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D-Wave sticks with its approach to quantum computing




Earlier this month, at the WebSummit conference in Lisbon, D-Wave and Volkswagen teamed up to manage a fleet of buses using a new system that, among other things, used D-Wave’s quantum technology to help generate the most efficient routes. While D-Wave’s 2000Q only played a small part in this process, it’s nevertheless a sign that quantum computing is slowly getting ready for production use and that D-Wave’s approach, somewhat controversial in its early days, is paying off.

Unlike other players in the quantum computing market, D-Wave always bet on quantum annealing as its core technology. This technology lends itself perfectly to optimization problems like the kind of routing problem the company tackled with VW, as well as sampling problems, which, in the context of quantum computing, are useful for improving machine learning models, for example. Depending on their complexity, some of these problems are nearly impossible to solve with classical computers (at least in a reasonable time).

Grossly simplified, with quantum annealing, you are building a system that almost naturally optimizes itself for the lowest energy state, which then represents the solution to your problem.

Microsoft, IBM, Rigetti and others are mostly focused on building gate-model quantum computers and they are starting to see results (with the exception of Microsoft, which doesn’t have a working computer just yet and is hence betting on partnerships for the time being). But this is also a far more complex problem. And while you can’t really compare these technologies qubit to qubit, it’s telling that D-Wave’s latest machines, the Advantage, will feature 5,000 qubits — while the state of the art among the gate-model proponents is just over 50. Scaling these machines up is hard, though, especially given that the industry is still trying to figure out how to manage the noise issues.

D-Wave remains the only major player that’s betting on annealing, but the company’s CEO Vern Brownell remains optimistic that this is the right approach. “We feel more strongly about our decision to do quantum annealing now that there are a few companies that actually have quantum computers that people can access,” he said in an interview earlier this month.

“We have customers, Volkswagen included, that have run problems against those other computers and seeing what they can actually do and it’s vastly different. Our capability is many orders of magnitude faster for most problems than what you can do with other quantum computers. And that is because of the choice of quantum annealing. And that is because quantum healing is more robust to errors.” Error correction, he argues, remains the fundamental problem, and will hamper the performance of these systems for the foreseeable future. “And in order to move into the enterprise or any kind of practical application, that error correction needs to be wrestled with,” he noted.


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On maximum-likelihood decoding with circuit-level errors




Leonid P. Pryadko

Department of Physics & Astronomy, University of California, Riverside, California 92521, USA

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Error probability distribution associated with a given Clifford measurement circuit is described exactly in terms of the circuit error-equivalence group, or the circuit subsystem code previously introduced by Bacon, Flammia, Harrow, and Shi. This gives a prescription for maximum-likelihood decoding with a given measurement circuit. Marginal distributions for subsets of circuit errors are also analyzed; these generate a family of related asymmetric LDPC codes of varying degeneracy. More generally, such a family is associated with any quantum code. Implications for decoding highly-degenerate quantum codes are discussed.

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Cited by

[1] Nicolas Delfosse, Ben W. Reichardt, and Krysta M. Svore, “Beyond single-shot fault-tolerant quantum error correction”, arXiv:2002.05180.

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On Crossref’s cited-by service no data on citing works was found (last attempt 2020-08-07 05:01:00).


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A robust W-state encoding for linear quantum optics




Madhav Krishnan Vijayan1, Austin P. Lund2, and Peter P. Rohde1

1Centre for Quantum Software & Information (UTS:QSI), University of Technology Sydney, Sydney NSW, Australia
2Centre for Quantum Computation & Communications Technology, School of Mathematics & Physics, The University of Queensland, St Lucia QLD, Australia

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Error-detection and correction are necessary prerequisites for any scalable quantum computing architecture. Given the inevitability of unwanted physical noise in quantum systems and the propensity for errors to spread as computations proceed, computational outcomes can become substantially corrupted. This observation applies regardless of the choice of physical implementation. In the context of photonic quantum information processing, there has recently been much interest in $textit{passive}$ linear optics quantum computing, which includes boson-sampling, as this model eliminates the highly-challenging requirements for feed-forward via fast, active control. That is, these systems are $textit{passive}$ by definition. In usual scenarios, error detection and correction techniques are inherently $textit{active}$, making them incompatible with this model, arousing suspicion that physical error processes may be an insurmountable obstacle. Here we explore a photonic error-detection technique, based on W-state encoding of photonic qubits, which is entirely passive, based on post-selection, and compatible with these near-term photonic architectures of interest. We show that this W-state redundant encoding techniques enables the suppression of dephasing noise on photonic qubits via simple fan-out style operations, implemented by optical Fourier transform networks, which can be readily realised today. The protocol effectively maps dephasing noise into heralding failures, with zero failure probability in the ideal no-noise limit. We present our scheme in the context of a single photonic qubit passing through a noisy communication or quantum memory channel, which has not been generalised to the more general context of full quantum computation.

► BibTeX data

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Sum-of-squares decompositions for a family of noncontextuality inequalities and self-testing of quantum devices




Debashis Saha, Rafael Santos, and Remigiusz Augusiak

Center for Theoretical Physics, Polish Academy of Sciences, Aleja Lotników 32/46, 02-668 Warsaw, Poland

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Violation of a noncontextuality inequality or the phenomenon referred to `quantum contextuality’ is a fundamental feature of quantum theory. In this article, we derive a novel family of noncontextuality inequalities along with their sum-of-squares decompositions in the simplest (odd-cycle) sequential-measurement scenario capable to demonstrate Kochen-Specker contextuality. The sum-of-squares decompositions allow us to obtain the maximal quantum violation of these inequalities and a set of algebraic relations necessarily satisfied by any state and measurements achieving it. With their help, we prove that our inequalities can be used for self-testing of three-dimensional quantum state and measurements. Remarkably, the presented self-testing results rely on weaker assumptions than the ones considered in Kochen-Specker contextuality.

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