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New NQIT Co-Director for User Engagement

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We are very pleased to welcome Evert Geurtsen as our new Co-Director for User Engagement, following the retirement of Tim Cook earlier this year.

Evert Geurtsen joins NQIT from his role as Head of Licensing and Ventures for the Physical Sciences at Oxford University Innovation (OUI). At OUI, where Evert worked since 2009, he and his team helped founders to start more than 50 new ventures, created the Oxford Startup Incubator and secured licence agreements for many Oxford inventions, software and other research outcomes, latterly also from the Humanities and Social Sciences Divisions.

Prior to Oxford, Evert’s career has included new product development roles and directorships at large and medium sized companies in the automotive industry including General Motors and Lotus Engineering. Evert has founded his own ventures and raised investment pioneering the introduction of affordable electric cars. Evert studied Automotive Engineering in the Netherlands and holds a MBA from Warwick University.

Evert Geurtsen now takes over our User Engagement programme, working along with NQIT’s Technology Associates Dr Iris Choi and Dr Rupesh Srivastava, to build links with industry, to ensure uptake of early spin-out technologies from NQIT work and to identify new opportunities for user-driven applications.

If you would like to speak with Evert Geurtson about developing a partnership with NQIT, please contact the User Engagement Team.

Source: https://www.nqit.ox.ac.uk/news/new-nqit-co-director-user-engagement

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Quantum

Sense, sensibility, and superconductors

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Jonathan Monroe disagreed with his PhD supervisor—with respect. They needed to measure a superconducting qubit, a tiny circuit in which current can flow forever. The qubit emits light, which carries information about the qubit’s state. Jonathan and Kater intensify the light using an amplifier. They’d fabricated many amplifiers, but none had worked. Jonathan suggested changing their strategy—with a politeness to which Emily Post couldn’t have objected. Jonathan’s supervisor, Kater Murch, suggested repeating the protocol they’d performed many times.

“That’s the definition of insanity,” Kater admitted, “but I think experiment needs to involve some of that.”

I watched the exchange via Skype, with more interest than I’d have watched the Oscars with. Someday, I hope, I’ll be able to weigh in on such a debate, despite working as a theorist. Someday, I’ll have partnered with enough experimentalists to develop insight.

I’m partnering with Jonathan and Kater on an experiment that coauthors and I proposed in a paper blogged about here. The experiment centers on an uncertainty relation, an inequality of the sort immortalized by Werner Heisenberg in 1927. Uncertainty relations imply that, if you measure a quantum particle’s position, the particle’s momentum ceases to have a well-defined value. If you measure the momentum, the particle ceases to have a well-defined position. Our uncertainty relation involves weak measurements. Weakly measuring a particle’s position doesn’t disturb the momentum much and vice versa. We can interpret the uncertainty in information-processing terms, because we cast the inequality in terms of entropies. Entropies, described here, are functions that quantify how efficiently we can process information, such as by compressing data. Jonathan and Kater are checking our inequality, and exploring its implications, with a superconducting qubit.

With chip

I had too little experience to side with Jonathan or with Kater. So I watched, and I contemplated how their opinions would sound if expressed about theory. Do I try one strategy again and again, hoping to change my results without changing my approach? 

At the Perimeter Institute for Theoretical Physics, Masters students had to swallow half-a-year of course material in weeks. I questioned whether I’d ever understand some of the material. But some of that material resurfaced during my PhD. Again, I attended lectures about Einstein’s theory of general relativity. Again, I worked problems about observers in free-fall. Again, I calculated covariant derivatives. The material sank in. I decided never to question, again, whether I could understand a concept. I might not understand a concept today, or tomorrow, or next week. But if I dedicate enough time and effort, I chose to believe, I’ll learn.

My decision rested on experience and on classes, taught by educational psychologists, that I’d taken in college. I’d studied how brains change during learning and how breaks enhance the changes. Sense, I thought, underlay my decision—though expecting outcomes to change, while strategies remain static, sounds insane.

Old cover

Does sense underlie Kater’s suggestion, likened to insanity, to keep fabricating amplifiers as before? He’s expressed cynicism many times during our collaboration: Experiment needs to involve some insanity. The experiment probably won’t work for a long time. Plenty more things will likely break. 

Jonathan and I agree with him. Experiments have a reputation for breaking, and Kater has a reputation for knowing experiments. Yet Jonathan—with professionalism and politeness—remains optimistic that other methods will prevail, that we’ll meet our goals early. I hope that Jonathan remains optimistic, and I fancy that Kater hopes, too. He prophesies gloom with a quarter of a smile, and his record speaks against him: A few months ago, I met a theorist who’d collaborated with Kater years before. The theorist marveled at the speed with which Kater had operated. A theorist would propose an experiment, and boom—the proposal would work.

Sea monsters

Perhaps luck smiled upon the implementation. But luck dovetails with the sense that underlies Kater’s opinion: Experiments involve factors that you can’t control. Implement a protocol once, and it might fail because the temperature has risen too high. Implement the protocol again, and it might fail because a truck drove by your building, vibrating the tabletop. Implement the protocol again, and it might fail because you bumped into a knob. Implement the protocol a fourth time, and it might succeed. If you repeat a protocol many times, your environment might change, changing your results.

Sense underlies also Jonathan’s objections to Kater’s opinions. We boost our chances of succeeding if we keep trying. We derive energy to keep trying from creativity and optimism. So rebelling against our PhD supervisors’ sense is sensible. I wondered, watching the Skype conversation, whether Kater the student had objected to prophesies of doom as Jonathan did. Kater exudes the soberness of a tenured professor but the irreverence of a Californian who wears his hair slightly long and who tattooed his wedding band on. Science thrives on the soberness and the irreverence.

Green cover

Who won Jonathan and Kater’s argument? Both, I think. Last week, they reported having fabricated amplifiers that work. The lab followed a protocol similar to their old one, but with more conscientiousness. 

I’m looking forward to watching who wins the debate about how long the rest of the experiment takes. Either way, check out Jonathan’s talk about our experiment if you attend the American Physical Society’s March Meeting. Jonathan will speak on Thursday, March 5, at 12:03, in room 106. Also, keep an eye out for our paper—which will debut once Jonathan coaxes the amplifier into synching with his qubit.

Source: https://quantumfrontiers.com/2020/02/23/sense-sensibility-and-superconductors/

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Approximating Hamiltonian dynamics with the Nyström method

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Alessandro Rudi1, Leonard Wossnig2,3, Carlo Ciliberto2, Andrea Rocchetto2,4,5, Massimiliano Pontil6, and Simone Severini2

1INRIA – Sierra project team, Paris, France
2Department of Computer Science, University College London, London, United Kingdom
3Rahko Ltd., London, United Kingdom
4Department of Computer Science, University of Texas at Austin, Austin, United States
5Department of Computer Science, University of Oxford, Oxford, United Kingdom
6Computational Statistics and Machine Learning, IIT, Genoa, Italy

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Abstract

Simulating the time-evolution of quantum mechanical systems is BQP-hard and expected to be one of the foremost applications of quantum computers. We consider classical algorithms for the approximation of Hamiltonian dynamics using subsampling methods from randomized numerical linear algebra. We derive a simulation technique whose runtime scales polynomially in the number of qubits and the Frobenius norm of the Hamiltonian. As an immediate application, we show that sample based quantum simulation, a type of evolution where the Hamiltonian is a density matrix, can be efficiently classically simulated under specific structural conditions. Our main technical contribution is a randomized algorithm for approximating Hermitian matrix exponentials. The proof leverages a low-rank, symmetric approximation via the Nyström method. Our results suggest that under strong sampling assumptions there exist classical poly-logarithmic time simulations of quantum computations.

► BibTeX data

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Cited by

[1] Ewin Tang, “Quantum-inspired classical algorithms for principal component analysis and supervised clustering”, arXiv:1811.00414.

[2] Juan A. Acebron, “A Monte Carlo method for computing the action of a matrix exponential on a vector”, arXiv:1904.12759.

[3] Nai-Hui Chia, András Gilyén, Tongyang Li, Han-Hsuan Lin, Ewin Tang, and Chunhao Wang, “Sampling-based sublinear low-rank matrix arithmetic framework for dequantizing quantum machine learning”, arXiv:1910.06151.

The above citations are from SAO/NASA ADS (last updated successfully 2020-02-20 15:40:36). The list may be incomplete as not all publishers provide suitable and complete citation data.

Could not fetch Crossref cited-by data during last attempt 2020-02-20 15:40:34: Could not fetch cited-by data for 10.22331/q-2020-02-20-234 from Crossref. This is normal if the DOI was registered recently.

Source: https://quantum-journal.org/papers/q-2020-02-20-234/

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Quantum

Extension of the Alberti-Ulhmann criterion beyond qubit dichotomies

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Michele Dall’Arno1,2, Francesco Buscemi3, and Valerio Scarani1,4

1Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, 117543, Singapore
2Faculty of Education and Integrated Arts and Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan
3Graduate School of Informatics, Nagoya University, Chikusa-ku, 464-8601 Nagoya, Japan
4Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore

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Abstract

The Alberti-Ulhmann criterion states that any given qubit dichotomy can be transformed into any other given qubit dichotomy by a quantum channel if and only if the testing region of the former dichotomy includes the testing region of the latter dichotomy. Here, we generalize the Alberti-Ulhmann criterion to the case of arbitrary number of qubit or qutrit states. We also derive an analogous result for the case of qubit or qutrit measurements with arbitrary number of elements. We demonstrate the possibility of applying our criterion in a semi-device independent way.

As soon as entanglement was recognised as a resource, theorists started studying the interconversions properties of this resource. The most famous such question is: given N copies of a state rho, how many copies N’ of the state rho’ can one obtain with local operations and classical communication? This question led to the definition of entanglement of formation (rho is the maximally entangled state), of distillation (rho’ is the maximally entangled state), to the discovery of inequivalent entanglement classes for multipartite systems… The amount of literature on this question is enormous.

Very little however is known about a different problem, the one we consider here. The question is whether a pair of states (rho,sigma) can be converted into another pair of states (rho’,sigma’). This question does not need to refer to entanglement: in fact, here we don’t consider composite systems, and consequently we don’t restrict the possible operations. A very simple answer would be the one that holds for classical probability distributions: Pair 1 can be converted into Pair 2, if all the statistics that can be observed with Pair 2 can also be observed with Pair 1. This conveys the idea that Pair 1 can do all that Pair 2 can do, and possibly more. This answer holds for two states of qubits (Alberti and Uhlmann, 1980), but counter-examples are known already when Pair 1 comprises qutrit states. In this paper, we prove that the classical-like characterisation still holds when Pair 1 is generalized to any family of qubit states, as soon as they can all be expressed with real coefficients, and Pair 2 is generalized to any family of qubit or, under certain hypotheses, qutrit, states. We also exploit a duality between states and measurements to present a similar characterisation of measurement devices.

► BibTeX data

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[20] M. Dall’Arno, F. Buscemi, A. Bisio, and A. Tosini, Data-driven inference, reconstruction, and observational completeness of quantum devices, arXiv:1812.08470.
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[21] F. Buscemi and M. Dall’Arno, Data-driven Inference of Physical Devices: Theory and Implementation, New J. Phys. 21, 113029 (2019).
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