Category Archives: Quantum measurement

Workshop ‘Scientific Realism about the Quantum State’: Leeds, 9 September 2015

University of Leeds
Centre for the History and Philosophy of Science

This workshop examines recent debates about the nature of the quantum state (e.g. epistemic vs. ontic conceptions of the quantum state) in the broader context of scientific (anti-)realism.

Speakers:

Simon Friederich (Groningen)
Ruth Kastner (UMD)
Owen Maroney (Oxford)
Chris Timpson (Oxford)
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New journal: International Journal of Quantum Foundations

A new open-access journal called ‘International Journal of Quantum Foundations’ has just been launched. The Editorial Board consists of many international luminaries and includes Lajos Diosi, Bernard d’Espagnat, Arthur Fine, Gordon N. Fleming, Olival Freire Jr., Sheldon Goldstein, Robert Griffiths, Hans Halvorson, Richard Healey, Basil Hiley, Don Howard, Roger Penrose, Carlo Rovelli, Maximilian Schlosshauer and H. Dieter Zeh. The aim of the journal is to provide a convenient online platform for researchers to debate and resolve the long-standing, controversial issues in the foundations of quantum theory. You may visit the journal website at http://www.ijqf.org/.

Good luck on the launch!

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Quantum measurement: reliable teleportation

This week, two papers were published in Nature that report experiments of so-called ‘deterministic’ quantum teleportation, i.e., teleportation using entangled states and classical communication which works with a much higher level of reliability than previous experiments. In other words, these new experiments are significantly more efficient in producing events of actual quantum teleportation. The first of these experiments, by Takeda et al., uses single-photon quantum bits in superposition states of different times of arrival. The second, by Steffen et al., works by means of a solid-state qubit. For a short write-up of the two experiments and their relevance, see the short article by Timothy Ralph.

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Quantum Measurement (4): PBR Theorem (again)

Sorry to be repetitive, but I was just preparing this entry as John Manchak uploaded the previous post… But I still go ahead as I would like to add a few details.

The arXiv preprint by second year graduate student Matthew F Pusey of the Controlled Quantum Dynamics group at Imperial College London and his supervisors Jonathan Barrett at Royal Halloway and Terry Rudolph also at Imperial offers a potentially groundbreaking result: on just rather mild assumptions, it must be the case that the quantum state is a real physical property of a quantum system. The paper is currently under review (for Nature, it seems), but Eugenie Samuel Reich has already collected a few quotes from leading quantum foundationalists on the online portal of Nature. And, as John also mentioned below, Matt Leifer offers a very accessible account of the result on his blog.

Let me state the three assumptions that go into the theorem. Continue reading

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Quantum Measurement (3): Superconductor-based tests of quantum mechanics

I must admit, I have never been tempted to delve deeply into the theory of superconductors. So when I came across a review article by J Q You and Franco Nori entitled “Atomic physics and quantum optics using superconducting circuits”, published in Nature on 30 June 2011, I was about to skip it. But then I discovered the box on page 595 which discusses tests of quantum mechanics based on macroscopic superconducting circuits and realized that this box at least deserves mention in this ongoing series on recent advances in measurements of quantum systems with implications for the foundations of quantum mechanics.

The box discusses three types of tests of quantum mechanics: Bell inequality, Leggett-Garg inequality, and Kochen-Specker theorem. While I disagree with some of the foundational gloss the authors give on these results (for instance, we get the usual spiel about hidden variables in introducing Bell’s theorem), the experiments they list testify to what a resource superconductors can be in foundations. There are no surprises, as all experiments listed confirm quantum mechanics.
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Quantum Measurement (2): Quantum correlations in indivisible systems

In a classical world, properties of physical systems do not depend on their own measurement, and also not on any other measurements made on the system. In this case, there are joint probability distributions for the system’s properties. In the language of quantum mechanics, such worlds can be described by ‘non-contextual’ hidden-variables models. That in a quantum-mechanical world such joint probability distributions sometimes cannot obtain was established by a series of deep theorems, essentially from Gleason (1957) to Kochen-Specker (1967). It has been previously shown, both in theory and in experiment, that there are simple cases of physical systems, such as pairs of qubits, which behave in accordance to quantum mechanics and thus in conflict with the classical assumption of joint probability distributions.
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Quantum Measurement (1): Directly measuring the wavefunction

There has recently been a number of quite astonishing experiments measuring quantum systems, some of which seem of relevance, or at least of interest, to philosophers of physics. So I have resolved to present some of these results in a loose series entitled ‘Quantum Measurement’. I plan not so much to offer the definitive philosophical appraisal of these experiments than simply to report the results and link to the relevant publications.

I start the new series with an article which appeared in Nature in June (Vol 474, 188-191; cf. also the lead article in the same Vol, 170f) and reports the results of a Canadian group presenting a method for directly measuring the quantum wavefunction!
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