913 research outputs found
Mini-Brayton heat source assembly development
The work accomplished on the Mini-Brayton Heat Source Assembly program is summarized. Required technologies to design, fabricate and assemble components for a high temperature Heat Source Assembly (HSA) which would generate and transfer the thermal energy for a spaceborne Brayton Isotope Power System (BIPS) were developed
Mars Spacecraft Power System Development Final Report
Development of optimum Mariner spacecraft power system for application to future flyby and orbiter mission
Hardness of Token Swapping on Trees
Given a graph where every vertex has exactly one labeled token, how can we most quickly execute a given permutation on the tokens? In (sequential) token swapping, the goal is to use the shortest possible sequence of swaps, each of which exchanges the tokens at the two endpoints of an edge of the graph. In parallel token swapping, the goal is to use the fewest rounds, each of which consists of one or more swaps on the edges of a matching. We prove that both of these problems remain NP-hard when the graph is restricted to be a tree.
These token swapping problems have been studied by disparate groups of researchers in discrete mathematics, theoretical computer science, robot motion planning, game theory, and engineering. Previous work establishes NP-completeness on general graphs (for both problems), constant-factor approximation algorithms, and some poly-time exact algorithms for simple graph classes such as cliques, stars, paths, and cycles. Sequential and parallel token swapping on trees were first studied over thirty years ago (as "sorting with a transposition tree") and over twenty-five years ago (as "routing permutations via matchings"), yet their complexities were previously unknown.
We also show limitations on approximation of sequential token swapping on trees: we identify a broad class of algorithms that encompass all three known polynomial-time algorithms that achieve the best known approximation factor (which is 2) and show that no such algorithm can achieve an approximation factor less than 2
The Many Facets of SDF-1α, CXCR4 Agonists and Antagonists on Hematopoietic Progenitor Cells
Stromal cell-derived factor-1alpha (SDF-1α) has pleiotropic effects on hematopoietic progenitor cells (HPCs). We have monitored podia formation, migration, proliferation, and cell-cell adhesion of human HPC under the influence of SDF-1α, a peptide agonist of CXCR4 (CTCE-0214), a peptide antagonist (CTCE-9908), and a nonpeptide antagonist (AMD3100). Whereas SDF-1α induced migration of CD34+ cells in a dose-dependent manner, CTCE-0214, CTCE-9908, and AMD3100 did not induce chemotaxis in this concentration range albeit the peptides CTCE-0214 and CTCE-9908 increased podia formation. Cell-cell adhesion of HPC to human mesenchymal stromal cells was impaired by the addition of SDF-1α, CTCE-0214, and AMD3100. Proliferation was not affected by SDF-1α or its analogs. Surface antigen detection of CXCR4 was reduced upon treatment with SDF-1α or AMD3100 and it was enhanced by CTCE-9908. Despite the fact that all these molecules target the same CXCR4 receptor, CXCR4 agonists and antagonists have selective effects on different functions of the natural molecule
Quantum interferences and gates with emitter-based coherent photon sources
Quantum emitters, such as atoms, defects in crystals, or quantum dots, are
excellent sources of indistinguishable single-photons for quantum technologies.
Upon coherent excitation, however, the emitted photonic state includes a vacuum
component in a quantum superposition with the one-photon component. This
feature has so far been largely disregarded in the framework of linear optical
computing. Here we experimentally and theoretically study how the presence of
photon-number coherence alters the foundation of photon-photon gates: the
Hong-Ou-Mandel interference. We show that the presence of vacuum coherence not
only introduces errors to standard photon indistinguishability measurements,
but also results in complex quantum interference phenomena. These phenomena
lead to additional entanglement that has profound impact on linear computing
schemes, as we illustrate by simulating a heralded gate. Our work reveals the
rich physics arising from photon-number coherence, which holds the potential to
become an asset in future quantum protocols.Comment: 15 pages, 8 figure
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