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: 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: 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: 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.

Preprint ❱

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: 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.