Environment-assisted quantum transport in a 10-qubit network

The way in which energy is transported through an interacting system governs fundamental properties in many areas of physics, chemistry, and biology. Remarkably, environmental noise can enhance the transport, an effect known as environment-assisted quantum transport (ENAQT). In this paper, we study ENAQT in a network of coupled spins subject to engineered static disorder and temporally varying dephasing noise. The interacting spin network is realized in a chain of trapped atomic ions and energy transport is represented by the transfer of electronic excitation between ions. With increasing noise strength, we observe a crossover from coherent dynamics and Anderson localization to ENAQT and finally a suppression of transport due to the quantum Zeno effect. We found that in the regime where ENAQT is most effective the transport is mainly diffusive, displaying coherences only at very short times. Further, we show that dephasing characterized by non-Markovian noise can maintain coherences longer than white noise dephasing, with a strong influence of the spectral structure on the transport effciency. Our approach represents a controlled and scalable way to investigate quantum transport in many-body networks under static disorder and dynamic noise.Comment: Mai

Estimation of the Quantum Fisher Information on a quantum processor

The quantum Fisher information (QFI) is a fundamental quantity in quantum physics and is central to the field of quantum metrology. It certifies quantum states that have useful multipartite entanglement for enhanced metrological tasks. Thus far, only lower bounds with finite distance to the QFI have been measured on quantum devices. Here, we present the experimental measurement of a series of polynomial lower bounds that converge to the QFI, done on a quantum processor. We combine advanced methods of the randomized measurement toolbox to obtain estimators that are robust against drifting errors caused uniquely during the randomized measurement protocol. We estimate the QFI for Greenberger-Horne-Zeilinger states, observing genuine multipartite entanglement and the Heisenberg limit attained by our prepared state. Then, we prepare the ground state of the transverse field Ising model at the critical point using a variational circuit. We estimate its QFI and investigate the interplay between state optimization and noise induced by increasing the circuit depth.Comment: 24 pages, 13 figure

Observation of Entangled States of a Fully Controlled 20-Qubit System

We generate and characterise entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the out-of-equilibrium dynamics of an Ising-type Hamiltonian, engineered via laser fields. Since the qubit-qubit interactions decay with distance, entanglement is generated at early times predominantly between neighbouring groups of qubits. We characterise entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighbouring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.Comment: 20 pages, 4 figure

Single-shot error mitigation by coherent Pauli checks

Generating samples from the output distribution of a quantum circuit is a ubiquitous task used as a building block of many quantum algorithms. Here we show how to accomplish this task on a noisy quantum processor lacking full-blown error correction for a special class of quantum circuits dominated by Clifford gates. Our approach is based on Coherent Pauli Checks (CPCs) that detect errors in a Clifford circuit by verifying commutation rules between random Pauli-type check operators and the considered circuit. Our main contributions are as follows. First, we derive a simple formula for the probability that a Clifford circuit protected by CPCs contains a logical error. In the limit of a large number of checks, the logical error probability is shown to approach the value ${\approx}7\epsilon n/5$, where $n$ is the number of qubits and $\epsilon$ is the depolarizing error rate. Our formula agrees nearly perfectly with the numerical simulation results. Second, we show that CPCs are well-suited for quantum processors with a limited qubit connectivity. For example, the difference between all-to-all and linear qubit connectivity is only a 3X increase in the number of CNOT gates required to implement CPCs. Third, we describe simplified one-sided CPCs which are well-suited for mitigating measurement errors in the single-shot settings. Finally, we report an experimental demonstration of CPCs with up to 10 logical qubits and more than 100 logical CNOT gates. Our experimental results show that CPCs provide a marked improvement in the logical error probability for the considered task of sampling the output distribution of quantum circuits.Comment: 30 pages, 20 figure

Demonstration of quantum volume 64 on a superconducting quantum computing system

We improve the quality of quantum circuits on superconducting quantum computing systems, as measured by the quantum volume, with a combination of dynamical decoupling, compiler optimizations, shorter two-qubit gates, and excited state promoted readout. This result shows that the path to larger quantum volume systems requires the simultaneous increase of coherence, control gate fidelities, measurement fidelities, and smarter software which takes into account hardware details, thereby demonstrating the need to continue to co-design the software and hardware stack for the foreseeable future.Comment: Fixed typo in author list. Added references [38], [49] and [52

Quantum computation and many-body physics with trapped ions

In den letzten zwei Dekaden hat sich die Quanteninformationswissenschaft zu einem blühenden Fachgebiet entwickelt, in dem grossartige Fortschritte, von theoretischer und experimenteller Seite her, in Richtung kommerziellen Anwendungen stattgefunden haben. In dieser Dissertationsschrift werden vier Experimente vorgestellt, die sich mit unterschiedlichen Aspekten der Quanteninformationswissenschaft beschäftigen. Als physikalische Plattform um Quanteninformation zu kodieren, dienen 40Ca+-Ionen, gefangen in einer makroskopischen, linearen Paulfalle, die mit Hilfe von Laserlicht kohärent manipuliert werden können. Aufbauend auf einem existierenden Experiment, wurden Techniken entwickelt und angewandt um lange Ionenketten kontrolliert zu manipulieren. Hierbei werden die Möglichkeiten des gegenwärtigen Aufbaus aufgezeigt. Zwei der Experimente, die in dieser Arbeit vorgestellt werden, beschäftigen sich mit der Quantensimulation von wechselwirkenden Vielteilchensystemen. Im ersten der beiden Experimente geht es um die erstmalige Beobachtung wie sich Verschränkung in einem solchen Vielteilchensystem ausbreitet. Des Weiteren wurde die Abhängigkeit dieser Ausbreitung für unterschiedliche Wechselwirkungslängen untersucht. Das nachfolgende Experiment befasst sich mit einer neu-entwickelten spektroskopischen Methode, um ebendiese wechselwirkenden Systeme auf ihre Eigenschaften zu untersuchen. Der Fokus des dritten Experimentes liegt auf einem spezifischen Model der Quantenrechnung, das sogenannte messbasierte Quantenrechner-Model. Dabei wurden die grundlegenden Bausteine des messbasierten Quantenrechners, sogenannte Cluster-Zustände, erstmalig deterministisch erzeugt. Darüber hinaus wurden Cluster-Zustände unterschiedlicher Größe im Hinblick auf Fehlerkorrekturcodes erzeugt, die, unseres Wissens nach, erstmalig nachweisen, dass längere Codewörter in der Tat Quanteninformation besser beschützen können, trotz der höheren Komplexität bei deren Herstellung. Das vierte und letzte Experiment erforscht eine grundlegende Art von Quantenkorrelationen in gemischten Zuständen, die man auch als “Quanten-Zwietracht” (Quantum discord) kennt. Hier wird die Frage untersucht, wie Quanten-Zwietracht durch unterschiedliches Rauschen erzeugt werden kann, genauer gesagt durch Amplitudenzerfall und korreliertes Magnetfeldrauschen. Hierbei wurde die Quanten-Zwietracht durch unterschiedliche Metriken quantifiziert. Im letzten Kapitel dieser Arbeit werden die Limitierungen des derzeitigen Aufbaues bezüglich langer Ionen-Ketten erläutert und wenn möglich, werden Lösungen diskutiert. Darüber hinaus werden offene Fragen vorgestellt, die in Zukunft untersucht werden müssen, um ein besseres Verständnis der Limitierungen zu erhalten. Zum Abschluss folgt ein kurzer Ausblick auf mögliche Verbesserungen des experimentellen Aufbaues und Ideen für zukünftige Projekte werden vorgestellt.Over the last two decades, quantum information science has made significant progress, both theoretically and experimentally, evolving into a prosperous field with potential commercial applications within the next decade. This PhD thesis reports on four different experiments, performed over the last few years, all investigating various aspects of quantum information science. 40Ca+-ions trapped in a macroscopic, linear Paul trap serve as a qubits encoding quantum information that are coherently manipulated with laser light fields. These four experiments utilise an existing experimental arrangement, adapted to allow coherent manipulation of long ion strings, thus demonstrating the capabilities of the current setup. Two of the experiments presented in this thesis are focused on quantum simulations of interacting many-body systems. In the first experiment, the propagation of entanglement in such a many-body system is experimentally observed for the very first time. Additionally, the systems response is investigated as the spatial range of the interactions is tuned. The following experiment employs a novel spectroscopic method for probing these interacting many-body systems. The third experiment focuses on quantum computation, specifically the measurement-based quantum computation approach. Here, the deterministic generation of cluster states in trapped ions is demonstrated for the first time. Moreover, certain cluster states are used to implement error correction codes of different sizes, granting, to the authors knowledge, the first experimental evidence that larger code words are indeed capable of better protecting quantum information, despite the higher complexity of their preparation. The fourth and last experiment explores a type of quantum correlation present in mixed states, known as quantum discord. The generation of quantum discord via two different, noisy processes -that is, amplitude damping and correlated magnetic field noise - is investigated, and the generated discord is quantified by different measures. In the last part of this thesis, the limitations of the current setup are presented and, if possible, potential solutions are discussed. Furthermore, open questions encountered in the experimental setup are addressed for future investigations in order to obtain a better understanding of further limitations. A brief outlook on possible improvements to the experimental setup, as well as ideas for future projects, conclude this manuscript.by Petar JurcevicAbweichender Titel laut Übersetzung der Verfasserin/des VerfassersZusammenfassung in deutscher SpracheUniversity of Innsbruck, Dissertation, 2017OeBB(VLID)169911