Biomedical subjects

David G Cory

Publications and source records attributed to David G Cory.

9 recordsLinked to original sources

PubMed2006

Single spin measurement using cellular automata techniques.

We analyze a conceptual approach to single-spin measurement. The method uses techniques from the theory of quantum cellular automata to correlate a large number of ancillary spins to the one to be measured. It has the distinct advantage of being efficient: under ideal conditions, it requires the application of only O((3)square root N)) steps (each requiring a constant number of rf pulses) to create a system of N correlated spins. Numerical simulations suggest that it is also, to a certain extent, robust against pulse errors, and imperfect initial polarization of the ancilla spin system.

Journal Article↗

PubMed2005

Experimental implementation of a concatenated quantum error-correcting code.

Concatenated coding provides a general strategy to achieve the desired level of noise protection in quantum information processing. We report the implementation of a concatenated quantum error-correcting code able to correct phase errors with a strong correlated component. The experiment was performed using liquid-state nuclear magnetic resonance techniques on a four spin subsystem of labeled crotonic acid. Our results show that concatenation between active and passive quantum error correction is a practical tool to handle realistic noise involving both independent and correlated errors.

Journal Article↗

PubMed2005

Xenon NMR measurements of permeability and tortuosity in reservoir rocks.

In this work we present measurements of permeability, effective porosity and tortuosity on a variety of rock samples using NMR/MRI of thermal and laser-polarized gas. Permeability and effective porosity are measured simultaneously using MRI to monitor the inflow of laser-polarized xenon into the rock core. Tortuosity is determined from measurements of the time-dependent diffusion coefficient using thermal xenon in sealed samples. The initial results from a limited number of rocks indicate inverse correlations between tortuosity and both effective porosity and permeability. Further studies to widen the number of types of rocks studied may eventually aid in explaining the poorly understood connection between permeability and tortuosity of rock cores.

Geological Phenomena↗

PubMed2005

NMR-propagator measurements in porous media in the presence of surface relaxation and internal fields.

We measure the probability distribution P(zeta)--the propagator--of molecular displacements on Stokes flow through a pack of microporous glass beads and a carbonate rock. An optimized sampling of q-space is introduced for the measurement of a P(zeta) and its first moment zeta. Our results delineate and provide an understanding of the experimental regimes where background fields and surface relaxation distort the measured propagators.

Carbonates↗

PubMed2004

Quantum process tomography of the quantum Fourier transform.

The results of quantum process tomography on a three-qubit nuclear magnetic resonance quantum information processor are presented and shown to be consistent with a detailed model of the system-plus-apparatus used for the experiments. The quantum operation studied was the quantum Fourier transform, which is important in several quantum algorithms and poses a rigorous test for the precision of our recently developed strongly modulating control fields. The results were analyzed in an attempt to decompose the implementation errors into coherent (overall systematic), incoherent (microscopically deterministic), and decoherent (microscopically random) components. This analysis yielded a superoperator consisting of a unitary part that was strongly correlated with the theoretically expected unitary superoperator of the quantum Fourier transform, an overall attenuation consistent with decoherence, and a residual portion that was not completely positive-although complete positivity is required for any quantum operation. By comparison with the results of computer simulations, the lack of complete positivity was shown to be largely a consequence of the incoherent errors which occurred over the full quantum process tomography procedure. These simulations further showed that coherent, incoherent, and decoherent errors can often be identified by their distinctive effects on the spectrum of the overall superoperator. The gate fidelity of the experimentally determined superoperator was 0.64, while the correlation coefficient between experimentally determined superoperator and the simulated superoperator was 0.79; most of the discrepancies with the simulations could be explained by the cumulative effect of small errors in the single qubit gates.

Journal Article↗

PubMed2004

Water suppression without signal loss in HR-MAS 1H NMR of cells and tissues.

In cell and tissue samples, water is normally three orders of magnitude more abundant than other metabolites. Thus, water suppression is required in the acquisition of NMR spectra to overcome the dynamic range problem and to recover metabolites that overlap with the broad baseline of the strong water resonance. However, the heterogeneous cellular environment often complicates water suppression and the strong coupling of water to membrane lipids interferes with the NMR detection of membrane associated lipid components. The widely used water suppression techniques including presaturation and double pulsed field gradient selective echo result in more than a 70% reduction in membrane associated lipid components in proton spectra of cells and tissues compared to proton spectra acquired in the absence of water suppression. A water suppression technique based on the combination of selective excitation pulses and pulsed field gradients is proposed to use in the acquisition of high resolution MAS NMR spectra of tissue specimens and cell samples. This pulse sequence methodology enables efficient water suppression for intact cells and tissue samples and eliminates signal loss from cellular metabolites.

Body Water↗

PubMed2003

Pseudo-random unitary operators for quantum information processing.

In close analogy to the fundamental role of random numbers in classical information theory, random operators are a basic component of quantum information theory. Unfortunately, the implementation of random unitary operators on a quantum processor is exponentially hard. Here we introduce a method for generating pseudo-random unitary operators that can reproduce those statistical properties of random unitary operators most relevant to quantum information tasks. This method requires exponentially fewer resources, and hence enables the practical application of random unitary operators in quantum communication and information processing protocols. Using a nuclear magnetic resonance quantum processor, we were able to realize pseudorandom unitary operators that reproduce the expected random distribution of matrix elements.

Journal Article↗

PubMed2002

Experimental implementation of the quantum baker's map.

This Letter reports on the experimental implementation of the quantum baker's map via a three bit nuclear magnetic resonance quantum information processor. The experiments tested the sensitivity of the quantum chaotic map to perturbations. In the first experiment, the map was iterated forward and then backward to provide benchmarks for intrinsic errors and decoherence. In the second set of experiments, the least significant qubit was perturbed in between the iterations to test the sensitivity of the quantum chaotic map to controlled perturbations. These experiments can be used to investigate existing theoretical predictions for quantum chaotic dynamics.

Journal Article↗

PubMedDate not supplied

A new probe of pore geometry: diffusive MASS NMR.

NMR methods are widely used to probe the structure and fluid dynamics of porous materials, as they are uniquely suited to these studies since NMR records the correlation of changing local magnetic fields over a time scale of ns to seconds. The local magnetic fields are established by local variations in the bulk magnetic susceptibility of the sample (and so are directly tied to the sample's local structure). The fluctuation in field that a spin sees is due to molecular transport (including molecular diffusion) through these local fields, and so reports on the length scales of structures and impediments to transport. We have developed a new set of methods DIFFUSIVE-MASS to provide a means of systematically varying the effective time scale of the measurement and thus the effective length scale. This new handle permits a detailed, microscopic picture of the structure and dynamics. Diffusive MASS NMR methods will permit a systematic set of methods and analysis for characterizing the chemistry, structure and fluid dynamics of the mobile phase in porous materials. The approach will be applicable to any diamagnetic material. In particular, the industry of oil discovery depends on understanding heterogeneous porous media.

Diffusion↗