Category Archives: Q+

Q+ Hangout: Daniel Gottesman

Here are the details of the next Q+ hangout. To watch live, visit this link at the appointed hour.

Date/time: Tue. May 20th 2014 2pm BST/UTC+1

Speaker: Daniel Gottesman (Perimeter Institute)

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: Chris Richardson

Here are the details of the next Q+ hangout.

Date/time: 22nd April 2014 2pm BST/UTC+1

Speaker: Chris Richardson (University of Liege)

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.

To watch the talk live, go to the event page at the appointed hour.

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Q+ Hangout: Tobias Fritz

Here are the details of the next Q+ hangout. To watch live, visit the event page at the appointed hour. Be aware of the fact that although North America has switched to DST, the UK has not yet and the time is still listed in GMT/UTC.

Date/time: Tuesday 25th March 2014 2pm GMT/UTC

Speaker: Tobias Fritz (Perimeter Institute)

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 arXiv:1212.4084.

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Q+ Hangout: Nicholas Brunner

Here are the details of the next Q+ hangout.

Date/time: Tue. 25th Feb. at 4pm GMT/UTC

Speaker: Nicholas Brunner (University of Geneva)

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.

To watch the talk live, visit the event page at the appointed hour.

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Q+ Hangout: Troels Frimodt Rønnow

Here are the details of the next Q+ hangout

Date: 28th January 2014

Time: 2pm UTC/GMT

Speaker: Troels Frimodt Rønnow (ETH Zurich)

Title: Quantum annealing on 503 qubits

Abastract: 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.

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

To keep up to date on the latest news about Q+ hangouts you can follow us on:

or visit our website at http://qplus.burgarth.de

Q+ Hangout: Mark Wilde

Here are the details of the next Q+ hangout.

Date/time: Tue. 26th Nov. 3pm GMT/UTC

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).

To watch the talk live, go to http://gplus.to/qplus at the appointed hour. To stay up to date on the latest news about Q+ hangouts you can follow us on:

or visit our website http://qplus.burgarth.de

Q+ Hangout: Renato Renner

Here are the details of the next Q+ hangout.

Date/Time: 29th October 2013 2pm GMT

Speaker: Renato Renner (ETH Zurich)

Title: Does freedom of choice imply that the wave function is real?

Abstract:

The question whether the quantum-mechanical wave function is “real” has recently attracted considerable interest. More precisely, the question is whether the wave function of a system is uniquely determined by any complete description of its “physical state”. In this talk I will present a simple and self-contained proof that this is indeed the case, under an assumption that one may term “freedom of choice”. It demands that arbitrary measurements can be applied to the system, and that these can be chosen independently of all parameters available at the time of measurement (with respect to any relativistic frame). A possible interpretation of this result is that the wave function of a system is as “objective or “real as any other complete description of the system’s state.
(This is based on unpublished work in collaboration with Roger Colbeck.)

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

Note that the change from daylight savings time to standard time will have happened in the UK, but not some other countries like the US and Canada. Therefore, your usual timezone calculation may be out by an hour, e.g. the talk is at 10am in East Coast US and Canada. Please check the time conversion for your location.

To keep up to date with the latest news on Q+ hangouts, you can follow us on:

or visit our website http://qplus.burgarth.de