Random Phase Fluctuations Research Articles

Atom interferometer using a free oscillation of Bose-Einstein Condensate on an atom chip

We have demonstrated a novel atom interferometer which uses a free oscillation of 87Rb BEC atoms in a magnetic harmonic potential on an atom chip. This interferometer can effectively suppress the dephasing due to the trap potential by applying two Bragg laser pulses with an time interval T equal to the oscillation period of the axial mode of the magnetic trap. We observed a high contrast (∼30 %) interference fringe signal for T = 58 ms. When we increased the time interval T to 97 ms, an interference fringe was washed out by the random phase fluctuation. We attributed this phase fluctuation to the external vibrational noise from the floor.

Modal Analysis of Path Length Sensitivity in Superposition Architectures for Coherent Laser Beam Combining

An analytical model is developed to describe the modal behavior of a class of coherent beam combining resonators based on laser beam superposition. This model is used to explore the interplay between modal discrimination and tolerance to gain element path length changes. Under certain conditions, the sensitivity to the random phase fluctuations of the individual gain elements can be substantially reduced.

Preparation of distilled and purified continuous-variable entangled states

Two independent experiments demonstrate that quantum entanglement that has been lost in decoherence processes can be recovered. For the first time such ’entanglement distillation’ has been achieved for states of light that are entangled in continuous variables, which should help to increase the distance over which quantum information can be distributed. The distribution of entangled states of light over long distances is a major challenge in the field of quantum information. Optical losses, phase diffusion and mixing with thermal states lead to decoherence and destroy the non-classical states after some finite transmission-line length. Quantum repeater protocols1,2, which combine quantum memory3, entanglement distillation4,5 and entanglement swapping6, were proposed to overcome this problem. Here we report on the experimental demonstration of entanglement distillation in the continuous-variable regime7,8,9. Entangled states were first disturbed by random phase fluctuations and then distilled and purified using interference on beam splitters and homodyne detection. Measurements of covariance matrices clearly indicate a regained strength of entanglement and purity of the distilled states. In contrast to previous demonstrations of entanglement distillation in the complementary discrete-variable regime10,11, our scheme achieved the actual preparation of the distilled states, which might therefore be used to improve the quality of downstream applications such as quantum teleportation12.

Mid-latitude amplitude scintillation of GPS signals and GPS performance slips

Degradation of transionospheric radio signals and operation failures during ionospheric disturbances constitute a crucial factor of space weather influence on radio engineering satellite systems performance. We found that during the main phase of strong magnetic storms in 2000–2003 when the auroral oval expands into mid-latitudes, its southern boundary develops a region with intense small-scale electron density irregularities. Such irregularities may cause strong amplitude scintillations of GPS signals at both GPS operating frequencies. The another consequence of it was significant random GPS signal phase fluctuations, breaking-down of signal tracking, and sharp increasing of GPS positioning errors as a result.

Soliton generation from randomly modulated return-to-zero pulses

We consider return-to-zero (RZ) pulses with random phase modulation propagating in a nonlinear channel (modelled by the integrable nonlinear Schrödinger equation, NLSE). We suggest two different models for the phase fluctuations of the optical field: (i) Gaussian short-correlated fluctuations and (ii) generalized telegraph process. Using the rectangular-shaped pulse form we demonstrate that the presence of phase fluctuations of both types strongly influences the number of solitons generated in the channel. It is also shown that increasing the correlation time for the random phase fluctuations affects the coherent content of a pulse in a non-trivial way. The result obtained has potential consequences for all-optical processing and design of optical decision elements.

Relaxation of synchronization on complex networks

We study collective synchronization in a large number of coupled oscillators on various complex networks. In particular, we focus on the relaxation dynamics of the synchronization, which is important from the viewpoint of information transfer or the dynamics of system recovery from a perturbation. We measure the relaxation time tau that is required to establish global synchronization by varying the structural properties of the networks. It is found that the relaxation time in a strong-coupling regime (K>Kc) logarithmically increases with network size N , which is attributed to the initial random phase fluctuation given by O(N-1/2) . After elimination of the initial-phase fluctuation, the relaxation time is found to be independent of the system size; this implies that the local interaction that depends on the structural connectivity is irrelevant in the relaxation dynamics of the synchronization in the strong-coupling regime. The relaxation dynamics is analytically derived in a form independent of the system size, and it exhibits good consistency with numerical simulations. As an application, we also explore the recovery dynamics of the oscillators when perturbations enter the system.

应用时间分集改善大气光通信性能

A laser beam propagating through turbulence experiences random amplitude and phase fluctuations, which can severely degrade the performance of free space optical communication systems. In this letter, time diversity is demonstrated as a technique which can decrease turbulence influence. Statistically, laser propagation along an atmospheric path is uncorrelated with an earlier-time path for a time interval greater than the atmospheric turbulence correlation time. To estimate time diversity system performance, a 2.2-km optical link is set up for comparing the fade probability of a system using time diversity with a system not using time diversity. The experimental results obtained under different turbulence conditions are shown which are in good agreement with the theory.

Scattering quantum random-walk search with errors

We analyze the realization of a quantum-walk search algorithm in a passive, linear optical network. The specific model enables us to consider the effect of realistic sources of noise and losses on the search efficiency. Photon loss uniform in all directions is shown to lead to the rescaling of search time. Deviation from directional uniformity leads to the enhancement of the search efficiency compared to uniform loss with the same average. In certain cases even increasing loss in some of the directions can improve search efficiency. We show that while we approach the classical limit of the general search algorithm by introducing random phase fluctuations, its utility for searching is lost. Using numerical methods, we found that for static phase errors the averaged search efficiency displays a damped oscillatory behavior that asymptotically tends to a nonzero value.

High visibility in an electronic Mach–Zehnder interferometer with random phase fluctuations

We present an original method to measure the visibility of interferences in an electronic Mach–Zehnder interferometer in the presence of low frequency phase fluctuations. We studied the visibility of the interferences as a function of the energy and showed a gaussian variation of the decoherence and/or phase averaging at finite energy. This gaussian variation leads to a visibility modulation with a single side lobe.

Stochastic convective transport in presence of fragmented dendrites in a solidifying binary melt

A comprehensive integrated model of stochastic convective transport in a solidifying binary melt is presented in this work. The detailed transport phenomena in the particle and bulk phases are coupled together through a stochastic Eulerian-Lagrangian formalism, capturing the physical mechanisms and consequences of particle agglomeration, de-agglomeration, and the underlying hydrodynamic interactions. The interactions between random thermo-fluidic fluctuations in the continuum carrier phase and the associated growth/dissolution of particle phases are modeled by employing a Langevin formalism. Representative case studies highlighting the implications of the dynamics of the fragmented dendrites on the overall convective patterns in an electromagnetically-stirred semi-solid binary melt are subsequently presented, so as to emphasize on the comprehensive physical bases and to illustrate the fundamental approach of the present predictive methodology.

Performance limitations of an optical heterodyne continuous-phase frequency-shift keying transmission system affected by polarization mode dispersion in a single-mode fiber

A theoretical analysis is developed to evaluate the conditional and average bit error rate (BER) performance degradation of an optical heterodyne continuous-phase frequency-shift keying (CPFSK) system caused by signal phase distortion due to polarization mode dispersion (PMD) in a single-mode fiber. The probability density function (pdf) of the random phase fluctuation due to PMD at the output of the receiver is determined analytically. The average BER performance results are evaluated at a bit rate of 10 Gb/s, considering Maxwellian distribution for the differential group delay (DGD) based on the pdf of the random phase fluctuation. The results show that the performance of the optical heterodyne CPFSK direct detection system suffers power penalties of 0.75, 1.95, 4.00, and 9.15 dB, corresponding to mean DGD of 20, 30, 40, and 60 ps, respectively, at a average BER of 10 -9 operating at 10 Gb/s. Furthermore, at increased values of the mean DGD, there occur BER floors above 10 -9 , which cannot be lowered by further increasing the signal power. It is noticed that BER floors occur at about 2×10 -8 , 10 -5 , and 2.5×10 -4 , corresponding to mean DGD of 60, 70, and 80 ps, respectively, at 10 Gb/s and a modulation index of 0.50. The effect of PMD is found to be more detrimental at higher modulation indexes and bit rates.

Statistical model for analyzing the dephasing effects in a one-dimensional scattering chain

Based on the phenomenological statistical model of dephasing introduced by Pala and Iannaccone [Phys. Rev. B 69, 235304 (2004)], we perform an analytical and numerical calculation of electron transmission through a one-dimensional (1D) scattering chain. With the introduction of random phase fluctuations into the computation of the scattering matrix, dephasing can be treated as a distribution phenomenon. By adopting the scattering matrix method combined with a Monte Carlo averaging procedure, we simulate the transition from a coherent transport to a fully incoherent transport in the 1D scattering chain. We also find the random-phase fluctuations closely related to the phase breaker model introduced by Mello et al. [Phys. Rev. B 61, 16570 (2000)].

Universal dynamical control of local decoherence for multipartite and multilevel systems

A unified theory is given of dynamically modified decay and decoherence of field-driven multilevel multipartite entangled states that are weakly coupled to zero-temperature baths or undergo random phase fluctuations. The theory allows for arbitrary local differences in their coupling to the environment. Due to such differences, the optimal driving-field modulation to ensure maximal fidelity is found to substantially differ from conventional “Bang–Bang” or π-phase flips of the single-qubit evolution.

Artifact removal in complex frequency domain optical coherence tomography with an iterative least-squares phase-shifting algorithm

We present what we believe to be a new computational algorithm for complex frequency domain optical coherence tomography that can effectively suppress artifacts that are caused by uncertainty in phase shift due to sample motion and errors in reference phases. The algorithm treats the phase-shifting values as additional unknowns, and we can determine their exact values by analyzing interference fringes using the numerical least-squares technique. A series of simulations and experiments prove that this algorithm can effectively remove strong mirror-image artifacts because it is unaffected by random phase fluctuation.

Self-Trapping of Spatially and Temporally Incoherent White Light in a Photochemical Medium

We report that a beam of spatially and temporally incoherent white light self-traps by initiating free-radical polymerization in an organosiloxane medium. Refractive index changes due to polymerization lead to the formation of a narrow channel waveguide that traps and guides the entire multimode, broadband beam without diffraction. The response time of the system, which is determined by the inherently slow rate of free-radical polymerization, exceeds by several orders of magnitude the femtosecond-scale random phase fluctuations that characterize white light. Self-trapping of incoherent light is possible in the photochemical medium because it responds to the time-averaged intensity profile of the white light beam.

The role of pump incoherence in continuous-wave supercontinuum generation

Supercontinuum generation can be achieved in the continuous-wave regime with a few watts of pump power launched into kilometer-long fibers. High power spectral density broadband light sources can be obtained in this way. Using a generalized nonlinear Schrödinger equation model and an ensemble averaging procedure that takes into account the partially-coherent nature of the pump laser, we fully explain for the first time the spectral broadening mechanisms underlying this process. Our simulations and experiments confirm that continuous-wave supercontinuum generation involve Raman soliton dynamics and dispersive waves in a way akin to pulsed supercontinua. The Raman solitons are however generated with a wide distribution of parameters because they originate from the random phase and intensity fluctuations associated with the pump incoherence. This soliton distribution is averaged out by experimental measurements, which explains the remarkable smoothness of experimental continuous-wave supercontinuum spectra.

The light curve of the semiregular variable L2 Puppis -- II. Evidence for solar-like excitation of the oscillations

We analyse visual observations of the pulsations of the red giant variable L 2 Puppis (L 2 Pup). The data cover 77 yr between 1927 and 2005, thus providing an extensive empirical base for characterizing properties of the oscillations. The power spectrum of the light curve shows a single mode resolved into multiple peaks under a narrow envelope. We argue that this results from stochastic excitation, as seen in solar oscillations, with a mode lifetime of about 5 yr. The random fluctuations in phase also support this idea. A comparison with X Cam, a true Mira star with the same pulsation period, and W Cyg, a true semiregular star, illustrates the basic differences in phase behaviours. The Mira shows very stable phase, consistent with excitation by the κ-mechanism, whereas W Cyg shows large phase fluctuations that imply stochastic excitation. We find L 2 Pup to be intermediate, implying that both mechanisms play a role in its pulsation. Finally, we also checked the presence of low-dimensional chaos and could safely exclude it.

Universal dynamical control of decay and decoherence for weak and strong system-bath coupling

A unified theory is given of dynamically modified decay and decoherence in driven two-level and multilevel quantum systems that are weakly coupled to arbitrary finite-temperature reservoirs and undergo random phase fluctuations. Criteria for the optimization of decoherence suppression and the limitations of this approach are obtained. For a driven qubit that is strongly coupled to the continuum edge of reservoir's spectrum, we demonstrate that only an appropriately ordered sequence of abrupt changes of the resonance frequency, near the continuum edge, can effectively protect the qubit state from decoherence.

Random-phase solitons in nonlinear periodic lattices.

We predict the existence of random phase solitons in nonlinear periodic lattices. These solitons exist when the nonlinear response time is much longer than the characteristic time of random phase fluctuations. The intensity profiles, power spectra, and statistical (coherence) properties of these stationary waves conform to the periodicity of the lattice. The general phenomenon of such solitons is analyzed in the context of nonlinear photonic lattices.

Statistical model of dephasing in mesoscopic devices introduced in the scattering matrix formalism

We propose a phenomenological model of dephasing in mesoscopic transport, based on the introduction of random-phase fluctuations in the computation of the scattering matrix of the system. A Monte Carlo averaging procedure allows us to extract electrical and microscopic device properties. We show that, in this picture, scattering matrix properties enforced by current conservation and time-reversal invariance still hold. In order to assess the validity of the proposed approach, we present simulations of conductance and magnetoconductance of Aharonov-Bohm rings that reproduce the behavior observed in experiments, in particular as far as aspects related to decoherence are concerned.