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Artificial Decoherence and its Suppression in NMR Quantum Computer  [PDF]
Yasushi Kondo,Mikio Nakahara,Shogo Tanimura
Physics , 2006,
Abstract: Liquid-state NMR quantum computer has demonstrated the possibility of quantum computation and supported its development. Using NMR quantum computer techniques, we observed phase decoherence under two kinds of artificial noise fields; one a noise with a long period, and the other with shorter random period. The first one models decoherence in a quantum channel while the second one models transverse relaxation. We demonstrated that the bang-bang control suppresses decoherence in both cases.
Simulating the Effect of Decoherence and Inaccuracies on a Quantum Computer  [PDF]
Kevin M. Obenland,Alvin M. Despain
Physics , 1998,
Abstract: A Quantum Computer is a new type of computer which can solve problems such as factoring and database search very efficiently. The usefulness of a quantum computer is limited by the effect of two different types of errors, decoherence and inaccuracies. In this paper we show the results of simulations of a quantum computer which consider both decoherence and inaccuracies. We simulate circuits which factor the numbers 15, 21, 35, and 57 as well as circuits which use database search to solve the circuit satisfaction problem. Our simulations show that the error rate per gate is on the order of 10^-6 for a trapped ion quantum computer whose noise is kept below pi/4096 per gate and with a decoherence rate of 10^-6. This is an important bound because previous studies have shown that a quantum computer can factor more efficiently than a classical computer if the error rate is of order 10^-6.
Hadamard NMR spectroscopy for two-dimensional quantum information processing and parallel search algorithms  [PDF]
T. Gopinath,Anil Kumar
Physics , 2006, DOI: 10.1016/j.jmr.2006.09.001
Abstract: Hadamard spectroscopy has earlier been used to speed-up multi-dimensional NMR experiments. In this work we speed-up the two-dimensional quantum computing scheme, by using Hadamard spectroscopy in the indirect dimension, resulting in a scheme which is faster and requires the Fourier transformation only in the direct dimension. Two and three qubit quantum gates are implemented with an extra observer qubit. We also use one-dimensional Hadamard spectroscopy for binary information storage by spatial encoding and implementation of a parallel search algorithm.
Individual Mechanisms of Nuclear Spin Decoherence in a Nanoscale GaAs NMR Device  [PDF]
Go Yusa,Norio Kumada,Koji Muraki,Yoshiro Hirayama
Physics , 2005,
Abstract: We study decoherence of nuclear spins in a nanoscale GaAs device based on resistively detected nuclear magnetic resonance (NMR). We demonstrate how the spin echo technique can be modified for our system, and this is compared to the damping of Rabi-type coherent oscillations. By selectively decoupling nuclear-nuclear and electron-nuclear spin, we determine decoherence rates due to individual mechanisms, namely, direct or indirect dipole coupling between different or like nuclides and electron-nuclear spin coupling. The data reveal that the indirect dipole coupling between Ga and As mediated by conduction electrons has the strongest contribution, whereas the direct dipole coupling between them has the smallest, reflecting the magic angle condition between the As-Ga bonds and the applied magnetic field.
Interpretation of Pulsed-Field-Gradient NMR in Terms of Molecular Displacements  [PDF]
George D. J. Phillies
Physics , 2011,
Abstract: The relationship for Pulsed-Field-Gradient NMR between the amplitude $I(t)$ of the spin echo and the molecular displacement ${\bf X}(t)$ is examined. $I(t)$ of a single species in a simple solution is determined by the mean-square displacement $\bar{X(t)^{2}}$. With polydisperse species, or molecular probes in complex fluids showing memory effects, $I(t)$ in general includes large contributions from all higher even moments $\bar{X(t)^{2n}}$. Conditions under which the NMR signal is indeed determined by the molecular mean-square displacement are noted. A diagnostic that sometimes identifies when these conditions are not met is presented.
Decoherence of quantum states and its suppression in ensemble large-scale solid state NMR quantum computers  [PDF]
A. A. Kokin
Physics , 2002, DOI: 10.1117/12.517916
Abstract: It is discussed the decoherence problems in ensemble large-scale solid state NMR quantum computer based on the array of P donor atoms having nuclear spin I = 1/2. It is considered here, as main mechanisms of decoherence for low temperature (< 0.1 K), the adiabatic processes of random modulation of qubit resonance frequency determined by secular part of nuclear spin interaction with electron spin of the basic atoms, with impurity paramagnetic atoms and also with nuclear spins of impurity diamagnetic atoms. It was made estimations of allowed concentrations of magnetic impurities and of spin temperature whereby the required decoherence suppression is obtained. It is discussed the random phase error suppression in the ensemble quantum register basic states.
Simulating decoherence behavior of a system in entangled state using nuclear magnetic resonance  [PDF]
Jingfu Zhang,Zhiheng Lu,Lu Shan,Zhiwei Deng
Physics , 2002,
Abstract: By choosing H nucleus in Carbon-13 labelled trichloroethylene as one qubit environment, and two C nuclei as a two-qubit system, we have simulated quantum decoherence when the system lies in an entangled state using nuclear magnetic resonance (NMR). Decoupling technique is used to trace over the environment degrees of freedom. Experimental results show agreements with the theoretical predictions. Our experiment scheme can be generalized to the case that environment is composed of multiple qubits.
Realizing the quantum baker's map on an NMR quantum computer  [PDF]
Todd A. Brun,Ruediger Schack
Physics , 1998, DOI: 10.1103/PhysRevA.59.2649
Abstract: By numerically simulating an implementation of the quantum baker's map on a 3-qubit NMR quantum computer based on the molecule trichloroethylene, we demonstrate the feasibility of quantum chaos experiments on present-day quantum computers. We give detailed descriptions of proposed experiments that investigate (a) the rate of entropy increase due to decoherence and (b) the phenomenon of hypersensitivity to perturbation.
The association of dimethylsulfoxide and model membranes studied by pulse-field gradient NMR  [PDF]
Anthony Kennedy,Carla J. Long,Peter J. Hmel,Rickey Hicks,Thomas J. Reid
Spectroscopy: An International Journal , 2004, DOI: 10.1155/2004/814213
Abstract: The use of dimethylsulfoxide (DMSO) as a cryoprotectant to reduce cellular injury during freezing is well known, however the intermolecular interactions between this amphiphilic molecule and biological membranes that form the basis of this protection are unknown. DMSO–dipalmitoylphosphatidylcholine (DPPC) vesicle interactions were investigated in pulsed-field gradient NMR (PFGNMR) experiments and spectra analysis allowed for the determination of self-diffusion coefficients for each species present. The mole fraction of DMSO associated with the DPPC vesicles was then calculated from the diffusion coefficients: the mole fraction increased from 14% to 42% as the membrane was heated from below to above the main phase transition temperature.
Les écoulements par RMN à gradient pulsé Pulsed Gradient Nmr Techniques for Studying Flows  [cached]
Lebon L.,Leblond J.
Oil & Gas Science and Technology , 2006, DOI: 10.2516/ogst:1998043
Abstract: Nous présentons ici les techniques de RMN à gradient pulsé qui permettent d'étudier les écoulements multiphasiques en canalisation ou en milieu poreux. Les principaux avantages sont de pouvoir travailler sur des milieux non transparents et d'accéder à des échelles de longueurs faibles. On montre qu'il est possible d'obtenir des informations locales sur l'écoulement, telles que le profil de vitesse et ses fluctuations dans les écoulements diphasiques, ou les cartes de distribution des probabilités de déplacement dans des échantillons poreux hétérogènes. Pulsed gradient NMR techniques are presented here. They allow the study of multiphase flow in pipes as well as porous media. The main advantages are the possibilities of studying non transparent media at small length scales. We show that it is possible to obtain local information on the fluid flow, such as velocity profiles in two phase systems, or maps of distribution of displacement probabilities in heterogeneous porous media.
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