# Publications

**Authors: **Marios Christodoulou and Carlo Rovelli

**Year:** 2020

The Bose-Marletto-Vedral experiment tests a non-relativistic quantum effect due to a gravitational interaction. It has received attention because it may soon be within observational reach in the lab. We observe here that: (i) in relativistic language the experiment tests an interference effect between proper-time intervals; (ii) the relevant difference of proper times is of the order of the Planck time if the masses of the particles in the experiment are of the order of the Planck mass (micrograms); (iii) the experiment might open a window on the structure of time at the Planck scale: if time differences are discrete at this scale —as quantum gravity research may suggest— the Planckian discreteness of time could show up as quantum levels of a measurable entanglement entropy.

**Authors: **Fabio D’Ambrosio, Marios Christodoulou, Pierre Martin-Dussaud, Carlo Rovelli and Farshid Soltani

**Year:** 2021

At the end of Hawking evaporation, the horizon of a black hole enters a physical region where quantum gravity cannot be neglected. The physics of this region has not been much explored. We characterise its physics and introduce a technique to study it.

**Authors: **Richard Howl, Vlatko Vedral, Marios Christodoulou and Carlo Rovelli

**Year:** 2020

Until recently, table-top tests of quantum gravity (QG) were thought to be practically impossible. However, due to a radical new approach to testing QG that uses principles of quantum information theory (QIT) and quantum technology, such tests now seem, remarkably, within sight. In particular, a promising test has been proposed where the generation of entanglement between two massive quantum systems, both in a superposition of two locations, would provide evidence of QG. In QIT, quantum information can be encoded in discrete variables, such as qubits, or continuous variables. The latter approach, called continuous-variable QIT (CVQIT), is extremely powerful as it has been very effective in applying QIT to quantum field theory. Here we apply CVQIT to QG, and show that another signature of QG would be the creation of non-Gaussianity, a continuous-variable resource that is necessary for universal quantum computation. In contrast to entanglement, non-Gaussianity can be applied to a single rather than multi-partite quantum system, and does not rely on local interactions. We use these attributes to describe a table-top test of QG that is based on just a single quantum system in a single location.

**Authors: **Hlér Kristjánsson, Giulio Chiribella, Sina Salek, Daniel Ebler and Matthew Wilson**Year:** 2019

A series of recent works has shown that placing communication channels in a coherent superposition of alternative configurations can boost their ability to transmit information. Instances of this phenomenon are the advantages arising from the use of communication devices in a superposition of alternative causal orders, and those arising from the transmission of information along a superposition of alternative trajectories. The relation among these advantages has been the subject of recent debate, with some authors claiming that the advantages of the superposition of orders could be reproduced, and even surpassed, by other forms of superpositions. To shed light on this debate, we develop a general framework of resource theories of communication. In this framework, the resources are communication devices, and the allowed operations are (a) the placement of communication devices between the communicating parties, and (b) the connection of communication devices with local devices in the parties’ laboratories. The allowed operations are required to satisfy the minimal condition that they do not enable communication independently of the devices representing the initial resources. The resource-theoretic analysis reveals that the aforementioned criticisms on the superposition of causal orders were based on an uneven comparison between different types of quantum superpositions, exhibiting different operational features.

**Authors: **Philippe Allard Guérin, Veronika Baumann, Flavio Del Santo and Časlav Brukner

**Year:** 2021

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

**Authors: **Giulia Rubino, Lee A. Rozema, Daniel Ebler, Hlér Kristjánsson, Sina Salek, Philippe Allard Guérin, Alastair A. Abbott, Cyril Branciard, Časlav Brukner, Giulio Chiribella and Philip Walther

**Year:** 2021

In quantum communication networks, wires represent well-defined trajectories along which quantum systems are transmitted. In spite of this, trajectories can be used as a quantum control to govern the order of different noisy communication channels, and such a control has been shown to enable the transmission of information even when quantum communication protocols through well-defined trajectories fail. This result has motivated further investigations on the role of the superposition of trajectories in enhancing communication, which revealed that the use of quantum control of parallel communication channels, or of channels in series with quantum-controlled operations, can also lead to communication advantages. Building upon these findings, here we experimentally and numerically compare different ways in which two trajectories through a pair of noisy channels can be superposed. We observe that, within the framework of quantum interferometry, the use of channels in series with quantum-controlled operations generally yields the largest advantages. Our results contribute to clarify the nature of these advantages in experimental quantum-optical scenarios, and showcase the benefit of an extension of the quantum communication paradigm in which both the information exchanged and the trajectory of the information carriers are quantum.

**Authors: **Pablo Arrighi, Marios Christodoulou and Amélia Durbec

**Year:** 2020

We provide a robust notion of quantum superpositions of graphs. Quantum superpositions of graphs crucially require node names for their correct alignment, as we demonstrate through a non-signalling argument. Nevertheless, node names are a fiducial construct, serving a similar purpose to the labelling of points through a choice of coordinates in continuous space. We explain that graph renamings are, indeed, a natively discrete analogue of diffeomorphisms. We show how to impose renaming invariance at the level of graphs and their quantum superpositions.

**Authors: **Jonathan Barrett, Robin Lorenz abd Ognyan Oreshkov**Year:** 2021

Causal reasoning is essential to science, yet quantum theory challenges it. Quantum correlations violating Bell inequalities defy satisfactory causal explanations within the framework of classical causal models. What is more, a theory encompassing quantum systems and gravity is expected to allow causally nonseparable processes featuring operations in indefinite causal order, defying that events be causally ordered at all. The first challenge has been addressed through the recent development of intrinsically quantum causal models, allowing causal explanations of quantum processes — provided they admit a definite causal order, i.e. have an acyclic causal structure. This work addresses causally nonseparable processes and offers a causal perspective on them through extending quantum causal models to cyclic causal structures. Among other applications of the approach, it is shown that all unitarily extendible bipartite processes are causally separable and that for unitary processes, causal nonseparability and cyclicity of their causal structure are equivalent.

Comments: 21 pages, 10 figures. Close to published version

Subjects: Quantum Physics (quant-ph)

Journal reference: Nature Communications 12, 885 (2021)

DOI: 10.1038/s41467-020-20456-x

Cite as: arXiv:2002.12157 [quant-ph]

(or arXiv:2002.12157v3 [quant-ph] for this version)

**Authors: **Marios Christodoulou, Andrea Di Biagio and Pierre Martin-Dussaud

**Year:** 2020

Time at the Planck scale (∼10−44 s) is an unexplored physical regime. It is widely believed that probing Planck time will remain for long an impossible task. Yet, we propose an experiment to test the discreteness of time at the Planck scale and show that it is not far removed from current technological capabilities.

**Authors: **Augustin Vanrietvelde and Giulio Chiribella **Year:** 2021

No universal circuit architecture can implement the coherently controlled version of a completely unknown unitary gate. Yet, universal coherent control of unknown gates has been realised in experiments, making use of a different type of initial resources. Here, we formalise the task achieved by these experiments, extending it to the control of arbitrary noisy channels, and to more general types of control involving higher dimensional control systems. For the standard notion of coherent control, we identify the information-theoretic resource for controlling an arbitrary quantum channel on a d-dimensional system: specifically, the resource is an extended quantum channel acting as the original channel on a d-dimensional sector of a (d+1)-dimensional system. Using this resource, arbitrary controlled channels can be built with a universal circuit architecture. We then extend the standard notion of control to more general notions, including control of multiple channels with possibly different input and output systems. Finally, we develop a theoretical framework, called supermaps on routed channels, which provides a compact representation of coherent control as an operation performed on the extended channels, and highlights the way the operation acts on different sectors.

**Authors:**Rafael Chaves, George Moreno, Emanuele Polino, Davide Poderini, Iris Agresti, Alessia Suprano, Mariana R. Barros, Gonzalo Carvacho, Elie Wolfe, Askery Canabarro, Robert W. Spekkens and Fabio Sciarrino

**Year:**2021 Bell’s theorem is typically understood as the proof that quantum theory is incompatible with local hidden variable models. More generally, we can see the violation of a Bell inequality as witnessing the impossibility of explaining quantum correlations with classical causal models. The violation of a Bell inequality, however, does not exclude classical models where some level of measurement dependence is allowed, that is, the choice made by observers can be correlated with the source generating the systems to be measured. Here we show that the level of measurement dependence can be quantitatively upper bounded if we arrange the Bell test within a network. Furthermore, we also prove that these results can be adapted in order to derive non-linear Bell inequalities for a large class of causal networks and to identify quantumly realizable correlations which violate them.

**Authors: **Farshid Soltani, Carlo Rovelli, Pierre Martin-Dussaud

**Year:** 2021

We derive an explicit expression for the transition amplitude from black to white hole horizon at the end of Hawking evaporation using covariant loop quantum gravity.

**Authors: **Andrea Di Biagio, Pietro Donà and Carlo Rovelli**Year:** 2020

It is often stated that quantum theory yields probabilities for outcomes of future measurements. This language reinforces a common misconception: that quantum dynamics is time oriented. In fact, the past of quanta is as undetermined as their future given the present. Here we address the apparent tension between the time-reversal invariance of elementary quantum physics and the intrinsic time orientation of the formulations of quantum theory used in the quantum information and quantum foundations communities. We show that the latter does not stem from a quantum arrow of time, but from the agent’s own thermodynamic arrow of time.

**Authors: **Matt Wilson, Giulio Chiribella

**Year:** 2021

Quantum supermaps provide a framework in which higher order quantum processes can act on lower order quantum processes. In doing so, they enable the definition and analysis of new quantum protocols and causal structures. Recently, key features of quantum supermaps were captured through a general categorical framework, which led to a framework of higher order process theories (HOPT). The HOPT framework models lower and higher order transformations in a single unified theory, with its mathematical structure shown to coincide with the notion of a closed symmetric monoidal category. Here we provide an equivalent construction of the HOPT framework from four simple axioms of process-theoretic nature. We then use the HOPT framework to establish connections between foundational features such as causality, determinism and signalling, alongside exploring their interaction with the mathematical structure of *-autonomy.

**Authors: **Augustin Vanrietvelde, Hlér Kristjánsson, Jonathan Barrett

**Year:** 2021

We argue that the quantum-theoretical structures studied in several recent lines of research cannot be adequately described within the standard framework of quantum circuits. This is in particular the case whenever the combination of subsystems is described by a nontrivial blend of direct sums and tensor products of Hilbert spaces. We therefore propose an extension to the framework of quantum circuits, given by \textit{routed linear maps} and \textit{routed quantum circuits}. We prove that this new framework allows for a consistent and intuitive diagrammatic representation in terms of circuit diagrams, applicable to both pure and mixed quantum theory, and exemplify its use in several situations, including the superposition of quantum channels and the causal decompositions of unitaries. We show that our framework encompasses the `extended circuit diagrams’ of Lorenz and Barrett [arXiv:2001.07774 (2020)], which we derive as a special case, endowing them with a sound semantics.

**Authors:**Iris Agresti, Davide Poderini, Beatrice Polacchi, Nikolai Miklin, Mariami Gachechiladze, Alessia Suprano, Emanuele Polino, Giorgio Milani, Gonzalo Carvacho, Rafael Chaves, Fabio Sciarrino

**Year:**2021 Since Bell’s theorem, it is known that the concept of local realism fails to explain quantum phenomena. Indeed, the violation of a Bell inequality has become a synonym of the incompatibility of quantum theory with our classical notion of cause and effect. As recently discovered, however, the instrumental scenario — a tool of central importance in causal inference — allows for signatures of nonclassicality that do not hinge on this paradigm. If, instead of relying on observational data only, we can also intervene in our experimental setup, quantum correlations can violate classical bounds on the causal influence even in scenarios where no violation of a Bell inequality is ever possible. That is, through interventions, we can witness the quantum behaviour of a system that would look classical otherwise. Using a photonic setup — faithfully implementing the instrumental causal structure and allowing to switch between the observational and interventional modes in a run to run basis — we experimentally observe this new witness of nonclassicality for the first time. In parallel, we also test quantum bounds for the causal influence, showing that they provide a reliable tool for quantum causal modelling.

**Authors:**Gonzalo Carvacho, Emanuele Roccia, Mauro Valeri, Francesco Basso Basset, Davide Poderini, Claudio Pardo, Emanuele Polino, Lorenzo Carosini, Michele B. Rota, Julia Neuwirth, Saimon F. Covre da Silva, Armando Rastelli, Nicolò Spagnolo, Rafael Chaves, Rinaldo Trotta, Fabio Sciarrino

**Year:**2021 Quantum networks play a crucial role for distributed quantum information processing, enabling the establishment of entanglement and quantum communication among distant nodes. Fundamentally, networks with independent sources allow for new forms of nonlocality, beyond the paradigmatic Bell’s theorem. Here we implement the simplest of such networks — the bilocality scenario — in an urban network connecting different buildings with a fully scalable and hybrid approach. Two independent sources using different technologies, respectively a quantum dot and a nonlinear crystal, are used to share photonic entangled state among three nodes connected through a 270 m free-space channel and fiber links. By violating a suitable non-linear Bell inequality, we demonstrate the nonlocal behaviour of the correlations among the nodes of the network. Our results pave the way towards the realization of more complex networks and the implementation of quantum communication protocols in an urban environment, leveraging on the capabilities of hybrid photonic technologies.

**Authors: **Veronika Baumann, Marius Krumm, Philippe Allard Guérin, Časlav Brukner

**Year:** 2022

One of the most fundamental open problems in physics is the unification of general relativity and quantum theory to a theory of quantum gravity. An aspect that might become relevant in such a theory is that the dynamical nature of causal structure present in general relativity displays quantum uncertainty. This may lead to a phenomenon known as indefinite or quantum causal structure, as captured by the process matrix formalism. Due to the generality of that framework, however, for many process matrices there is no clear physical interpretation. A popular approach towards a quantum theory of gravity is the Page-Wootters formalism, which associates to time a Hilbert space structure similar to spatial position. By explicitly introducing a quantum clock, it allows to describe time-evolution of systems via correlations between this clock and said systems encoded in history states. In this paper we combine the process matrix framework with a generalization of the Page-Wootters formalism in which one considers several agents, each with their own discrete quantum clock. We describe how to extract process matrices from scenarios involving such agents with quantum clocks, and analyze their properties. The description via a history state with multiple clocks imposes constraints on the implementation of process matrices and on the perspectives of the agents as described via causal reference frames. While it allows for scenarios where different definite causal orders are coherently controlled, we explain why certain noncausal processes might not be implementable within this setting.

**Authors: **Laurent Freidel, Roberto Oliveri, Daniele Pranzetti, Simone Speziale

**Year:** 2021

We propose an extension of the BMS group, which we refer to as Weyl BMS or BMSW for short, that includes, besides super-translations, local Weyl rescalings and arbitrary diffeomorphisms of the 2d sphere metric. After generalizing the Barnich-Troessaert bracket, we show that the Noether charges of the BMSW group provide a centerless representation of the BMSW Lie algebra at every cross section of null infinity. This result is tantamount to proving that the flux-balance laws for the Noether charges imply the validity of the asymptotic Einstein’s equations at null infinity. The extension requires a holographic renormalization procedure, which we construct without any dependence on background fields. The renormalized phase space of null infinity reveals new pairs of conjugate variables. Finally, we show that BMSW group elements label the gravitational vacua.

**Authors: **Hlér Kristjánsson, Wenxu Mao, Giulio Chiribella

**Year:** 2020

When a noisy communication channel is used multiple times, the errors occurring at different times generally exhibit correlations. Classically, these correlations do not affect the evolution of individual particles: a single classical particle can only traverse the channel at a definite moment of time, and its evolution is insensitive to the correlations between subsequent uses of the channel. In stark contrast, here we show that a single quantum particle can sense the correlations between multiple uses of a channel at different moments of time. In an extreme example, we show that a channel that outputs white noise when the particle is sent at a definite time can exhibit correlations that enable a perfect transmission of classical bits when the particle is sent at a superposition of two times. In contrast, we show that, in the lack of correlations, a single particle sent at a superposition of two times undergoes an effective channel with classical capacity of at most 0.16 bits. When multiple transmission lines are available, time correlations can be used to simulate the application of quantum channels in a coherent superposition of alternative causal orders, and even to provide communication advantages that are not accessible through the superposition of causal orders.

**Authors: ** Augustin Vanrietvelde, Hlér Kristjánsson, Jonathan Barrett

**Year:** 2020

We argue that the quantum-theoretical structures studied in several recent lines of research cannot be adequately described within the standard framework of quantum circuits. This is in particular the case whenever the combination of subsystems is described by a nontrivial blend of direct sums and tensor products of Hilbert spaces. We therefore propose an extension to the framework of quantum circuits, given by \textit{routed linear maps} and \textit{routed quantum circuits}. We prove that this new framework allows for a consistent and intuitive diagrammatic representation in terms of circuit diagrams, applicable to both pure and mixed quantum theory, and exemplify its use in several situations, including the superposition of quantum channels and the causal decompositions of unitaries. We show that our framework encompasses the `extended circuit diagrams’ of Lorenz and Barrett [arXiv:2001.07774 (2020)], which we derive as a special case, endowing them with a sound semantics.

**Authors: **Pablo Arrighi, Amélia Durbec, Matt Wilson

**Year:** 2021

The formalism of quantum theory over discrete systems is extended in two significant ways. First, tensors and traceouts are generalized, so that systems can be partitioned according to almost arbitrary logical predicates. Second, quantum evolutions are generalized to act over network configurations, in such a way that nodes be allowed to merge, split and reconnect coherently in a superposition. The hereby presented mathematical framework is anchored on solid grounds through numerous lemmas. Indeed, one might have feared that the familiar interrelations between the notions of unitarity, complete positivity, trace-preservation, non-signalling causality, locality and localizability that are standard in quantum theory be jeopardized as the partitioning of systems becomes both logical and dynamical. Such interrelations in fact carry through, albeit two new notions become instrumental: consistency and comprehension.

**Authors: ** Marios Christodoulou, Andrea Di Biagio, Markus Aspelmeyer, Časlav Brukner, Carlo Rovelli, Richard Howl

**Year:** 2022

Observing entanglement generation mediated by a local field certifies that the field cannot be classical. This information-theoretic argument is at the heart of the race to observe gravity-mediated entanglement in a `table-top’ experiment. Previous derivations of the effect assume the locality of interactions, while using an instantaneous interaction to derive the effect. We correct this by giving a first principles derivation of mediated entanglement using linearised gravity. The framework is Lorentz covariant — thus local — and yields Lorentz and gauge invariant expressions for the relevant quantum observables. For completeness we also cover the electromagnetic case. An experimental consequence of our analysis is the possibility to observe *retarded* mediated entanglement, which avoids the need of taking relativistic locality as an assumption. This is a difficult experiment for gravity, but could be feasible for electromagnetism. Our results confirm that the entanglement is dynamically mediated by the gravitational field.

**Authors: ** Richard Howl, Ali Akil, Hlér Kristjánsson, Xiaobin Zhao, Giulio Chiribella

**Year:** 2022

It has been theorized that when a quantum communication protocol takes place near a black hole, the spacetime structure induced by the black hole causes an inescapable and fundamental degradation in the protocol’s performance compared to if the protocol took place in flat spacetime. This is due to quantum information beyond the event horizon being inaccessible, introducing noise and degrading the entanglement resources of the protocol. However, despite black holes being a place where we expect quantum gravity to be integral, it has been assumed in these results that the black hole is a classical object with a classical spacetime. We show that when the quantum nature of a black hole and its spacetime are taken into account, their quantum properties can be used as resources to allay the degradation of entanglement caused by the event horizon, and thus improve the performance of quantum communication protocols near black holes. Investigating the resourceful nature of quantum gravity could be useful in better understanding the fundamental features of quantum gravity, just as the resourcefulness of quantum theory has revealed new insights into its foundations.

**Authors: ** Flaminia Giacomini, Časlav Brukner

**Year:** 2022

We challenge the view that there is a basic conflict between the fundamental principles of Quantum Theory and General Relativity, and in particular the fact that a superposition of massive bodies would lead to a violation of the Equivalence Principle. It has been argued that this violation implies that such a superposition must inevitably spontaneously collapse (like in the Diósi-Penrose model). We identify the origin of such an assertion in the impossibility of finding a local, classical reference frame in which Einstein’s Equivalence Principle would hold. In contrast, we argue that the formulation of the Equivalence Principle can be generalised so that it holds for reference frames that are associated to quantum systems in a superposition of spacetimes. The core of this new formulation is the introduction of a quantum diffeomorphism to such Quantum Reference Frames (QRFs). This procedure reconciles the principle of linear superposition in Quantum Theory with the principle of general covariance and the Equivalence Principle of General Relativity. Hence, it is not necessary to invoke a gravity-induced spontaneous state reduction when a massive body is prepared in a spatial superposition.

**Authors: ** Marius Krumm, Philippe Allard Guérin, Thomas Zauner, Časlav Brukner

**Year:** 2022

Quantum teleportation is a very helpful information-theoretic protocol that allows to transfer an unknown arbitrary quantum state from one location to another without having to transmit the quantum system through the intermediate region. Quantum states, quantum channels, and indefinite causal structures are all examples of quantum causal structures that not only enable advanced quantum information processing functions, but can also model causal structures in nonclassical spacetimes. In this letter, we develop quantum teleportation of arbitrary quantum causal structures, as formalized by the process matrix framework. Instead of teleporting all the physical degrees of freedom that implement the causal structure, the central idea is to just teleport the inputs to and outputs from the operations of agents. The communication of outcomes of Bell state measurements, which is necessary for deterministic quantum teleportation, is not possible for all causal structures that one might wish to investigate. To avoid this problem, we propose partially and fully post-selected teleportation protocols. We prove that our partially post-selected teleportation protocol is compatible with all quantum causal structures, including those that involve indefinite causal order.

**Authors: ** Pablo Arrighi, Amélia Durbec, Matt Wilson

**Year:** 2022

Tensors and traceouts are generalised, so that systems can be partitioned according to almost arbitrary logical predicates. One might have feared that the familiar interrelations between the notions of unitarity, complete positivity, trace-preservation, non-signalling causality, locality and localizability that are standard in quantum theory be jeopardized as the partitioning of systems becomes both logical and dynamical. Such interrelations in fact carry through, although a new notion, consistency, becomes instrumental.

**Authors: ** Jonathan Engle and Carlo Rovelli

**Year:** 2022

We shed some light on the reason why the accidental flatness constraint appears in certain limits of the amplitudes of covariant loop quantum gravity. We show why this constraint is harmless, by displaying how analogous accidental constraints appear in transition amplitudes of simple systems, when certain limits are considered.

**Authors: ** Matt Wilson, Giulio Chiribella and Aleks Kissinger

**Year:** 2022

We prove that the sequential and parallel composition rules for quantum channels are sufficient to axiomatize quantum supermaps from first this http URL do so we provide a simple definition of locally-applicable transformation, which can be stated for arbitrary symmetric monoidal categories, and so for arbitrary process theories and operational probabilistic theories. The definition can be rephrased in the language of category theory using the principle of naturality, and can be given an intuitive diagrammatic representation in terms of which all proofs are presented. In our main technical contribution, we use this diagrammatic representation to show that locally-applicable transformations on quantum channels are in one-to-one correspondence with deterministic quantum supermaps. This alternative characterization of quantum supermaps is proven to hold for supermaps on arbitrary convex subsets of channels, including as a special case the supermaps on non-signalling channels used in the study of quantum causal structure.

**Authors: ** Emily Adlam, Carlo Rovelli

**Year:** 2022

Relational quantum mechanics (RQM) is an interpretation of quantum mechanics based on the idea that quantum states describe not an absolute property of a system but rather a relationship between systems. In this article, we observe that there is a tension between RQM’s naturalistic emphasis on the physicality of information and the inaccessibility of certain sorts of information in current formulations of RQM. Therefore we propose a new postulate for RQM which requires that all of the information possessed by a certain observer is stored in physical variables of that observer and thus accessible by measurement to other observers, so observers can reach intersubjective agreement about quantum events which have occurred in the past. Based on this postulate, we suggest an ontology for RQM which upholds the principle that quantum states are always relational, but which also postulates a set of quantum events which are not strictly relational. We show that the new postulate helps address some existing objections to RQM and finally we address the Frauchiger-Renner experiment in the context of RQM.

**Authors: ** Emanuele Polino, Beatrice Polacchi, Davide Poderini, Iris Agresti, Gonzalo Carvacho, Fabio Sciarrino, Andrea Di Biagio, Carlo Rovelli and Marios Christodoulou

**Year:** 2022

Detecting gravity mediated entanglement can provide evidence that the gravitational field obeys quantum mechanics. We report the result of a simulation of the phenomenon using a photonic platform. The simulation tests the idea of probing the quantum nature of a variable by using it to mediate entanglement, and yields theoretical and experimental insights. We employed three methods to test the presence of entanglement: Bell test, entanglement witness and quantum state tomography. We also simulate the alternative scenario predicted by gravitational collapse models or due to imperfections in the experimental setup and use quantum state tomography to certify the absence of entanglement. Two main lessons arise from the simulation: 1) which–path information must be first encoded and subsequently coherently erased from the gravitational field, 2) performing a Bell test leads to stronger conclusions, certifying the existence of gravity mediated nonlocality.

**Authors: ** Giuseppe Di Pietra and Chiara Marletto

**Year:** 2022

A general entanglement-based witness of non-classicality has recently been proposed, which can be applied to testing quantum effects in gravity. This witness is based on generating entanglement between two quantum probes via a mediator. In this paper we provide a “temporal” variant of this witness, using a single quantum probe to assess the non-classicality of the mediator. Within the formalism of quantum theory, we show that if a system M is capable of inducing a coherent dynamical evolution of a quantum system Q, in the presence of a conservation law, then M must be non-classical. This argument supports witnesses of non-classicality relying on a single quantum probe, which can be applied to a number of open issues, notably in quantum gravity or quantum biology.

**Authors: ** Matt Wilson and Giulio Chiribella

**Year:** 2022

We provide a construction for holes into which morphisms of abstract symmetric monoidal categories can be inserted, termed the polyslot construction pslot[C], and identify a sub-class srep[C] of polyslots that are single-party representable. These constructions strengthen a previously introduced notion of locally-applicable transformation used to characterize quantum supermaps in a way that is sufficient to re-construct unitary supermaps directly from the monoidal structure of the category of unitaries. Both constructions furthermore freely reconstruct the enriched polycategorical semantics for quantum supermaps which allows to compose supermaps in sequence and in parallel whilst forbidding the creation of time-loops. By freely constructing key compositional features of supermaps, and characterizing supermaps in the finite-dimensional case, polyslots are proposed as a suitable generalization of unitary-supermaps to infinite dimensions and are shown to include canonical examples such as the quantum switch. Beyond specific applications to quantum-relevant categories, a general class of categorical structures termed path-contraction groupoids are defined on which the srep[C] and pslot[C] constructions are shown to coincide.