Signal-to-noise ratio in microelectrode-array-based electrochemical detectors

The signal-to-noise ratio at an electrode array depends on the electrode area, the perimeter-to-area ratio of the electroactive portion of the surface, the mass transfer coefficient of the analyte-electrode combination, the measurement bandwidth, and the sources and magnitudes of the noises. Simple models for chronoamperometry with an array in quiescent solution and for hydrodynamic current at an array in one wall of a rectangular conduit through which analyte-containing solution is following are given. Noises from seven sources, including environmental noises, are considered in a noise model. The signal and noise models are combined to yield a model for signal-to-noise ratio at array-based electrochemical detectors. There exists an optimum array density for a given area that depends on the noise power, noise resistance, the current density at a sparse array, and the current density at a solid electrode of the same area. Approximations that lead to simple expressions for the optimum electroactive area fraction and noise resistance lead to results that are in good agreement with more complex and less approximate calculations. Electrodes of millimeter dimensions consisting of about 1% active surface with electroactive "pieces" of micrometer dimensions are anticipated to yield detection limits of about 1 fmol injected into a typical packed-column liquid chromatograph. This corresponds to about 10(-10) M analyte in the detector and about an order of magnitude improvement over solid electrodes.     

Superconducting single-photon detector for the visible and infrared spectral range

Abstract

We present results on dark count rates and spectral sensitivities of superconducting single-photon detectors in the visible and near-infrared spectral range. The active detector element is a nanometre-sized (a few nanometres thick and less than 100nm wide) meander line carrying a supercurrent. The superconducting materials are NbN and Nb, respectively. The NbN detector exhibited a flat spectral sensitivity up to about 2.4μm. Fluctuations of the superconducting order parameter are considered as a major source of dark count events. A simple model and its limitations to explain the observed dark count rates is discussed.     

Quantum Random-number Generation and Key Sharing

Abstract

We review the status of interferometry-based quantum cryptography and compare photon-pair and faint-pulse schemes. The key technical limitations in both cases are the propagation losses and detector performance. We also discuss a simple approach to generating the random measurement bases used in quantum cryptography systems which exploits random partition at a beam splitter. This removes the need for active components in the receiver, reducing system complexity and losses.     

The Cryogenic Anticoincidence Detector for ATHENA-XMS

The TES cryogenic detectors, due to their high spectral resolution and imaging capability in the soft X-ray domain, are the reference devices for the next proposed space missions whose aims are to characterize the spectra of faint or diffuse sources. ATHENA is the re-scoped IXO mission, and one of its focal plane instrument is the X-ray Microcalorimeter Spectrometer (XMS) working in the energy range 0.3–10?keV. XMS will be able to achieve the proposed scientific goals if a background lower than 0.02?cts/cm²/s/keV is guaranteed. The studies performed by GEANT4 simulations depict a scenario where it is mandatory to use an active Anti-Coincidence (AC) to reduce the expected background in the L2 orbit down to the required level. This is possible using a cryogenic AC detector able to provide a rejection efficiency of about 99%. We are developing for this purpose a TES-based detector made by Silicon absorbers (total assembled area about 1?cm² and 300?μm thick) and sensed by a Ir:Au TES. All the work done for IXO is applicable to ATHENA, with more margins due to the smaller area required for the detector. Here we present the results obtained from different samples, as a step towards the final detector design.     

Spatial Signal Detection Using Continuous Shrinkage Priors

Abstract

Motivated by the problem of detecting changes in two-dimensional X-ray diffraction data, we propose a Bayesian spatial model for sparse signal detection in image data. Our model places considerable mass near zero and has heavy tails to reflect the prior belief that the image signal is zero for most pixels and large for an important subset. We show that the spatial prior places mass on nearby locations simultaneously being zero, and also allows for nearby locations to simultaneously be large signals. The form of the prior also facilitates efficient computing for large images. We conduct a simulation study to evaluate the properties of the proposed prior and show that it outperforms other spatial models. We apply our method in the analysis of X-ray diffraction data from a two-dimensional area detector to detect changes in the pattern when the material is exposed to an electric field.     

Optical heterodyne detection of random electromagnetic beams

Abstract

In this paper we present a general analysis for the optical heterodyne detection of random electromagnetic beams. To describe the ensemble of quasimonochromatic beams which are partially polarized and partially coherent, we use a recently developed matrix treatment. We derive an expression for the signal-to-noise ratio (SNR) in terms of the beam coherence polarization matrices of the beams on the detector surface. Numerical examples are given for the SNR variation in the case of partially polarized Gaussian Schell model beams and the optimum detection is discussed in terms of beam parameters of the local oscillator.     

Detection of nanoparticles using optical gradient forces

Abstract

We present a detection scheme for nanoscale particles based on the gradient force and torque near a tightly focused laser beam. The focus affects the path of nanoparticles passing by and a quadrant detector records the particle trajectory. A feedback system continuously adjusts the laser power and thereby prevents the particles from being trapped. Particle size and shape can be assessed by evaluating the time-trace of the quadrant detector signal.     

Development of a digital astronomical intensity interferometer: laboratory results with thermal light

Abstract

We present measurements of the second-order spatial coherence function of thermal light sources using Hanbury-Brown and Twiss interferometry with a digital correlator. We demonstrate that intensity fluctuations between orthogonal polarizations, or at detector separations greater than the spatial coherence length of the source, are uncorrelated but can be used to reduce systematic noise. The work performed here can readily be applied to existing and future Imaging Air-Cherenkov Telescopes used as star light collectors for stellar intensity interferometry to measure spatial properties of astronomical objects.     

Iron working at Zambana el Vato (TN), Italy

ABSTRACT

The workshop of Zambana el Vato (region Trentino, Northern Italy), is dated to the period between the 7th-6th and the 5th century BC. Iron working activities are clearly recognizable from the various finds. Among them there are working slag, heated clay, fragments of hearth or forge, hammerscale and more residues that can be referred to iron technology. A number of selected specimens were sectioned and mounted for photomicroscopy to identify the structure and some of the mounted samples were also examined by scanning electron microscopy (SEM) using both a back scattered electron detector and energy dispersive (EDS) x-ray analysis. This paper presents the results of these studies. The hearths were regularly repaired, as their fragments were found mixed with working slag. The hammerscale samples indicate that there were three iron-working areas. The fragments of forge with traces of tuyeres indicate that bellows were employed. Refining slag was identified among the debris. This is particularly significant as for the moment no iron refining centers are known in this area.     

Possibility of analyzing deuterium-tritium gas mixtures using a time-of-flight mass spectrometer

The use of a time-of-flight mass spectrometer, whose ion source and detector operate in a synchronous regime, is proposed for the chemical and isotope analysis of gas mixtures containing radioactive tritium. The experiments performed confirm that the detector background current caused by tritium β electrons scarcely influences the signal-to-noise ratio and does not diminish the accuracy of analyses. Pis’ma Zh. Tekh. Fiz. 23, 83–87 (October 26, 1997)     

Detector efficiency limits on quantum improvement

Abstract

Although the National Institute of Standards and Technology has measured the intrinsic quantum efficiency of Si and InGaAs avalanche photo diode (APD) materials to be above 98% by building an efficient compound detector, commercially available devices have efficiencies ranging between 15 and 75%. This means bandwidth, dark current, cost, and other factors are more important than quantum efficiency for existing applications. For non-classical correlated photon applications, the system's correlated signal-to-noise ratio is proportional to (ηN)^½ /(1 ? η)^½, rather than the classical signal-to-noise (ηN)^½. Consequently, the detector design trade space must be re-evaluated. This paper systematically examines the generic detection process, lays out the considerations needed for designing detectors for non-classical applications, and identifies the ultimate physical limits on quantum efficiency.     

On the Design of Čzerny-Turner Charge-coupled Device Spectrographs

Abstract

The problems of the optical design of a ?zerny-Turner spectrograph for use with a charge-coupled device detector are discussed, and theorems are derived which describe its performance. It is shown that a flat-field design is possible, with meridional coma corrected at one wavelength and insignificantly small in the whole field. At the same time, the focal plane can be made perpendicular to the median chief-ray of the output fan so that a focal reducer can be attached and the final camera focal ratio reduced to F/2 or less. A 1 m ?zerny-Turner spectrograph with a camera focal ratio of F/2 and with a dispersion of 0·35 Å pixel^?1 and corresponding resolution is described. An example is given of an airglow spectrum showing details of the OH Meinel band in the vicinity of the 6300 Å forbidden line of atomic oxygen.     

Nonlinear quantum optical computing via measurement

Abstract

We show how the measurement induced model of quantum computation proposed by Raussendorf and Briegel (2001, Phys. Rev. Letts., 86, 5188) can be adapted to a nonlinear optical interaction. This optical implementation requires a Kerr nonlinearity, a single photon source, a single photon detector and fast feed forward. Although nondeterministic optical quantum information proposals such as that suggested by KLM (2001, Nature, 409, 46) do not require a Kerr nonlinearity they do require complex reconfigurable optical networks. The proposal in this paper has the benefit of a single static optical layout with fixed device parameters, where the algorithm is defined by the final measurement procedure.     

Large-area scintillating-fiber time-of-flight/hodoscope detectors for particle astrophysics experiments

A new generation of detectors of ultra-heavy (UH) cosmic rays is being built which require the use of large-area, light-weight detector elements. In this paper, we present the design and operation of a new type of scintillating optical fiber detector, which can serve as both a time-of-flight (TOF) detector and coded hodoscope (i.e. trajectory detector). We demonstrate that this type of detector can make TOF measurements with a time resolution of 48 ps. We also present results of position measurements from a large area TOF/hodoscope that was flown on a high-altitude balloon flight in August, 1995. Finally, we discuss areas of modification and improvements that can be made to these detectors to enable them to be more useful for making UH cosmic ray measurements.     

On the possibility of detecting solar neutrinos with an 115In detector

The inverse β-decay of ¹¹⁵In provides a low-threshold direct-counting neutrino detector. Unfortunately, the radioactive background due to ¹¹⁵In β-decay may be a very grave obstacle in the development of a detector for pp solar neutrinos. We suggest that this background can be efficiently suppressed in searches for higher energy solar neutrinos, e.g. from Be, p-e-p and ⁸B. A signal/background ratio of 10–30 is expected.It may be possible to have a count rate of 0.2 events/d with a detector consisting of 10 tons of indium and about 15 m³ of liquid xenon. Unfortunately, pp neutrinos cannot be detected in this set-up.     

The superheated superconducting colloid: A new particle detector

The use of micron sized superconducting grains in the search for “dark matter” is discussed. Cosmions and massive neutrinos could produce count rates up to 10⁶ per kg of active detector per day, and the signal could be differentiated from the background by the modulation produced by the Earth's motion around the Sun.We report a radiation test of a small superheated superconducting colloid in which 90 KeV γ-rays were detected using an rf SQUID. Individual “flips” of 5 μm radius grains were observed with a signal/noise ratio of 10.     

Optical modelling of cone concentrators for positive and negative IR emitters

Abstract

InSb optical cone concentrators are modelled to assess their optical performance in positive and negative emission modes. The output distribution from the active layer is shown to be isotropic for thin active regions, tending towards Lambertian distribution as the layer thickness increases. Refraction from the emitting surface is shown to make the distribution more Lambertian. Optical efficiencies of straight-sided and Winston cone concentrators are modelled using a ray-tracing program, and those of straight-sided cones also determined using an analytical approximation, which allows use of the distribution derived earlier. The effect on the active layer thickness on the positive and negative emission is also determined. Normal light emitting diode structures are found to be poor positive emitters and to draw large currents when used as negative emitters. A Winston cone of area gain equal to n ² is found to be the best option for devices required to work in both positive and negative modes. Intermediate structures are also considered. The optimum active layer thickness is derived for three emitter configurations.     

Towards photostatistics from photon-number discriminating detectors

Abstract

We study the properties of a photodetector that has a numberresolving capability. In the absence of dark counts, due to its finite quantum efficiency, photodetection with such a detector can only eliminate the possibility that the incident field corresponds to a number of photons less than the detected photon number. We show that such a non-photon number-discriminating detector, however, provides a useful tool in the reconstruction of the photon number distribution of the incident field even in the presence of dark counts.     

Absolute linearity measurements on a PbS detector in the infrared

The nonlinearity characteristics of a commercially available thin-film photoconductive PbS detector were experimentally investigated in the infrared using the National Physical Laboratory detector linearity characterization facility. The deviation from linearity of this detector was shown to be significant even for relatively low values of radiant power incident on the active area of the detector. For example, the linearity factor was approximately 0.8 when 0.6 microW of radiant power at a wavelength of 2.2 microm was illuminating a spot of 1 mm in diameter on the active area of the PbS detector. These figures demonstrate the poor linearity characteristics of this detector and provide a warning to other users of PbS detection systems. The deviation from linearity was shown to be a function of the size of the spot being illuminated on the detector active area, as well as the wavelength of the incident radiation. The deviation from linearity was shown to be a function of irradiance illuminating the detector for irradiance values lower than 1 microW mm(-2).     

High-stability time-domain balanced homodyne detector for ultrafast optical pulse applications

Abstract

Low-noise, efficient, phase-sensitive time-domain optical detection is essential for foundational tests of quantum physics based on optical quantum states and the realization of numerous applications ranging from quantum key distribution to coherent classical telecommunications. Stability, bandwidth, efficiency, and signal-to-noise ratio are crucial performance parameters for effective detector operation. Here we present a high-bandwidth, low-noise, ultra-stable time-domain coherent measurement scheme based on balanced homodyne detection ideally suited to characterization of quantum and classical light fields in well-defined ultrashort optical pulse modes.