Can Quantum Correlations be Explained Causally?
Robert Spekkens Perimeter Institute for Theoretical Physics
PIRSA:14070026
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Inferring causal structure: a quantum advantage
Katja Ried Universität Innsbruck
Measuring the overhead of a quantum error correcting code
Austin Fowler Institute for Quantum Computing (IQC)
PIRSA:14070014Is scalable quantum error correction realistic? Some projects, thoughts and open questions.
Barbara Terhal Delft University of Technology
PIRSA:14070010Exploring N=1 theories of class S through Higgsing, dualizing and twisting
Jaewon Song University of California, San Diego
TBA
John Preskill California Institute of Technology (Caltech) - Division of Physics Mathematics & Astronomy
PIRSA:14070009Improving Mass Measurements Using Many-Body Phase Space
Can Kilic The University of Texas at Austin
Quantum codes – from experimental realizations to quantum foundations
Robert Raussendorf Leibniz University Hannover
PIRSA:14070008
Can Quantum Correlations be Explained Causally?
Robert Spekkens Perimeter Institute for Theoretical Physics
PIRSA:14070026There is a strong correlation between the sun rising and the rooster crowing, but to say that the one causes the other is to say more. In particular, it says that making the rooster crow early will not precipitate an early dawn, whereas making the sun rise early (for instance, by moving the rooster eastward) can lead to some early crowing. Intervening upon the natural course of events in this manner is a good way of discovering causal relations. Sometimes, however, we can't intervene, or we'd prefer not to. For instance, in trying to determine whether smoking causes lung cancer, we'd prefer not to force any would-be nonsmokers to smoke. Fortunately, there are some clever tricks that allow us to extract information about what causes what entirely from features of the observed correlations. One of these tricks was discovered by the physicist John Bell in 1964. In a groundbreaking paper, he used it to demonstrate the seeming impossibility of providing a causal explanation of certain quantum correlations. This revealed a fundamental tension between quantum theory and Einstein's theory of relativity --the two central pillars of modern physics. It is a tension that is still with us today.Inferring causal structure: a quantum advantage
Katja Ried Universität Innsbruck
A fundamental question in trying to understand the world -- be it classical or quantum -- is why things happen. We seek a causal account of events, and merely noting correlations between them does not provide a satisfactory answer. Classical statistics provides a better alternative: the framework of causal models proved itself a powerful tool for studying causal relations in a range of disciplines. We aim to adapt this formalism to allow for quantum variables and in the process discover a new perspective on how causality is different in the quantum world. Causal inference is a central task in the context of causal models: given observed statistics over a set of variables, one aims to infer how they are causally related. Yet in the seemingly simple case of just two classical variables, this is impossible (unless one makes additional assumptions). I will show how the analogous task for quantum variables can be solved. This quantum advantage is reminiscent of the advantages that quantum mechanics offers in computing and communication, and may lead to similarly rich insights. Our scheme is corroborated by data obtained in collaboration with Kevin Resch's experimental group. Time permitting, I will also address other applications of the quantum causal models. arXiv:1406.5036Spatially coupled quantum LDPC codes
PIRSA:14070015Spatially coupled LDPC were introduced by Felström and Zigangirov in 1999. They might be viewed in the following way, take several several instances of a certain LDPC code family, arrange them in a row and then mix the edges of the codes randomly among neighboring layers. Moreover fix the bits of the first and last layers to zero. It has soon been found out that iterative decoding behaves much better for this code than for the original LDPC code. A breakthrough occurred when it was proved by Kudekar, Richardson and Urbanke that these codes attain the capacity of all binary input memoryless output-symmetric channels.
All these nice features of classical spatially coupled LDPC codes suggest to study whether they have a quantum analogue. The fact that spatially coupled LDPC codes may afford to have large degrees and still perform well under iterative decoding would be quite interesting in the quantum setting, since by the very nature of the quantum construction of stabilizer codes the rows of the parity-check matrix of the quantum code have to belong to the code which is decoded by the iterative decoder. This implies that we should have rather large row weights to avoid severe error-floor phenomena and/or oscillatory behavior of iterative decoding which degrades significantly its performance.
With Andriyanova and Maurice, I showed last year that it is possible to come up with coupled versions of quantum LDPC codes that perform excellently under iterative decoding. For instance we have constructed a spatially coupled LDPC code family of rate $\approx \frac{1}{4}$ which performs well under iterative decoding even for noise values close to the hashing bound $p \approx 0.127$.
This represents a tremendous improvement over all previous known families of quantum LDPC codes of the same rate.
I will discuss in this talk what can be expected from this approach when these spatially coupled LDPC codes are used for performing fault tolerant computation.Measuring the overhead of a quantum error correcting code
Austin Fowler Institute for Quantum Computing (IQC)
PIRSA:14070014If one's goal is large-scale quantum computation, ultimately one wishes to minimize the amount of time, number of qubits, and qubit connectivity required to outperform a classical system, all while assuming some physically reasonable gate error rate. We present two examples of such an overhead study, focusing on the surface code with and without long-range interactions.Homological product codes
Sergey Bravyi IBM (United States)
PIRSA:14070013All examples of quantum LDPC codes known to this date suffer from a poor distance scaling limited by the square-root of the code length. This is in a sharp contrast with the classical case where good LDPC codes are known that combine constant encoding rate and linear distance. In this talk I will describe the first family of good quantum "almost LDPC" codes. The new codes have a constant encoding rate, linear distance, and stabilizers acting on at most square root of n qubits, where n is the code length. For comparison, all previously known families of good quantum codes have stabilizers of linear weight. The proof combines two techniques: randomized constructions of good quantum codes and the homological product operation from algebraic topology. We conjecture that similar methods can produce good quantum codes with stabilizer weight n^a for any a>0. Finally, we apply the homological product to construct new small codes with low-weight stabilizers.
This is a joint work with Matthew Hastings
Preprint: arXiv:1311.0885Gauge color codes
Hector Bombin PsiQuantum Corp.
PIRSA:14070012I will describe a new class of topological quantum error correcting codes with surprising features. The constructions is based on color codes: it preserves their unusual transversality properties but removes important drawbacks. In 3D, the new codes allow the effectively transversal implementation of a universal set of gates by gauge fixing, while error-dectecting measurements involve only 4 or 6 qubits. Furthermore, they do not require multiple rounds of error detection to achieve fault-tolerance.Is scalable quantum error correction realistic? Some projects, thoughts and open questions.
Barbara Terhal Delft University of Technology
PIRSA:14070010Exploring N=1 theories of class S through Higgsing, dualizing and twisting
Jaewon Song University of California, San Diego
We study a class of 4d N=1 SCFTs obtained from partial compactifications of 6d N=(2, 0) theory on a Riemann surface with punctures. We identify theories corresponding to curves with general type of punctures through nilpotent Higgsing and Seiberg dualities. The `quiver tails' of N=1 class S theories turn out to differ significantly from N=2 counterpart and have interesting properties. Various dual descriptions for such a theory can be found by using colored pair-of-pants decompositions. Especially, we find N=1 analog of Argyres-Seiberg duality for the SQCD with various gauge groups. We compute anomaly coefficients and superconformal indices to verify our proposal.TBA
John Preskill California Institute of Technology (Caltech) - Division of Physics Mathematics & Astronomy
PIRSA:14070009Improving Mass Measurements Using Many-Body Phase Space
Can Kilic The University of Texas at Austin
After the 7 and 8 TeV LHC runs, we have no conclusive evidence of physics beyond the Standard Model, leading us to suspect that even if new physics is discovered during run II, the number of signal events may be limited, making it crucial to optimize measurements for the case of low statistics. I will argue that phase space correlations between subsequent on-shell decays in a cascade contain additional information compared to commonly used kinematic variables, and this can be used to significantly improve the precision and accuracy of mass measurements. The improvement is connected to the properties of the volume element of many-body phase space, and is particularly relevant to the case of low signal statistics.Quantum codes – from experimental realizations to quantum foundations
Robert Raussendorf Leibniz University Hannover
PIRSA:14070008This talk is divided into two parts. In the first part, I discuss a scheme of fault-tolerant quantum computation for a web-like physical architecture of a quantum computer. Small logical units of a few qubits (realized in ion traps, for example) are linked via a photonic interconnect which provides probabilistic heralded Bell pairs [1]. Two time scales compete in this system, namely the characteristic decoherence time T_D and the typical time T_E it takes to provide a Bell pair. We show that, perhaps unexpectedly, this system can be used for fault-tolerant quantum computation for all values of the ratio T_D/T_E.
The second part of my talk is about something entirely different, namely the role of contextuality in quantum computation by magic state distillation. Recently, Howard et al. [2] have shown that contextuality is a necessary resource for such computation on qudits of odd prime dimension. Here we provide an analogous result for 2-level systems.
However, we require them to be rebits. [joint work with Jake Bian, Philippe Guerin and Nicolas Delfosse]
[1] C. Monroe, R. Raussendorf, A. Ruthven, K. R. Brown, P. Maunz4, L.-M.
Duan, and J. Kim, , Phys Rev A 89, 22317 (2014).
[2] Mark Howard, Joel Wallman, Victor Veitch & Joseph Emerson, Nature
doi:10.1038/nature13460 (2014).Cosmological Coincidence Problem
I will try to explain how cosmological coincidence of the two values, the matter energy density and the dark energy density, at the present epoch based on a single scalar field model whith a quartic potential, non-mimimally interacting with gravity. Dark energy in this model originates from the potential energy of the scalar field, which is sourced by the appearance of non-relativistic matter at the time z~ 10^10. No fine tuning of parameter are neccessary.