43,506 research outputs found
Quantum state tomography by continuous measurement and compressed sensing
The need to perform quantum state tomography on ever larger systems has
spurred a search for methods that yield good estimates from incomplete data. We
study the performance of compressed sensing (CS) and least squares (LS)
estimators in a fast protocol based on continuous measurement on an ensemble of
cesium atomic spins. Both efficiently reconstruct nearly pure states in the
16-dimensional ground manifold, reaching average fidelities FCS = 0.92 and FLS
= 0.88 using similar amounts of incomplete data. Surprisingly, the main
advantage of CS in our protocol is an increased robustness to experimental
imperfections
Scale Dependent Dimension of Luminous Matter in the Universe
We present a geometrical model of the distribution of luminous matter in the
universe, derived from a very simple reaction-diffusion model of turbulent
phenomena. The apparent dimension of luminous matter, , depends linearly
on the logarithm of the scale under which the universe is viewed: , where is a correlation length.
Comparison with data from the SARS red-shift catalogue, and the LEDA database
provides a good fit with a correlation length Mpc. The
geometrical interpretation is clear: At small distances, the universe is
zero-dimensional and point-like. At distances of the order of 1 Mpc the
dimension is unity, indicating a filamentary, string-like structure; when
viewed at larger scales it gradually becomes 2-dimensional wall-like, and
finally, at and beyond the correlation length, it becomes uniform.Comment: 6 pages, 2 figure
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