Quantum optimal control in quantum technologies. Strategic report on current status, visions and goals for research in Europe

Quantum optimal control, a toolbox for devising and implementing the shapes of external fields that accomplish given tasks in the operation of a quantum device in the best way possible, has evolved into one of the cornerstones for enabling quantum technologies. The last few years have seen a rapid evolution and expansion of the field. We review here recent progress in our understanding of the controllability of open quantum systems and in the development and application of quantum control techniques to quantum technologies. We also address key challenges and sketch a roadmap for future developments

Quantum optimal control in quantum technologies. Strategic report on current status, visions and goals for research in Europe

Quantum optimal control, a toolbox for devising and implementing the shapes of external fields that accomplish given tasks in the operation of a quantum device in the best way possible, has evolved into one of the cornerstones for enabling quantum technologies. The last few years have seen a rapid evolution and expansion of the field. We review here recent progress in our understanding of the controllability of open quantum systems and in the development and application of quantum control techniques to quantum technologies. We also address key challenges and sketch a roadmap for future developments.Comment: this is a living document - we welcome feedback and discussio

Enhancing the performance of quantum computers

Recent progress in quantum information technology suggests that we will soon be able to carry out computational tasks that are intractable on classical computers. In the common quantum circuit model that divides the underlying quantum algorithms into discrete gates, it is crucial to perform each of these gates with high accuracy. In this thesis we develop an effective model required to precisely describe interactions between superconducting qubits mediated by a resonator. An analytical technique to design optimal control shapes based on derivatives of some base waveform is reviewed, connecting it to related methodology in literature and completing with novel insights. Using this technique, we develop improved pulse sequences to entangle two Rydberg atoms via the Rydberg-blockade interaction and achieve fidelities beyond what previously appeared to be a fundamental limit. A modern optimal control algorithm that combines analytical pulse shapes with numerical optimization is used to study the generation of entanglement between trapped ions. Additionally, we focus on cooling superconducting qubits in adiabatic quantum computation where computational problems are solved by adiabatically changing the Hamiltonian of a quantum system. Errors due to imperfect adiabatic evolution and finite temperatures deteriorate performance in such protocols. We propose a novel approach to reducing these errors efficiently, overcoming limits of previous cooling schemes.Aktuelle Entwicklungen im Gebiet Quanteninformationstechnologie lassen erwarten, dass bald erste Rechnungen auf Quantencomputern durchgeführt werden, die auf klassischen Computern nicht ausführbar sind. Unterteilt man die zugrunde liegenden Quantenalgorithmen in individuelle Gatter, ist es nötig, diese Gatter mit hoher Genauigkeit auszuführen. In dieser Arbeit erarbeiten wir ein effektives Modell zur genauen Beschreibung der mittels eines Reso\-nators vermittelten Wechselwirkung zwischen supraleitenden Qubits. Eine analytische Technik zur präzisen Systemkontrolle mittels Ableitungen einer Funktion wird in neuartiger Form aufgearbeitet, in Bezug zu verwandten Methoden gesetzt und durch bisher unveröffentlichte Erkenntnisse vervoll\-ständigt. Anhand dessen entwickeln wir verbesserte Sequenzen zur Verschrän\-kung zweier Rydbergatome mittels der Rydberg-Blo\-cka\-de und überwinden bisher vermutete Einschränkungen. Ein moderner Algorithmus, der analy\-tische Pulsformen mit numerischer Optimierung kombiniert, wird genutzt, um die Verschränkung zwischen gefangenen Ionen zu untersuchen. Daneben untersuchen wir Kühlprozesse im adiabatischen Quantencomputing, bei dem Probleme durch adiabatisches Umschalten des Hamiltonians eines Quantensystems gelöst werden. Wir schlagen eine neue Methode vor, um in solchen Verfahren relevante Fehler aufgrund imperfekter Adiabatizität und endlicher Temperatur effektiv zu reduzieren und überwinden dabei Einschränkungen bisheriger Verfahren

Novel approaches to optomechanical transduction

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Coherent Control of Low Anharmonicity Systems for Superconducting Quantum Computing

This dissertation describes research to coherently control quantum states of superconducting devices. In the first project, the state of an 8 GHz 3D superconducting Al cavity at 20mK was manipulated to add a quantum of excitation. Preparing a harmonic resonator in a state with a well-defined number of excitations (Fock states) is not possible using one external classical drive. I generated Fock states by transferring a single excitation from a 5.5 GHz transmon qubit to a cavity using Stimulated Raman Adiabatic Passage (STIRAP). I also extended the STIRAP technique to put the cavity in higher Fock states, superpositions of Fock states, and Bell states between the qubit and the cavity. Master-equation simulations of the system’s density matrix were in good agreement with the data, and I obtained estimated fidelities of 89%, 68% and 43% for the first three Fock states, respectively. The second project involved implementing an entangling gate between two Al/AlOx/Al transmon qubits that were mounted in an Al cavity and cooled to 20mK. Pertinent system frequencies were as follows: one qubit was at 6.0 GHz, the other qubit at 6.8 GHz, the cavity at 7.7 GHz, and the qubit-qubit dispersive shift was -1MHz. By applying a specially-shaped pulse of duration tg = 907ns, I implemented a generalized CNOT gate using an all-microwave technique known as Speeding up Waveforms by Inducing Phases to Harmful Transitions (SWIPHT). Using quantum process tomography, I found that the gate fidelity was 80%–82%, close to the 87% fidelity expected from decoherence in the transmons during the gate time. Details of the device fabrication, device characterization, measurement techniques, and extensive modeling of device behavior are presented, along with chi-matrix characterization of single-qubit gates and SWIPHT gates

Superconducting qubits for quantum annealing applications

Over the last two decades, Quantum Annealing (QA) has grown to be a commercial technology with machines reaching the scale of 5000 interconnected qubits. Two reasons for this progress are the relative ease of implementing adiabatic Hamiltonian control and QA’s partial robustness against errors caused by decoherence. Despite the success of this approach to quantum computation, proving a scaling advantage over classical computation remains an elusive goal to this date. Different strategies are therefore being considered to boost the performance of quantum annealing. These include using more coherent qubit architectures and error-suppression to limit the effect of environmental noise, implementing non-stoquastic driver terms and tailored annealing schedules to enhance the success probability of the algorithm, and using many-body couplers to embed higher-order binary optimisation problems with less resource overhead. This thesis contributes to these efforts in two different ways. The first part provides a detailed numerical analysis and a physical layout for a threebody coupler for flux qubits based on ancillary spins. The application of the coupler in a coherence-signature QA Hamiltonian is also considered and the results of the simulated quantum evolution are compared to the outcomes of classical optimisation on the problem Hamiltonian showing that the classical algorithms cannot correctly reproduce the state distribution at the end of QA. In the second part of the thesis, we develop a numerical method for mapping the Hamiltonian of a composite superconducting circuit to an effective many-qubit Hamiltonian. By overcoming drawbacks of standard reduction methods, this protocol can be used to guide the design of non-stoquastic and many-body Hamiltonian terms, as well as to get a more precise evaluation of the QA schedule parameters, which can greatly improve the outcomes of the optimisation. This numerical work is accompanied by a proposal for an experimental verification of the predictions of the reduction protocol and by some preliminary experimental results

QUANTUM CONTROL AND MEASUREMENT ON FLUXONIUMS

Superconducting circuit is a promising platform for quantum computing and quantum simulation. A number of efforts have been made to explore the physics in transmon systems and optimize the qubit performance. Compared to transmon, fluxonium is a relatively new type of qubit and attracts more attention recently due to its high coherence time and large anharmonicity. In this thesis, we summarize recent progress toward high fidelity two-qubit gate and readout for fluxonium qubits. We report improved fluxonium coherence either in cavity or cavityless environment. In the former case, we demonstrate single-shot joint readout for two fluxonium qubits and explore various two-qubit gate schemes such as controlled-Z(CZ) gate, controlled-phase(CP) gate, bSWAP gate and cross-resonance(CR) gate. The CZ gate realized by near-resonantly driving the high transitions exhibits 99.2% fidelity from randomized benchmarking. A continuous CP gate set can be implemented by off-resonantly driving the high transitions and shows an average 99.2% fidelity from the cross-entropy benchmarking technique. Other gates involving only computational states are also explored to further improve the gate fidelity, which can take advantage of the high coherence of the fluxonium lower levels. In the cavityless environment, we demonstrate fluorescence shelving readout with 1.7 MHz radiative decay rate for the readout transition while maintaining 52 us coherence time for the qubit transition. Our research explores the basic elements for fluxonium-based quantum processors. The results suggest that fluxonium can be an excellent candidate for not only universal quantum computation but also quantum network and quantum optics studies

Understanding Quantum Technologies 2022

Understanding Quantum Technologies 2022 is a creative-commons ebook that provides a unique 360 degrees overview of quantum technologies from science and technology to geopolitical and societal issues. It covers quantum physics history, quantum physics 101, gate-based quantum computing, quantum computing engineering (including quantum error corrections and quantum computing energetics), quantum computing hardware (all qubit types, including quantum annealing and quantum simulation paradigms, history, science, research, implementation and vendors), quantum enabling technologies (cryogenics, control electronics, photonics, components fabs, raw materials), quantum computing algorithms, software development tools and use cases, unconventional computing (potential alternatives to quantum and classical computing), quantum telecommunications and cryptography, quantum sensing, quantum technologies around the world, quantum technologies societal impact and even quantum fake sciences. The main audience are computer science engineers, developers and IT specialists as well as quantum scientists and students who want to acquire a global view of how quantum technologies work, and particularly quantum computing. This version is an extensive update to the 2021 edition published in October 2021.Comment: 1132 pages, 920 figures, Letter forma