# Weekly Papers on Quantum Foundations (49)

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

Echoes from the Abyss: Evidence for Planck-scale structure at black hole horizons. (arXiv:1612.00266v1 [gr-qc])

on 2016-12-03 3:23am GMT

Authors: Jahed AbediHannah DykaarNiayesh Afshordi

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of $8 M \log M$ (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the “look elsewhere” effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at $2.9\sigma$ significance level (corresponding to false detection probability of 1 in 270). Future data releases from LIGO collaboration, along with more physical echo templates, will definitively confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in ${\it firewall}$ or ${\it fuzzball}$ paradigms.

Time in quantum cosmology. (arXiv:1612.00353v1 [gr-qc])

on 2016-12-03 3:23am GMT

Authors: Martin BojowaldTheodore Halnon

A cosmological model with two global internal times shows that time reparameterization invariance, and therefore covariance, is not guaranteed by deparameterization. In particular, it is impossible to derive proper-time effective equations from a single deparameterized model if quantum corrections from fluctuations and higher moments are included. The framework of effective constraints shows how proper-time evolution can consistently be defined in quantum cosmological systems, such that it is time reparameterization invariant when compared with other choices of coordinate time. At the same time, it allows transformations of moment corrections in different deparameterizations of the same model, indicating partial time reparameterization of internal-time evolution. However, in addition to corrections from moments such as quantum fluctuations, also factor ordering corrections may appear. The latter generically break covariance in internal-time formulations. Fluctuation effects in quantum cosmology are therefore problematic, in particular if derivations are made with a single choice of internal time or a fixed physical Hilbert space.

Rudolf Haag’s legacy of Local Quantum Physics and reminiscences about a cherished teacher and friend. (arXiv:1612.00003v1 [math-ph])

on 2016-12-03 3:23am GMT

Authors: Bert Schroer

After some personal recollectioms about Rudolf Haag and his thoughts which led him to “Local Quantum Physics”, the present work recalls his ideas about scattering theory, the relation between local observables and localized fields and his contributions to the physical aspects of modular operator theory which paved the way for an intrisic understanding of quantum causal localization in which fields “coordinatize” the local algebras. The paper ends with the presentation of string-local fields whose construction and use in a new renormalization theory for higher spin fields is part of an ongoing reformulation of gauge theory in the conceptual setting of Haag’s LQP.

[Feature] Quest for qubits

Science: Current Issue

on 2016-12-02 12:00am GMT

Building a quantum computer has gone from a far-off dream of a few university scientists to an immediate goal for some of the world’s biggest companies. Tech giants Intel, Microsoft, IBM, and Google are all plowing tens of millions of dollars into quantum computing, which aims to harness quantum mechanics to vastly accelerate computation. Yet the contenders are betting on different technological horses: No one yet knows what type of quantum logic bit, or qubit, will power a practical quantum computer. Google, often considered the field’s leader, has signaled its choice: tiny, superconducting circuits. Its group has built a nine-qubit machine and hopes to scale up to 49 within a year—an important threshold. At about 50 qubits, many say a quantum computer could achieve “quantum supremacy” and do something beyond the ken of a classical computer, such as simulating molecular structures in chemistry and materials science, or solving problems in cryptography. Small startup company ionQ, a decided underdog, is sticking with its preferred technology: trapped ions. Author: Gabriel Popkin

Entanglement Entropy in Causal Set Theory. (arXiv:1611.10281v1 [hep-th])

on 2016-12-01 1:49pm GMT

Authors: Rafael D. SorkinYasaman K. Yazdi

Entanglement entropy is now widely accepted as having deep connections with quantum gravity. It is therefore desirable to understand it in the context of causal sets, especially since they provide in a natural manner the UV cutoff needed to render entanglement entropy finite. Defining entropy in a causal set is not straightforward because the type of canonical hypersurface-data on which definitions of entanglement typically rely is not available in a causal set. Instead, we will appeal to a more global expression given in arXiv:1205.2953 which, for a gaussian scalar field, expresses the entropy of a spacetime region in terms of the field’s correlation function within that region. Carrying this formula over to the causal set, one obtains an entanglement entropy which is both finite and of a Lorentz invariant nature. Herein we evaluate this entropy for causal sets of 1+1 dimensions, and specifically for order-intervals (“causal diamonds”) within the causal set, finding in the first instance an entropy that obeys a (spacetime) volume law instead of the expected (spatial) area law. We find, however, that one can obtain an area law by truncating the eigenvalues of a certain “Pauli-Jordan” operator that enters into the entropy formula. In connection with these results, we also study the “entropy of coarse-graining” generated by thinning out the causal set, and we compare it with what one obtains by similarly thinning out a chain of harmonic oscillators, finding the same, “universal” behaviour in both cases.

Detecting gravitational decoherence with clocks: Limits on temporal resolution from a classical channel model of gravity. (arXiv:1611.09919v1 [quant-ph])

on 2016-12-01 1:49pm GMT

Authors: Kiran KhoslaNatacha Altamirano

The notion of time is given a different footing in Quantum Mechanics and General Relativity, treated as a parameter in the former and being an observer dependent property in the later. From a operational point of view time is simply the correlation between a system and a clock, where an idealized clock can be modelled as a two level systems. We investigate the dynamics of clocks interacting gravitationally by treating the gravitational interaction as a classical information channel. In particular, we focus on the decoherence rates and temporal resolution of arrays of $N$ clocks showing how the minimum dephasing rate scales with $N$, and the spatial configuration. Furthermore, we consider the gravitational redshift between a clock and massive particle and show that a classical channel model of gravity predicts a finite dephasing rate from the non-local interaction. In our model we obtain a fundamental limitation in time accuracy that is intrinsic to each clock.

Probing Spontaneous Wave-Function Collapse with Entangled Levitating Nanospheres. (arXiv:1611.09989v1 [quant-ph])

on 2016-12-01 8:41am GMT

Authors: Jing ZhangTiancai ZhangJie Li

Wave function collapse models are considered as the modified theories of standard quantum mechanics at the macroscopic level. By introducing nonlinear stochastic terms in the Schr\”odinger equation, these models make predictions, differently from those of standard quantum mechanics, that it is fundamentally impossible to prepare macroscopic systems in macroscopic superpositions. The validity of these models can only be examined by experiments and hence efficient protocols for this kind of experiments are highly needed. Here we provide a protocol that is able to probe the postulated collapse effect by means of the entanglement of the center-of-mass motion of two nanospheres optically trapped in a Fabry-P\’erot cavity. We show that the collapse noise results in large reduction of the steady-state entanglement and the entanglement, with and without the collapse effect, shows distinguishable scalings with certain system parameters, which can be used to unambiguously determine the effect of these models.

Weak Value, Quasiprobability and Bohmian Mechanics

Latest Results for Foundations of Physics

on 2016-12-01 12:00am GMT

Abstract

We clarify the significance of quasiprobability (QP) in quantum mechanics that is relevant in describing physical quantities associated with a transition process. Our basic quantity is Aharonov’s weak value, from which the QP can be defined up to a certain ambiguity parameterized by a complex number. Unlike the conventional probability, the QP allows us to treat two noncommuting observables consistently, and this is utilized to embed the QP in Bohmian mechanics such that its equivalence to quantum mechanics becomes more transparent. We also show that, with the help of the QP, Bohmian mechanics can be recognized as an ontological model with a certain type of contextuality.

Quantum physics: Record set for linked photons

Nature – Issue – nature.com science feeds

on 2016-11-30 12:00am GMT

Quantum physics: Record set for linked photons

Nature 540, 7631 (2016). doi:10.1038/540011b

Particles that have linked quantum states, known as ‘entangled’ particles, can affect each other’s states even if they are physically separated. Now scientists have set a record by entangling ten photons — two more than achieved previously.Entangled particles should one day enable quantum computing

Collapse or no collapse? What is the best ontology of quantum mechanics in the primitive ontology framework?

Philsci-Archive: No conditions. Results ordered -Date Deposited.

on 2016-11-29 9:05pm GMT

Esfeld, Michael (2016) Collapse or no collapse? What is the best ontology of quantum mechanics in the primitive ontology framework? [Preprint]

Spontaneous collapse: A solution to the measurement problem and a source of the decay in mesonic systems

PRA: Fundamental concepts

on 2016-11-28 3:00pm GMT

Author(s): Kyrylo Simonov and Beatrix C. Hiesmayr

Dynamical reduction models propose a solution to the measurement problem in quantum mechanics: the collapse of the wave function becomes a physical process. We compute the predictions to decaying and flavor-oscillating neutral mesons for the two most promising collapse models, the QMUPL (quantum mec…

[Phys. Rev. A 94, 052128] Published Mon Nov 28, 2016