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Video of the Q+ Hangout with Lorenzo Maccone
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Dear All,

(We have ONE more slot left: if you want to join, please comment below; you can always watch the livestream & offline recording otherwise)

After a long gap, we're happy to announce the first Q+ hangout of the year! As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment. We also encourage individuals interested in active participation---which typically involves asking questions after the talk---to join the hangout. Otherwise you can watch on the livestream.

Lorenzo Maccone, Universita' di Pavia,

Title: Quantum Time

Abstract:
We give a consistent quantum description of time, based on Page and Wootters' conditional probabilities mechanism, that overcomes the criticisms that were raised against similar previous proposals. In particular we show how the model allows to reproduce the correct statistics of sequential measurements performed on a system at different
times. This is joint work with Vittorio Giovannetti and Seth Lloyd and
is based on arXiv:1504.04215.
Q+ Hangout: Lorenzo Maccone
Tue, June 9, 9:00 AM

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thanku for sharing the video  :)
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Dear All, we're happy to announce the next Q+ hangout as below! This is at an unusual time but should be well suited for people in America. As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment. We also encourage individuals interested in active participation---which typically involves asking questions after the talk---to join the hangout. Otherwise you can watch on the livestream.

Howard Wiseman, Griffith University

Title: After 50 years, Bell's Theorem Still Reverberates

Abstract:
Fifty years ago this month, Belfast-born physicist John Bell submitted for publication a paper [1] which has been described as “the most profound discovery in science” [2]. However, its significance is still much disputed by physicists and philosophers [3, 4].
I will explain what is so puzzling about the types of correlations Bell introduced, by a specific example based on [5]. (For those well-versed in Bell inequalities this may still be of pedagogical interest.)
But what exactly do these Bell-type correlations violate? Bell’s original answer [1] was the joint assumptions of determinism and locality. His later answer [6] was the single assumption of local causality (which, confusingly, he sometimes also called locality). Different ‘camps’ of physicists – operationalists and realists respectively – prefer the different versions of Bell’s theorem.

Which of Bell’s notions, locality or local causality, expresses the causal structure of Einstein’s theory of relativity? I will argue for the answer: neither [3,4]. Both notions require an additional causal assumption, and the one required for local causality is a stronger one. I will discuss how the different assumptions fit with the ideologies of the two camps, and how they can best be reconciled.

[1]  J. S. Bell, “On the Einstein-Podolsky-Rosen paradox”, Physics 1, 195-200 (1964).
[2]  H. P. Stapp, “Are superluminal connections necessary?”, Nuovo Cim. 40B, 191 (1977).
[3]  H. M. Wiseman, “The two Bell’s theorems of John Bell”, J. Phys. A 47, 424001 (2014) (Invited Review for Special Issue, 50 years of Bell’s theorem)
[4]  H. M. Wiseman, “Bell’s theorem still reverberates”, Nature 510, 467-9 (2014).
[5] P. K. Aravind, “Bell’s theorem without inequalities and only two distant observers”,  Found. Phys. Lett. 15, 397 (2002).
[6]  J. S. Bell, “The Theory of Local Beables”, Epistemological Lett. 9, 11-24 (1976).
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Q+ Hangout: Howard Wiseman
Wed, November 26, 2014, 5:00 PM
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In a paper, Results in Physics 4, 2014 71-82, it was demonstrated that CHSH is a flawed argument. Despite the objections uttered in a PubPeer discussion it is clear that statistically one can no longer employ CHSH as a not contaminated way to test whether or not LHV models may explain the quantum correlation. 
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We'd like to announce the next Q+ hangout.  As usual, please leave a comment below if you would like to join the hangout itself.  Everyone else can watch on the livestream.

Title: Fault-tolerant quantum computation with constant overhead

Abstract: The threshold theorem for fault tolerance tells us that it is possible to build arbitrarily large reliable quantum computers provided the error rate per physical gate or time step is below some threshold value. Most research on the threshold theorem so far has gone into optimizing the tolerable error rate under various assumptions, with other considerations being secondary. However, for the foreseeable future, the number of qubits may be an even greater restriction than error rates. The overhead, the ratio of physical qubits to logical qubits, determines how expensive (in qubits) a fault-tolerant computation is. Earlier results on fault tolerance used a large overhead which grows even larger (albeit slowly) with the size of the computation. I show that it is possible in principle to do fault-tolerant quantum computation with the overhead constant in the size of the computation, and with a low constant at that. The result depends on recent progress on quantum low-density parity check codes.
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Q+ Hangout: Daniel Gottesman (Perimeter Institute)
Tue, May 20, 2014, 9:00 AM
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Hi, Q+. Will there be more talks in the future? Thanks for all of your efforts before. 

Regards,
Qi
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Since we don't have many group reservations, we are opening up the possibility of booking a seat in the hangout to individuals, particularly those who work on related areas and might have good questions for Tobias.  If you are an individual who would like to be in the hangout rather than just watching the livestream then please comment below.
 
We'd like to announce the next Q+ hangout.  As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment on the event page.  Everyone else can watch on the livestream.

Title: A Combinatorial Approach to Nonlocality and Contextuality

Abstract:
Most work on contextuality so far has focused on specific examples and concrete proofs of the Kochen-Specker theorem, while general definitions and theorems about contextuality are sparse. For example, it is commonly believed that nonlocality is a special case of contextuality, but what exactly does this mean? After a brief discussion of previous work, I will introduce our "device-independent" approach to contextuality based on the mathematics of test spaces and explain how nonlocality is indeed a special case of contextuality. This work builds on the graph-theoretic approach of Cabello, Severini and Winter by improving on several of its shortcomings and merging it with the work of Foulis and Randall on test spaces. Our results include:

(1) various relationships to graph invariants, similar to CSW;
(2) a proof that our set of quantum models cannot be characterized by a graph invariant;
(3) a proof that the set of all models satisfying the Consistent Exclusivity principle at any number of copies is not convex;
(4) new results on the Shannon capacity of graphs;
(5) an "inverse sandwich conjecture" with ramifications for C*-algebra theory and quantum logic.

This talk is based on http://arXiv.org/abs/1212.4084
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Q+ Hangout: Tobias Fritz (Perimeter Institute)
Tue, March 25, 2014, 10:00 AM
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OK, +Elie Wolfe.  You are in.
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Title: Dimension of Physical Systems

Abstract:
The dimension of a physical system refers loosely speaking to the number of degrees of freedom relevant to describe it. Here we ask how quantum theory compares to more general models (such as Generalized Probabilistic Theories) from the point of view of dimension. This gives insight to information processing and thermodynamics in GPTs.
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Q+ Hangout: Nicolas Brunner
Tue, February 25, 2014, 1:30 PM
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A reminder about the Q+ Hangout by Lorenzo Maccone on 'Quantum Time' tomorrow!
 
Dear All,

(We have ONE more slot left: if you want to join, please comment below; you can always watch the livestream & offline recording otherwise)

After a long gap, we're happy to announce the first Q+ hangout of the year! As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment. We also encourage individuals interested in active participation---which typically involves asking questions after the talk---to join the hangout. Otherwise you can watch on the livestream.

Lorenzo Maccone, Universita' di Pavia,

Title: Quantum Time

Abstract:
We give a consistent quantum description of time, based on Page and Wootters' conditional probabilities mechanism, that overcomes the criticisms that were raised against similar previous proposals. In particular we show how the model allows to reproduce the correct statistics of sequential measurements performed on a system at different
times. This is joint work with Vittorio Giovannetti and Seth Lloyd and
is based on arXiv:1504.04215.
Q+ Hangout: Lorenzo Maccone
Tue, June 9, 9:00 AM

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Look from a different persp
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A reminder about the Q+ hangout by Howard Wiseman tomorrow. It's Bell's theorem: Come one, come all.
 
Dear All, we're happy to announce the next Q+ hangout as below! This is at an unusual time but should be well suited for people in America. As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment. We also encourage individuals interested in active participation---which typically involves asking questions after the talk---to join the hangout. Otherwise you can watch on the livestream.

Howard Wiseman, Griffith University

Title: After 50 years, Bell's Theorem Still Reverberates

Abstract:
Fifty years ago this month, Belfast-born physicist John Bell submitted for publication a paper [1] which has been described as “the most profound discovery in science” [2]. However, its significance is still much disputed by physicists and philosophers [3, 4].
I will explain what is so puzzling about the types of correlations Bell introduced, by a specific example based on [5]. (For those well-versed in Bell inequalities this may still be of pedagogical interest.)
But what exactly do these Bell-type correlations violate? Bell’s original answer [1] was the joint assumptions of determinism and locality. His later answer [6] was the single assumption of local causality (which, confusingly, he sometimes also called locality). Different ‘camps’ of physicists – operationalists and realists respectively – prefer the different versions of Bell’s theorem.

Which of Bell’s notions, locality or local causality, expresses the causal structure of Einstein’s theory of relativity? I will argue for the answer: neither [3,4]. Both notions require an additional causal assumption, and the one required for local causality is a stronger one. I will discuss how the different assumptions fit with the ideologies of the two camps, and how they can best be reconciled.

[1]  J. S. Bell, “On the Einstein-Podolsky-Rosen paradox”, Physics 1, 195-200 (1964).
[2]  H. P. Stapp, “Are superluminal connections necessary?”, Nuovo Cim. 40B, 191 (1977).
[3]  H. M. Wiseman, “The two Bell’s theorems of John Bell”, J. Phys. A 47, 424001 (2014) (Invited Review for Special Issue, 50 years of Bell’s theorem)
[4]  H. M. Wiseman, “Bell’s theorem still reverberates”, Nature 510, 467-9 (2014).
[5] P. K. Aravind, “Bell’s theorem without inequalities and only two distant observers”,  Found. Phys. Lett. 15, 397 (2002).
[6]  J. S. Bell, “The Theory of Local Beables”, Epistemological Lett. 9, 11-24 (1976).
This Hangout On Air is hosted by Q+. The live video broadcast will begin soon.
Q&A
Preview
Live
Q+ Hangout: Howard Wiseman
Wed, November 26, 2014, 5:00 PM
Hangouts On Air - Broadcast for free

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Q+

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After a long break we are ready to announce the next Q+ hangout. As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment. We also encourage individuals interested in active participation---which typically involves asking questions after the talk---to join the hangout. Otherwise you can watch on the livestream.


SPEAKER: Marco Piani, University of Strathclyde


TITLE: Usefulness of entanglement and steering in the discrimination of physical processes

ABSTRACT: Not all entangled states are created equal: they are all special, but some are more special than others. In particular, this is true in an operational characterization of quantum correlations based on their usefulness in the discrimination of physical processes. We will discuss how every entangled state of a probe-ancilla composite system is useful as a resource for the problem of minimum-error channel discrimination. We will then focus on the subset of entangled states that exhibit steering. The latter is the entanglement-based quantum effect that embodies the “spooky action at a distance” disliked by Einstein and scrutinized by Einstein, Podolsky, and Rosen. We prove that, for any fixed steerable state, there are instances of a generalization of the channel discrimination problem, which we dub quantum subchannel discrimination, where such a state allows a correct discrimination with strictly higher probability than in absence of entanglement, even when measurements are restricted to local measurements aided by one-way communication. On the other hand, unsteerable states are useless under such a restriction, even when entangled. We also prove that the above steering advantage can be exactly quantified in terms of the steering robustness, which is a natural measure of the steerability exhibited by the state.


Joint work with J. Watrous, arXiv:1406.0530
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Q+ Hangout: Marco Piani
Mon, October 27, 2014, 10:00 AM
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Requesting a seat. Thanks.
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We'd like to announce the next Q+ hangout.  As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment.  Otherwise you can watch on the livestream.

Title: On the Uncertainty of the Ordering of Nonlocal Wavefunction Collapse when Relativity is Considered

Abstract: The temporal measurement order and therefore the originator of the instantaneous collapse of the wavefunction of a spatiality entangled particle pair can change depending on the reference frame of an observer. This can lead to a paradox in which its seems that both measurements collapsed the wavefunction before the other. We resolve this paradox by demonstrating how attempting to determine the order of measurement of the entangled pair introduces uncertainty which makes the measurement order impossible to know.
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Q+ Hangout: Chris Richardson (University of Liege)
Tue, April 22, 2014, 9:00 AM
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+Ravi Kunjwal OK done
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We'd like to announce the next Q+ hangout.  As usual, if you are watching with a group and want to reserve a seat in the hangout then leave a comment on the event page.  Everyone else can watch on the livestream.

Title: A Combinatorial Approach to Nonlocality and Contextuality

Abstract:
Most work on contextuality so far has focused on specific examples and concrete proofs of the Kochen-Specker theorem, while general definitions and theorems about contextuality are sparse. For example, it is commonly believed that nonlocality is a special case of contextuality, but what exactly does this mean? After a brief discussion of previous work, I will introduce our "device-independent" approach to contextuality based on the mathematics of test spaces and explain how nonlocality is indeed a special case of contextuality. This work builds on the graph-theoretic approach of Cabello, Severini and Winter by improving on several of its shortcomings and merging it with the work of Foulis and Randall on test spaces. Our results include:

(1) various relationships to graph invariants, similar to CSW;
(2) a proof that our set of quantum models cannot be characterized by a graph invariant;
(3) a proof that the set of all models satisfying the Consistent Exclusivity principle at any number of copies is not convex;
(4) new results on the Shannon capacity of graphs;
(5) an "inverse sandwich conjecture" with ramifications for C*-algebra theory and quantum logic.

This talk is based on http://arXiv.org/abs/1212.4084
This Hangout On Air is hosted by Q+. The live video broadcast will begin soon.
Q&A
Preview
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Q+ Hangout: Tobias Fritz (Perimeter Institute)
Tue, March 25, 2014, 10:00 AM
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If there are any spots left, I'd like one for the theory group in Geneva
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***FOR THE LIVE VIDEO STREAM GO HERE: https://plus.google.com/events/c8qnklo1cuv48l5cnjmajnv2tak

We'd like to announce the next Q+ hangout.  As usual, if you are watching with a group and would like to reserve a seat in the hangout then leave a comment.  Everyone else can watch on the livestream on the event page.

Title: Dimension of Physical Systems

Abstract:
The dimension of a physical system refers loosely speaking to the number of degrees of freedom relevant to describe it. Here we ask how quantum theory compares to more general models (such as Generalized Probabilistic Theories) from the point of view of dimension. This gives insight to information processing and thermodynamics in GPTs.
Q+ Hangout: Nicholas Brunner (University of Geneva)
Tue, February 25, 2014, 11:00 AM EST
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Thanks for the great talk. 
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Tagline
Q+ hangouts: international online seminars and discussions on quantum theory
Introduction
Q+ is a series of online seminars on quantum information and quantum foundations that use Google+ hangouts.  Seminars currently happen once a month and are organized by Daniel Burgarth and Matthew Leifer.

Follow this page for seminar announcements and to see the hangout.
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