Phase Coherence Properties Research Articles

Comparison of maximum-likelihood mapping methods for gravitational-wave backgrounds

Detection of a stochastic background of gravitational waves is likely to occur in the next few years. Beyond searches for the isotropic component of SGWBs, there have been various mapping methods proposed to target anisotropic backgrounds. Some of these methods have been applied to data taken by the Laser Interferometer Gravitational-wave Observatories (LIGO) and Virgo. Specifically, these directional searches have focused on mapping the intensity of the signal on the sky via maximum likelihood solutions. We compare this intensity mapping approach to a previously proposed, but never employed, amplitude-phase mapping method to understand whether this latter approach may be employed in future searches. We build up our understanding of the differences between these two approaches by analysing simple toy models of time-stream data, and run mock-data mapping tests for the two methods. We find that the amplitude-phase method is only applicable to the case of a background which is phase-coherent on large scales or, at the very least, has an intrinsic coherence scale that is larger than that of the detector. Otherwise, the amplitude-phase mapping method leads to a loss of overall information, with respect to both phase and amplitude. Since we do not expect these phase-coherent properties to hold for any of the gravitational-wave background signals we hope to detect in the near future, we conclude that intensity mapping is the preferred method for such backgrounds.

A pressure decomposition framework for aeroacoustic analysis of turbulent jets

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Theory (MPT) approach of (Doak, 1989), to split pressure fluctuations into their hydrodynamic, acoustic and entropic, collectively designated fluid-thermodynamic (FT), components. Key advantages are that the approach is applicable everywhere in the jet i.e, not restricted to the near-acoustic field, and does not need user-defined thresholds. The effectiveness of the technique is demonstrated by analyzing the flowfields of three simulated jets, to encompass moderate-compressible to supersonic conditions. The statistical properties and wavepacket dynamics of each pressure component, and their relationships with the unsplit pressure are elaborated. The acoustic pressure field has the form of a wavepacket that attenuates downstream and whose modal analysis reveals low-rank behavior. At each Mach number examined, the acoustic pressure also identifies the relative prominence of each of three components: i) waves with upstream propagating energy content (negative group velocity), ii) supersonically traveling radiating downstream waves, and iii) subsonically convected evanescent waves, which follow the convection pattern of hydrodynamic eddies in the turbulent region. With increasing Mach number, the radiating and convected bands of energy move closer to each other. The hydrodynamic pressure also displays a wavepacket structure, but its features are different: it displays large-scale subsonically convected structures even past the core collapse region. Thus, in the turbulent region of the jet, the acoustic pressure displays smaller integral time scales of fluctuations than the hydrodynamic component. The acoustic pressure field, which includes a zero-crossing in its radial profiles, displays larger wavelengths than the hydrodynamic pressure field, correlates better with the near-field pressure signal and captures the radiated component of noise, especially at shallow angles. These properties make it a suitable field for informing pressure-based wavepacket models for jet noise.

Observation of long phase-coherence length in epitaxial La-doped CdO thin films

The search for long electron phase-coherence length, which is the length that an electron can keep its quantum wavelike properties, has attracted considerable interest in the last several decades. Here, we report the long phase-coherence length of \ensuremath{\sim}3.7 \ensuremath{\mu}m in La-doped CdO thin films at 2 K. Systematical investigations of the La doping and the temperature dependences of the electron mobility and the electron phase-coherence length reveal contrasting scattering mechanisms for these two physical properties. Furthermore, these results show that the oxygen vacancies could be the dominant scatters in CdO thin films that break the electron phase coherence, which would shed light on further investigation of phase-coherence properties in oxide materials.

Shape dependence and anisotropic finite-size scaling of the phase coherence of three-dimensional Bose-Einstein-condensed gases

We investigate the equilibrium phase-coherence properties of Bose-condensed particle systems, focusing on their shape dependence and finite-size scaling (FSS). We consider three-dimensional (3D) homogeneous systems confined to anisotropic L x L x L_a boxes, below the BEC transition temperature $T_c$. We show that the phase correlations develop peculiar anisotropic FSS for any $T<T_c$, in the large-$L$ limit keeping the ratio \lambda = L_a/L^2 fixed. This phenomenon is effectively described by the 3D spin-wave (SW) theory. Its universality is confirmed by quantum Monte Carlo simulations of the 3D Bose-Hubbard model in the BEC phase. The phase-coherence properties of very elongated BEC systems, \lambda>>1, are characterized by a coherence length \xi_a \sim A_t \rho_s/T where A_t is the transverse area and \rho_s is the superfluid density.

COHERENT EVENTS AND SPECTRAL SHAPE AT ION KINETIC SCALES IN THE FAST SOLAR WIND TURBULENCE

ABSTRACT In this paper we investigate spectral and phase coherence properties of magnetic fluctuations in the vicinity of the spectral transition from large, magnetohydrodynamic to sub-ion scales using in situ measurements of the Wind spacecraft in a fast stream. For the time interval investigated by Leamon et al. (1998) the phase coherence analysis shows the presence of sporadic quasi-parallel Alfvén ion cyclotron (AIC) waves as well as coherent structures in the form of large-amplitude, quasi-perpendicular Alfvén vortex-like structures and current sheets. These waves and structures importantly contribute to the observed power spectrum of magnetic fluctuations around ion scales; AIC waves contribute to the spectrum in a narrow frequency range whereas the coherent structures contribute to the spectrum over a wide frequency band from the inertial range to the sub-ion frequency range. We conclude that a particular combination of waves and coherent structures determines the spectral shape of the magnetic field spectrum around ion scales. This phenomenon provides a possible explanation for a high variability of the magnetic power spectra around ion scales observed in the solar wind.

Multimode waveguide speckle patterns for compressive sensing.

Compressive sensing (CS) of sparse gigahertz-band RF signals using microwave photonics may achieve better performances with smaller size, weight, and power than electronic CS or conventional Nyquist rate sampling. The critical element in a CS system is the device that produces the CS measurement matrix (MM). We show that passive speckle patterns in multimode waveguides potentially provide excellent MMs for CS. We measure and calculate the MM for a multimode fiber and perform simulations using this MM in a CS system. We show that the speckle MM exhibits the sharp phase transition and coherence properties needed for CS and that these properties are similar to those of a sub-Gaussian MM with the same mean and standard deviation. We calculate the MM for a multimode planar waveguide and find dimensions of the planar guide that give a speckle MM with a performance similar to that of the multimode fiber. The CS simulations show that all measured and calculated speckle MMs exhibit a robust performance with equal amplitude signals that are sparse in time, in frequency, and in wavelets (Haar wavelet transform). The planar waveguide results indicate a path to a microwave photonic integrated circuit for measuring sparse gigahertz-band RF signals using CS.

Future shallow water low frequency (100–1000 Hz) acoustic signal propagation physics studies

The measurement and prediction of the phase and amplitude properties of acoustic signals (1 to 1000 Hz) propagating in the dynamic shallow water (30 to 500 m water depths) environment has been a focus of the underwater acoustic research community for nearly 20 years. The majority of the ocean studies have occurred during late spring to summertime oceanic conditions when the sound speed variability was influenced by nonlinear mode 1 internal wavefields. Sound speed variability during the summer-to-winter and winter-to-summer water column transition periods will cause variability in the phase coherent properties of acoustic signals. Few experimental studies explicitly focused on quantifying the variability of phase coherent properties of acoustic signals during these periods have been performed. Illustration of the types of fluid processes that will perturb the sound speed structure during the fall-to-winter transition periods will be presented. Included will be mode 1 and mode 2 internal wave perturbation of the sound speed field as well as the locations and spatial scales of sound speed variability caused by interleaving water masses of varying temperature and salinity (Spice). Fall-to-winter transition sound speed fields will be contrasted with summer time sound speed conditions. [Work supported by the Office of Naval Research.]

Coherent nonlocal correlations in Andreev interferometers

Andreev interferometers, hybrid normal–superconducting loops, are well suited for studying phase-coherent effects, as they combine the robust quantum phase of a superconductor with a finite resistance normal section where phase-dependent transport properties can be easily measured. While they have previously been used to demonstrate local, phase-coherent properties, they may also be integrated into structures where nonlocal phase coherence occurs. In this paper, we use Andreev interferometer devices to experimentally establish the existence of nonlocal phase coherence between two normal metals linked by a superconductor. The generation of phase-coherent nonlocal signals is brought about by producing a nonequilibrium quasiparticle distribution in the normal section of the interferometer and tuning the phase with an external magnetic flux through the loop. Quasiclassical modeling of our experiment shows that the nonequilibrium distribution, coupled with the flux, leads to an induced supercurrent that is not constant along the length of the interferometer's normal section. The supercurrent–quasiparticle current conversion that occurs in this section is manifested in the production of flux-dependent nonlocal voltages through the mechanisms of crossed Andreev reflection and elastic cotunneling.

Magnetoresistance and Phase Breaking Behavior of a Nitrogen Doped Multi-Walled Carbon Nanotube

The low temperature magnetoresistance (MR) has been studied in a nitrogen doped multi-walled carbon nanotube (CNxMWNT) with a four terminal resistance measurement. The phase coherent properties of electron transport in the CNxMWNT have been deduced from results of quantum-transport analysis. A zero-field peak of the weak localization can be observed in the low-temperature MR, however, there exhibits a clear boundary between the phase braking processes around 10 K. The phase coherence has been analyzed and the quasi-one-dimensional properties have been discussed in weakly localized metallic transport with a low temperature MR in terms of quantum interference.

Finite-temperature Effect in Phase Transition to Superfluidity for Bose–Einstein Condensates in a 1-D Optical Lattice

We experimentally study the phase transition of (87)Rb Bose-Einstein condensates adiabatically loaded to a combined trap or a 1D optical lattice and a magnetic trap. The phase coherence property of this system is probed by recording the interference pattern of the expanded atomic cloud suddenly released from the combined trap. We show that as the temperature is sufficiently low (below the critical temperature T(C)), an interference pattern has a "peak on a peak" feature which is a true signature of macroscopic superfluid states. The normal gas only contributes to the broad background and three wide peaks in an interference pattern. These observations qualitatively agree with the recent theoretical predictions (Diener et al. in Phys. Rev. Lett. 98:180404, 2007; Kato et al. in Nat. Phys. 4:617, 2008). We also computed both the critical temperature and the interference pattern for a practical combined trap as ours in the tight-binding limit, and the numerical results are consistent with our experimental observations.

Bose–Einstein Condensates in a One-Dimensional Optical Lattice: from Superfluidity to Number-Squeezed States

We study the phase coherence property of Bose−Einstein condensates confined in a one-dimensional optical lattice formed by a standing-wave laser field. The lattice depth is determined using a method of Kapitza–Dirac scattering between a condensate and a short pulse lattice potential. Condensates are then adiabatically loaded into the optical lattice. The phase coherence property of the confined condensates is reflected by the interference patterns of the expanded atomic cloud released from the optical lattice. For weak lattice, nearly all of the atoms stay in a superfluid state. However, as the lattice depth is increased, the phase coherence of the whole condensate sample is gradually lost, which confirms that the sub-condensates in each lattice well have evolved into number-squeezed states.

Single parameter description of seafloors for shallow oceans

For a shallow ocean, sound transmission beyond short range is dominated by seafloor interactions at small grazing angles, for which the loss in dB on each bottom reflection may be approximated by a function F dB/radian which is linear with grazing angle. Acoustic inversion techniques have been shown to be able to obtain the value of F for a particular frequency. The suitability of this single parameter F as a seafloor descriptor has now been studied for an extensive range of seafloor types. It is shown that the phase-incoherent transmission loss (TL) obtained using the parameter F is not greatly different to TL predicted using complete knowledge of a particular seafloor, for a shallow ocean. Further, if the phase angle for a seafloor reflection is linked to the parameter F via a simple approximation, the phase-coherent properties of the shallow water interference field may be estimated to an accuracy which is surprising. This paper reviews the relevant theory and presents comparisons between TL predicted using full geoacoustic parameters versus TL based on the single parameter, for both uniform half-space and layered seafloor types.

Magneto-resistance peaks and phase breaking behaviour in a thin multi-walled carbon nanotube

The low temperature magneto-resistance (MR) has been studied in a single thin multi-walled carbon nanotube (MWNT) field effect transistor (FET). The phase coherent properties of electron transport in the FET have been deduced from results of quantum-transport studies. A large zero-field peak is observed in the low-temperature MR, and exhibits superimposed oscillatory components. Phase coherence is discussed, and the carrier concentration is also estimated, by analyzing these results in terms of quantum interference.

Super-Poissonian shot noise as a measure of dephasing in closed quantum dots

Electron-electron interactions play a major role in determining the low-temperature rate of phase loss of electrons in mesoscopic systems. The study of the dephasing rate is expected to contribute to the understanding of the many-body nature of such systems. Closed quantum dots are of special interest in this respect, due to theoretical predictions suggesting a possible transition temperature below which the dephasing rate vanishes. This prediction has attracted much attention, since closed quantum dots are prime candidates for storage units in quantum computers, and thus their phase coherence properties are of great importance. However, an effective method for measuring the dephasing rate within a closed quantum dot is still lacking. Here, we study two-level systems and show that the Fano factor has a sharp peak as a function of the chemical potential, the location of which can be simply related to the dephasing rate. We thus suggest to use the properties of the Fano factor peak in the super-Poissonian regime as a probe for the dephasing rate.

Ultracold Atoms in a Disordered Crystal of Light: Towards a Bose Glass

We use a bichromatic optical lattice to experimentally realize a disordered system of ultracold strongly interacting 87Rb bosons. In the absence of disorder, the atoms are pinned by repulsive interactions in the sites of an ideal optical crystal, forming one-dimensional Mott-insulator states. We measure the excitation spectrum of the system as a function of disorder strength and characterize its phase-coherence properties with a time-of-flight technique. Increasing disorder, we observe a broadening of the Mott-insulator resonances and the transition to a state with vanishing long-range phase coherence and a flat density of excitations, which suggest the formation of a Bose-glass phase.

Phase Coherence of an Atomic Mott Insulator

We investigate the phase coherence properties of ultracold Bose gases in optical lattices, with special emphasis on the Mott insulating phase. We show that phase coherence on short length scales persists even deep in the insulating phase, preserving a finite visibility of the interference pattern observed after free expansion. This behavior can be attributed to a coherent admixture of particle-hole pairs to the perfect Mott state for small but finite tunneling. In addition, small but reproducible kinks are seen in the visibility, in a broad range of atom numbers. We interpret them as signatures for density redistribution in the shell structure of the trapped Mott insulator.

Bose-Einstein condensation, phase coherence, and two-fluid behavior in 4He.

It is shown that the assumption that the many particle wave functions describing superfluid 4He have some phase coherence properties of correlated basis functions provides a new explanation of the link between Bose-Einstein condensation and two fluid behavior. This assumption also implies that the condensate fraction is proportional to the superfluid fraction, in agreement with experiment, and provides a simple quantitative explanation of the anomalous reduction in spatial order and liquid density, observed as the temperature is lowered in superfluid 4He.

Macroscopic quantum tunnelling in rotating Bose–Einstein condensates

In this paper we investigate various macroscopic quantum tunnellingphenomena and the phase coherence property of rotating Bose–Einsteincondensates. Based on the two-mode approximation, three differentcases are studied. When the rotating frequency is independent of timet, by usingthe SU(2)coherent state path integral method, we discuss two cases: a superposition of twomacroscopic states and a Josephson-type ground state. In addition, the specificconditions to reach the two cases are obtained. We also discuss the nonlinearLandau–Zener effect in the dynamic case in which the rotating frequency varies withtime t.

Matched-field processing gain degradation caused by tidal flow over continental shelf bathymetry.

Temporally variable, range dependent sound-speed profiles measured during ebb flow and estimated for slack flow are used to quantify the variability of matched-field signal-processing gain degradation in shallow water propagation channels controlled by tidally driven stratified flow over variable bathymetry. Calculations along a 9.3 km range establish phase changes in the acoustic signal as the primary cause of a 3-9 dB degradation in the coherent matched-field processing output of a full water column vertical array. The work indicates that over a tidal cycle acoustic signal properties and matched-field processing gain can be expected to change continuously in a shallow water stratified channel that has bathymetry variability. Acoustic signals propagating in such tidal flow-controlled environments may be expected to display repeatable (over successive tidal cycles) and predictable changes in their phase coherent properties. These results suggest that matched-field processor replica fields used in the shelf/slope propagation environment will have to be updated regularly during a tidal cycle to maintain maximum processor gain.

Quantum Coherence of the Ground State of a Mesoscopic Ring

We discuss the phase coherence properties of a mesoscopic normal ring coupled to an electric environment via Coulomb interactions. This system can be mapped onto the Caldeira–Leggett model with a flux dependent tunneling amplitude. We show that depending on the strength of the coupling between the ring and the environment the free energy can be obtained either by a Bethe ansatz approch (for weak coupling) or by using a perturbative expression which we derive here (in the case of strong coupling). We demonstrate that the zero-point fluctuations of the environment can strongly suppress the persistent current of the ring below its value in the absence of the environment. This is an indication that the equilibrium fluctuations in the environment disturb the coherence of the wave functions in the ring. Moreover the influence of quantum fluctuations can induce symmetry breaking seen in the polarization of the ring and in the flux induced capacitance.