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Posters (QCEW01)

Shanon Vuglar (University of New South Wales)
Nina Hadis Amini (Stanford University)
 Design of Coherent Quantum Observers for Linear Quantum Systems

Quantum versions of control problems are often more difficult than their classical counterparts because of the additional constraints imposed by quantum dynamics. For example, the quantum LQG and quantum H(infinity) optimal control problems remain open. To make further rogress, new, systematic and tractable methods need to be developed. This paper gives three algorithms for designing coherent quantum observers, i.e. quantum systems that are connected to a quantum plant and their outputs provide information about the internal state of the plant. Importantly, coherent quantum observers avoid measurements of the plant outputs. We compare our coherent quantum observers with a classical (measurement-based) observer by way of an example involving an optical cavity with thermal and vacuum noises as inputs.

Christian Kraglund Andersen (Aarhus Universitet)
 Circuit QED flip-flop with all-microwave switching

Microwave electronics constitutes an area of research aimed primarily towards the use of high-speed components and circuits for communication and sensing, while digital logic is difficult to implement with all-microwave technologies. We introduce a microwave driven circuit composed of superconducting resonators and qubits which shows a bistable behaviour, and we present a simple mechanism that allows single- or few-photon microwave pulses to work as Set- and Reset-signals that switch the circuit between its stable modes. The resulting system constitutes an ultra-low-energy Set-Reset flip-flop, and we show that its memory lifetime far exceeds the lifetime of states stored in any of its separate components.

Constantin Brif (Sandia National Laboratories)
 Using Optimal Control Theory to Improve Adiabatic Quantum Trajectories

In adiabatic quantum computation, a slow change of a time-dependent control function (or functions) is employed to interpolate between an initial and final Hamiltonian, in order to keep the system in the instantaneous ground state. When the evolution time is finite, the degree of adiabaticity (quantified in this work as the average ground-state population during evolution) depends on the particulars of a dynamic trajectory associated with a given set of control functions. We use quantum optimal control theory with a composite objective functional to numerically search for controls that achieve the target final state with a high fidelity while simultaneously maximizing the degree of adiabaticity. Exploring properties of optimal adiabatic trajectories in model systems elucidates the dynamic mechanisms that suppress unwanted excitations from the ground state. Specifically, we discover that the use of multiple control functions makes it possible to access a rich set of dynamic tr ajectories, some of which attain a significantly improved performance (in terms of both fidelity and adiabaticity) through the increase of the energy gap during most of the evolution time [1].

Sandia is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

[1] C. Brif, M. D. Grace, M. Sarovar, and K. C. Young, New J. Phys. 16, 065013 (2014).

Yameng Cao (Imperial College London)
 Ultrafast Generation of Coherent Photons in a Charge Noisy Quantum Dot
Kazunori Miyata (University of Tokyo)
Akira Furusawa (University of Tokyo)
 Demonstration of Dynamic Squeezing Gate for Continuous-Variable Quantum Information Processing
Michèle Heurs (Leibniz Universität Hannover)
Ousama Houhou (Université Constantine 1)
Michael Hush (University of Nottingham)
 Ignorance is bliss: General and robust cancellation of decoherence via no-knowledge quantum feedback

A “no-knowledge” measurement of an open quantum system yields no information about any system observable; it only returns noise input from the environment. Surprisingly, measuring nothing is most advantageous. We prove that a system undergoing no-knowledge monitoring has reversible noise, which can be cancelled by directly feeding back the measurement signal. We show how no-knowledge feedback control can be used to cancel decoherence in an arbitrary quantum system coupled to a Markovian reservoir. Since no-knowledge feedback does not depend on the system state or Hamiltonian, such decoherence cancellation is guaranteed to be general, robust and can operate in conjunction with any other quantum control protocol. As an application, we show that no-knowledge feedback could be used to improve the performance of dissipative quantum computers subjected to local loss.

Jan Jeske (RMIT University)
Spatially correlated decoherence

Despite rapid progress in quantum engineering, one of the main hindrances in many experiments is the diffculty of isolating the quantum system from the surrounding environment and its fluctuations. This environmental noise perturbation induces dephasing and relaxation processes into the systems. With ever growing experimental and computational capabilities, larger systems at smaller spatial distances are being studied. Simple models of spatially uncorrelated noise are therefore becoming increasingly questionable. We developed new modelling techniques for partially and fully spatially correlated noise and present the novel dephasing and relaxation behaviour dependent on noise correlation length. Effects include super- and subradiance and -decoherence, effective couplings, dark states and new scalings of decoherence rates with number of entangled particles. As a consequence in quantum metrology the best-possible (i.e. Heisenberg-) precision scaling can be achieved in the presence of spatially correlated noise. Furthermore noise correlations can reinstate quantum transport dynamics which is destroyed by uncorrelated noise, facilitate exciton transport in light-harvesting biological systems and influence macroscopic material properties such as the magnetisation in a quantum ferromagnet.

Mads Kock Pedersen (Aarhus Universitet)
Zaki Leghtas (Yale University)
Luca Mazzarella (Università degli Studi di Padova)
Zlatko Minev (Yale University)
Casey Myers (University of Queensland)
Michael Peterer (University of Oxford)
Ksenia Samburskaya (St. Petersburg State Polytechnical University)
 Efficient Storage of Nonclassical Light in Parallel Quantum Memory Based on Scheme of Atomic Levels

Present study focuses on the development of the theory in continuous variables for efficient storage and retrieval of multimode squeezed and entangled light. We consider quantum memory protocol based on atoms with ^-configuration of energy levels. We estimate degree in which light  preserves squeezing during the full process of writing and read-out of a light pulse. We demonstrate that proposed scheme preserves squeezing in the retrieved light. The goal is to devise theoretically optimized and experimentally realizable schemes for the transmission of squeezed states of light, and assess their performance against the set benchmarks under realistic conditions.

Marcilio Santos (University of Aberdeen)
Rebecca Schmidt (University of Nottingham)
Cooperative effects of external control and dissipation in open quantum systems

Coherent optimal control of non-Markovian open quantum systems is crucial in tailored-matter such as quantum information processing. In general, the presence of dissipative reservoirs is considered as detrimental to quantum coherence and entanglement. However, tailored control pulses may change the role of a heat bath from being destructive on quantum resources to an asset promoting them [1]. Here we show that the cooperative interplay between optimal control signals and a dissipative medium may indeed induce phenomena such as entropy reduction (cooling) [1,2] and creation of bi-partite entanglement [3].

[1] R. S., A. Negretti, J. Ankerhold, T. Calarco and J.T. Stockburger, PRL 107, 130404 (2011)
[2] R. S., S. Rohrer, J.T. Stockburger and J. Ankerhold Phys. Scr. T151, 014034 (2012)
[3] R. S., J.T. Stockburger, and J. Ankerhold, PRA 88, 052321 (2013)

Thomas Schulte-Herbrueggen (Technische Universität München)
Pierre Six (Mines Paris Tech)
Vivishek Sudhir (EPFL - Ecole Polytechnique Fédérale de Lausanne)
Antoine Tilloy (École Normale Supérieure)
Agung Trisetyarso (Telkom University)
Brian Vlastakis (Yale University)
Jingbo Wang (University of Western Australia)
Controlled quantum walk, potential application, and physical realization

Quantum walk represents a generalised version of the well-known classical random walk. Regardless of their apparent connection, the dynamics of a quantum walk is often non-intuitive and far deviate from its classical counterpart. A multi-particle quantum walk provides an even richer dynamical system due to intrinsic quantum correlations such as entanglement. In this poster, we will present our recent studies on controlled quantum walks along graphs of arbitrarily complex topology, including directed and weighted graphs with/without traps and sinks. Current research is suggesting potential applications across a whole range of different fields, of which we will consider in particular network analysis, graph isomorphism, quantum page-ranking, quantum games, pattern recognition, quantum search, as well as quantum simulation of chemical and biological processes. We will also discuss several physical implementation schemes for quantum walks of arbitrary complexity.

University of Cambridge Research Councils UK
    Clay Mathematics Institute The Leverhulme Trust London Mathematical Society Microsoft Research NM Rothschild and Sons