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Continuous-time quantum computing and simulation: perspectives and challenges

Scientific meeting

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Theo Murphy international scientific meeting organised by Dr Viv Kendon, Sir Peter Knight FRS and Professor Andrew Daley.

CCD image of atoms in a quantum gas microscope. Credits: Stefan Kuhr and Bruno Peaudecerf, University of Strathclyde.

Developing experiments capable of useful simulations of quantum systems and computation using native Hamiltonians needs highly optimised hardware and software, to extract the best performance from state-of-the-art quantum engineering. This meeting brings together experts from different research areas to share ideas and further develop UK strength in this important future quantum technology.

The schedule of talks and speaker biographies are available below. Speaker abstracts will be available closer to the meeting. Recorded audio of the presentations will be available on this page after the meeting has taken place.

Poster session

There will be a poster session at 16:15 on Wednesday 2 October 2019. If you would like to apply to present a poster please submit your title, proposed abstract (not more than 200 words and in third person), author list, name of the proposed presenter and authors' institutions to the Scientific Programmes team no later than Wednesday 28 August 2019. Please include the text "Quantum computing: poster abstract" in the subject heading. Please note that places are limited and are selected at the scientific organisers' discretion. Poster abstracts will only be considered if the presenter is registered to attend the meeting.

Attending this event

This is a residential conference, which allows for increased discussion and networking.

  • Free to attend
  • Advance registration essential 
  • Catering and accommodation available to purchase during registration

Enquiries: contact the Scientific Programmes team


Event organisers

Select an organiser for more information

Schedule of talks

02 October

09:00-12:30

Session 1

4 talks Show detail Hide detail

Chairs

Professor Stefan Kuhr, University of Strathclyde, UK

09:05-09:35 Quantum simulation with ultracold atoms in optical lattices

Dr Ulrich Schneider, University of Cambridge, UK

Abstract

During the last fifteen years, ultracold atoms in optical lattices have emerged as very versatile and powerful Quantum Simulators to study the many-body physics of interacting particles in periodic potentials. Not only can they faithfully reproduce many prototypical effects from condensed matter physics, they also enable radically new systems with fascinating physics and hold promise for wider quantum information applications. After a brief review of fundamental properties and key experiments that already reach far beyond what can be computed classically, this talk will present an outlook into current and coming developments for realizing more complex lattice geometries.

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09:35-09:45 Discussion

09:45-10:15 Rydberg quantum optics

Professor Charles Adams, Durham University, UK

Abstract

This talk will begin with a brief review of the attractive features of Rydberg atoms for applications in quantum technology (CS Adams et al., arXiv:1907:09231), with a particular emphasis on single photon sources, photon gates and continuous-time quantum processors. Subsequently, the details of two experimental platforms being pursued in Durham will be discussed: First arrays of light-matter interfaces with quasi-deterministic control of photon-photon interactions (H Busche et al., Nature Phys.), and second arrays of individual Sr atoms (NC Jackson et al., arXiv:1904:03233).

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 Quantum simulations and quantum networks with trapped ions

Dr Ben Lanyon, Institut für Quantenoptik und Quanteninformation & University of Innsbruck, Austria

Abstract

In the first half Dr Ben Lanyon will present a trapped-ion quantum simulator. His approach is based on a 1D string of trapped atomic calcium ions, between which his group can turn on tunable-range interactions using lasers. He achieved full individual qubit (ion) control and entangled states for up to 20 qubits (N Friis et al., Phys. Rev. X., 2018). Dr Lanyon presents the system capabilities, challenges and recent results on extending the system to 50 qubits. In the second half Dr Lanyon will present his recent results on interfacing these registers of trapped ions with travelling photons. In particular, using cavity-QED techniques the group achieve on demand entanglement between the ion-qubit state and a travelling photon with probability of over 50%. Secondly, he observes that the entanglement remains after the photon travels over 50km of optical fibre (V. Krutyanskiy et al., npj Quantum Information, 2019). This opens up the possibility of entangling these registers of ions, hundreds of kilometres apart and more.

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11:30-11:45 Discussion

11:45-12:15 Engineering programmable spin interactions with atoms and photons

Emily Davis, Stanford University, USA

Abstract

Photon-mediated interactions among atoms coupled to an optical cavity are a powerful tool for engineering quantum many-body Hamiltonians. Emily will present observations of dynamics of spins evolving under continuously tunable Heisenberg models, where the relative strength and sign of spin-exchange and Ising couplings are controllable parameters. The interaction dynamics manifest as rotations of large effective spins in a mean-field picture, as well as a spin-mixing process seeded by quantum fluctuations, which in principle generates a highly entangled twin Fock state. The optical access afforded by a near-concentric cavity geometry enables spatially-dependent addressing and imaging with micron-scale resolution, allowing the implementation of initial states designed for Hamiltonian tomography. Whereas the single-mode cavity most naturally mediates all-to-all couplings, she will also discuss progress in generalizing to control the distance-dependence of the interactions.

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12:15-12:30 Discussion

12:30-13:30

Lunch

13:30-16:15

Session 2

3 talks Show detail Hide detail

13:30-14:00 Quantum optimisation

Dr Wolfgang Lechner, University of Innsbruck, Austria

Abstract

With the current progress in building quantum devices we have now entered the era of NISQ (near-term noisy intermediate scale qubit). It is a main open question whether quantum devices with qubits that are operated in a noisy regime and in the absence of error correction can achieve any advantage compared to classical computation. Dr Wolfgang Lechner will discuss recent developments in adiabatic quantum computing or quantum annealing which aims at solving classical optimisation problems with noisy qubits and Hamiltonian dynamics. In particular, he will discuss non-adiabatic method to improve adiabatic quantum computation. 

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14:00-14:15 Discussion

14:15-14:45 Continuous time quantum computing beyond adiabatic: quantum walks and fast quenches

Dr Nicholas Chancellor, Durham University, UK

Abstract

While the adiabatic theorem provides a useful theoretical handle to understand quantum computing in continuous time, solving hard problems adiabatically would require an exponentially long runtime and therefore unless P=NP will require either an exponentially long coherence time or a mechanism to restore coherence. On the other hand, algorithms which only succeed with an exponentially small probability may still be useful on more realistic devices, for which coherence time either does not scale, or scales only mildly. One example of such an algorithm is a continuous time quantum walk applied to hard optimisation problems, where a system is evolved with a fixed (in time) Hamiltonian. Dr Nicholas Chancellor finds that for small Sherrington-Kirkpatrick spin glasses, such an algorithm delivers a scaling which is less than the square root of the system size expected from Grover like simple search dynamics, he attributes this superior performance to the fact that the energy landscape of real optimization problems is correlated, and provide evidence that these correlations are crucial to being able to build practical algorithms, and argue why the quantum walk algorithm. In hindsight this is somewhat unsurprising, since these correlations are necessary for any classical algorithm to perform better than random guessing. Dr Chancellor also discusses the underlying dynamics which allow this and other far-from-adiabatic algorithms to work. Finally he discusses extensions of the work, which include a technique his group have called pre-annealing for which they observe scaling of their continuous time quantum algorithm which is competitive with a cutting edge gate based hybrid quantum/classical gate model algorithms. It is likely that by adding a hybrid component these algorithms could be made more efficient and beat the current state of the art.

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14:45-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 Continuous variables quantum complex networks

Dr Valentina Parigi, Laboratoire Kastler Brossel, Sorbonne Université, France

Abstract

Experimental procedures based on optical frequency combs and parametric processes are able to produce quantum states of light involving large numbers of modes -in the frequency and time domain- that can be mapped and analyzed in terms of quantum complex networks.  The protocols, along with mode selective and multimode homodyne measurements, in fact, allow for the implementation of reconfigurable entanglement structures that can go beyond the regular geometry of cluster states and implement graphs with more complex topology. Quantum complex networks, mimicking real-world structures, can then be explored to study quantum transport and tailored quantum communication and information protocols.  Additional mode-selective non-Gaussian operations have been recently demonstrated. When applied to the graph structure entanglement properties and non-Gaussian features are spread out with particular geometrical properties.

 

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16:00-16:15 Discussion

16:15-18:00

Poster session

03 October

09:00-12:30

Session 3

4 talks Show detail Hide detail

Chairs

Professor Sougato Bose, University College London, UK

09:00-09:30 Quantum many-body physics with arrays of single Rydberg atoms

Dr Thierry Lahaye, Institut d’Optique, CNRS, France

Abstract

This talk will present recent progress in the use of arrays of single atoms held in optical tweezers, and interacting strongly with each other when excited to Rydberg states. This platform provides an almost arbitrary control over the geometry of single-atom assemblies in 1, 2 and 3 dimensions, with up to about 100 atoms, and is ideal to realise quantum spin models with various types of interactions, such as the Ising or the XY models.

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09:30-09:45 Discussion

09:45-10:15 Quantum dots for quantum simulations

Dr Ruth Oulton, University of Bristol, UK

Abstract

Quantum dots, quantum emitters in a semiconductor matrix, are most often proposed as a bright and efficient source of single photons for many quantum technologies. And, as Professor Ruth Oulton has demonstrated previously, a near-perfect single photon source goes hand-in-hand with potentially deterministic interactions with input photons. However, it is the spin degree of freedom in their ground state which gives them the greatest scope for quantum simulations. Photons input into a QD device can be entangled with the long coherence time spin system, and protocols to produce entangle chains of photons (1D cluster states) have already been demonstrated. Professor Oulton will discuss how one may entangle very long coherence time photons with the spin. Reflecting the long photon from a quantum dot spin precessing in a magnetic field results in a phase modulation of the photon wave function in time, with periodic entanglement resulting. Professor Oulton will discuss the potential of spin-photon entangled states as building blocks for analogue and digital quantum simulations.

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 Quantum annealing with superconducting flux qubits

Professor Paul Warburton, University College London, UK

Abstract

Quantum annealing makes less stringent demands on qubit coherence than gate-based approaches, thereby enabling proof-of-principle demonstrations of annealers with around 2000 superconducting flux qubits.  Furthermore by capacitively shunting the flux qubit and reducing the circulating current one can achieve both high coherence and low leakage, making the flux qubit an excellent approximation to a two-level quantum system. Nevertheless most measurements on experimental annealers are plagued by noise, and the role of coherence in quantum annealing is not currently understood. Professor Paul Warburton will describe his group’s experimental and analytical work on both understanding coherence in flux qubit annealers and how to optimise their use for real-world applications in the presence of noise. They have used the Schrieffer-Wolf transformation to extract the Pauli coefficients from quantum circuit models and developed this technique to investigate non-stoquastic Hamiltonians arising from simultaneous inductive and capacitive qubit interactions. They have analysed the extent to which Landau-Zener-Stückelberg oscillations can be used as a coherence metric in the context of quantum annealing. The group has also developed a new method for embedding real-world problems with high qubit connectivity onto hardware graphs of limited degree and show experimentally that this method outperforms rival embedding techniques for annealers in the presence of noise. The research is based upon work supported by EPSRC (grant reference EP/R020159/1) and the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via the US Army Research Office contract W911NF-17-C-0050. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, or the US Government. 

 

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11:30-11:45 Discussion

11:45-12:15 Energy-landscape shaping for quantum simulation with cold atoms and in semiconductors

Dr Sophie Shermer, Swansea University, UK

Abstract

Energy landscape shaping is a way to alter the natural evolution of a quantum system to achieve certain objectives utilising the continuous evolution of the system instead of applying discrete quantum gates and dynamic control. Using spin networks as an abstract model system Dr Sophie Shermer will discuss how to design energy landscapes to control information flow between nodes in various networks. Energy landscape design will be formulated as an optimal control problem and in terms of linear feedback control systems. Various solutions to the optimal control problems arising will be examined in terms of robustness. Robustness of the evolution with regard to uncertainty in system parameters, initial conditions and environmental effects such as decoherence is crucial, and the development of better tools inspired by classical engineering is essential for robust quantum technology. Dr Shermer will discuss classical engineering approaches to robustness and the challenges in applying them to quantum systems, as well as some promising results suggesting that classical limits on robustness need not apply to the latter. Possible implementations of energy landscape control using cold atoms trapped in optical lattices as experimental testbeds for pseudo-spin networks will also be considered.

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12:30-13:30

Lunch

13:30-17:00

Session 4

4 talks Show detail Hide detail

13:30-14:00 Variational quantum algorithms for nonlinear problems

Professor Dieter Jaksch, University of Oxford, UK

Abstract

Professor Dieter Jaksch will discuss how nonlinear problems including nonlinear partial differential equations can be efficiently solved by variational quantum computing. This is achieved by utilising multiple copies of variational quantum states to treat nonlinearities efficiently and by introducing tensor networks as a programming paradigm. He will demonstrate the key concepts of the algorithm using the nonlinear Schrödinger equation as a canonical example. He will present numerical results showing that the variational quantum ansatz can be exponentially more efficient than matrix product states and present experimental proof-of-principle results obtained on an IBM Q device.

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14:00-14:15 Discussion

14:15-14:45 Quantum-assisted machine learning in near-term quantum devices

Dr Alejandro Perdomo-Ortiz, Zapata Computing, Canada

Abstract

With quantum computing technologies nearing the era of commercialisation and quantum advantage, machine learning (ML) has been proposed as one of the promising killer applications. Despite significant effort, there has been a disconnect between most quantum ML proposals, the needs of ML practitioners, and the capabilities of near-term quantum devices towards a conclusive demonstration of a meaningful quantum advantage in the near future. In this talk, Dr Alejandro Perdomo-Ortiz provides concrete examples of intractable ML tasks that could be enhanced with near-term devices. He argues that to reach this target, the focus should be on areas where ML researchers are struggling, such as generative models in unsupervised and semi-supervised learning, instead of the popular and more tractable supervised learning tasks. Alejandro focuses on hybrid quantum-classical approaches and illustrates some of the key challenges he foresees for near-term implementations. He will present as well recent experimental implementations of these quantum ML models in both, superconducting-qubit and ion-trap quantum computers.

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14:45-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 Analogue and digital simulation - parallels and differences

Dr Steve Brierley, Riverlane, UK

Abstract

There are many parallels between digital and analogue quantum simulation. For example until very recently, the best digital simulation algorithms used quantum random walks. Using this and other examples, Dr Steve Brierley will explore the similarities and differences between digital and analogue simulation with a view to what each community might learn from the other.

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16:00-16:15 Discussion

16:15-17:00 Panel discussion

Continuous-time quantum computing and simulation: perspectives and challenges

Theo Murphy international scientific meeting organised by Dr Viv Kendon, Sir Peter Knight FRS and Professor Andrew Daley.

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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