Authors:
Auffeves, A; Grangier, P. Pages: 20170311 - 20170311 Abstract: We develop the point of view where quantum mechanics results from the interplay between the quantized number of ‘modalities’ accessible to a quantum system, and the continuum of ‘contexts’ that are required to define these modalities. We point out the specific roles of ‘extracontextuality’ and ‘extravalence’ of modalities, and relate them to the Kochen–Specker and Gleason theorems.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0311 Issue No:Vol. 376, No. 2123 (2018)

Authors:Rovelli C. Pages: 20170312 - 20170312 Abstract: Quantum mechanics is not about ‘quantum states’: it is about values of physical variables. I give a short fresh presentation and update on the relational perspective on the theory, and a comment on its philosophical implications.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0312 Issue No:Vol. 376, No. 2123 (2018)

Authors:DAriano G. M. Pages: 20170313 - 20170313 Abstract: Causality has never gained the status of a ‘law’ or ‘principle’ in physics. Some recent literature has even popularized the false idea that causality is a notion that should be banned from theory. Such misconception relies on an alleged universality of the reversibility of the laws of physics, based either on the determinism of classical theory, or on the multiverse interpretation of quantum theory, in both cases motivated by mere interpretational requirements for realism of the theory. Here, I will show that a properly defined unambiguous notion of causality is a theorem of quantum theory, which is also a falsifiable proposition of the theory. Such a notion of causality appeared in the literature within the framework of operational probabilistic theories. It is a genuinely theoretical notion, corresponding to establishing a definite partial order among events, in the same way as we do by using the future causal cone on Minkowski space. The notion of causality is logically completely independent of the misidentified concept of ‘determinism’, and, being a consequence of quantum theory, is ubiquitous in physics. In addition, as classical theory can be regarded as a restriction of quantum theory, causality holds also in the classical case, although the determinism of the theory trivializes it. I then conclude by arguing that causality naturally establishes an arrow of time. This implies that the scenario of the ‘block Universe’ and the connected ‘past hypothesis’ are incompatible with causality, and thus with quantum theory: they are both doomed to remain mere interpretations and, as such, are not falsifiable, similar to the hypothesis of ‘super-determinism’.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0313 Issue No:Vol. 376, No. 2123 (2018)

Authors:Zurek W. H. Pages: 20170315 - 20170315 Abstract: I compare the role of the information in classical and quantum dynamics by examining the relation between information flows in measurements and the ability of observers to reverse evolutions. I show that in the Newtonian dynamics reversibility is unaffected by the observer’s retention of the information about the measurement outcome. By contrast—even though quantum dynamics is unitary, hence, reversible—reversing quantum evolution that led to a measurement becomes, in principle, impossible for an observer who keeps the record of its outcome. Thus, quantum irreversibility can result from the information gain rather than just its loss—rather than just an increase of the (von Neumann) entropy. Recording of the outcome of the measurement resets, in effect, initial conditions within the observer’s (branch of) the Universe. Nevertheless, I also show that the observer’s friend—an agent who knows what measurement was successfully carried out and can confirm that the observer knows the outcome but resists his curiosity and does not find out the result—can, in principle, undo the measurement. This relativity of quantum reversibility sheds new light on the origin of the arrow of time and elucidates the role of information in classical and quantum physics. Quantum discord appears as a natural measure of the extent to which dissemination of information about the outcome affects the ability to reverse the measurement.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0315 Issue No:Vol. 376, No. 2123 (2018)

Authors:Vaccaro J. A. Pages: 20170316 - 20170316 Abstract: Advances in our understanding of the physical universe have dramatically affected how we view ourselves. Right at the core of all modern thinking about the universe is the assumption that dynamics is an elemental feature that exists without question. However, ongoing research into the quantum nature of time is challenging this view: my recently introduced quantum theory of time suggests that dynamics may be a phenomenological consequence of a fundamental violation of time reversal symmetry. I show here that there is consistency between the new theory and the block universe view. I also discuss the new theory in relation to the human experience of existing in the present moment, able to reflect on the past and contemplate a future that is yet to happen.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: atomic and molecular physics, quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0316 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Compagno, G; Castellini, A, Lo Franco, R. Pages: 20170317 - 20170317 Abstract: Here, we discuss a particle-based approach to deal with systems of many identical quantum objects (particles) that never employs labels to mark them. We show that it avoids both methodological problems and drawbacks in the study of quantum correlations associated with the standard quantum mechanical treatment of identical particles. The core of this approach is represented by the multiparticle probability amplitude, whose structure in terms of single-particle amplitudes we derive here by first principles. To characterize entanglement among the identical particles, this new method uses the same notions, such as partial trace, adopted for non-identical ones. We highlight the connection between our approach and second quantization. We also define spin-exchanged multipartite states which contain a generalization of W states to identical particles. We prove that particle spatial overlap plays a role in the distributed entanglement within multipartite systems and is responsible for the appearance of non-local quantum correlations.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0317 Issue No:Vol. 376, No. 2123 (2018)

Authors:Unruh W. G. Pages: 20170320 - 20170320 Abstract: It is argued that it is best not to think of quantum mechanics as non-local, but rather that it is non-realistic.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0320 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Krämer, L; del Rio, L. Pages: 20170321 - 20170321 Abstract: Within a global physical theory, a notion of locality allows us to find and justify information-processing primitives, like non-signalling between distant agents. Here, we propose exploring the opposite direction: to take agents as the basic building blocks through which we test a physical theory, and recover operational notions of locality from signalling conditions. First, we introduce an operational model for the effective state spaces of individual agents, as well as the range of their actions. We then formulate natural secrecy conditions between agents and identify the aspects of locality relevant for signalling. We discuss the possibility of taking commutation of transformations as a primitive of physical theories, as well as applications to quantum theory and generalized probability frameworks. This ‘it from bit’ approach establishes an operational connection between local actions and local observations, and gives a global interpretation to concepts like discarding a subsystem or composing local functions.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: mathematical physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0321 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Grangier, P; Auffeves, A. Pages: 20170322 - 20170322 Abstract: It is often said that quantum and classical randomness are of different nature, the former being ontological and the latter epistemological. However, so far the question of ‘What is quantum in quantum randomness'’, i.e. what is the impact of quantization and discreteness on the nature of randomness, remains to be answered. In a first part, we make explicit the differences between quantum and classical randomness within a recently proposed ontology for quantum mechanics based on contextual objectivity. In this view, quantum randomness is the result of contextuality and quantization. We show that this approach strongly impacts the purposes of quantum theory as well as its areas of application. In particular, it challenges current programmes inspired by classical reductionism, aiming at the emergence of the classical world from a large number of quantum systems. In a second part, we analyse quantum physics and thermodynamics as theories of randomness, unveiling their mutual influences. We finally consider new technological applications of quantum randomness that have opened up in the emerging field of quantum thermodynamics.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0322 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Conrad, J; Chamberland, C, Breuckmann, N. P, Terhal, B. M. Pages: 20170323 - 20170323 Abstract: We explore a distance-3 homological CSS quantum code, namely the small stellated dodecahedron code, for dense storage of quantum information and we compare its performance with the distance-3 surface code. The data and ancilla qubits of the small stellated dodecahedron code can be located on the edges respectively vertices of a small stellated dodecahedron, making this code suitable for three-dimensional connectivity. This code encodes eight logical qubits into 30 physical qubits (plus 22 ancilla qubits for parity check measurements) in contrast with one logical qubit into nine physical qubits (plus eight ancilla qubits) for the surface code. We develop fault-tolerant parity check circuits and a decoder for this code, allowing us to numerically assess the circuit-based pseudo-threshold.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum computing, quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0323 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Braunstein, S. L; Pirandola, S. Pages: 20170324 - 20170324 Abstract: The prevalent view that evaporating black holes should simply be smaller black holes has been challenged by the firewall paradox. In particular, this paradox suggests that something different occurs once a black hole has evaporated to one-half its original surface area. Here, we derive variations of the firewall paradox by tracking the thermodynamic entropy within a black hole across its entire lifetime and extend it even to anti-de Sitter space–times. Our approach sweeps away many unnecessary assumptions, allowing us to demonstrate a paradox exists even after its initial onset (when conventional assumptions render earlier analyses invalid). The most natural resolution may be to accept firewalls as a real phenomenon. Further, the vast entropy accumulated implies a deep firewall that goes ‘all the way down’ in contrast with earlier work describing only a structure at the horizon.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics, relativity, thermodynamics, cosmology PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0324 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Dahlberg, A; Wehner, S. Pages: 20170325 - 20170325 Abstract: Stabilizer states form an important class of states in quantum information, and are of central importance in quantum error correction. Here, we provide an algorithm for deciding whether one stabilizer (target) state can be obtained from another stabilizer (source) state by single-qubit Clifford operations (LC), single-qubit Pauli measurements (LPM) and classical communication (CC) between sites holding the individual qubits. What is more, we provide a recipe to obtain the sequence of LC+LPM+CC operations which prepare the desired target state from the source state, and show how these operations can be applied in parallel to reach the target state in constant time. Our algorithm has applications in quantum networks, quantum computing, and can also serve as a design tool—for example, to find transformations between quantum error correcting codes. We provide a software implementation of our algorithm that makes this tool easier to apply. A key insight leading to our algorithm is to show that the problem is equivalent to one in graph theory, which is to decide whether some graph G' is a vertex-minor of another graph G. The vertex-minor problem is, in general, -Complete, but can be solved efficiently on graphs which are not too complex. A measure of the complexity of a graph is the rank-width which equals the Schmidt-rank width of a subclass of stabilizer states called graph states, and thus intuitively is a measure of entanglement. Here, we show that the vertex-minor problem can be solved in time O( G 3), where G is the size of the graph G, whenever the rank-width of G and the size of G' are bounded. Our algorithm is based on techniques by Courcelle for solving fixed parameter tractable problems, where here the relevant fixed parameter is the rank width. The second half of this paper serves as an accessible but far from exhausting introduction to these concepts, that could be useful for many other problems in quantum information.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0325 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Gisin, N; Fröwis, F. Pages: 20170326 - 20170326 Abstract: Quantum non-locality has been an extremely fruitful subject of research, leading the scientific revolution towards quantum information science, in particular, to device-independent quantum information processing. We argue that the time is ripe to work on another basic problem in the foundations of quantum physics, the quantum measurement problem, which should produce good physics in theoretical, mathematical, experimental and applied physics. We briefly review how quantum non-locality contributed to physics (including some outstanding open problems) and suggest ways in which questions around macroscopic quantumness could equally contribute to all aspects of physics.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0326 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Agnesi, C; Vedovato, F, Schiavon, M, Dequal, D, Calderaro, L, Tomasin, M, Marangon, D. G, Stanco, A, Luceri, V, Bianco, G, Vallone, G, Villoresi, P. Pages: 20170461 - 20170461 Abstract: Recent interest in quantum communications has stimulated great technological progress in satellite quantum technologies. These advances have rendered the aforesaid technologies mature enough to support the realization of experiments that test the foundations of quantum theory at unprecedented scales and in the unexplored space environment. Such experiments, in fact, could explore the boundaries of quantum theory and may provide new insights to investigate phenomena where gravity affects quantum objects. Here, we review recent results in satellite quantum communications and discuss possible phenomena that could be observable with current technologies. Furthermore, stressing the fact that space represents an incredible resource to realize new experiments aimed at highlighting some physical effects, we challenge the community to propose new experiments that unveil the interplay between quantum mechanics and gravity that could be realizable in the near future.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: optics, quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2017.0461 Issue No:Vol. 376, No. 2123 (2018)

Authors:Zurek W. H. Pages: 20180107 - 20180107 Abstract: The emergence of the classical world from the quantum substrate of our Universe is a long-standing conundrum. In this paper, I describe three insights into the transition from quantum to classical that are based on the recognition of the role of the environment. I begin with the derivation of preferred sets of states that help to define what exists—our everyday classical reality. They emerge as a result of the breaking of the unitary symmetry of the Hilbert space which happens when the unitarity of quantum evolutions encounters nonlinearities inherent in the process of amplification—of replicating information. This derivation is accomplished without the usual tools of decoherence, and accounts for the appearance of quantum jumps and the emergence of preferred pointer states consistent with those obtained via environment-induced superselection, or einselection. The pointer states obtained in this way determine what can happen—define events—without appealing to Born’s Rule for probabilities. Therefore, pk= k 2 can now be deduced from the entanglement-assisted invariance, or envariance—a symmetry of entangled quantum states. With probabilities at hand, one also gains new insights into the foundations of quantum statistical physics. Moreover, one can now analyse the information flows responsible for decoherence. These information flows explain how the perception of objective classical reality arises from the quantum substrate: the effective amplification that they represent accounts for the objective existence of the einselected states of macroscopic quantum systems through the redundancy of pointer state records in their environment—through quantum Darwinism.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2018.0107 Issue No:Vol. 376, No. 2123 (2018)

Authors:
Popescu, S; Sainz, A. B, Short, A. J, Winter, A. Pages: 20180111 - 20180111 Abstract: We construct a quantum reference frame, which can be used to approximately implement arbitrary unitary transformations on a system in the presence of any number of extensive conserved quantities, by absorbing any back action provided by the conservation laws. Thus, the reference frame at the same time acts as a battery for the conserved quantities. Our construction features a physically intuitive, clear and implementation-friendly realization. Indeed, the reference system is composed of the same types of subsystems as the original system and is finite for any desired accuracy. In addition, the interaction with the reference frame can be broken down into two-body terms coupling the system to one of the reference frame subsystems at a time. We apply this construction to quantum thermodynamic set-ups with multiple, possibly non-commuting conserved quantities, which allows for the definition of explicit batteries in such cases.This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’. Keywords: quantum physics PubDate: 2018-05-28T10:26:18-07:00 DOI: 10.1098/rsta.2018.0111 Issue No:Vol. 376, No. 2123 (2018)