# Weekly Papers on Quantum Foundations (25)

This is a list of this week’s papers on quantum foundations published in the various journals or uploaded to the preprint servers such as arxiv.org and PhilSci Archive.

Hints of dynamical vacuum energy in the expanding Universe. (arXiv:1506.05793v1 [gr-qc])

on 2015-6-20 8:19am GMT

Recently there have been claims on model-independent evidence of dynamical dark energy. Herein we consider a fairly general class of cosmological models with a time-evolving cosmological term of the form $\Lambda(H)=C_0+C_H H^2+C_{\dot{H}} \dot{H}$, where $H$ is the Hubble rate. These models are well motivated from the theoretical point of view since they can be related to the general form of the effective action of quantum field theory in curved spacetime. Consistency with matter conservation can be achieved by letting the Newtonian coupling $G$ change very slowly with the expansion. We solve these dynamical vacuum models and fit them to the wealth of expansion history and linear structure formation data. The results of our analysis show a significantly better agreement as compared to the concordance $\Lambda$CDM model, thus supporting the possibility of a dynamical cosmic vacuum.

Delayed Choice Contextuality: A way to rule out Contextual Hidden Variables. (arXiv:1506.05493v1 [quant-ph])

on 2015-6-20 8:19am GMT

A PhD student is locked inside a box, imitating a quantum system by mimicking the measurement statistics of any viable observable nominated by external observers. Inside a second box lies a genuine quantum system. Either box can be used to pass a test for contextuality – and from the perspective of an external observer, be operationally indistinguishable. There is no way to discriminate between the two boxes based on the output statistics of any contextuality test. This poses a serious problem for contextuality tests to be used as viable tests for device independent quantumness, and severely limits realistic use of contextuality as an operational resource. Here we rectify this problem by building experimental techniques for distinguishing a contextual system that is genuinely quantum, and one that mimics it through clever use of hidden variables.

Multiparticle entanglement as an emergent phenomenon. (arXiv:1506.05766v1 [quant-ph])

on 2015-6-20 8:19am GMT

The question whether global entanglement of a multiparticle quantum system can be inferred from local properties is of great relevance for the theory of quantum correlations as well as for experimental implementations. We present a method to systematically find quantum states, for which the two- or three-body marginals do not contain any entanglement, nevertheless, the knowledge of these reduced states is sufficient to prove genuine multiparticle entanglement of the global state. Our method shows that the emergence of global entanglement from separable local quantum states occurs frequently in various scenarios. Finally, we present examples where global entanglement can be proven from marginals, even if entanglement cannot be localized in the marginals with measurements on the other parties.

A Survey of Physical Principles Attempting to Define Quantum Mechanics. (arXiv:1506.05515v1 [quant-ph])

on 2015-6-20 8:19am GMT

Authors: Gary OasJ. Acacio de Barros

Quantum mechanics, one of the most successful theories in the history of science, was created to account for physical systems not describable by classical physics. Though it is consistent with all experiments conducted thus far, many of its core concepts (amplitudes, global phases, etc.) can not be directly accessed and its interpretation is still the subject of intense debate, more than 100 years since it was introduced. So, a fundamental question is why this particular mathematical model is the one that nature chooses, if indeed it is the correct model. In the past two decades there has been a renewed effort to determine what physical or informational principles define quantum mechanics. In this paper, recent attempts at establishing reasonable physical principles are reviewed and their degree of success is tabulated. An alternative approach using joint quasi-probability distributions is shown to provide a common basis of representing most of the proposed principles. It is argued that having a common representation of the principles can provide intuition and guidance to relate current principles or advance new principles. The current state of affairs, along with some alternative views are discussed.

Causal and causally separable processes. (arXiv:1506.05449v1 [quant-ph])

on 2015-6-18 2:47am GMT

Authors: Ognyan OreshkovChristina Giarmatzi

We develop rigorous notions of causality and causal separability in the process framework introduced in [Oreshkov, Costa, Brukner, Nat. Commun. 3, 1092 (2012)], which describes correlations between separate local experiments without a prior assumption of causal order between them. We consider the general multipartite case and take into account the possibility for dynamical causal order between the local experiments. Starting from a general definition of causality, we derive an iteratively formulated canonical decomposition of multipartite causal processes, and show that for a fixed number of settings and outcomes for each party, the respective probabilities form a polytope whose facets define causal inequalities. In the case of quantum processes, we investigate the link between causality and the theory-dependent notion of causal separability, which we here extend to the multipartite case based on concrete principles. We show that causality and causal separability are not equivalent in the multipartite case by giving an example of a physically admissible tripartite quantum process that is causal but not causally separable. We also show that there exist causally separable (and hence causal) quantum processes that become non-causal if extended by supplying the parties with entangled ancillas. This motivates the concepts of extensibly causal and extensibly causally separable processes, for which the respective property remains invariant under extension. We fully characterize the class of extensibly causally separable processes in terms of simple conditions on the form of the process matrix, which generalize the form of bipartite causally separable process matrices. We show that the processes realizable by classically controlled quantum circuits are extensibly causally separable and conjecture that the reverse also holds.

Are strings the aether of our time?. (arXiv:1506.05361v1 [quant-ph])

on 2015-6-18 2:47am GMT

Authors: Glenn Eric Johnson

Descriptions of relativistic quantum physics that derive from quantizations of classical physics require additional technical properties and these technical conjectures exclude interaction in example realizations. In this essay, uniquely quantum mechanical examples are discussed to illustrate realizations made available by displacing classical concepts from relativistic quantum physics.

How gravity kills Schrödinger’s cat

Nature News & Comment

on 2015-6-17 12:00am GMT

Theorists argue that warped space-time prevents quantum superpositions of large-scale objects.

Nature News doi: 10.1038/nature.2015.17773

Disturbance in weak measurements and the difference between quantum and classical weak values

PRA: Fundamental concepts

on 2015-6-16 2:00pm GMT

Author(s): Asger C. Ipsen

The role of measurement-induced disturbance in weak measurements is of central importance for the interpretation of the weak value. Uncontrolled disturbance can interfere with the postselection process and make the weak value dependent on the details of the measurement process. Here we develop the c…

[Phys. Rev. A 91, 062120] Published Tue Jun 16, 2015

A classical Deutsch-Jozsa algorithm. (arXiv:1506.04627v1 [quant-ph])

on 2015-6-16 9:20am GMT

In 1985, David Deutsch challenged the Church-Turing thesis by stating that his quantum model of computation “could, in principle, be built and would have many remarkable properties not reproducible by any Turing machine”. While this is thought to be true in general, there is usually no way of knowing that the corresponding classical algorithms are the best possible solutions. Here we provide an efficient classical version of the Deutsch-Jozsa algorithm, which was one of the first examples of quantum computational speed-up. Our conclusion is that the quantum Deutsch-Josza algorithm owes its speed-up to resources that are not necessarily quantum-mechanical, and when compared with this new classical solution, offers no speed-up at all.

Quantum mechanics: knocking at the gates of mathematical foundations. (arXiv:1506.04511v1 [quant-ph])

on 2015-6-16 9:20am GMT

The Weltanschauung emerging from quantum theory clashes profoundly with our classical concepts. Quantum characteristics like superposition, entanglement, wave-particle duality, nonlocality, contextuality are difficult to reconcile with our everyday intuition. In this article I survey some aspects of quantum foundations and discuss intriguing connections with the foundations of mathematics.

Polynomial Bell inequalities. (arXiv:1506.04325v1 [quant-ph])

on 2015-6-16 9:20am GMT

Authors: Rafael Chaves

It is a recent realization that many of the concepts and tools of causal discovery in machine learning are highly relevant to problems in quantum information, in particular quantum nonlocality. The crucial ingredient in the connection between both fields is the tool of Bayesian networks, a graphical model used to reason about probabilistic causation. Indeed, Bell’s theorem concerns a particular kind of a Bayesian network and Bell inequalities are a special case of linear constraints following from such models. It is thus natural to look for generalized Bell scenarios involving more complex Bayesian networks. The problem, however, relies on the fact that such generalized scenarios are characterized by polynomial Bell inequalities and no current method is available to derive them beyond very simple cases. In this work, we make a significant step in that direction, providing a general and practical method for the derivation of polynomial Bell inequalities in a wide class of scenarios, applying it to a few cases of interest. We also show how our construction naturally gives rise to a notion of non-signalling in generalized networks.

Quantum entanglement. (arXiv:1506.04262v1 [physics.hist-ph])

on 2015-6-16 9:20am GMT

Expository paper providing a historical survey of the gradual transformation of the “philosophical discussions” between Bohr, Einstein and Schr\”odinger on foundational issues in quantum mechanics into a quantitative prediction of a new quantum effect, its experimental verification and its proposed (and loudly advertised) applications. The basic idea of the 1935 paper of Einstein-Podolsky-Rosen (EPR) was reformulated by David Bohm for a finite dimensional spin system. This allowed John Bell to derive his inequalities that separate the prediction of quantum entanglement from its possible classical interpretation. We reproduce here their later (1971) version, reviewing on the way the generalization (and mathematical derivation) of Heisenberg’s uncertainty relations (due to Weyl and Schr\”odinger) needed for the passage from EPR to Bell. We also provide an improved derivation of the quantum theoretic violation of Bell’s inequalities. Soon after the experimental confirmation of the quantum entanglement (culminating with the work of Alain Aspect) it was Feynman who made public the idea of a quantum computer based on the observed effect.

From the Kochen-Specker theorem to noncontextuality inequalities without assuming determinism. (arXiv:1506.04150v1 [quant-ph])

on 2015-6-16 9:20am GMT

Authors: Ravi KunjwalRobert W. Spekkens

The Kochen-Specker theorem demonstrates that it is not possible to reproduce the predictions of quantum theory in terms of a hidden variable model where the hidden variables assign a value to every projector deterministically and noncontextually. A noncontextual value-assignment to a projector is one that does not depend on which other projectors – the context – are measured together with it. Using a generalization of the notion of noncontextuality that applies to both measurements and preparations, we propose a scheme for deriving inequalities that test whether a given set of experimental statistics is consistent with a noncontextual model. Unlike previous inequalities inspired by the Kochen-Specker theorem, we do not assume that the value-assignments are deterministic and therefore in the face of a violation of our inequality, the possibility of salvaging noncontextuality by abandoning determinism is no longer an option. Our approach is operational in the sense that it does not presume quantum theory: a violation of our inequality implies the impossibility of a noncontextual model for any operational theory that can account for the experimental observations, including any successor to quantum theory.

Resolving the vacuum fluctuations of an optomechanical system using an artificial atom

Nature Physics – AOP – nature.com science feeds

on 2015-6-15 12:00am GMT

Nature Physics. doi:10.1038/nphys3365

Authors: F. Lecocq, J. D. Teufel, J. Aumentado & R. W. Simmonds

Heisenberg’s uncertainty principle results in one of the strangest quantum behaviours: a mechanical oscillator can never truly be at rest. Even at a temperature of absolute zero, its position and momentum are still subject to quantum fluctuations. However, direct energy detection of the oscillator in its ground state makes it seem motionless, and in linear position measurements detector noise can masquerade as mechanical fluctuations. Thus, how can we resolve quantum fluctuations? Here, we parametrically couple a micromechanical oscillator to a microwave cavity to prepare the system in its quantum ground state and then amplify the remaining vacuum fluctuations into real energy quanta. We monitor the photon/phonon-number distributions using a superconducting qubit, allowing us to resolve the quantum vacuum fluctuations of the macroscopic oscillator’s motion. Our results further demonstrate the ability to control a long-lived mechanical oscillator using a non-Gaussian resource, directly enabling applications in quantum information processing and enhanced detection of displacement and forces.