# Seminars (QCEW01)

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Event When Speaker Title Presentation Material
QCEW01 4th August 2014
10:00 to 10:30
Quantum Dissipation and Control through Feedback Networks
We discuss the role of dissipation in the analysis and design of quantum control systems, and in particular how the approach of control by interconnection” due to Jan Willems can be implemented in the quantum domain utilizing the formalism of quantum feedback network. Our results include an in?nitesimal characterization of the dissipation property, and review concepts such as the Positive Real and Bounded Real Lemmas in this context. We also discuss recent work on stabilization of open quantum systems.
QCEW01 4th August 2014
10:30 to 11:10
Quantum control of an optomechanical system with a single photon
Co-authors: S Basiri-Esfahani (The University of Queensland), C Myers (The University of Queensland), J Combes (The University of Queensland)

Progress in the technology of optomechanical systems is reaching the point where the mechanical action of a single photon can be significant. In this talk I will describe how single photon states can be used to control the quantum state of a mechanical resonator coupled via radiation pressure to the light in a cavity. The photons can be used as a quantum probe of the mechanical state . This enables the engineering of non classical states of the mechanical oscillator with applications to metrology and experiments to look for gravitational decoherence.
QCEW01 4th August 2014
11:45 to 12:25
Signal Flows in Non-Markovian Quantum Networks
With rapid progresses in quantum technologies, quantum devices will be possibly networked at a much larger scale in the near future, over which effective analysis and synthesis are very demanding. In this paper, we construct signal flow graphs for linear quantum networks that are more concise than the traditional block diagram representation. Starting from two interconnected linear quantum systems, we present a bidirectional signal flow graph that describes the quantum action and backaction between them. Such signal flow graph can be naturally applied to non-Markovian systems with colored quantum noise inputs, and thus can include more general linear components such as spectral filters and dispersive quantum waveguides. These components can be integrated to build complex feedback networks via interconnections or serial products, where either direct or indirect coherent feedback loops are envisioned as bidirectional flows of quantum signals. Finally, we introduce the Riegle 9;s matrix gain rule to calculate transfer functions between source and sink nodes in a linear quantum network with multiple overlapping feedback loops. The theory provides a basis for synthesizing non-Markovian linear quantum feedback networks in the frequency-domain.
QCEW01 4th August 2014
13:30 to 14:10
J-S Li Optimal Pulse Design in Quantum Control: An Ensemble Control Perspective
Designing and implementing time-varying electromagnetic pulses to manipulate the time-evolution of a large quantum ensemble is a long-standing problem in quantum control and an indispensable step that enables many cutting-edge quantum technologies. In practice, such pulse designs are made significantly more challenging because the values of parameters that characterize the dynamics of the quantum ensemble may show variation, so that the system Hamiltonian is not uniform over the ensemble. For example, nuclear magnetic resonance applications often suffer from imperfections such as inhomogeneity in the static magnetic field and in the applied radio-frequency field, variation in the dissipation rates of spins as well as dispersion in their Larmor frequency due to chemical shifts. A good pulse design strategy must be robust to these effects, and such variations need to be considered in the modeling and pulse design stages in order for theoretical predictions to match experimental outcomes.

In this talk, ensemble control-theoretic approaches for optimal pulse design in quantum control will be introduced. A new method that integrates Lie algebras with polynomial approximation for analyzing controllability of spin ensemble systems will be presented. In addition, robust computational methods for optimal pulse synthesis will also be presented, which include a unified computational method based on multidimensional pseudospectral approximations and an optimization-free iterative algorithm based on the singular value decomposition. Commonly used pulses in various fields of quantum control developed by these computational methods will be illustrated, and, moreover, experimental realizations of these optimal pulses will be shown to demonstrate the robustness and applicability of these newly developed methods.

QCEW01 4th August 2014
14:15 to 14:55
Hybrid quantum optomechanics: a single spin coupled to a nano-oscillator
Interfacing a mechanical resonator with a purely quantum object is a promising route toward the generation of macroscopic non-classical states. We consider here an hybrid system composed of a single NV spin qubit coupled to a nano-resonator. The theoretical framework of the coupling mechanism will be detailed and the rst experimental signatures of the coupled dynamics will be presented.
QCEW01 4th August 2014
15:30 to 16:10
HM Wiseman Quantum State Smoothing
Co-author: Ivonne Guevara (Griffith University)

Under noisy observations, estimation theory allows one to infer the state of the measured system, if its a priori statistics are given. In the continuous time situation, three different types of estimation can be distinguished: filtering, which is estimating of the state at time t from earlier records; retro-filtering, which is estimating the state at time t from later records; and smoothing, which is estimating the state at time t from both earlier and later records. Of the three, smoothing allows the greatest precision. This theory has been well developed in classical systems, but its application to quantum systems has only recently begun to be explored. Previous works have used the term “quantum smoothing” to mean estimating classical parameters, [Tsang, Phys. Rev. Lett 102, 250403 (2009)], which is essentially classical smoothing in which the noisy observation of the classical parameters is mediated by a quantum system. Here we introduce quantum state smoothin g, where the state of a partially observed open quantum system itself is smoothed. We achieve this by applying classical smoothing to a hypothetical unobserved noisy measurement record correlated with the stochastic dynamics ("quantum trajectories") of the system, induced by that hypothetical measurement. Using the formalism of linear quantum trajectories, we simulate quantum state smoothing for a simple system, and study how the choice of unravelling for the true observation of the system affects how well the unobserved results can be estimated, and hence how effective is the quantum state smoothing. Our investigations shed new light on the nature of open quantum systems and the applicability of classical concepts.

QCEW01 4th August 2014
16:15 to 16:55
M Plenio Diamond Quantum Devices: Control & Signal Processing Methods for Quantum Sensing and Quantum Simulation
In this lecture I will present work towards achieving control of NV centers in diamonds and show how this can be used to create quantum spin sensors that may achieve nuclear spin sensitivity, hybrid devices for spin-motion coupling and enhanced sensing of pressure, electric fields as well as a novel design for a quantum simulator. These ideas benefit from the use of control techniques to manage noise as well as methods from signal processing to extract information from noisy experimental signals.
QCEW01 5th August 2014
09:45 to 10:25
A Direct Coupling Coherent Quantum Observer
This paper considers the problem of constructing a direct coupling quantum observer for a closed linear quantum system. The proposed observer is shown to be able to estimate some but not all of the plant variables in a time averaged sense. A simple example and simulations are included to illustrate the properties of the observer.
QCEW01 5th August 2014
10:30 to 11:10
Taking Control of Superconducting Qubits
Electronic circuits which exhibit quantum mechanical phenomena—superposition and entanglement, in particular—promise a new generation of computers capable of solving currently intractable problems, secure communication, precision metrology, detectors with unparalleled sensitivity, and an efficient route for synthesizing new materials. One of the fundamental challenges, however, in realizing quantum machines is to sustain coherence over a time interval practical for performing coherent operations or computation. Until now, boosting coherence has involved hardware development to minimize coupling to a dissipative environment which typically transforms a quantum superposition into a classical state. Recent advances in the development of robust quantum-noise-limited microwave amplifiers and quantum bits with lifetimes in excess of 100 microseconds have enabled the use of feedback to actively suppress decoherence. In particular, we have been able to tailor the dissipa tive environment, either via measurement or excitation pulses, to stabilize quantum superposition states and coherent oscillations as well as track the evolution of single and two qubit states.
QCEW01 5th August 2014
11:45 to 12:25
Robust Engineering of Correlations and Symmetries on Quantum Networks
In many quantum information applications, ranging from computation to communication, a key step is the ability to engineer some states with given correlations on multipartite systems (or "quantum network"), where the interaction betweens the parts are limited by locality constraints and control capabilities. We here focus on two specific tasks and some robust, randomized protocols to achieve them. The first one is the asymptotic preparation of a given entangled pure state: we provide a test to check if the task is feasible under the existing constraints, as well as randomized design methods for controlled open-system dynamics. The second is the symmetrization of the network state with respect to the subsystem permutation group: we show that this problem can be tackled with methods borrowed from classical "consensus" problems. Motivating examples and applications will be introduced. In both cases, asymptotic convergence to the target can be proved under mi nimal assumptions on the network topology and the way the local dynamics are selected.
QCEW01 5th August 2014
13:30 to 14:10
Non-Markovian and nonlinear quantum input-output response analysis
Co-authors: Yu-xi Liu (Tsinghua University), Rebing Wu (Tsinghua University), Kurt Jacobs (Boston University), Tzyh-Jong Tarn (Washington University in Saint Louis), Franco Nori (RIKEN)

Quantum input-output response analysis is a useful method for modeling the dynamics of complex quantum networks, such as those for communication or quantum control via cascade connections. Non-Markovian and nonlinear effects are expected to be important in networks realized using mesoscopic circuits. Here we first extend the Markovian input-output network formalism to non-Markovian networks, and apply it to various examples. Second, as an natural extension of the input-output formalism of Gardiner and Collet, we develop a new approach based on the so-called quantum Volterra series which can greatly reduce the computational complexity of nonlinear quantum input-output analysis. By this method, we can ignore the internal dynamics of the input-output system and attribute all the information we need to a series of scalar kernal functions. This method can be used to describe the quantum network with both nonlinear components and Bogoliubov components such as quantum amplifers whic h cannot be modelled by the existing methods such as Hudson-Parthasarathy model and quantum transfer function model.

QCEW01 5th August 2014
14:15 to 14:55
A General Filter-Function Approach to Noise Filtering in Open-Loop Quantum Control
Hamiltonian engineering via open-loop quantum control provides a versatile and experimentally validated framework for precisely manipulating a broad class of non-Markovian dynamical evolutions of interest, with applications ranging from dynamical decoupling and dynamically corrected quantum gates to noise spectroscopy and quantum simulation. In this context, transfer-function techniques directly motivated by control engineering have proved invaluable for obtaining a transparent picture of the controlled dynamics in the frequency domain and for quantitatively analyzing control performance. In this talk, I will show how to construct a general filter-function approach, which overcomes the limitations of the existing formalism. The key insight is to identify a set of "fundamental filter functions", whose knowledge suffices to construct arbitrary filter functions in principle and to determine the minimum "filtering order" that a given control protocol can guar antee. Implications for dynamical control in multi-qubit systems and/or in the presence of non-Gaussian noise will be discussed.
QCEW01 5th August 2014
15:30 to 16:10
Microwaves and phonons in 1D transmission lines: Giant Cross-Kerr effect, QND Photon Detection and Giant Atoms
In this talk, I’ll discuss the physics of microwave photons moving in a coplanar wave-guide (1D transmission line) interacting with one or more artificial atoms. Compared to the optical regime, the microwave regime allows for strong and stable coupling of the photons to (artificial) atoms. In particular, I’ll discuss the possibility of using the giant cross-Kerr effect for QND detection of propagating microwave photons. Motivated by recent experiments, I’ll also discuss what happens when the microwave photons are replaced by surface acoustic wave (SAW) phonons. The phonon velocity is five orders of magnitude slower, implying that the atom is now substantially larger than the wavelength for its spontaneous emission.
QCEW01 5th August 2014
16:15 to 16:55
Input-Output Device Modelling for Quantum Information Systems
Recent decades have seen significant progress in quantum information and computation, both at the abstract level of qubits as well as in the laboratory, where rudimentary proof of concept devices and small scale systems have been demonstrated. However, while commercial QKD systems are becoming viable, useful large scale quantum computers appear to be some distance away. The problem of scaling up to large interconnected quantum information processing systems remains a major challenge for quantum technology researchers. This talk will discuss a number of aspects of modelling that are relevant to this scale up issue, with a particular emphasis on input-output models. We review pertinent features of hierarchical modelling for classical information systems, and discuss their implications for quantum information. The talk reports on some recent model development for quantum memories and nonclassical sources.
QCEW01 6th August 2014
09:45 to 10:25
T Stace Cross-Kerr non-linearities for quantum non-demolition measurement
Various proposals exist that exploit cross-Kerr non-linearities between electromagnetic fields, for quantum non-demolition (QND) measurements, and other related protocols. Typically, these proposals assume the existence of such non-linearities, based on a presumption that the internal dynamics of the medium can be eliminated from the problem. I will discuss some recent work in which we explicitly include the medium. We show that homodyne detection of a probe field, plus classical signal processing, is sufficient to yield an itinerant microwave photon counter with fidelities above 90%. Our analysis has implications for the validity of a wider range of proposed cross-Kerr based quantum information protocols, which may need revision in consequence.
QCEW01 6th August 2014
10:30 to 11:10
Quantum control of superconducting circuits
We develop an efficient scheme that allows for arbitrary operations on a cavity mode using a strongly dispersive qubit-cavity interaction and time-dependent drives. The scheme can readily be implemented using circuit QED systems. Moreover, the scheme can be extended for quantum error correction to protect information encoded in photonic cat states.
QCEW01 6th August 2014
11:45 to 12:25
Preserving quantum coherence of spins in the presence of noises
In this talk, I will discuss various control schemes for protecting quantum coherence of electron spins against noises in solid-state environments. I will also present a few applications of the coherence protection in quantum information processing and in ultrasensitive magnetometry.

This work is supported by Hong Kong Research Grants Council and CUHK Focused Investments Scheme.
QCEW01 6th August 2014
13:30 to 14:10
Quantum control of magnon modes in ferromagnet
Concepts and technologies of quantum coherent control were initially developed in microscopic systems such as atoms, nuclear and electron spins. However, triggered by the emergence of quantum information science, they have recently been extended toward more macroscopic degrees of freedom such as collective excitations in solid. The most pronounced example is electromagnetic excitations in superconducting circuits, in which superconducting qubits have been realized by exploiting the nonlinearity provided by Josephson junctions. The artificial two-level systems, or atoms, coupled with superconducting resonators and other quantum systems have been enjoying rich physics and applications of circuit quantum electrodynamics (circuit QED) and hybrid quantum systems.

Spin-wave excitations (magnons) in magnetic materials are another well-known collective excitation which is commonly studied in magnetism and spintronics. Sometimes they have a long lifetime, for example, in the typical ferromagnetic insulator, yttrium iron garnet (YIG).

We have investigated ferromagnetic resonance of a mm-scale YIG sphere at low temperature (10 mK) and low power (-140 dBm). The sphere is placed in a microwave cavity resonator made of oxygen-free copper and biased with a static field of about 0.3 T. We demonstrate coherent coupling between the magnon mode (the Kittel mode) with the microwave cavity mode (10 GHz), even in the quantum limit where both the average magnon and photon numbers are less than one.

We also discuss how to couple the magnon mode with a superconducting qubit via a resonator. Coherent coupling with a qubit enables quantum control and measurement of the magnon excitations and thus opens a field of quantum magnonics.

This work was done in collaboration with Y. Tabuchi, S. Ishino, T. Ishikawa, R. Yamazaki, and K. Usami.
QCEW01 6th August 2014
14:30 to 16:00
QHDL Tutorial
QCEW01 7th August 2014
09:00 to 09:40
Robust Quantum Control for Quantum Information Systems
Co-authors: Matthew Grace (Sandia National Laboratories), Constantin Brif (Sandia National Laboratories), Hersch Rabitz (Princeton University)

In a 1985 paper in Optics News entitled Quantum Mechanical Computers,'' Richard Feynman described how a computer could be built upon the mathematical principles of quantum mechanics. But he also heralded the difficulties in an actual physical implementation: This computer seems to be very delicate and these imperfections may produce considerable havoc.'' This talk will describe our on-going efforts to alleviate the potential havoc'' by appealing to robust control design, both model-based and data-based. The model-based approach relies on uncertainty modeling, i.e., values of parameters and noise sources in the model are unknown but contained in bounded sets. Sequential convex programming (SCP) is used to find controls which maximize fidelity despite the uncertainty. From an available set of input/observable pairs, the same type of model is used to estimate fidelity as well as refine knowledge about uncertain parameters, thereby leading to a more robust control providing increased performance.

QCEW01 7th August 2014
09:40 to 10:20
Model realization and model reduction for quantum systems
Co-authors: Jun Zhang (Shanghai Jiao Tong University) & Akshat Kumar (Sandia National Laboratories)

I will describe the application of two classic notions from linear systems theory to the quantum domain: (i) model realization, the notion of constructing a dynamical model from input-output data only, and (ii) model reduction, the notion of developing compressed descriptions of system dynamics.

First I will detail how model realization methods can be used to develop a technique for estimating parameters in quantum Hamiltonians directly from input-output data (arXiv:1401.5780). This method is particularly advantageous in scenarios with restricted system access and nontrivial prior information.

In the second part of the talk I will present methods that allow for construction of reduced order models for quantum dynamics based on identifying invariant subspaces of Hamiltonians (arXiv:1406.7069). These methods reduce the burden of simulating some models of quantum many-body dynamics.

QCEW01 7th August 2014
10:50 to 11:30
C Santori Towards large-scale nonlinear photonic circuits
Co-authors: Jason S. Pelc (Hewlett-Packard Laboratories), Raymond G. Beausoleil (Hewlett-Packard Laboratories), Nikolas Tezak (Stanford University), Ryan Hamerly (Stanford University), Hideo Mabuchi (Stanford University)

This talk will describe work at HP Labs to design and build photonic circuits on semiconductor platforms that use Kerr, carrier-based and thermal nonlinearities to perform all-optical logic. We have developed a semi-classical model to simulate quantum noise and spontaneous switching in circuits containing hundreds of components. With this model we can address basic questions such as the minimum switching energy needed to avoid errors, or whether the circuits have adequate digital signal restoration. Recently, we have been working to reduce the complexity and improve tolerance to fabrication variations in the circuit designs. I will also summarize our recent experimental efforts on all-optical logic devices.
QCEW01 7th August 2014
11:30 to 12:30
Nonlinear optomechanical measurement of mechanical motion
Co-authors: George A. Brawley (Australian Centre for Engineered Quantum Systems, University of Queensland), Michael R. Vanner (Australian Centre for Engineered Quantum Systems, University of Queensland), Silvan Schmid (Department of Micro- and Nanotechnology, Technical University of Denmark, Kongens Lyngby, Denmark ), Anja Boisen (Department of Micro- and Nanotechnology, Technical University of Denmark, Kongens Lyngby, Denmark )

An important goal in all facets of quantum optics is to be able to perform precise measurements of non-linear observables. This allows measurement-based non-classical state preparation, which has been applied to great success in various physical systems, and also provides a route for quantum information processing with otherwise linear interactions. In cavity optomechanics much progress has been made using a linear interaction and measurement, but observation of nonlinear degrees-of-freedom, such as phonon number, remains outstanding. Here we report the observation of position-squared thermal motion of a micro-mechanical resonator by exploiting the optical non-linearity of the radiation pressure interaction. Using this measurement, we conditionally prepare classical bi-modal mechanical states of motion with feature sizes well below 100 pm. Future improvements to our approach will allow the preparation of quantum superposition states, which can be used to experimentally explor e collapse models of the wavefunction and the potential for mechanical-resonator based quantum-metrology applications.

•http://www.equs.org/ - ARC Centre of Excellence for Engineered Quantum Systems

•http://www.physics.uq.edu.au/QOlab/ - Queensland Quantum Optics Lab

QCEW01 7th August 2014
13:30 to 13:35
J Leeks Welcome and Introduction
QCEW01 7th August 2014
13:35 to 13:45
Quantum Technologies Special Interest Group and Funding Opportunities
QCEW01 7th August 2014
13:45 to 14:15
Emerging Quantum Technologies
QCEW01 7th August 2014
14:15 to 14:45
Quantum Technologies From Science to Innovation
QCEW01 7th August 2014
14:45 to 15:15
Maturing Quantum Technologies: a National Lab Perspective
QCEW01 7th August 2014
15:30 to 16:00
Quantum Technology: Supplying the Picks and Shovels
QCEW01 7th August 2014
16:00 to 16:30
Some Quantum Technology Challenges for Defence
QCEW01 7th August 2014
16:30 to 17:00
Questions & Open Discussion
QCEW01 8th August 2014
09:45 to 10:25
Filtering noise in quantum systems
Co-authors: Alex Soare (ARC Centre for Engineered Quantum Systems, University of Sydney), Harrison Ball (ARC Centre for Engineered Quantum Systems, University of Sydney), David Hayes (ARC Centre for Engineered Quantum Systems, University of Sydney), Xinglong Zhen (ARC Centre for Engineered Quantum Systems, University of Sydney), MC Jarratt (ARC Centre for Engineered Quantum Systems, University of Sydney), Jarrah Sastrawan (ARC Centre for Engineered Quantum Systems, University of Sydney)

Instabilities due to extrinsic interference are routinely faced in systems engineering, and a common solution is to rely on a broad class of filtering techniques in order to afford stability to intrinsically unstable systems. For instance, electronic systems are frequently designed to incorporate electrical filters composed of, e.g. RLC components, in order to suppress the effects of out-of-band fluctuations that interfere with desired performance. Quantum coherent systems are now moving to a level of complexity where challenges associated with realistic time-dependent noise are coming to the fore. In this talk we present work using the theory of quantum control engineering and experiments with trapped ions to demonstrate the construction of noise filters which are specifically designed to mitigate the effect of realistic time-dependent fluctuations on qubits during useful operations. Starting with desired filter characteristics and the Walsh basis functions, we use a com bination of analytic design rules and numeric search to construct time-domain noise filters tailored to a desired state transformation. We describe experiments validating the generalized filter-transfer function framework for arbitrary quantum control operations, and demonstrate that it can be leveraged as an effective tool for developing robust control protocols. We describe how these filtering approaches can be extended to multi-qubit gates and even complex quantum control tasks at the algorithmic level.
QCEW01 8th August 2014
10:30 to 11:10
Dynamics of an oscillator bounded in energy by a Zeno-like effect
Co-authors: Landry Bretheau (Ecole Normale Superieure, Paris, France), Philippe Campagne-Ibarcq (Ecole Normale Superieure, Paris, France), Emmanuel Flurin (Ecole Normale Superieure, Paris, France), Francois Mallet (Ecole Normale Superieure, Paris, France)

In the strong coupling regime of circuit-QED, a qubit can be used as a photocounter. Leading to a binary response only, it is possible to probe whether or not the cavity has N photons for any number N we choose. By repeatedly asking this question, it is possible to freeze the evolution of the cavity mode by Zeno effect into either the number state |N> or the space generated by all the other states. Allowing resonant driving and starting from the ground state of the cavity, this Zeno blockade is thus similar to a transformation of the oscillator into an N/2 spin. Interestingly, one can also produce the same blockade without any dissipation or readout by driving the qubit at the resonance frequency it exhibits when the cavity is in state |N>. We have realized this blockade using superconducting circuits and measured the photon number distribution and Wigner function of the cavity mode as a function of time, while driving the effective N/2 spin. Interesting non-classical s tates develop during the evolution.
QCEW01 8th August 2014
11:45 to 12:25
Squeezing and cubic phase gates and the related technologies
I will show our research activities toward universal optical quantum information processing.