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Title:

Decoherence, control, and encoding of coupled solid-state quantum bits

Description:

In this thesis the decoherence properties, gate performance, control of solid-state quantum bits (qubits), and novel design proposals for solid-state qubits analogous to quantum optics are investigated. The qubits are realized as superconducting nanocircuits or quantum dot systems. The thesis elucidates both very appealing basic questions, like ...

In this thesis the decoherence properties, gate performance, control of solid-state quantum bits (qubits), and novel design proposals for solid-state qubits analogous to quantum optics are investigated. The qubits are realized as superconducting nanocircuits or quantum dot systems. The thesis elucidates both very appealing basic questions, like the generation and detection of deeply nonclassical states of the electromagnetic field, i.e., single photon Fock states, in the solid-state, but also presents a broad range of different strategies to improve the scalability and decoherence properties of solid-state qubit setups. Minimize

Publisher:

Ludwig-Maximilians-Universität München

Year of Publication:

2005-11-14

Document Type:

Dissertation ; NonPeerReviewed

Subjects:

Fakultät für Physik

Fakultät für Physik Minimize

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http://edoc.ub.uni-muenchen.de/4604/

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Title:

Decoherence and gate performance of coupled solid state qubits

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Description:

Solid state quantum bits are promising candidates for the realization of a {\em scalable} quantum computer. However, they are usually strongly limited by decoherence due to the many extra degrees of freedom of a solid state system. We investigate a system of two solid state qubits that are coupled via $\sigma_z^{(i)} \otimes \sigma_z^{(j)}$ type...

Solid state quantum bits are promising candidates for the realization of a {\em scalable} quantum computer. However, they are usually strongly limited by decoherence due to the many extra degrees of freedom of a solid state system. We investigate a system of two solid state qubits that are coupled via $\sigma_z^{(i)} \otimes \sigma_z^{(j)}$ type of coupling. This kind of setup is typical for {\em pseudospin} solid-state quantum bits such as charge or flux systems. We evaluate decoherence properties and gate quality factors in the presence of a common and two uncorrelated baths coupling to $\sigma_z$, respectively. We show that at low temperatures, uncorrelated baths do degrade the gate quality more severely. In particular, we show that in the case of a common bath, optimum gate performance of a CPHASE gate can be reached at very low temperatures, because our type of coupling commutes with the coupling to the decoherence, which makes this type of coupling attractive as compared to previously studied proposals with $\sigma_y^{(i)} \otimes \sigma_y^{(j)}$ -coupling. Although less pronounced, this advantage also applies to the CNOT gate. ; Comment: 15 pages, 16 figures, 1 table, submitted to Phys. Rev. A Minimize

Year of Publication:

2002-12-16

Document Type:

text

Subjects:

Condensed Matter - Mesoscale and Nanoscale Physics ; Condensed Matter - Superconductivity ; Quantum Physics

Condensed Matter - Mesoscale and Nanoscale Physics ; Condensed Matter - Superconductivity ; Quantum Physics Minimize

DDC:

190 Modern western philosophy *(computed)*

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Title:

Decoherence of a two-qubit system with a variable bath coupling operator

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Description:

We examine the decoherence of an asymmetric two-qubit system that is coupled via a tunable interaction term to a common bath or two individual baths of harmonic oscillators. The dissipative dynamics are evaluated using the Bloch-Redfield formalism. It is shown that the behaviour of the decoherence effects is affected mostly by different symmetri...

We examine the decoherence of an asymmetric two-qubit system that is coupled via a tunable interaction term to a common bath or two individual baths of harmonic oscillators. The dissipative dynamics are evaluated using the Bloch-Redfield formalism. It is shown that the behaviour of the decoherence effects is affected mostly by different symmetries between the qubit operator which is coupled to the environment and temperature, whereas the differences between the two bath configurations are very small. Moreover, it is elaborated that small imperfections of the qubit parameters do not lead to a drastic enhancement of the decoherence rates. ; Comment: 10 pages, 5 figures Minimize

Year of Publication:

2005-04-29

Document Type:

text

Subjects:

Condensed Matter - Mesoscale and Nanoscale Physics ; Condensed Matter - Superconductivity ; Quantum Physics

Condensed Matter - Mesoscale and Nanoscale Physics ; Condensed Matter - Superconductivity ; Quantum Physics Minimize

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Title:

Intrinsic phonon decoherence and quantum gates in coupled lateral quantum dot charge qubits

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Description:

Recent experiments by Hayashi et al. [Phys. Rev. Lett. 91, 226804 (2003)] demonstrate coherent oscillations of a charge quantum bit (qubit) in laterally defined quantum dots. We study the intrinsic electron-phonon decoherence and gate performance for the next step: a system of two coupled charge qubits. The effective decoherence model contains p...

Recent experiments by Hayashi et al. [Phys. Rev. Lett. 91, 226804 (2003)] demonstrate coherent oscillations of a charge quantum bit (qubit) in laterally defined quantum dots. We study the intrinsic electron-phonon decoherence and gate performance for the next step: a system of two coupled charge qubits. The effective decoherence model contains properties of local as well as collective decoherence. Decoherence channels can be classified by their multipole moments, which leads to different low-energy spectra. It is shown that due to the super-Ohmic spectrum, the gate quality is limited by the single-qubit Hadamard gates. It can be significantly improved, by using double-dots with weak tunnel coupling. ; Comment: 11 pages, 7 figures Minimize

Year of Publication:

2005-07-07

Document Type:

text

Subjects:

Condensed Matter - Mesoscale and Nanoscale Physics ; Quantum Physics

Condensed Matter - Mesoscale and Nanoscale Physics ; Quantum Physics Minimize

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Title:

Macroscopic Einstein-Podolsky-Rosen pairs in superconducting circuits

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Description:

We propose an efficient approach to prepare Einstein-Podolsky-Rosen (EPR) pairs in currently existing Josephson nanocircuits with capacitive couplings. In these fixed coupling circuits, two-qubit logic gates could be easily implemented while, strictly speaking, single-qubit gates cannot be easily realized. For a known two-qubit state, conditiona...

We propose an efficient approach to prepare Einstein-Podolsky-Rosen (EPR) pairs in currently existing Josephson nanocircuits with capacitive couplings. In these fixed coupling circuits, two-qubit logic gates could be easily implemented while, strictly speaking, single-qubit gates cannot be easily realized. For a known two-qubit state, conditional single-qubit operation could still be designed to evolve only the selected qubit and keep the other qubit unchanged; the rotation of the selected qubit depends on the state of the other one. These conditional single-qubit operations allow to deterministically generate the well-known Einstein-Podolsky-Rosen pairs, represented by EPR-Bell (or Bell) states. Quantum-state tomography is further proposed to experimentally confirm the generation of these states. The decays of the prepared EPR pairs are analyzed using numerical simulations. Possible application of the generated EPR pairs to test Bell's Inequality is also discussed. ; Comment: small changes in text Minimize

Year of Publication:

2005-08-03

Document Type:

text

Subjects:

Quantum Physics ; Condensed Matter - Superconductivity

Quantum Physics ; Condensed Matter - Superconductivity Minimize

DDC:

115 Time *(computed)*

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