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Matthew Leifer
Works at Perimeter Institute for Theoretical Physics
Attended University of Bristol
Lives in Brampton, Ontario, Canada
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Matthew Leifer

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I recently received an email with lots of bold and italics from one of the editors of Nature Physics about their Insight on the Foundations of Quantum Mechanics, which is available for free during April.  I assume the excessive use of bold and italics was supposed to make me more likely to share this on the interwebs, so here it is.  Despite my aversion to marketing emails, it looks quite good.
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Matthew Leifer

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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
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Q+ Hangout: Tobias Fritz (Perimeter Institute)
Tue, March 25, 10:00 AM
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Matthew Leifer

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Engaged!
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Congratulations, Matt!!!
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New paper dance: http://arxiv.org/abs/1401.7996
Title: ψ-epistemic models are exponentially bad at explaining the distinguishability of quantum states
Abstract:
The status of the quantum state is perhaps the most controversial issue in the foundations of quantum theory. Is it an epistemic state (state of knowledge) or an ontic state (state of reality)? In realist models of quantum theory, the epistemic view asserts that nonorthogonal quantum states correspond to overlapping probability measures over the true ontic states. This naturally accounts for a large number of otherwise puzzling quantum phenomena. For example, the indistinguishability of nonorthogonal states is explained by the fact that the ontic state sometimes lies in the overlap region, in which case there is nothing in reality that could distinguish the two states. For this to work, the amount of overlap of the probability measures should be comparable to the indistinguishability of the quantum states. In this letter, I exhibit a family of states for which the ratio of these two quantities must be ≤2de−cd in Hilbert spaces of dimension d that are divisible by 4. This implies that, for large Hilbert space dimension, the epistemic explanation of indistinguishability becomes implausible at an exponential rate as the Hilbert space dimension increases.
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And you got cited at the QIP conference already. (You can probably guess which talk!) :-)
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I don't think I've ever seen freezing rain like this. A lot of trees have lost branches in the neighborhood.
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Matthew Leifer

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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, 9:00 AM
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Matthew Leifer

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Can you bear to go to the APS March meeting?
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Peaking in on the superconducting sessions? 
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Matthew Leifer

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Please leave any comments on the original post.
 
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, 11:00 AM EST
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We'd like to invite you to the next Q+ hangout with Troels Frimodt Rønnow from ETH Zurich.

TITLE: Quantum annealing on 503 qubits

ABSTRACT: Quantum speedup refers to the advantage of quantum devices can  over classical ones in solving  classes of computational problems. In this talk we show how to correctly define and measure quantum speedup in experimental devices. We show how to avoid issues that might mask or fake quantum speedup. As illustration we will compare the performance of a D-Wave Two quantum annealing device on random spin glass instances to simulated classical and quantum annealers, and other classical solvers.

If you are sharing this talk with a group of people, you can reserve a seat in the hangout in the thread below. Otherwise, simply come back to this page at the above date, and you will see a livestream here.
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Some none D-Wave affiliated counter spin to Scott's blog :-)
http://wp.me/p2lHU6-LQ
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We'd like to invite you to the next Q+ hangout with Mark Wilde! Please note this event takes place one hour later than our usual seminar time. 

If you are sharing this talk with a group of people, you can reserve a seat in the hangout in the thread below. Otherwise, simply come back to this page at the above date, and you will see a livestream here.

Speaker: Mark Wilde
Louisiana State University 

Title: Strong Converse Theorems in Quantum Information Theory

Abstract: One of the main goals in quantum information theory is to establish the capacity of a quantum channel for communicating various kinds of information, whether it be bits or qubits. While several communication capacities of quantum channels are now known, the characterization of capacity in many of these cases is often limited to it being a threshold that determines the rates at which reliable communication is or is not possible. While this characterization might be satisfactory
for some purposes, it leaves open the possibility for a trade-off between communication rate and error probability (that is, one might think that it would be possible to send data at a higher rate by allowing for errors to occur some of the time). However, we now know that such a trade-off is not possible for many channels and capacities of interest. That is, many researchers have now established that a strong converse theorem holds for several channels and capacities, so that as soon as the communication rate exceeds capacity, it is guaranteed that the error probability converges to one in the limit of large blocklength, no matter what strategy the sender and receiver employ. These strong converse theorems strengthen the interpretation and our understanding of capacity as a very sharp dividing line between rates for which asymptotically perfect communication is possible and rates for which an error is guaranteed to occur (analogous to a phase transition in statistical physics). This Q+ talk will review much of the progress in establishing strong converse theorems for several channels and their communication capacities in quantum information theory. 

Joint work with Bhaskar Roy Bardhan (LSU Baton Rouge), Manish K. Gupta (LSU Baton Rouge), Naresh Sharma (TIFR Mumbai), Dong Yang (UAB Barcelona), and Andreas Winter (UAB Barcelona)
Q+ hangout: Mark Wilde
Tue, November 26, 2013, 10:00 AM

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People
In his circles
275 people
Have him in circles
2,768 people
Dan Hagon's profile photo
Education
  • University of Bristol
    Ph.D. Applied Mathematics, 2000 - 2003
  • University of Cambridge
    MaST Applied Mathematics and Theoretical Physics, 1999 - 2000
  • University of Manchester
    B.Sc. Physics with Theoretical Physics, 1996 - 1999
  • Ashmole School
    Secondary School, 1989 - 1996
  • Monkfrith School
    Primary School, 1982 - 1989
Basic Information
Gender
Male
Other names
Matt Leifer
Work
Occupation
Researcher in Theoretical Physics
Employment
  • Perimeter Institute for Theoretical Physics
    Long term visitor, 2013 - present
  • University College London
    Research Associate, 2010 - 2011
  • University of Waterloo
    Postdoctoral Fellow, 2007 - 2008
  • Perimeter Institute for Theoretical Physics
    Associate Postdoctroal Fellow, 2007 - 2008
  • University of Cambridge
    Research Assistant, 2006 - 2006
  • University of Bristol
    Research Assistant, 2003 - 2003
  • Perimeter Institute for Theoretical Physics
    Postdoctoral Fellow, 2004 - 2006
Places
Map of the places this user has livedMap of the places this user has livedMap of the places this user has lived
Currently
Brampton, Ontario, Canada
Previously
Waterloo, Ontario, Canada - London, UK - Manchester, UK - Cambridge, UK - Bristol, UK
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