High-sensitivity imaging with multi-mode twin beams

Twin entangled beams produced by single-pass parametric down-conversion (PDC) offer the opportunity to detect weak amount of absorption with an improved sensitivity with respect to standard techniques which make use of classical light sources. We propose a differential measurement scheme which exploits the spatial quantum correlation of type II PDC to image a weak amplitude object with a sensitivity beyond the standard quantum limit imposed by shot-noise.Comment: 13 pages, 8 figure

Simple model for 1/f noise

We present a simple stochastic mechanism which generates pulse trains exhibiting a power law distribution of the pulse intervals and a $1/f^\alpha$ power spectrum over several decades at low frequencies with $\alpha$ close to one. The essential ingredient of our model is a fluctuating threshold which performs a Brownian motion. Whenever an increasing potential $V(t)$ hits the threshold, $V(t)$ is reset to the origin and a pulse is emitted. We show that if $V(t)$ increases linearly in time, the pulse intervals can be approximated by a random walk with multiplicative noise. Our model agrees with recent experiments in neurobiology and explains the high interpulse interval variability and the occurrence of $1/f^\alpha$ noise observed in cortical neurons and earthquake data.Comment: 4 pages, 4 figure

Non-classical Photon Statistics For Two-mode Optical Fields

The non-classical property of subpoissonian photon statistics is extended from one to two-mode electromagnetic fields, incorporating the physically motivated property of invariance under passive unitary transformations. Applications to squeezed coherent states, squeezed thermal states, and superposition of coherent states are given. Dependences of extent of non-classical behaviour on the independent squeezing parameters are graphically displayed.Comment: 15 pages, RevTex, 5 figures, available by sending email to [email protected]

Social change and cycling as a form of sustainable transportation : the behavior-policy interaction in a medium-sized developing city

Thesis (M.C.P.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning; and, (S.M. in Transportation)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2008.Includes bibliographical references (p. 163-170).In developing countries, growth frequently parallels increasing motorization rates, and visions of mobility are often centered on the private automobile as the most flexible form of personal transportation and a symbol of increasing wealth. The pursuit of mobility in this form has severe social, environmental, and economic consequences, some of which can be mitigated by the promotion of alternative modes of transportation Cycling and other forms of human-powered transportation have benign environmental effects, improve physical health, and can positively affect the psychology of people who, by choosing this mode, are more active and spend time outside and engaged in their environment. Given these benefits, it not surprising that some medium-sized, developing cities are including cycling in their transportation plans.These cities experience significant barriers, however, in promoting cycling as a form of sustainable transportation. Much of the challenge involves effectively utilizing planning processes and tools, which are often imported and applied in a context where they were not designed to be used, to elicit more sustainable transportation behavior in the midst of rapid change. Solutions that come from these planning processes and tools (particularly infrastructure and other engineering- focused solutions) can be ineffective in promoting an alternative form of transportation.Addressing these shortcomings to elicit a change in behavior toward cycling as a form of sustainable transportation requires a new combination of planning tools and processes that can produce effective solutions.(cont.) In this thesis, I propose a three-step approach to induce behavioral change toward cycling, including: gaining an understanding of the barriers to bicycle use through attitude and perception analysis, improving the planning process through a visioning and backcasting exercise, and assessing and selecting the most appropriate modeling tools for the short-term and long-term promotion of cycling. I apply this approach using a single case study: Chihuahua, Mexico. Although I am unable to show a change in behavior in Chihuahua, initial results indicate that this adjusted approach does have promise in terms of inducing more behavior associated with sustainable transportation practices.by Tegin L. Teich.S.M.in TransportationM.C.P

Quantum optical coherence tomography with dispersion cancellation

We propose a new technique, called quantum optical coherence tomography (QOCT), for carrying out tomographic measurements with dispersion-cancelled resolution. The technique can also be used to extract the frequency-dependent refractive index of the medium. QOCT makes use of a two-photon interferometer in which a swept delay permits a coincidence interferogram to be traced. The technique bears a resemblance to classical optical coherence tomography (OCT). However, it makes use of a nonclassical entangled twin-photon light source that permits measurements to be made at depths greater than those accessible via OCT, which suffers from the deleterious effects of sample dispersion. Aside from the dispersion cancellation, QOCT offers higher sensitivity than OCT as well as an enhancement of resolution by a factor of 2 for the same source bandwidth. QOCT and OCT are compared using an idealized sample.Comment: 19 pages, 4 figure

Characterisations of Classical and Non-classical states of Quantised Radiation

A new operator based condition for distinguishing classical from non-classical states of quantised radiation is developed. It exploits the fact that the normal ordering rule of correspondence to go from classical to quantum dynamical variables does not in general maintain positivity. It is shown that the approach naturally leads to distinguishing several layers of increasing nonclassicality, with more layers as the number of modes increases. A generalisation of the notion of subpoissonian statistics for two-mode radiation fields is achieved by analysing completely all correlations and fluctuations in quadratic combinations of mode annihilation and creation operators conserving the total photon number. This generalisation is nontrivial and intrinsically two-mode as it goes beyond all possible single mode projections of the two-mode field. The nonclassicality of pair coherent states, squeezed vacuum and squeezed thermal states is analysed and contrasted with one another, comparing the generalised subpoissonian statistics with extant signatures of nonclassical behaviour.Comment: 16 pages, Revtex, One postscript Figure compressed and uuencoded Replaced, minor changes in eq 4.30 and 4.32. no effect on the result

Modeling and simulation of heterogeneous real-time systems based on a deterministic discrete event model

Abstract In this paper, an approach to system-level modeling and simulation of a class of heterogeneous real-time systems the timing behaviour of which can be m o deled by deterministic discrete event systems is described. Examples of systems we consider are self-timed systems, synchronously clocked systems, and mixed asynchronous/synchronous systems. Our model is based on several extensions to the model of timed marked graphs. Basically, we augment this model by adding new schedule constraints such that we can express simultaneity, synchronicity, nite buering as well as arbitrary combinations of min-and max-constraints. We prove that these extensions allow ecient timing analysis and we show how to simulate realistic systems using the Ptolemy [3] design system

Dynamics of a Quantum Control-Not Gate for an Ensemble of Four-Spin Molecules at Room Temperature

We investigate numerically a single-pulse implementation of a quantum Control-Not (CN) gate for an ensemble of Ising spin systems at room temperature. For an ensemble of four-spin ``molecules'' we simulate the time-evolution of the density matrix, for both digital and superpositional initial conditions. Our numerical calculations confirm the feasibility of implementation of quantum CN gate in this system at finite temperature, using electromagnetic $\pi$-pulse.Comment: 7 pages 3 figure

Entangled Electronic States in Multiple Quantum-Dot Systems

We present an analytically solvable model of $P$ colinear, two-dimensional quantum dots, each containing two electrons. Inter-dot coupling via the electron-electron interaction gives rise to sets of entangled ground states. These ground states have crystal-like inter-plane correlations and arise discontinously with increasing magnetic field. Their ranges and stabilities are found to depend on dot size ratios, and to increase with $P$.Comment: To appear in Physical Review B (in press). RevTeX file. Figures available from [email protected]

From quantum cellular automata to quantum lattice gases

A natural architecture for nanoscale quantum computation is that of a quantum cellular automaton. Motivated by this observation, in this paper we begin an investigation of exactly unitary cellular automata. After proving that there can be no nontrivial, homogeneous, local, unitary, scalar cellular automaton in one dimension, we weaken the homogeneity condition and show that there are nontrivial, exactly unitary, partitioning cellular automata. We find a one parameter family of evolution rules which are best interpreted as those for a one particle quantum automaton. This model is naturally reformulated as a two component cellular automaton which we demonstrate to limit to the Dirac equation. We describe two generalizations of this automaton, the second of which, to multiple interacting particles, is the correct definition of a quantum lattice gas.Comment: 22 pages, plain TeX, 9 PostScript figures included with epsf.tex (ignore the under/overfull \vbox error messages); minor typographical corrections and journal reference adde