14 research outputs found
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 , where is the number
of qubits and 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 blĂŒhenden 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 beschĂ€ftigen. Als physikalische Plattform um Quanteninformation zu kodieren, dienen 40Ca+-Ionen, gefangen in einer makroskopischen, linearen Paulfalle, die mit Hilfe von Laserlicht kohĂ€rent manipuliert werden können. Aufbauend auf einem existierenden Experiment, wurden Techniken entwickelt und angewandt um lange Ionenketten kontrolliert zu manipulieren. Hierbei werden die Möglichkeiten des gegenwĂ€rtigen Aufbaus aufgezeigt. Zwei der Experimente, die in dieser Arbeit vorgestellt werden, beschĂ€ftigen sich mit der Quantensimulation von wechselwirkenden Vielteilchensystemen. Im ersten der beiden Experimente geht es um die erstmalige Beobachtung wie sich VerschrĂ€nkung in einem solchen Vielteilchensystem ausbreitet. Des Weiteren wurde die AbhĂ€ngigkeit dieser Ausbreitung fĂŒr unterschiedliche WechselwirkungslĂ€ngen 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-ZustĂ€nde, erstmalig deterministisch erzeugt. DarĂŒber hinaus wurden Cluster-ZustĂ€nde unterschiedlicher GröĂe im Hinblick auf Fehlerkorrekturcodes erzeugt, die, unseres Wissens nach, erstmalig nachweisen, dass lĂ€ngere Codewörter in der Tat Quanteninformation besser beschĂŒtzen können, trotz der höheren KomplexitĂ€t bei deren Herstellung. Das vierte und letzte Experiment erforscht eine grundlegende Art von Quantenkorrelationen in gemischten ZustĂ€nden, die man auch als âQuanten-Zwietrachtâ (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 bezĂŒglich langer Ionen-Ketten erlĂ€utert und wenn möglich, werden Lösungen diskutiert. DarĂŒber hinaus werden offene Fragen vorgestellt, die in Zukunft untersucht werden mĂŒssen, um ein besseres VerstĂ€ndnis der Limitierungen zu erhalten. Zum Abschluss folgt ein kurzer Ausblick auf mögliche Verbesserungen des experimentellen Aufbaues und Ideen fĂŒr zukĂŒnftige 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 Ăbersetzung der Verfasserin/des VerfassersZusammenfassung in deutscher SpracheUniversity of Innsbruck, Dissertation, 2017OeBB(VLID)169911