravity Fields Using the Very Long Baseline Atom Interferometer (Vlbai) Facility

One of the scientific objectives of the Very Long Baseline Atom Interferometry (VLBAI) facility in Hannover is to investigate how gravity affects quantum objects such as macroscopically delocallized atomic wave functions. Using the 10 m baseline we plan to position additional test masses at 15 cm from the atoms. Including and removing the additional test mass will allow us to perform a differential measurement in order to determine the gravitational influence of the test mass on the atomic wave function. For this measurement to be possible, a series of technical requirements have to be met. For example: launching an ultracold sample of atoms with sub nanokelvin effective energies, and giving the atoms a differential momentum sufficient to macroscopically delocallize the wave function. This contribution focuses on the progress in the facility during the past year, including the prototype system for positioning the masses with mm accuracy, demonstrating atom interferometry, and the plans to achieve the full potential of the facility. These include the progress towards achieving highly delocallized matter waves by the manipulation of rubidium atoms utilizing purely optical potentials for matter wave lensing, and control of the kinematics of the atoms for manipulation with Bragg beam splitting processes and Bloch oscillations for launch.

Closing Remarks

A Conservative Semiclassical Theory of Gravity

We argue that semiclassical gravity can be made consistent if quantum systems source gravity only when they participate in non-gravitational interactions that lead to environment-induced decoherence. Outside such decoherence events, systems do not contribute with their stress-energy to the semiclassical equations, so regions lacking these interactions may remain (approximately) flat. The proposal is testable by probing the gravitational field sourced by systems, which should depend entirely on environment-induced decoherence; by gravity not mediating entanglement in the Bose–Marletto–Vedral (BMV) experiment; and by how reversibility of the initial state in this experiment would depend solely on this decoherence, distinguishing it from competing approaches. We propose a specific kind of decoherence-inducing interaction that leads systems to source gravity: it models decoherence as chains of causally ordered, localized interactions between quantum matter fields, selecting the states and observables that source gravity. We argue that these interactions lead to the emergence of gravity. One way to see this is to note that these chains consist of timelike and lightlike separated events, whose causal order determines the metric up to a local conformal factor (Hawking–King–McCarthy–Malament theorem), and that when the number of events can be associated with the four-volume of spacetime, it provides the remaining information to fix the metric. Another way to understand this emergence is to note that these events can be correlated to varying degrees, so that the metric can be understood as arising from these interactions. This framework is conservative: it does not modify standard quantum theory while providing a consistent semiclassical theory of gravity. It may also explain why the vacuum need not gravitate, provide a semiclassical estimate of the cosmological constant, and allow for a time-varying cosmological “constant”.

Probing Holographic Wormholes Through Quantum Information in Deformed Double-Scaled SYK Model

Quantum information in holographic systems provides crucial insights to understand the emergence of the bulk geometry from a dual boundary theory. In this talk, we study very explicitly how notions of quantum complexity and algebraic entanglement entropy can help us deduce the bulk dual theory of new families of deformations of the SYK model. The deformations are defined such that after ensemble averaging and in a double-scaling limit, the theory is described by a transfer matrix encoding the recurrence relations of basic orthogonal polynomials in the q-Askey scheme. For certain families of deformations in the semiclassical limit at finite temperature, the chord number (encoding Krylov complexity) corresponds to the length of an Einstein-Rosen bridge connecting an End-Of-The-World brane to an anti-de Sitter asymptotic boundary. By increasing one of the deformation parameters, the models eventually exhibit discrete energy levels, signaling a new geometric transition in the bulk. Via the SYK-Schur duality, Krylov complexity also admits a representation-theoretic interpretation as the spread of the SU(2) spin in the index of an \mathcal{N} = 2 SU(2) gauge theory. We study the operator algebras of the deformed theories. The algebras can be type II_1 or type I_∞ factors, depending on the operators that are included. The entanglement entropy between the type II_1 algebras for a pure state manifests as an extremal surface through the Ryu-Takayanagi formula. We discuss connections between our results and the emergence of baby universes in the bulk.

Measurement of common-phase fluctuation in cold-atomic quantum simulators

Studying the dynamics of quantum many-body systems is often constrained by the limitations in probing relevant observables, especially in continuous systems. A powerful method to gain information about such systems is the reconstruction of local currents from the continuity equation. We show that this approach can be used to extract the common phase fluctuation of adjacent Bose gases. We validate our technique numerically and demonstrate its effectiveness by analyzing data from selected experiments simulating 1D quantum field theories with two parallel 1D Bose gases. This analysis reveals the previously hidden sector of the sum mode of the phase, which is important for studying entanglement and long-time thermalization of the system. Our method is general and can be applied to other cold atom systems with spatial phase gradients, thereby expanding the scope and capabilities of cold-atomic quantum simulators.

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TBA

Transition rates and their applications in accelerated single qubit for fermionic spinor field coupling

In this work, we investigate the interaction between a uniformly accelerated single qubit and a fermionic spinor field. Here we consider both the massless and the massive fermionic spinor fields. The qubit-field interaction occurs over a finite time and was evolved via perturbation theory. This approach yields the transition probability rates, from which we subsequently evaluate the quantum coherence of an Unruh-DeWitt (UDW) detector initially prepared in a qubit state. Our findings reveal that the UDW detector responds more when coupled with the fermionic field, and consequently, quantum coherence (for the fermionic case) degrades much more rapidly when compared to the case of the qubit linearly coupled with the scalar field. Moreover, the analysis suggests that particle mass plays a protective role against Unruh-induced decoherence as the rest mass energy becomes comparable to the detector's energy-level spacing, the detector's excitation probability and response decreases, which leads to the mitigation of quantum coherence degradation in accelerated quantum systems.

Memory Effects and Entanglement Dynamics of Finite time Acceleration

We investigate particle detectors undergoing acceleration for a finite duration in flat spacetime. We construct a smooth trajectory that is inertial in the distant past and future but closely approximates uniform acceleration over a finite interval, recovering the standard Rindler results in an appropriate limit. Using this framework, we study the response of an Unruh–DeWitt detector and show that finite-time acceleration induces memory effects, reflected in the loss of complete positivity divisibility and quantified using Fisher information. Extending to two detectors, we find that although transition rates are affected, both total correlations and harvested entanglement return smoothly to their initial values after acceleration, remaining insensitive to memory effects.

Particle creation by entanglement entropy

We investigate how entanglement entropy can drive particle creation, deriving explicit relations between entropy and the radiated particle spectrum, the total number of particles, and the total energy. Particle production is computed for scenarios that include accelerated motion, black hole evaporation, and beta decay, validating against known results while also extending them. We focus primarily on the low-entropy limit (analogous to non-relativistic motion), but also examine cases of significant particle production arising from harmonic cycles. The results establish an explicit operational link between information flow and matter creation, providing a concrete demonstration of 'it from bit'.

Lunch Break

Hyperbolic recoil and the Unruh effect at CERN-NA63

In this talk, we examine the high energy channeling radiation data sets from the CERN-NA63 experiment using ultra relativistic synchrotron emission. To incorporate recoil, we examine the standard quasi-classical formalism as well as develop a formalism which includes the Unruh effect by utilizing a hyperbolic recoil acceleration, based on conservation of momentum, in the classical synchrotron trajectory. We also perform an asymptotic radiation time scale analysis which predicts a photon energy threshold, beyond which the Unruh effect dominates. We then compare the classical, quasi-classical, and Unruh synchrotron theories to the data. We find that above threshold, the Unruh effect saturates the spectrum of all data sets.

The relativistic reason for quantum probability amplitudes

We show that the quantum-mechanical probability distribution involving complex probability amplitudes can be derived from three natural conditions imposed on a relativistically invariant probability function describing the motion of a particle that can take multiple paths simultaneously. The conditions are: (i) pairwise Kolmogorov additivity, (ii) time symmetry, and (iii) Bayes' rule. The resulting solution, parameterized by a single constant, is the squared modulus of a sum of complex exponentials of the relativistic action, thereby recovering the Feynman path-integral formulation of quantum mechanics.

Foundations of Relational Quantum Field Theory - Scalars

"We develop foundations for a relational approach to quantum field theory (RQFT) based on the operational quantum reference frames (QRFs) framework considered in a relativistic setting. Unlike other efforts in combining QFT with QRFs, we use the latter to provide novel mathematical and conceptual foundations for the former. We focus on scalar fields in Minkowski spacetime and discuss the emergence of relational local observables and fields from the consideration of Poincaré-covariant frame observables defined over the space of inertial reference frames. We recover a relational notion of Poincaré covariance, with transformations on the system directly linked to the state preparations of the QRF. We introduce and analyse various causality conditions, and construct an explicit example of a covariant scalar relational quantum field which is causal relative to operationally meaningful preparations of a relativistic QRF. If time permits, we will show how the theory makes direct contact with established foundational approaches to QFT: we demonstrate that the vacuum expectation values derived within our framework reproduce many of the essential properties of Wightman functions, carry out a detailed comparison of the proposed formalism with Wightman QFT with the frame smearing functions describing the QRF’s localisation uncertainty playing the role of the Wightmanian test functions, and show how the properties of algebras generated by relational local observables suitably extend the core axioms of Algebraic QFT. This work is an early step in revisiting the mathematical foundations of QFT from a relational and operational perspective."

Mass Dependence of the Araki-Uhlmann Relative Entropy Across Dimensions

The Araki-Uhmann relative entropy is the Quantum Field Theory generalization of the relative entropy from Quantum Mechanics. Previous works in (1+1)-D have shown a monotonic decay of the relative entropy between a coherent state and the vacuum with increasing mass. In this work, we extend this discussion to (1+2)- and (1+3)-D spacetimes, taking advantage of the fact that the relative entropy admits a closed expression in terms of the smeared Pauli-Jordan distribution which is sensitive to both the mass and the spacetime dimensionality. We find that the relative entropy decays monotonically in both (1+1)-, (1+2)- and (1+3)-D with an oscillating amplitude as the mass increases.

Metrology of open quantum systems from emitted radiation

We explore the task of learning about the dynamics of a Markovian open quantum system by monitoring the information it radiates into its environment. For an open system with Hilbert space dimension D, the quantum state of the emitted radiation can be described as a temporally ordered matrix-product state (MPS). We provide simple analytical expressions for the quantum Fisher information (QFI) of the radiation state, which asymptotically scales linearly with the sensing time unless the open system has multiple steady states. We characterize the crossovers in QFI near dynamical phase transitions, emphasizing the role of temporal correlations in setting the asymptotic rate at which QFI increases. We discuss when optimal sensing is possible with instantaneously measured radiation

Accessing and preparing spacelike vacuum entanglement in scalar quantum fields

"Though known to be present, the accessibility of spacelike vacuum entanglement capable of being a fundamental resource for quantum information processing has remained in question at distances beyond the scale of vacuum fluctuations in massive fields. Being UV-finite and capable of characterizing spacelike entanglement of the many-body vacuum, we begin by identifying the logarithmic negativity as a necessary and sufficient measure of entanglement [2]. We then leverage it to define an entanglement class [1] that enables the derivation of exact and optimal detector profiles, demonstrating that the exponentially decaying accessible entanglement persists across all separations in the scalar field vacuum [2]. Furthermore, allowing measurements and classical communication with the external volume enables access to pure spacelike quantum correlations. We identify a lower bound to the maximum of this pure quantum correlation that decays exponentially slower with separation than the two-point correlation functions [3]. By developing further techniques capable minimizing this entanglement [1] through a semi-definite conic geometry [3], we find an upper bound to the entanglement of formation between detection regions that decays exponentially faster than the two-point correlation functions. These results illustrate how deeper understanding of spacelike vacuum entanglement can inspire to new technical advances in many-body Gaussian quantum information. 1. B. Gao and N. Klco, Partial-transpose-guided entanglement classes and minimum noise filtering in many-body Gaussian quantum systems, Phys. Rev. A 109, 062413 (2024). 2. B. Gao and N. Klco, Detecting spacelike vacuum entanglement at all distances and promoting negativity to a necessary and sufficient entanglement measure in many-body regimes, Phys. Rev. A 112, 012430 (2025). 3. B. Gao and N. Klco, Finite Gaussian assistance protocols and a conic metric for extremizing spacelike vacuum entanglement, Phys. Rev. A 113, 012430 (2026)."