Showing papers on "Coherence (physics) published in 1992"

Quantum Brownian motion in a general environment: Exact master equation with nonlocal dissipation and colored noise.

Abstract: We use the influence functional path-integral method to derive an exact master equation for the quantum Brownian motion of a particle linearly coupled to a general environment (ohmic, subohmic, or supraohmic) at arbitrary temperature and apply it to study certain aspects of the loss of quantum coherence.

Three-dimensional sensing of rough surfaces by coherence radar

Abstract: We introduce a three-dimensional sensor designed primarily for rough objects that supplies an accuracy that is limited only by the roughness of the object surface. This differs from conventional optical systems in which the depth accuracy is limited by the aperture. Consequently, our sensor supplies high accuracy with a small aperture, i.e., we can probe narrow crevices and holes. The sensor is based on a Michelson interferometer, with the rough object surface serving as one mirror. The small coherence length of the light source is used. While scanning the object in depth, one can detect the local occurrence of interference within the speckles emerging from the object. We call this method coherence radar.

Chaos in semiconductor lasers with optical feedback: theory and experiment

Abstract: The authors present a detailed theoretical and experimental investigation of the nonlinear dynamics of a semiconductor laser with optical feedback. The results show that the coherence collapsed state is a chaotic attractor and that chaos is reached for increasing feedback level through a quasi-periodic route interrupted by frequency locking. Furthermore, the coexistence of two attractors, associated with the same external cavity mode, but having different relaxation oscillation frequencies, is demonstrated and explained. >

High-speed optical coherence domain reflectometry.

Abstract: We describe a high-speed optical coherence domain reflectometer. Scan speeds of 40 mm/s are achieved with a dynamic range of >90 dB and a spatial resolution of 17 μm. Two applications are presented: the noninvasive measurement of anterior eye structure in a rabbit inυiυo and the characterization of reflections and interelement spacing in a multielement lens.

Coherent submillimeter-wave emission from charge oscillations in a double-well potential.

Abstract: We directly observe the electromagnetic radiation emitted by electrons coherently oscillating between the two wells of a semiconductor coupled-quantum-well structure. Using time-resolved coherent detection of the submillimeter-wave radiation from these spatial charge oscillations, we trace up to fourteen oscillations at 1.5 THz before phase relaxation destroys the coherence of the oscillating wave packet. In addition to the oscillatory electromagnetic signal, we also observe an instantaneous signal from electric-field-induced optical rectification in the semiconductor structure.

Resonantly enhanced refractive index without absorption via atomic coherence

Abstract: An enhancement of the index of refraction accompanied by vanishing absorption is shown to be possible in an ensemble of phase-coherent atoms (``phaseonium''). A survey of various possible schemes in which coherence is established by certain coherent or incoherent methods is given, and the main results are compared and contrasted. In particular, the influence of processes such as Doppler broadening that degrade coherence is discussed.

Observation of a "quantum eraser": A revival of coherence in a two-photon interference experiment.

Abstract: We have observed an effect known as a quantum eraser, using a setup similar to one previously employed to demonstrate a violation of Bell's inequalities. In this effect, an interfering system is first rendered incoherent by making the alternate Feynman paths which contribute to the overall process distinguishable; with our apparatus this is achieved by placing a half wave plate in one arm of a Hong-Ou-Mandel interferometer so as to rotate the polarization of the light in that arm by 90\ifmmode^\circ\else\textdegree\fi{}. This adds information to the system, in that polarization is a new parameter which serves to label the path of a given photon, even after a recombining beam splitter. The quantum ``eraser'' removes this information from the state vector, after the output port of the interferometer, but in time to cause interference effects to reappear upon coincidence detection. For this purpose, we use two polarizers in front of our detectors. We present experimental results showing how the degree of erasure (which determines the visibility of the interference) depends on the relative orientation of the polarizers, along with theoretical curves. In addition, we show how this procedure may do more than merely erase, in that the act of ``pasting together'' two previously distinguishable paths can introduce a new relative phase between them.

A simple intensity noise reduction technique for optical low-coherence reflectometry

Abstract: It is shown that by attenuating the reference power in an optical low-coherence reflectometry (OLCR) measurement, the reflection sensitivity can be improved, even though, in many other types of optical measurements, sensitivity is improved as optical power is increased. The difference is due to the presence of inherent intensity noise associated with low-coherence sources, which can dominate over shot noise at optical powers that are as low as 1 mu W. A reflection sensitivity of -146 dB is demonstrated using this technique. >

One-atom lasers.

Abstract: One-atom lasers are important because their governing equations can be solved exactly, even with a quantized field. We present a fully quantum-mechanical treatment of one-atom lasers modeled by quantum-optical master equations. These are solved numerically without any significant approximations. We show that laser action is possible with one atom, and that it might be achievable experimentally. Laser action is characterized by the dominance of stimulated emission over spontaneous emission. We use the one-atom laser model to investigate, without approximation, some interesting generic laser phenomena. Under certain conditions lasers produce intensity squeezed light, and then the laser linewidth increases with the pumping rate, in contrast with standard lasers. We also report ``self-quenching'' behavior: lasers with incoherent pumping out of the lower laser level turn off when the pumping is sufficiently fast because the coherence between the laser levels is destroyed.

Quantum transport in small disordered samples from the diffusive to the ballistic regime

Abstract: The authors review conductance effects in small samples at low temperatures where quantum confinement and quantum interference are significant perturbations on the classical Drude conductance. In disordered materials, the elastic scattering of the carriers from impurities leads to random conductance fluctuations or resistance fluctuations. The fluctuations arise because of interference among the scattered waves, and they are random and sample specific because the impurity potential is. The fluctuations appear in response to changes in many extrinsic parameters such as the carrier density, the applied measuring current, external electric fields and external fields. The interface fluctuations have consequences for much larger samples, particularly in flicker noise, even though quantum coherence is obtained only over regions much smaller than the sample size, in completely phase coherent conductors a number of purely quantum effects are observed, including non-local response and Aharonov-Bohm effects. Other 'applications' of the quantum fluctuations include studies of reciprocity (which is related to time-reversal symmetry) and of the effects of the measurement probes on a quantum system. Interestingly, these are two areas where disagreements remain with theoretical calculations. The size and correlation scale of the fluctuations, however, are mainly in agreement with theoretical calculations of the same quantities, although one or two other small disagreements of detail remain. In very clean semiconductor heterostructures, the mean free path length between scattering events is large enough to allow for studies of ballistic transport that reveal a variety of conductance anomalies that result from device shape (as opposed to fortuitous placement of impurities as in the metals). These ballistic effects are reviewed briefly and connection is made to the effects of disorder in ballistic systems, and experiments on disordered metal samples are reviewed in detail.

Experimental Test of Bell's Inequality for Energy and Time

Abstract: An interference experiment with photon pairs produced by degenerate parametric fluorescence is performed. The pairs are split and directed to two spatially separated Michelson interferometers. The interferometers are set to path differences which cause time delays far exceeding the coherence time of the light. We investigate photon coincidences between the output arms of the interferometers and observe nonlocal fourth-order interferences in the absence of second-order interferences. Maximum visibilities up to 86% are obtained, leading to a violation of Bell's inequality for energy and time by several standard deviations.

Imaging through scattering media with holography

Abstract: Various holographic methods for imaging through scattering media such as biological tissue are described. The methods utilize light of either reduced spatial coherence or reduced temporal coherence.

A new passive synthetic aperture technique for towed arrays

Abstract: An algorithm that synthesizes apertures in the beam domain using FFT transformations and performs coherent processing of subaperture signals at successive time intervals is presented. Experimental tests of the algorithm show that for ocean environments with spatial coherence longer than the synthetic aperture length and for signals with temporal coherence longer than the required acquisition time, a synthetic array gain is achieved which roughly corresponds to the length of an equivalent fully populated array. In the experiments, transducer generated CW with phase stability and pseudorandom signals were used. Limitations on the spatial and temporal coherence were introduced only by the medium, the temporal coherence of the pseudorandom signal, and the shape and stability of the line array used. >

Many-body coherence effects in conduction through a quantum dot in the fractional quantum Hall regime.

Abstract: We study a quantum dot in the fractional quantum Hall regime by solving the problem exactly for up to eight electrons at filling factors between 1 and 1/3. Many-body coherence in the fractional regime strongly suppresses the resonant conductance through a weakly coupled dot below the integer regime values. In particular, using the edge-wave theory of excitations, we predict that, at low temperatures and small bias voltages, all conductance peaks are lowered by a factor of 1/N at v=1/3 and that at v=2/3 odd and even peaks are suppressed differently

Investigating decoherence in a simple system.

Abstract: I present the results of some simple calculations designed to study the loss of quantum coherence. The relevant physical issues are briefly reviewed, and then a very simple "toy" model is analyzed. Exact solutions are found using numerical techniques. The type of decoherence exhibited by the model can be changed by varying a coupling strength. I study the system from two points of view. One, the Schmidt paths approach, is closely related to the conventional approach of studying decoherence by checking the form of the density matrix. The consistent histories approach is also used, and the relationship between the two approaches is explored.

Propagation of electromagnetic waves through a turbulent atmosphere

Abstract: Reviews the major advances achieved in the studies of EM wave propagation in turbulent media. The greatest attention has been directly to the problem of strong fluctuations of intensity. The theoretical results of this treatise include: the path integration technique, the predominant correlation method, the two-scale method, the hybrid approach to scattering from small inhomogeneities in the presence of large ones, the theory of coherent channels in backscattering, and the concept of partially determinate processes and fields. New physical effects described in the review include the rise of the coherence radius in intermittent turbulence, backscatter enhancement, doubling the variance of phase, incidence angles, group delay, and frequency shifts in the double passage of waves through turbulent media, the residual scintillation effect, the long correlation effect in backscattering, the partial reversal of wavefronts in turbulent media, the magic cap effect in reflection from phase conjugated mirrors, the strong localization of UHF radio waves in stochastic tropospheric waveguides, the effect of penetration in caustic shadows, and enhanced translucence. New fields of experimental research have been outlined, such as scattering from vegetation and snow covers, and coherent structures in turbulence, and new objects of theoretical analysis have been elicited: random dislocations of wavefronts, fractal and critical wave phenomena, and the predictability of random fields.

On using coherence to measure distortion in hearing aids.

Abstract: Coherence is a frequency‐domain measure of the degree to which the output of a system is linearly related to the system input. The signal‐to‐distortion ratio (SDR), where the distortion term includes all nonlinear effects and noise in the system, can be computed from the coherence. The coherence estimate, however, is subject to sources of variance and bias that reduce the accuracy of the measured SDR. The origins of the variance and bias and their effects on distortion measurements are presented. New procedures for reducing the variance and bias effects are described, and the processing effectiveness is demonstrated for a simulated hearing‐aid response.

Optical Low Coherence Reflectometry with 1.9 Mu-M Spatial-Resolution

Abstract: An optical low coherence reflectometer is presented which uses an all-fibre Michelson interferometer with the fluorescence light of a Ti:Al2O3 crystal as a light source. The broad spectrum of the fluorescence allows a maximal spatial resolution of 1.9 μm to be reached, with a dynamic range of 80 dB.

Measurement of the spatial coherence of a soft-x-ray laser.

Abstract: The spatial coherence of a neonlike selenium x-ray laser operating at 206 and 210 \AA{} has been measured using a technique based on partially coherent x-ray diffraction. The time-integrated spatial coherence of the selenium x-ray laser was determined to be equivalent to that of a quasimonochromatic spatially incoherent disk source whose diameter is comparable to the line focus of the visible-light laser pumping the x-ray laser. The spatial coherence was improved by narrowing the line focus width.

Spectroscopic signatures of phase transitions in a charge-density-wave system: 1T-TaS2.

Abstract: Photoelectron spectroscopy with high-energy resolution has been utilized to investigate the charge-density wave (CDW) in 1T-TaS2 between 20 and 360 K. Constant binding-energy curves reveal discontinuities in the temperature dependence of the photoelectron spectral function rho(E), and demonstrate that sudden modifications of the electronic structure, namely, in the vicinity of the Fermi level E(F), mark the first-order CDW transitions. In the commensurate phase, below 180 K, rho(E(F)) reflects the formation of a correlation pseudogap. On disordered surfaces, however, the long-range coherence typical of the commensurate phase is lost, and a normal metallic behavior is recovered.

Gaussian Schell-model beams generated with synthetic acousto-optic holograms

Abstract: The recently introduced concept of a synthetic acousto-optic hologram [ J. Appl. Phys.67, 49 ( 1990)] is applied to convert a Gaussian laser beam into a partially coherent anisotropic Gaussian Schell-model (AGSM) beam. Real-time reconfigurability of the coherence properties is achieved by this technique, which features scattering of the laser beam by an electronically synthesized, digitally phase-modulated volume grating that propagates in an acousto-optic Bragg cell. The coherence and intensity distributions of the fields obtained by different types of phase modulation are investigated theoretically. We demonstrate some particularly interesting AGSM sources and fields: a secondary elliptical AGSM source with a circularly symmetric far-field intensity distribution and an AGSM field that retains the eccentricity of its intensity profile in the propagation through any centrosymmetric (paraxial) optical system.

Achromatic holographic configuration for 100-nm-period lithography

Abstract: For the fabrication of large-area, spatially coherent gratings with periods of 100 nm or less, a grating interferometer is preferred over a conventional holographic configuration because of the limited coherence of available sources. Using a configuration that employs two matched fused silica phase gratings and an ArF excimer laser, we obtain high-quality 100-nm gratings in polymethyl methacrylate. We analyze the conditions for achieving high-contrast fringes with such an achromatic holographic configuration and show that the depth of focus depends only on the spatial coherence of the source. We also describe a highly accurate method for calculating the diffraction efficiency of the phase gratings as a function of polarization, incidence angle, and grating structure.

Principles of superfluid-helium gyroscopes.

Abstract: In this paper we describe how quantum coherence of superfluid helium provides a mechanism by which very small rotations can substantially modify the flow in a toroidal container. The specific modifications to that flow are discussed. For $^{4}\mathrm{He}$, we explain how the rotationally induced flow can be detected by monitoring the apparent phase-slip critical current. The rotational resolution is limited by stochastic processes related to the nucleation of phase slips. This type of superfluid-helium gyroscope (SHEG) is an analog of the rf superconducting quantum interference device (SQUID). We also show how the large coherence length of $^{3}\mathrm{He}$ can be utilized to lead to a rotationally induced interference pattern. Changes in this pattern can permit the detection of very small rotational motion. This type of SHEG is analogous to the dc SQUID. In appendixes, electrical circuits equivalent to the SHEG are described, as are certain constraints on rotational sensitivity imposed by external measuring devices.

Product operators and coherence transfer in multiple‐pulse NMR experiments

Abstract: A practical introduction to coherence and coherence transfer pathways in NMR is provided using the density matrix and product-operator formalisms. The concept of coherence in NMR is first introduced using the density matrix formalism. Pictorial and quantitative representations of coherence are provided using product operators in the Cartesian basis. A simple transformation from the Cartesian basis to a spherical basis is used to follow coherence order during multiple-pulse experiments with coherence transfer pathways. The rules are introduced for designing phase cycles to eliminate unwanted coherences and pathways. An appendix contains a computer program written in Mathematica for performing product-operator calculations interactively.