Weak Coherent Signal Research Articles

Multidimensional mode-separable frequency conversion for high-speed quantum communication

Quantum frequency conversion (QFC) of photonic signals preserves quantum information while simultaneously changing the signal wavelength. A common application of QFC is to translate the wavelength of a signal compatible with the current fiber-optic infrastructure to a shorter wavelength more compatible with high quality single-photon detectors and optical memories. Recent work has investigated the use of QFC to manipulate and measure specific temporal modes (TMs) through tailoring of the pump pulses. Such a scheme holds promise for multidimensional quantum state manipulation that is both low loss and re-programmable on a fast time scale. We demonstrate the first QFC temporal mode sorting system in a four-dimensional Hilbert space, achieving a conversion efficiency and mode separability as high as 92% and 0.84, respectively. A 20-GHz pulse train is projected onto 6 different TMs, including superposition states, and mode separability with weak coherent signals is verified via photon counting. Such ultrafast high-dimensional photonic signals could enable long-distance quantum communication with high rates.

Photon statistics of pulse-pumped four-wave mixing in fiber with weak signal injection**Project supported by the National Natural Science Foundation of China (Grant No. 11527808), the State Key Development Program for Basic Research of China (Grant No. 2014CB340103), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120032110055), the Natural Science Foundation of Tianjin, China

We study the photon statistics of pulse-pumped four-wave mixing in fibers with weak coherent signal injection by measuring the intensity correlation functions of individual signal and idler fields. The experimental results show that the intensity correlation function of individual signal (idler) field decreases with the intensity of signal injection. After applying narrow band filter in signal (idler) band, the value of decreases from 1.9 ± 0.02 (1.9 ± 0.02) to 1.03 ± 0.02 (1.05 ± 0.02) when the intensity of signal injection varies from 0 to 120 photons/pulse. The results indicate that the photon statistics changes from Bose–Einstein distribution to Poisson distribution. We calculate the intensity correlation functions by using the multi-mode theory of four-wave mixing in fibers. The theoretical curves well fit the experimental results. Our investigation will be useful for mitigating the crosstalk between quantum and classical channels in a dense wavelength division multiplexing network.

Polarization Analysis with 3He Spin Filters for Separating Coherent from Incoherent Scattering in Soft Matter Studies

In soft matter small angle neutron scattering (SANS) studies at large Q values, incoherent scattering becomes the dominant signal. In the Q-range of interest to this work, from 0.2 A −1 to about 1.0 A −1 , the coherent scattering from the typical protein or polymer in a D2 Ob uffer solution inevitably drops one to two orders of magnitude or more below the total scattering. Even after careful and accurate subtraction of the measured D2 Ob uffer scattering, the remaining corrected, i.e. sample-only, signal will still be dominated by diffuse incoherent scattering from hydrogen in the sample itself. This is the exact region of interest when one wishes to probe the structural changes in ”living” proteins caused by interactions and motions related to function. To further complicate the problem, there is strong motivation to measure this Q-regime at very low concentrations because it has been shown with wide angle X-ray scattering that proteins can undergo concentration-dependent structural changes that rapidly increase below concentrations of about 5% [1] motivating the study of protein solutions at ever lower concentrations. In this case the signal from the protein will inevitably become much less than the scattering of the D2 Ob uffer solution it is contained in. Polarization analysis offers the opportunity to separate the weak coherent signal from the larger incoherent signal and perhaps enable measurements under the conditions described above. This paper will address the issues associated with the correct separation of coherent and incoherent scattering for soft matter samples. We have performed tests measurements on KWS2 which show the viability of the method on a protonated α-lactalbumin solution at 2.5% (1 mm thick) and 0.25% (2 mm thick) concentrations in a D2 Ob uffer solution. Additionally describe a the method of implementation using 3 He spin filters, some practical considerations, and future plans for a dedicated device at the JCNS. c � 2012 for authors transferring the copyright to Elsevier use the following: Elsevier BV. Selection and/or peer-review under responsibility of the organizing committee for PNCMI 2012.

Accurate Estimation of the Number of Weak Coherent Signals

Estimating the number of signals (NIS) is an important goal in array signal processing, such as direction-of-arrival (DOA) estimation. A common approach for solving this problem is to use an eigenvalue of the array covariance matrix and information criterion, such as the Akaike information criterion (AIC) and minimum description length (MDL). However they suffer serious degradation, when the incoming signals are coherent. To estimate the NIS of the coherent signals impinging on a uniform linear array (ULA), a method for estimating the number of signals without eigendecomposition (MENSE) is proposed. The accuracy of the NIS estimation performance of MENSE is superior to the other algorithms equipped with preprocessing such as the spatial smoothing preprocessing (SSP) and forward/backward spatial smoothing techniques (FBSS) to decorrelate the coherency of signals. Instead of using SSP or FBSS preprocessing, MENSE uses the Hankel correlation matrices. The Hankel correlation matrices can not only decorrelate the coherency of signals but also suppress the influence of noise. However, in severe conditions like low signal-to-noise ratio (SNR) or a closely spaced signals impinging on a ULA, the NIS estimation metric of MENSE has some bias which causes estimation error. In this paper, we pay attention to the multiplicity defined by the ratio of the geometric mean to the arithmetic mean. Accordingly, we propose a new estimation metric that has less bias than that in MENSE. The Computer simulation results show that the proposed method is superior to MENSE in the above severe conditions.

Dynamical theory of polariton amplifiers.

We present the theory of the dynamics of the polariton amplifier in the region of small polariton densities. We give an analytical solution for the polariton condensate density matrix and show that the formation of a coherent quantum state is possible. Once the condensate is formed, the coherence becomes macroscopically long living. Polariton amplifier represents, therefore, an optical memory element, where the input weak coherent signal can be amplified and kept.

Generation of mesoscopic quantum superpositions through Kerr-stimulated degenerate downconversion

A two-step interaction scheme involving (2) and (3) nonlinear media is suggested for the generation of Schrodinger cat-like states of a single-mode optical field. In the first step, a weak coherent signal undergoes a self-Kerr phase modulation in a (3) crystal, leading to a Kerr kitten, namely a microscopic superposition of two coherent states with opposite phases. In the second step, such a Kerr kitten enters a (2) crystal and, in turn, plays the role of a quantum seed for stimulated phase-sensitive amplification. The output state in the above-threshold regime consists in a quantum superposition of mesoscopically distinguishable squeezed states, i.e. an optical cat-like state. The whole setup does not rely on conditional measurements, and is robust against decoherence, as only weak signals interact with the Kerr medium.

Stochastic Resonance in a Globally Coupled Neuronal Network

The phenomenon of stochastic resonance in a globally coupled neuronal network has been studied via numerical simulation. The ability of the network to detect weak (subthreshold) periodic signals can be enhanced within a range of noise intensities. There exists a long plateau in the curve of the output signal-to-noise ratio versus noise intensity, which is saturated when the number of neurons in the network N ≥300. Such stochastic resonance without tuning may enable us to interpret the strong capability of nervous systems to distinguish weak coherent signals from environmental noise.

深部地殻探査に向けた反射法地震探査実験概報

The seismic profiling experiment using vibrator sources was conducted at Hidaka, Hokkaido, to clarify the deep geological structure in this area and to estimate an applicability and field parameters of the seismic survey in geologically complex area and in noisy circumstances. The preliminary results show that the diversity stacking suppresses burst noises like trafic ones and that the band-limited deconvolution filter seems to eliminate high frequency noise. Although the obtained seismic data are so noisy, a careful velocity analysis discriminates weak coherent signals and, therefore, provides a velocity structure in this area. The velocity structure is shown to be in harmony with the gravity data and surface geology in this area.

A Technique for Lowering the Noise Threshold of Conventional Frequency, Phase and Envelope Demodulators

Envelope, phase and frequency demodulators exhibit a stronger-signal capture phenomenon which enables the noise to suppress the signal when the signal is the weaker of the two, and the signal to suppress the noise when the reverse is true. The presence of the noise thus imposes a threshold that must be exceeded by the signal for proper envelope, phase or frequency demodulation. A general discussion is presented of the relative signal and noise content of the envelope, phase and frequency of the resultant of an amplitude- or exponent-modulated sinusoid plus random-fluctuation noise, when one or the other is heavily dominant. An analysis is then presented of the effect of regenerative feedback around an amplitude limiter upon a sinusoid plus random noise in order to define the conditions under which a CW-type signal will be enhanced over a strong random-fluctuation noise. It is shown that an oscillating limiter whose output maintains a high degree of coherence in the presence of a pure random-noise excitation will boost a wide class of relatively weak coherent signals into predominance over the noise when such a signal is added to the noise. Substantial threshold reductions which have been demonstrated in the laboratory are thereby made possible.