Weekly Papers on Quantum Foundations (3)

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.

A strict experimental test of macroscopic realism in a superconducting flux qubit. (arXiv:1601.03728v1 [quant-ph])

quant-ph updates on arXiv.org

on 2016-1-16 8:48am GMT

Authors: George C. KneeKosuke KakuyanagiMao-Chuang YehYuichiro MatsuzakiHiraku ToidaHiroshi YamaguchiAnthony J. LeggettWilliam J. Munro

Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent fashion, while largely preserving a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which attempt to provide a physical mechanism for wavefunction collapse, and thereby aim to solve the quantum measurement problem. Here we describe and implement an experimental protocol capable of constraining theories of this class, and show how it is related to the original Leggett-Garg test, yet more noise tolerant and conceptually transparent. By conducting a set of simple ‘prepare, shuffle, measure’ tests in a superconducting flux qubit, we rule out (by over 77 standard deviations) those theories which would deny coherent superpositions of 170 nA currents over a 9 ns timescale. Further, we address the ‘clumsiness loophole’ by determining classical disturbance in a set of control experiments.

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Distribution of the Time at Which an Ideal Detector Clicks. (arXiv:1601.03715v1 [quant-ph])

quant-ph updates on arXiv.org

on 2016-1-16 8:48am GMT

Authors: Roderich Tumulka

We consider the problem of computing, for a detector waiting for a quantum particle to arrive, the probability distribution of the time at which the detector clicks, from the initial wave function of the particle in the non-relativistic regime. Although the standard rules of quantum mechanics offer no operator for the time of arrival, quantum mechanics makes an unambiguous prediction for this distribution, defined by first solving the Schrodinger equation for the big quantum system formed by the particle of interest, the detector, a clock, and a device that records the time when the detector clicks, then making a quantum measurement of the record at a very late time, and finally using the distribution of the recorded time. This leads to question whether there is also a practical, simple rule for computing this distribution, at least approximately (i.e., for an idealized detector). We argue here in favor of a rule based on a 1-particle Schrodinger equation with a certain (absorbing) boundary condition at the ideal detecting surface, first considered by Werner in 1987. We present a novel derivation of this rule and describe how it arises as a limit of a “soft” detector represented by an imaginary potential.

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Bounds on Collapse Models from Matter-Wave Interferometry. (arXiv:1601.03672v1 [quant-ph])

quant-ph updates on arXiv.org

on 2016-1-16 8:48am GMT

Authors: Marko TorošAngelo Bassi

Matter-wave interferometry is a direct test of the quantum superposition principle for massive systems, and of collapse models. This is in contrast to non-interferometric tests, which are currently able to test collapse models more efficiently. However these bounds are in general model-dependent. Here we show that the bounds placed by matter-wave interferometry do not depend significantly on the type of collapse model. We also compute the current bounds, provided by the KDTL 2013 experiment of Arndt’s group.

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[Feature] The social life of quarks

Science: Current Issue

on 2016-1-15 12:00am GMT

Particle physicists at Europe’s CERN laboratory in Switzerland say they have observed bizarre new cousins of the protons and neutrons that make up the atomic nucleus. Protons and neutrons consist of other particles called quarks, bound by the strong nuclear force. By smashing particles at high energies, physicists have blasted into fleeting existence hundreds of other quark-containing particles. Until recently, all contained either two or three quarks. But since 2014, researchers working with CERN’s Large Hadron Collider have also spotted four- and five-quark particles. Such tetraquarks and pentaquarks could require physicists to rethink their understanding of quantum chromodynamics, or they could have less revolutionary implications. Researchers hope that computer simulations and more collider studies will reveal how the oddball newcomers are put together, but some wonder whether experiments will ever provide a definitive answer. Author: Adrian Cho

 

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