Influence of Source Parameters on the Polarization Properties of Beams for Practical Free-Space Quantum Key Distribution

Polarization encoding has been extensively used in quantum key distribution (QKD) implementations along free-space links. However, the calculation model to characterize channel transmittance and quantum bit error rate (QBER) for free-space QKD has not been systematically studied. As a result, it is often assumed that misalignment error is equal to a fixed value, which is not theoretically rigorous. In this paper, we investigate the depolarization and rotation of the signal beams resulting from spatially-dependent polarization effects of the use of curved optics in an off-axis configuration, where decoherence can be characterized by the Huygens–Fresnel principle and the cross-spectral density matrix (CSDM). The transmittance and misalignment error in a practical free-space QKD can thus be estimated using the method. Furthermore, the numerical simulations clearly show that the polarization effect caused by turbulence can be effectively mitigated when maintaining good beam coherence properties.

Propagation features of a class of beams radiated from pseudo-Schell model vectorial sources

Pseudo Schell-model sources were introduced and studied in the scalar domain as those partially coherent sources that present a degree of coherence depending on the difference between the radial coordinates of the two considered points. In this work we study the propagation features of a class of beams radiated from sources of this kind, but endowed with a vectorial nature. The polarization pattern and the degree of polarization are not uniform across the beam section but remain invariant upon free paraxial propagation. On the other hand, their coherence and irradiance features are also non-uniform but in this case they change at each z-plane. The field characteristics can be varied on acting on the free parameters of the cross-spectral density matrix of the source.

Seismic noise monitoring of a small rock block collapse test

SUMMARY We tested the capability of seismic noise to monitor the stability conditions of a small rock block that we forced to fail in four following stages. Ambient vibrations were recorded with a broad-band 3C seismometer placed on top of the block and were processed to analyse their spectral and polarization characteristics with diverse algorithms. To analyse the spectral content of the records, we applied the multitaper method while seismic noise polarization features were investigated by means of the singular value decomposition of the Hermitian spectral density matrix. Numerical modelling was found to add limited value because of the uncertainty in estimating correctly spatial and mechanical features of the rock bridges between the block and the rock mass. Nevertheless, a modelling exercise we performed is in agreement with previous post-failure observations according to which unstable rocks may be coupled to the stable rock mass by rock bridges covering only a few per cent of the total surface of the fractures. Our analyses confirm that, when approaching final collapse, there is a trend of the block eigenmodes towards lower frequencies and show that polarized bands become narrower.

Synthesizing General Electromagnetic Partially Coherent Sources from Random, Correlated Complex Screens

We present a method to generate any genuine electromagnetic partially coherent source (PCS) from correlated, stochastic complex screens. The method described here can be directly implemented on existing spatial-light-modulator-based vector beam generators and can be used in any application which utilizes electromagnetic PCSs. Our method is based on the genuine cross-spectral density matrix criterion. Applying that criterion, we show that stochastic vector field realizations (corresponding to a desired electromagnetic PCS) can be generated by passing correlated Gaussian random numbers through “filters” with space-variant transfer functions. We include step-by-step instructions on how to generate the electromagnetic PCS field realizations. As an example, we simulate the synthesis of a new electromagnetic PCS. Using Monte Carlo analysis, we compute statistical moments from independent optical field realizations and compare those to the corresponding theory. We find that our method produces the desired source—the correct shape, polarization, and coherence properties—within 600 field realizations.

Abstract TP158: Direct Path Functional Connectivity Changes in Stroke Survivors’ Brain Motor Areas Using a Brain Computer Interface Intervention for Upper Extremity Recovery

Objective: This research seeks to identify changes in frequency specific directional flow of information transmission (functional connectivity) as a result of BCI intervention. Direct path functional connectivity (isolated effective coherence (iCOH)) changes between motor-related brain areas (Brodmann areas (BA) 1-7) are thought to relate to electrophysiological signatures of motor learning. Methods: N=16 right hemisphere stroke survivors participated in 9-15 sessions with the BCI. Participants executed hand movements prompted by visual cues on a monitor concordantly with the corresponding audio instructions (e.g., Left, Right, Rest). The ‘screening’ sessions (i.e. pre and post BCI intervention) contained two runs, each consisting of 15 trials for rest, left hand, and right hand movements (i.e., 5 trials for each of the three conditions, the order of trials in a run was random). iCOH is based on formulating a multivariate autoregressive model from time series measurements, and calculating the corresponding partial coherences after setting all irrelevant connections to zero, according to Pascual-Marqui et al. 2014. From the spectral density matrix (including Mu[8-12 Hz] and Beta[18-26 Hz] ranges) obtained from estimated signals in the selected ROI, the partial coherences between any pair of nodes can be calculated. The t-statistics was performed for iCOH values between post and pre of the impaired (left) hand movement trials and thresholded at p=0.05( t=2.13, uncorrected). Results: In the Mu band, iCOH decreased pre to post from contralesional BA 4 to ipsilesional BA 5 and increased from ipsilesional BA 4 to ipsilesional BA 6. In Beta band, iCOH decreased pre to post from ipsilesional BA 5 to contralesional BA 7 and increased from ipsilesional BA 4 and & contralesional BA 7 to ipsilesional BA 6 and also from ipsilesional BA 1,2,3 to contralesional BA 4. Conclusion: There is a consistent change in the direction of information flow, as measured by iCOH, toward the ipsilesional motor and pre-motor areas (BA 4,6). Significant iCOH increases are observed in both Mu and Beta from ipsilesional BA 4 to ipsilesional BA 6 suggesting an increase in participation of motor brain areas associated with motor planning and execution with participation in BCI intervention.

CLEAN beamforming for the enhanced detection of multiple infrasonic sources

SUMMARY The detection and characterization of signals of interest in the presence of (in)coherent ambient noise is central to the analysis of infrasound array data. Microbaroms have an extended source region and a dynamical character. From the perspective of an infrasound array, these coherent noise sources appear as interfering signals that conventional beamform methods may not correctly resolve. This limits the ability of an infrasound array to dissect the incoming wavefield into individual components. In this paper, this problem will be addressed by proposing a high-resolution beamform technique in combination with the CLEAN algorithm. CLEAN iteratively selects the maximum of the f/k spectrum (i.e. following the Bartlett or the Capon method) and removes a percentage of the corresponding signal from the cross-spectral density matrix. In this procedure, the array response is deconvolved from the f/k spectral density function. The spectral peaks are retained in a ‘clean’ spectrum. A data-driven stopping criterion for CLEAN is proposed, which relies on the framework of Fisher statistics. This allows the construction of an automated algorithm that continuously extracts coherent energy until the point is reached that only incoherent noise is left in the data. CLEAN is tested on a synthetic data set and is applied to data from multiple International Monitoring System infrasound arrays. The results show that the proposed method allows for the identification of multiple microbarom source regions in the Northern Atlantic that would have remained unidentified if conventional methods had been applied.