1,673 research outputs found
An NMR Analog of the Quantum Disentanglement Eraser
We report the implementation of a three-spin quantum disentanglement eraser
on a liquid-state NMR quantum information processor. A key feature of this
experiment was its use of pulsed magnetic field gradients to mimic projective
measurements. This ability is an important step towards the development of an
experimentally controllable system which can simulate any quantum dynamics,
both coherent and decoherent.Comment: Four pages, one figure (RevTeX 2.1), to appear in Physics Review
Letter
All scale-free networks are sparse
We study the realizability of scale free-networks with a given degree
sequence, showing that the fraction of realizable sequences undergoes two
first-order transitions at the values 0 and 2 of the power-law exponent. We
substantiate this finding by analytical reasoning and by a numerical method,
proposed here, based on extreme value arguments, which can be applied to any
given degree distribution. Our results reveal a fundamental reason why large
scale-free networks without constraints on minimum and maximum degree must be
sparse.Comment: 4 pages, 2 figure
Experimental Implementation of Logical Bell State Encoding
Liquid phase NMR is a general purpose test-bed for developing methods of
coherent control relevant to quantum information processing. Here we extend
these studies to the coherent control of logical qubits and in particular to
the unitary gates necessary to create entanglement between logical qubits. We
report an experimental implementation of a conditional logical gate between two
logical qubits that are each in decoherence free subspaces that protect the
quantum information from fully correlated dephasing.Comment: 9 Pages, 5 Figure
A Method for Modeling Decoherence on a Quantum Information Processor
We develop and implement a method for modeling decoherence processes on an
N-dimensional quantum system that requires only an -dimensional quantum
environment and random classical fields. This model offers the advantage that
it may be implemented on small quantum information processors in order to
explore the intermediate regime between semiclassical and fully quantum models.
We consider in particular system-environment couplings which
induce coherence (phase) damping, though the model is directly extendable to
other coupling Hamiltonians. Effective, irreversible phase-damping of the
system is obtained by applying an additional stochastic Hamiltonian on the
environment alone, periodically redressing it and thereby irreversibliy
randomizing the system phase information that has leaked into the environment
as a result of the coupling. This model is exactly solvable in the case of
phase-damping, and we use this solution to describe the model's behavior in
some limiting cases. In the limit of small stochastic phase kicks the system's
coherence decays exponentially at a rate which increases linearly with the kick
frequency. In the case of strong kicks we observe an effective decoupling of
the system from the environment. We present a detailed implementation of the
method on an nuclear magnetic resonance quantum information processor.Comment: 12 pages, 9 figure
Spintronics and Quantum Dots for Quantum Computing and Quantum Communication
Control over electron-spin states, such as coherent manipulation, filtering
and measurement promises access to new technologies in conventional as well as
in quantum computation and quantum communication. We review our proposal of
using electron spins in quantum confined structures as qubits and discuss the
requirements for implementing a quantum computer. We describe several
realizations of one- and two-qubit gates and of the read-in and read-out tasks.
We discuss recently proposed schemes for using a single quantum dot as
spin-filter and spin-memory device. Considering electronic EPR pairs needed for
quantum communication we show that their spin entanglement can be detected in
mesoscopic transport measurements using metallic as well as superconducting
leads attached to the dots.Comment: Prepared for Fortschritte der Physik special issue, Experimental
Proposals for Quantum Computation. 15 pages, 5 figures; typos corrected,
references adde
A new method for building protein conformations from sequence alignments with homologues of known structure
We describe a largely automatic procedure for building protein structures from sequence alignments with homologues of known structure. This procedure uses simple rules by which multiple sequence alignments can be translated into distance and chirality constraints, which are then used as input for distance geometry calculations. By this means one obtains an ensemble of conformations for the unknown structure that are compatible with the rules employed, and the differences among these conformations provide an indication of the reliability of the structure prediction. The overall approach is demonstrated here by applying it to several Kazal-type trypsin inhibitors, for which experimentally determined structures are available. On the basis of our experience with these test problems, we have further predicted the conformation of the human pancreatic secretory trypsin inhibitor, for which no experimentally determined structure is presently available.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29506/1/0000593.pd
Multivariate calibration approach for quantitative determination of cell-line cross contamination by intact cell mass spectrometry and artificial neural networks
Cross-contamination of eukaryotic cell lines used in biomedical research represents a highly relevant problem. Analysis of repetitive DNA sequences, such as Short Tandem Repeats (STR), or Simple Sequence Repeats (SSR), is a widely accepted, simple, and commercially available technique to authenticate cell lines. However, it provides only qualitative information that depends on the extent of reference databases for interpretation. In this work, we developed and validated a rapid and routinely applicable method for evaluation of cell culture cross-contamination levels based on mass spectrometric fingerprints of intact mammalian cells coupled with artificial neural networks (ANNs). We used human embryonic stem cells (hESCs) contaminated by either mouse embryonic stem cells (mESCs) or mouse embryonic fibroblasts (MEFs) as a model. We determined the contamination level using a mass spectra database of known calibration mixtures that served as training input for an ANN. The ANN was then capable of correct quantification of the level of contamination of hESCs by mESCs or MEFs. We demonstrate that MS analysis, when linked to proper mathematical instruments, is a tangible tool for unraveling and quantifying heterogeneity in cell cultures. The analysis is applicable in routine scenarios for cell authentication and/or cell phenotyping in general
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