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1
Nasser, Metwally Aly Mohamed. "Entangled qubit pairs". Diss., [S.l.] : [s.n.], 2002. http://edoc.ub.uni-muenchen.de/archive/00000083.
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Fay, Aurélien. "Couplage variable entre un qubit de charge et un qubit de phase". Phd thesis, Université Joseph Fourier (Grenoble), 2008. http://tel.archives-ouvertes.fr/tel-00310131.
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Nous avons étudié la dynamique quantique d'un circuit supraconducteur constitué d'un SQUID dc couplé à un transistor à paires de Cooper fortement asymétrique (ACPT). Le SQUID dc est un qubit de phase contrôlé par un courant de polarisation et un champ magnétique. L'ACPT est un qubit de charge contrôlé par un courant de polarisation, un champ magnétique et une tension de la grille.Nous avons mesuré par spectroscopie micro-onde les premiers niveaux d'énergie du circuit couplé en fonction des paramètres de contrôle. Les mesures des états quantiques des qubits de charge et de phase sont réalisées par une mesure d'échappement du SQUID dc avec une impulsion de flux nanoseconde appliquée dans celui-ci. La mesure de l'ACPT utilise un nouveau processus quantique : l'état excité de l'ACPT est transféré adiabatiquement vers l'état excité du SQUID durant l'impulsion de flux.
Notre circuit permet de manipuler indépendamment chaque qubit tout comme il permet d'intriquer les états quantiques des deux circuits. Nous avons observé des anti-croisements des niveaux d'énergie des deux qubits lorsqu'ils sont mis en résonance. Le couplage a été mesuré sur une large gamme de fréquence, pouvant varier de 60 MHz à 1.1 GHz. Nous avons réussi à obtenir un couplage variable entre le qubit de charge et le qubit de phase. Nous avons analysé théoriquement la dynamique quantique de notre circuit. Cette analyse a permis de bien expliquer le couplage variable mesuré par une combinaison entre un couplage Josephson et un couplage capacitif entre les deux qubits.
3
Fay, Aurélien. "Couplage variable entre un qubit de charge et un qubit de phase". Phd thesis, Grenoble 1, 2008. http://www.theses.fr/2008GRE10071.
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Nous avons étudié la dynamique quantique d'un circuit supraconducteur constitué d'un SQUID dc couplé à un transistor à paires de Cooper fortement asymétrique (ACPT). Le SQUID dc est un qubit de phase contrôlé par un courant de polarisation et un champ magnétique. L'ACPT est un qubit de charge contrôlé par un courant de polarisation, un champ magnétique et une tension de la grille. Nous avons mesuré par spectroscopie micro-onde les premiers niveaux d'énergie du circuit couplé en fonction des paramètres de contrôle. Les mesures des états quantiques des qubits de charge et de phase sont réalisées par une mesure d'échappement du SQUID dc avec une impulsion de flux nanoseconde appliquée dans celui-ci. La mesure de l'ACPT utilise un nouveau processus quantique : l'état excité de l'ACPT est transféré adiabatiquement vers l'état excité du SQUID durant l'impulsion de flux. Notre circuit permet de manipuler indépendamment chaque qubit tout comme il permet d'intriquer les états quantiques des deux circuits. Nous avons observé des anti-croisements des niveaux d'énergie des deux qubits lorsqu'ils sont mis en résonance. Le couplage a été mesuré sur une large gamme de fréquence, pouvant varier de 60 MHz à 1. 1 GHz. Nous avons réussi à obtenir un couplage variable entre le qubit de charge et le qubit de phase. Nous avons analysé théoriquement la dynamique quantique de notre circuit. Cette analyse a permis de bien expliquer le couplage variable mesuré par une combinaison entre un couplage Josephson et un couplage capacitif entre les deux qubitsWe have studied the quantum dynamics of a superconducting circuit based on a dc-SQUID coupled to a highly asymmetric Cooper pair transistor (ACPT). The dc-SQUID is a phase qubit controlled by a bias current and magnetic field. The ACPT is a charge qubit controlled by a bias current, magnetic flux and gate voltage. We have measured by microwave spectroscopy the lowest quantum levels of the coupled circuit as a function of the bias parameters. Quantum state measurements of the phase and charge qubit are achieved by an escape measurement on the dc SQUID with a nanosecond flux pulse applied to it. The measurement of the ACPT state consist of a new quantum process: the excited state of the ACPT is adiabatically transferred to the excited state of the SQUID during the flux pulse. Our circuit enables the independent manipulation of each qubit as well as the entanglement of the quantum states of the two circuits. We observe avoided level crossings between the two qubits when they are put in resonance. The coupling strength is measured over a large frequency range and varies from 60 MHz to 1. 1 GHz. In this coupled circuit, we succeed to realize a tunable coupling between the charge and the phase qubit. We have analyzed theoretically the quantum dynamics of our circuit. This analysis explains well the measured tunable coupling strength by a combination of a capacitive and a Josephson coupling between the two qubits
4
Palomaki, Tauno A. "Dc SQUID phase qubit". College Park, Md.: University of Maryland, 2008. http://hdl.handle.net/1903/8575.
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Thesis (Ph. D.) -- University of Maryland, College Park, 2008.Thesis research directed by: Dept. of Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
5
Viehmann, Oliver. "Multi-qubit circuit quantum electrodynamics". Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-160998.
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Aiello, Clarice Demarchi. "Qubit dynamics under alternating controls". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/93053.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 111-117).
In this thesis, we discuss two problems of quantum dynamics in the presence of alternating controls. Alternating controls arise in many protocols designed to extend the duration over which a qubit is a useful computational resource. This is accomplished by control sequences that either retard decoherence, or that accomplish a quantum operation in as short a time as possible. The first problem tackles the use of a composite-pulse control sequence known as 'rotary-echo' for quantum magnetometry purposes. The sequence consists in the continuous drive of a qubit, with field phases that alternate at specific intervals. We implement such a magnetometry protocol using an electronic qubit in diamond, and experimentally confirm the flexibility yielded by the tuning of sequence parameters that achieves a good compromise between decoherence resilience and sensitivity. The second problem theoretically investigates the time-optimal evolution of a qubit in the case of a restricted control set composed of alternating rotations around two non-parallel axes on the Bloch sphere. Using accessible algebraic methods, we show that experimental parameters, such as the angle between the two rotation axes, restrict the necessary structure of time-optimal sequences. We propose to implement such an evolution through alternate driving as an advantageous alternative to the slow, noisy direct addressing of a nuclear qubit anisotropically hyperfine-coupled to an electronic spin in diamond.
by Clarice Demarchi Aiello.
Ph. D.
7
Convertini, Luciana. "Simulazione numerica di qubit a superconduttori di tipo transmon: dal layout al gate a singolo qubit". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2022.
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In questo lavoro di tesi è stato messo a punto un insieme di modelli e strumenti di simulazione numerica che consentono nel loro insieme l’analisi completa di un sistema formato da qubit a superconduttore di tipo transmon per il calcolo quantistico. Appoggiandosi agli elementi noti della teoria del transmon, l’analisi parte dalla definizione del layout da cui, mediante simulazioni elettromagnetiche, si estraggono le matrici di capacità ed induttanza dei vari componenti. Questo consente la costruzione dell’Hamiltoniano del sistema, noto il quale si possono eseguire le simulazioni nel dominio del tempo delle operazioni dei vari gate, una volta definiti gli opportuni segnali di controllo. Per l’implementazione del flusso di simulazione si sono utilizzati sia strumenti software open-source (Qiskit Metal, FastCap, QuTip), sia script Python sviluppati allo scopo. A dimostrazione della funzionalità dell’ambiente così creato viene presentata l’analisi di un transmon accoppiato a risonatori e linee di trasmissione, tramite il quale si realizza un quantum gate a singolo qubit di tipo X.
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Narla, Anirudh. "Flying Qubit Operations in Superconducting Circuits". Thesis, Yale University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10783459.
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The quantum non-demolition (QND) measurement process begins by entangling the system to be measured, a qubit for example, with an ancillary degree of freedom, usually a system with an infinite-dimensional Hilbert space. The ancilla is amplified to convert the quantum signal into a measurable classical signal. The continuous classical signal is recorded by a measurement apparatus; a discrete measurement outcome is recovered by thresholding the integrated signal record. Measurements play a central role in technologies based on quantum theory, like quantum computation and communication. They form the basis for a wide range of operations, ranging from state initialization to quantum error correction. Quantum measurements used for quantum computation must satisfy three essential requirements of being high fidelity, quantum non-demolition and efficient. Satisfying these criteria necessitates control over all the parts of the quantum measurement process, especially generating the ancilla, entangling it with the qubit and amplifying it to complete the measurement.
For superconducting quantum circuits, a promising platform for realizing quantum computation, a natural choice for the ancillae are modes of microwave-frequency electromagnetic radiation. In the paradigm of circuit quantum electrodynamics (cQED) with three-dimensional circuits, the most commonly used ancillae are coherent states, since they are easy to generate, process and amplify. Using these flying coherent states, we present results for achieving QND measurements of transmon qubits with fidelities of F> 0.99 and efficiencies of η = 0.56 ± 0.01. By also treating the measurement as a more general quantum operation, we use the ancillae as carriers of quantum information to generate remote entanglement between two transmon qubits in separate cavities. By using microwave single photons as the flying qubits, it is possible to generate remote entanglement that is robust to loss since the generation of entanglement is uniquely linked to a particular measurement outcome. We demonstrate, in a single experiment, the ability to efficiently generate and detect single microwave photons and use them to generate robust remote entanglement between two transmon qubits. This operation forms a crucial primitive in modular architectures for quantum computation. The results of this thesis extend the experimental toolbox at the disposal to superconducting circuits. Building on these results, we outline proposals for remote entanglement distillation as well as strategies to further improve the performance of the various tools.
9
Weber, Steven Joseph. "Quantum Trajectories of a Superconducting Qubit". Thesis, University of California, Berkeley, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3686046.
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In quantum mechanics, the process of measurement is intrinsically probabilistic. As a result, continuously monitoring a quantum system will randomly perturb its natural unitary evolution. An accurate measurement record documents this stochastic evolution and can be used to reconstruct the quantum trajectory of the system state in a single experimental iteration. We use weak measurements to track the individual quantum trajectories of a superconducting qubit that evolves under the competing influences of continuous weak measurement and Rabi drive. We analyze large ensembles of such trajectories to examine their characteristics and determine their statistical properties. For example, by considering only the subset of trajectories that evolve between any chosen initial and final states, we can deduce the most probable path through quantum state space. Our investigation reveals the rich interplay between measurement dynamics, typically associated with wavefunction collapse, and unitary evolution. Our results provide insight into the dynamics of open quantum systems and may enable new methods of quantum state tomography, quantum state steering through measurement, and active quantum control.
10
Bader, Samuel James. "Higher levels of the transmon qubit". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/92701.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 91-95).
This thesis discusses recent experimental work in measuring the properties of higher levels in transmon qubit systems. The first part includes a thorough overview of transmon devices, explaining the principles of the device design, the transmon Hamiltonian, and general Circuit Quantum Electrodynamics concepts and methodology. The second part discusses the experimental setup and methods employed in measuring the higher levels of these systems, and the details of the simulation used to explain and predict the properties of these levels.
by Samuel James Bader.
S.B.
11
Tufarelli, T. "Qubit-controlled displacements in Markovian environments". Thesis, University College London (University of London), 2012. http://discovery.ucl.ac.uk/1344094/.
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Cammack, Helen Mary. "Coherence protection in coupled qubit systems". Thesis, University of St Andrews, 2018. http://hdl.handle.net/10023/16457.
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Decoherence is a major barrier to the implementation of quantum technologies. Theoretical techniques for understanding decoherence in composite systems have traditionally been focused on systems with distinguishable emission spectra, where measuring the frequency of an emitted photon allows one to determine which process took place. Here the photon contains information about the state of the system. On the other hand, systems with indistinguishable spectra do not necessarily completely reveal information about the state of the system when a photon is emitted. It can be impossible to say for certain which of two nearly degenerate transitions has occurred just by measuring the photon's frequency. It is then possible to preserve information within the system throughout the decay process. In this Thesis we show that indistinguishable spectra can lead to protected coherences within one part of a coupled quantum system, even as another part decays. We develop a zero-temperature exact approach for modelling such systems, and compare it to the microscopically derived Born-Markov master equation. This comparison helps us to understand the range of validity of the Markovian approximation. We use this understanding to extend the master equation approach to finite temperature within the Markovian regime, and we compare its high temperature results to a semiclassical model. We examine the physical conditions required for coherence protection, and remarkably we find that heating the system can improve coherence protection. Similarly, increasing the decay rate of the unprotected part of the coupled system can also enhance the coherence of the protected part. These effects are the results of linewidth broadening and thus greater spectral indistinguishability. The findings in this Thesis are of interest to both those seeking to engineer hybrid quantum systems and those seeking to develop theoretical techniques for dealing with the decoherence of composite quantum systems.
13
Kelly, Stephen C. "EXPLORATION OF QUBIT ASSISTED CAVITY OPTOMECHANICS". Miami University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=miami1408097717.
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Rubens, William. "On the black hole/qubit correspondence". Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/9594.
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We present the evidence for and intriguing black hole/qubit correspondence. This correspondence will map the entanglement classification of three and four qubits over to the BPS and extremal classification of black holes in the STU model. We will start by looking at BPS black holes and use a variety of means to classify them and calculate their orbits. We will discover that three qubits, or more accurately, three real qubits will exhibit exactly the same structure as the black holes. This will allow us to identify the entropy of the black hole with the entanglement of the qubits. A mathematical framework known as the Freudenthal triple system will be used to classify both systems. We will be able to use the wrapped branes picture of the black holes as an explanation of the binary nature of the qubit. We will then develop this correspondence further and use the mathematics of nilpotent orbits and the Kostant-Sekiguchi correspondence to directly map the classification of extremal black holes to the entanglement classification of four qubits. We will discover that the classification of four qubits is related to the distinct orbits that exists of the SL(2,C)4 on nilpotent (2, 2, 2, 2). We will also discover that the extremal black holes of the STU model correspond to nilpotent orbits of the Lie algebra so4,4. We will then use the Kostant-Sekiguchi correspondence as a diffeomorphism between these two types of orbits.
15
Wernz, Johannes. "Dekohärenz gekoppelter Spin- und Qubit-Systeme". [S.l. : s.n.], 2003. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB10761312.
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Perez, Veitia Andrzej. "Local Entanglement Generation in Two-Qubit Systems". Scholarly Repository, 2010. http://scholarlyrepository.miami.edu/oa_dissertations/476.
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We study the entanglement of two-qubit systems resulting from local interactions with spatially extended bosonic systems. Our results apply to the case where the initial state of the bosonic system is represented by a statistical mixture of states with fixed particle number. In particular, we derive and discuss necessary conditions to generate entanglement in the two-qubit system. We also study the scenario where the joint system is initially in its ground state and the interaction is switched on adiabatically. Using time independent perturbation theory and the adiabatic theorem, we show conditions under which the qubits become entangled as the joint system evolves into the ground state of the interacting theory
17
Keitch, Benjamin. "A Quantum Memory Qubit in Calcium-43". Thesis, University of Oxford, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487270.
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The hyperfine ground states of a 43Ca+ trapped ion are proposed as a memory qubit. The experimental details to implement such a qubit are presented. These include the design and implementation of a pulse-sequencer to produce pulse sequences for single qubit rotations and other timed experimental sequences. A Raman laser system running at 396nm with a frequency offset of 3.2GHz, based on optically injected violet diode lasers, is presented along with an analysis of its performance. A laser-locking scheme based on the Pound-Drever-Hall method has been built and the design and results are presented here. The dominant limitation on the coherence of the memory qubit, namely fluctuations in the ambient magnetic field, is studied and quantified. The coherence of a superposition of the ground hyperfine clock states of 43Ca+, 11) == SI/2(F =4, MF =0) and IT) == SI/2 (F = 3, MF = 0) is measured using the Ramsey technique. The coherence of the same state after implementing a spin-echo pulse sequence is also measured. The coherence time (T2) for the memory qubit, using the Ramsey technique is found to be 1.2(2)s. With the implementation of a spin-echo sequence, bounds are put on the spin-echo coherence-time as : 2 min ;S T.fE ;S 10 min. The difference in these two results leads to the conclusion that the main source of decoherence is due to magnetic field drift. The 43Ca+ memory qubit has a coherence time that is 104 times greater than current gate times, which is sufficient to reach a regime where fault-tolerant quantum information processing should be possible.
18
Low, Guang Hao. "Quantum signal processing by single-qubit dynamics". Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/115025.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 117-125).
Quantum computation is the most powerful realizable model of computation, and is uniquely positioned to solve specialized problems intractable to classical computers. This quantum advantage arises from directly exploiting the strangeness of quantum mechanics that is fundamental to reality. As such, one expects our understanding of quantum processes in physical systems to be indispensable to the design and execution of quantum algorithms. We present quantum signal processing, which exploits the dynamics of simple quantum systems to perform non-trivial computations. Such systems applied as computational modules in larger quantum algorithms, offer a natural physical alternative to standard tasks such as the calculation of elementary functions with integer arithmetic. The quantum advantage of this approach, based on simple physics, is of significant practical relevance. In cases, arbitrary bits of precision may be emulated using only constant space. Moreover, the simplicity and performance of quantum signal processing is such that it is the final missing ingredient for realizing a number of optimal quantum algorithms, particularly in Hamiltonian simulation. Quantum signal processing realizes a useful fusion of analog and digital models of quantum computation. At the physical level, we focus on how even a simple two-level system - the qubit, computes through optimal discrete-time quantum control. Whereas quantum control is typically used to synthesize unitary quantum gates, we solve the synthesis problem of unitary quantum functions with a fully characterization of achievable functions, and efficient techniques for their implementation. This furnishes a surprisingly rich framework in the analog model of quantum computation for computing functions. The generality of this model is realized by many applications, often with no modification, to quantum algorithms designed for digital quantum computers, in particular for matrix manipulation. In this manner, we solve a number of open problem related to optimal amplitude amplification algorithms, optimally computing on matrices with a quantum computer, and the simulation of physical systems.
by Guang Hao Low.
Ph. D.
19
Xiao, Jiayang. "Investigating Entanglement Transformations in Three-qubit States". OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1752.
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This thesis studies the manipulation of entanglement in three-qubit quantum systems. I consider the operational setting in which the qubits are distributed to three spatially separated parties. The parties act locally on their quantum systems and share classical communication with one another, a scenario commonly called local operations and classical communication (LOCC). In the LOCC setting, there are two different classes of entanglement in multipartite systems, called the GHZ and W classes, which are inequivalent in the sense that states from one class cannot be transformed into the other without the consumption of additional entanglement. In this thesis, I first show that the LOCC conversion of certain GHZ and W-class states becomes possible by using only one additional ebit (“entangled bit”) of shared entanglement. In many cases, this can be proven as the minimal amount of needed entanglement. I then consider the problem of one-way communication transformations from general three-qubit states into two-qubit maximally entangled states, known as EPR states. An optimal protocol in terms of success probability is provided for W-class states. The success probability of this protocol coincides with the optimal success probability if two of the parties are allowed to act jointly within the same laboratory. In other words, forcing the locality constraint on all three parties does not weaken their capabilities for obtaining bipartite entanglement when starting from a W-class state. I also present that this property holds for certain types of GHZ-class states as well.
20
Conway, Lamb Ian. "Cryogenic Control Beyond 100 Qubits". Thesis, The University of Sydney, 2016. http://hdl.handle.net/2123/17046.
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Quantum computation has been a major focus of research in the past two decades, with recent experiments demonstrating basic algorithms on small numbers of qubits. A large-scale universal quantum computer would have a profound impact on science and technology, providing a solution to several problems intractable for classical computers. To realise such a machine, today's small experiments must be scaled up, and a system must be built which provides control and measurement of many hundreds of qubits. A device of this scale is challenging: qubits are highly sensitive to their environment, and sophisticated isolation techniques are required to preserve the qubits' fragile states. Solid-state qubits require deep-cryogenic cooling to suppress thermal excitations. Yet current state-of-the-art experiments use room-temperature electronics which are electrically connected to the qubits. This thesis investigates various scalable technologies and techniques which can be used to control quantum systems. With the requirements for semiconductor spin-qubits in mind, several custom electronic systems, to provide quantum control from deep cryogenic temperatures, are designed and measured. A system architecture is proposed for quantum control, providing a scalable approach to executing quantum algorithms on a large number of qubits. Control of a gallium arsenide qubit is demonstrated using a cryogenically operated FPGA driving custom gallium arsenide switches. The cryogenic performance of a commercial FPGA is measured, as the main logic processor in a cryogenic quantum control system, and digital-to-analog converters are analysed during cryogenic operation. Recent work towards a 100-qubit cryogenic control system is shown, including the design of interconnect solutions and multiplexing circuitry. With qubit fidelity over the fault-tolerant threshold for certain error correcting codes, accompanying control platforms will play a key role in the development of a scalable quantum machine.
21
Silveri, M. (Matti). "Nonlinear and stochastic driving of a superconducting qubit". Doctoral thesis, University of Oulu, 2013. http://urn.fi/urn:isbn:9789526201184.
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Abstract
The topic of this thesis is superconducting electric circuits. Technical advances have made possible the experimental study of Josephson junction based circuit elements which sustain quantum mechanical properties long enough to be denoted as quantum devices. The quantum state can be controlled with electronic variables and measured using standard electrical setups. The research is motivated by the possibility to examine quantum phenomena in circumstances that can be customized, prospects of new quantum devices, and the development of quantum information processing.
This thesis presents theoretical studies on the nonlinear and stochastic driving of a superconducting quantum two-level system (qubit). We first investigate the energy level shifts a single-Cooper-pair transistor under large amplitude driving realized via the inherently nonlinear Josephson energy by using an external magnetic flux. The effective driving field substantially deviates from a circular polarization and linear coupling. The energy level shifts are compared to the cases of a vanishing and a weak driving field, measured as the Stark shift and the generalized Bloch-Siegert shift, respectively. We describe criteria for the natural basis of the analytical and the numerical calculations. In addition to that, we develop a formalism based on the Floquet method for the weak probe measurement of the strongly driven qubit.
In the latter part of the thesis research, we study utilization of a stochastic driving field whose time evolution is not regular but follows probabilistic laws. We concentrate on the motional averaging phenomenon and show that it can be measured with an unparalleled accuracy by employing a flux-modulated transmon qubit. As the stochastically modulated qubit is simultaneously measured with a moderate driving field, we develop a theoretical description accounting the possible interference effects between the modulation and the drive. The comparison with experimental results shows good agreement. Motional averaging phenomenon can be applied to estimate the properties of fluctuation processes occurring in qubits, e.g., the quasiparticle tunneling or the photon shot noise. Resting on the motional averaging, we anticipate that the qubit dephasing times can be improved if one can accelerate the dynamics of two-level fluctuators.
We apply a semiclassical formalism where the qubit is treated with quantum mechanical concepts whereas the driving fields are classical. In the solution procedure, the numerical results support the main analytical understanding. As the theoretical results are extensively compared to reflection measurements, we construct an explicit connection between the dynamics of the studied quantum devices and the measured reflection coefficient.
22
Tarlton, James Edward. "Probing qubit memory errors at the 10⁻⁵ level". Thesis, Imperial College London, 2018. http://hdl.handle.net/10044/1/60650.
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Trapped atomic ions are a promising candidate system for developing a quantum computer. All elementary operations for quantum information processing have al- ready been achieved in trapped-ion systems with errors below the “fault-tolerant threshold”, meaning that these errors are sufficiently small to be removed with quan- tum error correction. However, the preservation of quantum information over the short timescales that are relevant for quantum computing has not, to our knowledge, been measured before in trapped-ion qubits. Previous investigations have measured decoherence over long times and extrapolated an exponential decay model to short times. We directly measure the qubit memory error rate over short timescales, which is made possible by the high-fidelity single-qubit gates and state preparation and mea- surement in our system. Our qubit is a hyperfine “atomic clock” transition in 43Ca+, and gates are applied with near-field microwaves. We find that the assumption of exponential decay of qubit state coherence does not apply in our system. The data is fit well by a model of 1/f frequency noise with a low-frequency cutoff. The small- est memory error that we measure is 3(1)×10−6 over 2ms. The time at which our qubit memory error is 1×10−3, an important threshold for viable quantum error correction, is ≈400ms. We also present a design study into a new ion trap electrode geometry for applying near-field microwave two-qubit gates. This design features an ‘S’-shaped meander electrode to passively null the microwave field at the ions’ locations. It has improved isolation between the meander and other electrodes compared to previous work on this, which should reduce the sensitivity of the microwave field distribution to the boundary conditions of these electrodes. We show that the trap would allow for ion chains to be trapped, transported and split with feasible DC and RF voltages.
23
Waegell, Mordecai. "Nonclassical Structures within the N-qubit Pauli Group". Digital WPI, 2013. https://digitalcommons.wpi.edu/etd-dissertations/150.
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Structures that demonstrate nonclassicality are of foundational interest in quantum mechanics, and can also be seen as resources for numerous applications in quantum information processing - particularly in the Hilbert space of N qubits. The theory of entanglement, quantum contextuality, and quantum nonlocality within the N-qubit Pauli group is further developed in this thesis. The Strong Kochen-Specker theorem and the structures that prove it are introduced and explored in detail. The pattern of connections between structures that show entanglement, contextuality, and nonlocality is explained. Computational search algorithms and related tools were developed and used to perform complete searches for minimal nonclassical structures within the N-qubit Pauli group up to values of N limited by our computational resources. Our results are surveyed and prescriptions are given for using the elementary nonclassical structures we have found to construct more complex types of such structures. Families of nonclassical structures are presented for all values of N, including the most compact family of projector-based parity proofs of the Kochen-Specker theorem yet discovered in all dimensions of the form 2N, where N>=2. The applications of our results and their connection with other work is also discussed.
24
Lee, Janice C. (Janice Cheng-Yee) 1978. "Resonant readout of a superconducting persistent current qubit". Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/37845.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 211-218).
Superconducting Josephson junction devices rank among the best candidates for realizing a quantum computer. While the coherent control of quantum dynamics has been demonstrated in these solid-state, macroscopic quantum systems, a major challenge has been to increase the coherence times for these qubits. With an objective to reduce the level of readout-induced decoherence, this thesis work focuses on a resonant readout scheme developed for a niobium persistent-current (PC) qubit. This non-dissipative readout approach detects the flux state of the qubit by sensing a change in the Josephson inductance of a SQUID magnetometer. By incorporating the SQUID inductor in a high-Q resonant circuit, we distinguished the flux states of the qubit as a shift in the resonant frequency at 300 mK. The nonlinearity due to the Josephson inductance has characteristic effects on the resonant behavior of the readout circuit. We observed novel manifestation of this nonlinearity given the high quality factor of the resonance. The readout circuit was characterized in the linear as well as the nonlinear regime for its potential use as a bifurcation amplifier. Numerical simulations based on Josephson-junction circuits were also performed to understand the observed nonlinearity in the resonant behavior.
by Janice C. Lee.
Ph.D.
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Tian, Lin 1971. "A superconducting flux QuBit : measurement, noise and control". Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/8483.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.Includes bibliographical references (p. 197-213).
A superconducting quantum bit can be made with three nano-scale Josephson junctions connected in series. In this thesis, various aspects of this qubit are studied. It is shown numerically that the qubit behaves as a giant magnetic dipole with quantum tunneling between the two qubit states. The natural coupling between qubits plus the manipulation on a single qubit state provides the building blocks for universal quantum computing. The state of the qubit can be determined by measuring its flux with a SQUID. The measurement efficiency and measurement-induced decoherence are investigated. A coherent transition assisted scheme is designed for a projective measurement on the qubit. A general method is developed to study qubit decoherence by environmental noise. The dynamic control approach is applied for preventing off-resonant leakage during gate operations and for de-coupling the qubit from noise.
by Lin Tian.
Ph.D.
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Berns, David Marc. "Large amplitude driving of a persistent current qubit". Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45451.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 181-190).
In this thesis, the persistent current qubit in the presence of large amplitude microwave radiation is studied. Three main results are presented in this work. A new coherent quasi classical regime has been observed, where coherent quantum dynamics persist even while transitions between energy levels are caused by many photon modes simultaneously. A new theoretical treatment of this regime has been developed, and remarkable agreement between theory and experiment is observed. Also presented is a novel application of strong driving, where unwanted excited state population is cooled to the ground state by utilizing a second avoided crossing. This method of cooling, via a third, ancillary qubit level, is analogous to atomic sideband cooling. Cooling from 400mK to 3mK has been achieved. Finally, a new type of spectroscopy is presented, where an entire manifold of quantum levels is characterized with a single driving frequency, by studying the amplitude dependence of the qubit's behavior. Characterization of energy level spacings reaching 120GHz is achieved with radiation on the order of 0.1GHz.
by David Marc Berns.
Ph.D.
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Cornelio, Marcio Fernando. "Estados emaranhados quânticos tri-partidos com um qubit". Universidade de São Paulo, 2008. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-01092008-161014/.
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Estudamos o emaranhamento quântico de estados puros emaranhados tri-partidos quando uma das partes é um qubit. Apresentamos um método para encontrar as decomposições do estado tri-partido mais simples do que sucessivas decomposições de Schmidt. Esse método permite encontrar uma grande quantidade de diferentes famílias de estados emaranhados tri-partidos. Essas famílias são classificadas de acordo com dimensão dos blocos de Jordan de uma matriz obtida do estado emaranhado. Além disso, também demonstramos que estados pertencentes a famílias distintas não podem ser convertidos um no outro por operações locais estocásticas com comunicação clássica (SLOCC). No caso de dois estados pertencentes à mesma família, obtemos condições necessárias e su cientes para sabermos se estes podem ser convertidos um no outro por SLOCC. No caso de serão, também podemos obter a operação do tipo SLOCC que realiza a conversão.We study the quantum entanglement of tripartite pure states when one of the parties is a qubit. We present a method to find the decompositions of tripartite entangled states which are simpler than two successive Schmidt decompositions. We will find many distinct families of entangled states with distinct decompositions. These families are classified according to the dimensions of the Jordan blocks of a matrix obtained from the entangled state. Furthermore, we show that states belonging to distinct families can not be converted into each other by stochastic local operations and classical communication (SLOCC). In case of two states belonging to the same family, we nd necessary and su?cient conditions to convert one state to the other. We can also find the SLOCC which realizes this conversion.
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Ткачов, В. А. "11 qubit quantum computers with cloud computing access". Thesis, Київський національний університет технологій та дизайну, 2018. https://er.knutd.edu.ua/handle/123456789/10795.
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Schauer, Floyd [Verfasser], i Dominique [Akademischer Betreuer] Bougeard. "Realizing spin qubits in 28Si/SiGe: heterostructure gating, qubit decoherence and asymmetric charge sensing / Floyd Schauer ; Betreuer: Dominique Bougeard". Regensburg : Universitätsbibliothek Regensburg, 2021. http://d-nb.info/1225935849/34.
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Cerfontaine, Pascal [Verfasser], Jörg Hendrik [Akademischer Betreuer] Bluhm i David P. [Akademischer Betreuer] DiVincenzo. "High-fidelity single- and two-qubit gates for two-electron spin qubits / Pascal Cerfontaine ; Jörg Hendrik Bluhm, David P. DiVincenzo". Aachen : Universitätsbibliothek der RWTH Aachen, 2019. http://d-nb.info/1211487806/34.
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Friedrich, Marion. "Vom Neuron zum Qubit auf den Spuren des Bewusstseins". Marburg Tectum-Verl, 2007. http://deposit.d-nb.de/cgi-bin/dokserv?id=3034725&prov=M&dok_var=1&dok_ext=htm.
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Friedrich, Marion. "Vom Neuron zum Qubit : auf den Spuren des Bewusstseins /". Marburg : Tectum-Verl, 2008. http://deposit.d-nb.de/cgi-bin/dokserv?id=3034725&prov=M&dokv̲ar=1&doke̲xt=htm.
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Olaya-Castro, Alexandra. "Dynamics of quantum correlations in multi-qubit-cavity systems". Thesis, University of Oxford, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.419329.
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Cooper, Rachel S. G. "A study of magnetic interactions and potential qubit synthesis". Thesis, University of Manchester, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492932.
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Since the discovery that it is possible for single molecules to retain their magnetisation in the absence of a magnetic field, this area of research has been considered to be of immense interest research groups from around the world have been attempting to reproduce this phenomenon using a wide range of transition metals and different bridging ligands, each group utilising different synthetic routes in an attempt to produce large nuclearity clusters with large negative spin anisotropy.
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Griffith, Elias James. "Characterisation and Rapid Purification of a Superconducting Charge Qubit". Thesis, University of Liverpool, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485993.
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The work of this thesis focuses on stochastic methods for the simulation of superconducting charge qubits, also known as Cooper pair boxes, which are promising candidates for large scale quantum information processing. To aid non-physicists, a brief outline of the structure of quantum mechanics is provided using the· Dirac formalism. Using the so called 'Backreaction effect', we consider if any information can be obtained regarding the qubit behaviour, through detecting the changes observed in the frequency spectrum of the coupled biasing circuitry, modelled as a dissipative oscillator circuit. The process of modelling dissipative quantum systems is described, however an alternative approach called 'quantum trajectories' is used rather than the traditional Calderia-Leggett model, as the time evolution of a single 'trajectory' represents the evolution of an individual system coupled to a noisy environment. Through the noise generated by an excited qubit, the energy level structure of the qubit can be probed with a microwave drive field, by observing the noise power within the biasing circuit. We consider a biasing circuit of unusually high resonant frequency which can drive the qubit, this creates frequency splitting features that would not normally be observed. Weak measurement is also examined as this is closely related to the stochastic 'quantum trajectories', where the measurement is recorded by the observer rather than lost to an environment. Weakly measuring a qubit does not completely collapse it and therefore 'quantum feedback' may be employed to alter the qubit controls favourably. In particular we consider the problem of purifying a weakly measured system rapidly; given a qubit in the completely mixed state what is the best feedback to become confident in the actual qubit state quickly. There are two optimal feedback protocols proposed by Jacobs [1] and by Wiseman and Ralph [2] for purifying qubits that have ideal controls. However, we adapt these protocols for the charge qubit, whose finite Hamiltonian resources and non-zero ax tunnelling term means the Bloch vector can not be easily held in the optimal location.
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Neumann, Christian [Verfasser]. "Isotopically-enriched 28Si heterostructures for qubit devices / Christian Neumann". München : Verlag Dr. Hut, 2017. http://d-nb.info/1149579919/34.
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Farrell, Matthew W. "Nonadiabatic control of a superconducting qubit via strong driving". Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/44466.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (leaf 43).
Quantum computation offers the promise of speeding up many calculations that are intractable on classical computers, including but not limited to factoring and the simulation of quantum mechanical systems. Quantum computation is achieved by replacing the bits of a classical computer with quoits. Qubits generalize bits by allowing not only the classical states of zero and one, but also any arbitrary superposition of zero and one. These qubits are implemented as two-state systems by mapping the classical one and zero states to two orthogonal quantum states. The qubits are then manipulated by varying the Hamiltonian of the two-state systems with time. The standard method to manipulate a two-state system is to drive it weakly using Rabi dynamics. This approach is ineffective for a large scale quantum computer because the rotation is slow, and decoherence breaks the fragile state before the computation can be completed. To address this problem, we developed a method to rapidly rotate a qubit by an arbitrary angle. This is achieved by abandoning Rabi oscillations, and instead using a strong, rapidly changing field to coherently rotate the spin. We rapid drive the system through an avoided crossing and back again by giving the on diagonal term of the Hamiltonian a parabolic time dependence. In this paper, I contrast the standard method of spin rotation via Rabi oscillations with our protocol. Then, I discuss the various numerical simulations used to evaluate our protocol. Finally, I present some experimental evidence suggesting the protocol will be effective when implemented. Then, I discuss experimental findings and computational results of our method. We found regions of parameter space that allow a qubit to be rapidly rotated by any angle from zero to nearly ~r. This new protocol for arbitrary qubit rotation is a significant improvement over techniques relying on Rabi oscillations, reducing the time needed to transition qubits.
(cont.) Our protocol deserves further study and refinement for its potential to speed up and, thusly, reduce the problem of decoherence in quantum computation.
by Matthew W. Farrell.
S.B.
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Chen, Zilong 1981. "Towards qubit noise spectroscopy by quantum bang-bang control". Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/32721.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, June 2004.Includes bibliographical references (p. 89-91).
Quantum bang-bang control is a method of suppressing decoherence in qubits [VKL99, VL98]. To date, mathematically rigorous treatments of quantum bang-bang control offered little intuition. To complement existing approaches and to seek better understanding, I present intuitive pictures to think about quantum bang-bang control. In addition, I develop a formalism for treating phase noise moments of a qubit under quantum bang-bang control. Although the desired purpose of quantum bang-bang control is to remove noise, it is conceivable that it can be used to infer information about the noise process and coupling on a qubit. By using a simple random rotation model of single qubit dephasing, I demonstrate how quantum bang-bang control can distinguish between dephasing under different stochastic processes. I also show how quantum bang-bang control can determine noise coupling in a toy model where noise couples to the qubit via a fixed noise axis. These two demonstrations indicate the potential of quantum bang-bang control as a tool for qubit noise spectroscopy.
by Zilong Chen.
S.B.
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Nakada, Daniel Yuki 1975. "Fabrication and measurement of a niobium persistent current qubit". Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/18049.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (p. 192-201).
Recent successes with superconducting Josephson junction qubits make them prime candidates for the implementation of quantum computing. This doctoral thesis details the study of a niobium Josephson junction circuit for quantum computing applications. The thesis covers two main areas: 1) the fabrication of sub-micron niobium Josephson junction devices using a Nb/Al/A1Ox/Nb trilayer process and 2) measurements of unique quantum properties of a superconducting device proposed as a quantum bit--the Persistent Current (PC) qubit. The thesis discusses the fabrication of niobium Josephson junction devices which is integral to the design and measurement of the circuit. The devices were fabricated at MIT Lincoln Laboratory using optical projection lithography to define features. A technique to produce more uniform critical-current densities across a wafer is developed within the scope of the thesis. We also introduce experimental work on the PC qubit performed at dilution refrigerator temperatures (T [approximately] 12mK). Microwave spectroscopy was used to map the energy level separation between macroscopic quantum states of the qubit system. We measured the intrawell energy relaxation time [tau]d between quantum levels in this particular device. The intrawell relaxation measurements are important in determining whether a promising decoherence time can be achieved in Nb-based Josephson devices, which has a more mature fabrication process compared to other superconducting fabrication processes.
by Daniel Yuki Nakada.
Ph.D.
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Strömberg, Philip, i Karlsson Vera Blomkvist. "4-qubit Grover's algorithm implemented for the ibmqx5 architecture". Thesis, KTH, Skolan för elektroteknik och datavetenskap (EECS), 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-229797.
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Quantum computing is a quickly growing field of research thanks to recent hardware advances. The quantum mechanical properties of quantum computers allow them to solve certain families of problems faster than classical computers. A quantum algorithm solving such a problem is Grover's algorithm, which finds an element in an unordered set faster than any classical search algorithm. In this paper an implementation of a 4-qubit Grover's algorithm for the IBM Q computer ibmqx5 is presented. Executing the implementation on an ibmqx5 simulator yield results in line with the theoretically optimal results. The accuracy of the ibmqx5 simulation results compared to the ibmqx5 execution results suggests that current hardware is not yet suitable,due to required complexity of the circuits for a 4-qubit Grover implementation.Kvantteknik är ett snabbt växande forskningsområde tack vare att nödvändig hårdvara förbättrats i rask takt. De kvantmekaniska egenskaperna hos kvantdatorer tillåter vissa familjer av problem att lösas snabbare på dessa än på klassiska datorer. En algoritm för kvantdatorer som löser ett sådant problem är Grover's algoritm, vilken hittar ett element i en oordnad mängd snabbare än vad någon sökalgoritm för klassiska datorer gör. I denna rapport presenteras en implementation av en 4-qubit Grover's algoritm för IBM Q-datorn ibmqx5. Vid exekvering av implementationen på en ibmqx5-simulator erhålls resultat som är i linje med teoretiskt optimala resultat. En jämförelse av korrektheten hos resultaten från ibmqx5-simulatorn och resultaten från ibmqx5-datorn tyder på att nuvarande hårdvara inte är lämplig för kretsar av komplexiteten nödvändig för en 4-qubit Grover implementation.
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Hoffer, Cole R. "Superconducting qubit readout pulse optimization using deep reinforcement learning". Thesis, Massachusetts Institute of Technology, 2021. https://hdl.handle.net/1721.1/130691.
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Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, February, 2021Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 101-106).
Quantum computers promise to solve specific problems significantly faster than classical computers. Superconducting quantum processors comprising more than 50 qubits can achieve quantum supremacy, the ability to outperform existing classical computers for particular problems. However, to build a useful quantum computer, the quantum processor's constituent components such as their control and readout must be very well-calibrated. Qubit-state readout of contemporary superconducting quantum processors is a significant error source. In an efficient, frequency-multiplexed readout of multiple qubits, effects such as drive cross-talk increase the complexity of optimal readout pulse shapes, requiring computationally intensive methods to discover high-fidelity pulse shapes. In this thesis, we extend existing readout optimization methods to work in multi-qubit environments and present a new pulse shaping optimization module using deep reinforcement learning. Compared to conventional readout methods in a simulated environment, we are able to reduce required readout pulse lengths by over 63% in single-qubit environments and by over 57% in multi-qubit environments. In addition to discussing how the deep reinforcement learning pulse shaping module will be used in experimental contexts, we also evaluate the future generalized use of deep reinforcement learning methods in quantum computing.
by Cole R. Hoffer.
M. Eng.
M.Eng. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science
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Dilley, Daniel Jacob. "An Insight on Nonlocal Correlations in Two-Qubit Systems". OpenSIUC, 2016. https://opensiuc.lib.siu.edu/theses/2069.
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In this paper, we introduce the motivation for Bell inequalities and give some background on two different types: CHSH and Mermin's inequalities. We present a proof for each and then show that certain quantum states can violate both of these inequalities. We introduce a new result stating that for four given measurement directions of spin, two respectively from Alice and two from Bob, which are able to produce a violation of the Bell inequality for an arbitrary shared quantum state will also violate the Bell inequality for a maximally entangled state. Then we provide another new result that characterizes all of the two-qubit states that violate Mermin's inequality.
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Sousa, Delano Klinger Alves. "O produto de quartebits não gera um qubit entrelaçado". Universidade Federal do Ceará, 2017. http://www.repositorio.ufc.br/handle/riufc/22860.
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SOUZA, D. K. A. O produto de quartebits não gera um qubit entrelaçado. 2017. 69 fl. Dissertação (Mestrado em Engenharia Elétrica e de Computação), Campus Sobral, Universidade Federal do Ceará, Sobral, 2017.Submitted by Programa de Pós-Graduação Engenharia Elétrica e de Computação (secretaria_ppgeec@sobral.ufc.br) on 2017-05-23T20:35:40Z No. of bitstreams: 1 2017_dis_dkasouza.pdf: 525869 bytes, checksum: 5bde9906f2ed9c3bd7ea2a782eb61a36 (MD5)
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The quaterbit is the smallest unit of information of Quaternion quantum mechanics. Their study brings to light fundamental problems of Quantum Mechanics that also have implications in Information Theory and Quantum Computation. In this work, it was investigated whether a pair of quaterbits may be entangled in Quantum Mechanics of complex numbers. To verify this hypothesis, two arbitrary quaterbits were used as singlets and we did the tensorial product. From this tensorial product, we establish the necessary conditions for the state to be only in Complex Quantum Mechanics. Within this procedure, two particular cases were analyzed: pure state and a simple case of mixed state. For pure states, we nd that the resulting separable bipartite qubit is not entangled, and therefore we conclude that the tensor product of two pure quaterbits does not result in a bipartite pure qubit entangled. Finally, we consider a mixed state of disentangled quaterbits (with some restrictions) and establish the conditions so that it lies only in Complex Quantum Mechanics, and therefore we conclude by using the Wootters concurrence that the state is disentangled.
O quaterbit é a menor unidade de informação da mecânica quântica quaterniônica. Seu estudo traz a tona problemas fundamentais da Mecânica Quântica que também têm implicações na Teoria da Informação e Computação Quântica. Neste trabalho, foi investigado se um par de quaterbits não entrelaçados podem estar entrelaçados na Mecânica Quântica dos números complexos. Para verificar esta hipótese, foram utilizados dois quaterbits arbitrários como singletos e, em seguida, tomamos o produto tensorial. A partir deste produto tensorial, estabelecemos as condições necessárias para que o estado esteja somente na Mecânica Quântica Complexa. Dentro desse procedimento, dois casos particulares foram analisados: estado puro e um caso simples de estado misto. Para estados puros, achamos que o qubit bipartido resultante ainda não estão entrelaçado, e, portanto, concluímos que o produto tensorial de dois quaterbits puros não resulta num qubit bipartido puro entrelaçado. Por último, consideramos um estado misto de quaterbits não entrelaçado (com algumas restrições) e estabelecemos as condições para que o mesmo esteja somente na Mecânica Quântica Complexa, e, portanto concluímos por meio da concurrência de Wootters que o estado continua desentrelaçado.
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Rogers, Nick. "Designing a Macroscopic Singlet-Triplet Qubit In a Linear Array of Quantum Dots Embedded In Nanowires". Thesis, Université d'Ottawa / University of Ottawa, 2016. http://hdl.handle.net/10393/34209.
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In this thesis I present a theory of a macroscopic singlet-triplet qubit in quantum dots embedded in nanowires, each containing 4 electrons and together simulating an artficial Haldane gap material. A Haldane gap material exhibits a 4-fold degenerate ground state separated by an energy gap from excitations. The ground state is equivalent to a degenerate spin-singlet and -triplet state. The 4 degenerate states exhibit the characteristics of spins-1/2 localized on either end of the chain. These states may be used as a coded qubit for quantum information processing.
Using the effective mass approximation, I calculate single-particle energy levels of one and two quantum dots in a quantum wire. Using these energy levels I compute the Coulomb matrix elements of the interacting Hamiltonian. Using configuration interaction I demonstrate that the ground state of a quantum dot with 4 electrons is a spin-1 state. I then show that the two dot system behaves approximately like two spin-1 objects interacting via an antiferromagnetic Heisenberg Hamiltonian. While the Heisenberg model is approximate, the two dots have a spin-0 ground-state, indicating antiferromagnetic coupling. I then present a simpler spin model to illustrate the physical parameters which control this interaction. Finally, I present a brief solution to the Heisenberg Hamiltonian for finite spin-chains, and show how one can manipulate the singlet-triplet combined ground state of the spin-chain via localized magnetic field, realizing a singlet-triplet qubit in a macroscopic semiconductor device.
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Escott, Christopher Colin Electrical Engineering & Telecommunications Faculty of Engineering UNSW. "Modelling of phosphorus-donor based silicon qubit and nanoelectronic devices". Publisher:University of New South Wales. Electrical Engineering & Telecommunications, 2008. http://handle.unsw.edu.au/1959.4/41470.
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Modelling of phosphorus donor-based silicon (Si:P) qubit devices and mesoscopic single-electron devices is presented in this thesis. This theoretical analysis is motivated by the use of Si:P devices for scalable quantum computing. Modelling of Si:P single-electron devices (SEDs) using readily available simulation tools is presented. The mesoscopic properties of single and double island devices with source-drain leads is investigated through ion implantation simulation (using Crystal-TRIM), 3D capacitance extraction (FastCap) and single-electron circuit simulation (SIMON). Results from modelling two generations of single and double island Si:P devices are given, which are shown to accurately capture their charging behaviour. The trends extracted are used to forecast limits to the reduction in size of this Si:P architecture. Theoretical analysis of P2+:Si charge qubits is then presented. Calculations show large ranges for the SET measurement signal, Δq, and geometric ratio factor, α, are possible given the 'top-down' fabrication procedure. The charge qubit energy levels are calculated using the atomistic simulator NEMO 3-D coupled to TCAD calculations of the electrostatic potential distribution, further demonstrating the precise control required over the position of the donors. Theory has also been developed to simulate the microwave spectroscopy of P2+:Si charge qubits in a decohering environment using Floquet theory. This theory uses TCAD finite-volume modelling to incorporate realistic fields from actual device gate geometries. The theory is applied to a specific P2+:Si charge qubit device design to study the effects of fabrication variations on the measurement signal. The signal is shown to be a sensitive function of donor position. Design and analysis of two different spin qubit architectures concludes this thesis. The first uses a high-barrier Schottky contact, SET and an implanted P donor to create a double-well suitable for implementation as a qubit. The second architecture is a MOS device that combines an electron reservoir and SET into a single structure, formed from a locally depleted accumulation layer. The design parameters of both architectures are explored through capacitance modelling, TCAD simulation, tunnel barrier transmission and NEMO 3-D calculations. The results presented strengthen the viability of each architecture, and show a large Δq (> 0.1e) can be expected.
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Viehmann, Oliver [Verfasser], i Florian [Akademischer Betreuer] Marquardt. "Multi-qubit circuit quantum electrodynamics / Oliver Viehmann. Betreuer: Florian Marquardt". München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2013. http://d-nb.info/1042147558/34.
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Laflamme, Catherine. "Weak qubit measurement with a nonlinear cavity: beyond perturbation theory". Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=110503.
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In this thesis we consider the quantum measurement properties of a driven cavity with a Kerr-type nonlinearity where the signal to be measured is a dispersively coupled qubit. Focusing on an operating regime that is near a bifurcation point,we calculate analytic expressions for the backaction dephasing rate and measurement rate beyond leading-order perturbation theory. This is done by using a phase-space approach which accounts for cavity noise squeezing. We consider two different methods to obtain an analytic result: the first is using an approximation common in the literature, the cavity dynamics are linearized with respect to the average photon number of the uncoupled cavity. With this linearization the physics of the nonlinear cavity maps to that of a degenerate parametric amplifier (DPA) with a detuned pump. The second method takes into account the displacement of the average cavity state when the qubit is in one of its eigenstates. Linearizing the cavity with respect to this {\it displaced} state takes this method beyond the DPA approach. Surprisingly, we find that increasing the coupling strength beyond the regime described by leading-order perturbation theory (i.e. linear response) allows one to come significantly closer to the quantum limit on the measurement efficiency. Moreover, we show that the approach which includes this displacement effect results in different behavior when evaluating how close one can approach the quantum limit. This provides a general framework for investigating quantum measurement with driven nonlinear systems beyond weak coupling. Finally, we interpret these results in terms of the non-Gaussian photon number fluctuations of the nonlinear cavity. Our results are relevant to recent experiments using superconducting microwave circuits to study quantum measurement.Dans cette thèse nous abordons des questions relatives à une cavité resonante nonlinéaire couplée à un qubit en régime dispersif. Nous nous concentrons sur un regime qui est proche d'un point de bifurcation et nous calculons des expressions analytiques pour le taux de mesures et le taux du déphasage. Nos calculs sont fait au-delà de la théorie de la perturbation et sont valides pour des couplages généraux. Nous utilisons une méthode qui tient en compte du resserrement du bruit dans la cavité.Nous considerons deux façons differentes pour obtenir un résultat analaytique. La première est une approximation courante qui consiste à linéariser les équations du mouvement grâce à un nombre élevé de photons dans la cavité. Avec cette approximation la dynamique du systeme devient celle d'un amplificateur parametrique dégeneré (DPA) avec pompe desaccordée.La deuxième façon tient compte de la modification de l'état moyen de la cavité en fonction de l'état propre du qubit. La cavité est linéarizée en funtion de ce nouvel état, ce qui rend la méthode plus précis que la première. Étonnamment, ces méthodes nous indiquent qu'il est préférable d'avoir un couplage plus élevé que celui permit par la théorie des perturbations afin de s'approcher de la limite quantique. Cette façon nous donne un cadre général pour étudier des systems quantiques nonlinéares.Finalement, nous interprètons nos résultats en termes des fluctuations non-Gaussiens du nombre de photons dans la cavité. Nos résultats sont pertinents dans le cadre d'expériences récentes, en particulier pour des circuits supraconducteurs.
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Yan, Fei Ph D. Massachusetts Institute of Technology. "Coherence characterization with a superconducting flux qubit through NMR approaches". Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82442.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 191-200).
This thesis discusses a series of experimental studies that investigate the coherence properties of a superconducting persistent-current or flux qubit, a promising candidate for developing a scalable quantum processor. A collection of coherence characterization experiments and techniques that originate from the field of nuclear magnetic resonance (NMR) are implemented. In particular, one type of dynamical decoupling techniques that uses refocusing pulses to recover coherence is successfully realized for the first time. This technique is further utilized as a noise spectrum analyzer in the megahertz range, by which a 1/f-type dependence is observed for the flux noise. Then, a novel method of performing low-frequency noise spectroscopy is developed and successfully implemented. New techniques used in the readout scheme and data processing result in an improved spectral range and signal visibility over conventional methods. The observed power law dependence below kilohertz agrees with separate measurements at higher frequencies. Also, the noise is found to be temperature independent. Finally, a robust noise spectroscopy method is presented, where the spin-locking technique is employed to extract noise information by measuring the driven-evolution longitudinal relaxation. This technique shows improved accuracy over other methods, due to its insensitivity to low-frequency noise. Spectral signatures of coherent fluctuators are resolved, and further confirmed in a time-domain spin-echo experiment.
by Fei Yan.
Ph.D.
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Song, Qiao, Swati Singh, Keye Zhang, Weiping Zhang i Pierre Meystre. "One qubit and one photon: The simplest polaritonic heat engine". AMER PHYSICAL SOC, 2016. http://hdl.handle.net/10150/622670.
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Hybrid quantum systems can often be described in terms of polaritons. These are quasiparticles formed of superpositions of their constituents, with relative weights depending on some control parameter in their interaction. In many cases, these constituents are coupled to reservoirs at different temperatures. This suggests a general approach to the realization of polaritonic heat engines where a thermodynamic cycle is realized by tuning this control parameter. Here we discuss what is arguably the simplest such engine, a single qubit coupled to a single photon. We show that this system can extract work from feeble thermal microwave fields. We also propose a quantum measurement scheme of the work and evaluate its backaction on the operation of the engine.
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Dahanayake, Duminda. "The role of supersymmetry in the black hole/qubit correspondence". Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/6060.
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This thesis explores the numerous relationships between the entropy of black hole solutions in supergravity and the entanglement of multipartite systems in quantum information theory: the so-called black hole/qubit correspondence. We examine how, through the correspondence, the dyonic charges in the entropy of supersymmetric black hole solutions are directly matched to the state vector coefficients in the entanglement measures of their quantum information analogues. Moreover the Uduality invariance of the black hole entropy translates to the stochastic local operations and classical communication (SLOCC) invariance of the entanglement measures. Several examples are discussed, with the correspondence broadening when the supersymmetric classification of black holes is shown to match the entanglement classification of the qubit/qutrit analogues. On the microscopic front, we study the interpretation of D-brane wrapping configurations as real qubits/qutrits, including the matching of generating solutions on black hole and qubit sides. Tentative generalisations to other dimensions and qubit systems are considered. This is almost eclipsed by more recent developments linking the nilpotent U-duality orbit classi cation of black holes to the nilpotent classi cation of complex qubits. We provide preliminary results on the corresponding covariant classi cation. We explore the interesting parallel development of supersymmetric generalisations of qubits and entanglement, complete with two- and three-superqubit entanglement measures. Lastly, we briefly mention the supergravity technology of cubic Jordan algebras and Freudenthal triple systems (FTS), which are used to: 1) Relate FTS ranks to threequbit entanglement and compute SLOCC orbits. 2) Define new black hole dualities distinct from U-duality and related by a 4D/5D lift. 3) Clarify the state of knowledge of integral U-duality orbits in maximally extended supergravity in four, five, and six dimensions.