Ti/Au TES to Discriminate Single Photons

Single photon detectors are fundamental devices in several scientific fields, like in optical science and technology. We introduce the state of the art of Transition Edge Sensors, developed and characterized at INRIM, to detect photons from optical to near-infrared wavelength range (406–1570?nm). Both the capability to resolve up to 29 incident photons in a single pulse and to discriminate up to 2 photons with an energy resolution of 0.18?eV have been reached.     

Collection efficiency of a single optical fiber in turbid media

If a single optical fiber is used for both delivery and collection of light, two major factors affect the measurement of collected light: (1) the light transport in the medium that describes the amount of light that returns to the fiber and (2) the light coupling to the optical fiber that depends on the angular distribution of photons entering the fiber. We focus on the importance of the latter factor and describe how the efficiency of the coupling depends on the optical properties of the medium. For highly scattering tissues, the efficiency is well predicted by the numerical aperture (NA) of the fiber. For lower scattering, such as in soft tissues, photons arrive at the fiber from deeper depths, and the coupling efficiency could increase twofold to threefold above that predicted by the NA.     

Detection and characterization of optical inhomogeneities with diffuse photon density waves: a signal-to-noise analysis

Diffusing photons provide information about the optical properties of turbid media. In biological tissues these optical properties may be correlated to physiological parameters, enabling one to probe effectively the physiological states of tissue for abnormalities such as tumors and hemorrhages. We show that positional uncertainty in the source and detector lead to significant random errors that degrade the optical information available from diffusing photons. We investigate the limits for the detection, localization, and characterization of optical inhomogeneities by using diffusing photons as a probe. Although detection is sufficient for tumor screening, full characterization of the optical properties is desirable for specification of the tumor. Our findings in model breast systems with realistic signal-to-noise ratios indicate that tumors as small as 0.3 cm in diameter can be unambiguously detected; however, simultaneous determination of tumor size and optical properties is possible only if its diameter is of the order of 1.0 cm or larger. On the other hand, if a priori information about the size (optical properties) is available, then the optical properties (size) of tumors as small as 0.3 cm in diameter can be determined.     

Near-infrared laser tomographic imaging with right-angled scattered coherent light using an optical heterodyne-detection-based confocal scanning system

We demonstrate, what is to the best of our knowledge, a novel optical tomographic method for the visualization of the inner structure of scattering media such as biological tissue in the near-infrared region. We constructed a scanning confocal imaging system with a cross-axes arrangement using optical fibers. This system is based on the optical heterodyne technique and enables the detection of very weak coherence photons that are generated in the spatially restricted confocal region and scattered laterally (90 degrees ) against an incident beam. To evaluate the fundamental imaging capabilities of the system, we assessed measurements from scattering phantoms composed of an Intralipid suspension with varying volume concentrations. The results of this study demonstrate that the right-angled scattered light adheres to the Lambert-Beer law and that the present system can detect light propagating through a distance of approximately 31l of the mean free path. An inclusion as small as 100 microm can be discriminated in a scattering media with an optical thickness of 4. We investigated the potential of the proposed system for imaging biological tissues in preliminary experiments using samples of chicken breast tissue.     

Two-dimensional near-infrared transillumination imaging of biomedical media with a chromium-doped forsterite laser

Transillumination images of objects hidden in normal and cancerous human breast tissues and bovine, porcine, and gallinaceous (chicken) tissues as well as model-random-scattering media were recorded with 1250-nm light from a chromium-doped forsterite laser. A Fourier space gate and a polarization gate were used to sort out image-bearing photons and discriminate against multiply scattered image-blurring photons. Better contrast, higher spatial resolution, and deeper penetration of samples were achieved for imaging with 1250-nm light than those obtained at shorter wavelengths, such as 1064 nm from a neodymium-doped yttrium aluminum garnet (YAG) laser. Better contrast and higher resolution were also obtained when the object was imaged through normal human breast tissue than through cancerous breast tissue. Images with marked distinction between fatty and fibrous human breast tissues were obtained when the Cr:forsterite laser was tuned to 1225 nm, a wavelength that resonates with an optical absorption band of breast fat tissues. Imaging with linearly polarized light revealed that the image quality depends significantly on the orientation of the polarization of the incident light with respect to the fibers in the bovine tissue.     

Measurement of the local optical properties of turbid media by differential path-length spectroscopy

We report on the development of an optical-fiber-based diagnostic tool with which to determine the local optical properties of a turbid medium. By using a single fiber in contact with the medium to deliver and detect white light, we have optimized the probability of detection of photons scattered from small depths. The contribution of scattered light from greater depths to the signal is measured and subtracted with an additional fiber, i.e., a collection fiber, to yield a differential backscatter signal. Phantoms demonstrate that, when photons have large mean free paths compared with the fiber diameter, single scattering dominates the differential backscatter signal. When photons have small mean free paths compared with the fiber diameter, the apparent path length of the photons that contribute to the differential backscatter signal becomes approximately equal to 4/5 of the fiber diameter. This effect is nearly independent of the optical properties of the sample under investigation.     

Vertex/propagator model for least-scattered photons traversing a turbid medium

The least-scattered photons that arrive at a detector through highly scattering tissues have the potential to image internal structures, functions, and status with high imaging resolution. In contrast, optical diffusing tomography is based on the use of the late-arriving photons, which have been diffusely scattered, leading to very low imaging resolution. A good model of the early-arriving photons, i.e., the least-scattered photons, may have a significant effect on the development of imaging algorithms and a further understanding of imaging mechanisms within current high-resolution optical-imaging techniques. We describe a vertex/propagator approach that attempts to find the probabilities for least-scattered photons traversing a scattering medium, based on analytical expressions for photon histories. The basic mathematical derivations for the model are outlined, and the results are discussed and found to be in very good agreement with those from the Monte Carlo simulations.     

Generating non-classical light inside an optical cavity by depositing photons

The creation of non-classical states of light is an interesting problem, that we solve sending atoms through an optical cavity. We show that it is possible to add or subtract many photons from a cavity field by interacting it resonantly with a two-level atom. The atom, after entangling with the field inside the cavity and exiting it, may be measured in one of the Schmidt states, producing a multiphoton process (in the sense that can add or annihilate more photons than a single transition allows), i.e. adding or subtracting several photons from the cavity field. By plotting the quadratures and the Husimi Q-function, we also show that the non-classical state produced by such measurements is a squeezed state.     

Generation of four-atom entangled decoherence-free states by interference of polarized photons

We propose a protocol to generate four-atom entangled decoherence-free states trapped in distant cavities by using the interference of polarized photons. The protocol uses the effects of quantum statistics of indistinguishable photons emitted by the atoms inside optical cavities. In the Lamb–Dicke limit, the protocol does not require the simultaneous click of detectors. This makes the protocol more realizable in experiments.     

Quantum interferometry with distorted wave packets

We herein study the rate of coincidences in a Hong–Ou–Mandel interferometer exit whose arms have been supplemented by two optical cavities which reflect the photons generated by parametric down conversion. The fourth-order correlation function at the beam-splitter exit is calculated. In the regime where the coherence length of the photons is larger than the cavities lengths, photon coalescence and anti-coalescence interferences are observed. In particular, complete anti-coalescence between photons generated by parametric down conversion Type I is evidenced. The photons reshaping as they are reflected by the cavities are also studied and path diagrams for the indistinguishable processes that lead to quantum interference are presented.     

Single-photon detection beyond 1 µm: performance of commercially available germanium photodiodes

Germanium avalanche photodiodes (APD's) working biased above the breakdown voltage detect single optical photons in the near-infrared wavelength range. We give guidelines for the selection of devices suitable for photon-counting applications among the commercial samples, and we discuss in detail how the devices should be operated to achieve the best performance, both in terms of noise-equivalent power (NEP) and the timing-equivalent bandwidth. We introduce the driving electronics and we show that, in the measurements of fast optical signals, the adoption of single-photon techniques is very favorable, notwithstanding that presently available photodiodes are not designed for this purpose. On the contrary, in the detection of cw signals, the lower NEP values achieved in photon counting may not be sufficient to justify the replacement of conventional analog p-i-n germanium detectors, which offer comparable performance with a definitely larger sensitive area. Finally, we show that, by properly choosing theoperating conditions, some selected APD's achieve an 85-ps time resolution in the detection of optical photons at a 1.3-μm wavelength, which corresponds to a timing-equivalent bandwidth of 1.8 GHz. To the best of our knowledge, this time resolution is the lowest reported to date for single-photon detectors in the near infrared.     

Simulating the effects of multiple scattering on images of dense sprays and particle fields

Most optical measurements in turbid media (including sprays, fogs, particulate and colloidal suspensions) assume single scattering of the detected photons. Multiple scattering introduces error, which has been quantified in very few systems. To quantify this error, we have written a flexible Monte Carlo photon transport simulation code capable of handling any three-dimensional geometry. Simulations of planar laser spray imaging with large, nonabsorbing particles show that up to 50% of the photons reaching the camera are multiply scattered. Because forward scattering dominates, the image is affected little. For particles with more absorption or with size closer to the wavelength of the light than those we have simulated, the effects are expected to be more serious.     

When AES blinks: introducing optical side channel

The authors present a short note describing the newly emerging optical side channel. The basic idea of the channel is very simple - many parts of the integrated circuits consist of transistors that represent one of the two logical states 0 or 1.When the state changes, there is some light that is emitted in the form of a few photons. A device employing the method which is able to detect these photons (called picosecond imaging circuit analysis) is available in several laboratories, for example, in the French space agency CNES. From the point of view of the cryptanalyst, once the optical side channel information is available for a specific cipher on a device, it is possible to identify deep inner states that should not be revealed. In fact, it turns out that for an outdated and unprotected 0.8 mm PIC16F84A microcontroller it is possible to recover the AES secret key directly during the initial AddRoundKey operation as the side channel can distinguish the individual key bits being XORed to the plaintext.     

声光延迟效应用于消除微弱背景光噪声

本文提出一种基于声光延迟效应消除脉冲光信号中微弱背景光噪声的方法,利用两个串联的声光调制器组成的光学开关有效地抑制了与信号光同频率的背景光,信号背景改善比达13 dB.其中,第一个声光调制器的作用是利用其对脉冲光的非线性光学延迟效应实现信号光与背景光在时序上的分离.第二个声光调制器将时域上已经和脉冲信号光分开的背景光消...     

Quantum cryptography using optical fibers

Quantum cryptography permits the transmission of secret information whose security is guaranteed by the uncertainty principle. An experimental system for quantum crytography is implemented based on the linear polarization of single photons transmitted by an optical fiber. Polarization-preserving optical fiber and a feedback loop are employed to maintain the state of polarization. Error rates of less than 0.5% are obtained.     

An integrated active-quenching circuit for single-photon avalanchediodes

We introduce the first integrated active quenching circuit (I-AQC) that drives an avalanche photodiode (APD) above its breakdown voltage, in order to detect single photons. Based on the I-AQC, we developed a compact and versatile photon-counting module suitable for applications in which very weak optical signals have to be detected, as for instance, photon correlation spectroscopy, luminescence measurements, and laser ranging. The prototype photon-counting module features quenching pulses up to 60 V amplitude, minimum dead time of less than 36 ns, corresponding to a saturated counting rate exceeding 25 Mcounts/s, user-controllable hold-off time, for reducing the afterpulsing effect, and nanosecond gating capability. The power dissipation is 60 mW in stand-by conditions, and the module size is less than 1 cm×2 cm     

Semi-analytical Monte Carlo simulation for time-resolved light propagating in multilayered turbid media

In this study, a Monte Carlo (MC) method for time-resolved light scattering from multilayered turbid media (SMCML) has been developed. This method is particularly suitable for simulating light backscattering from layered media and receiving the time-resolved signal in a finite sensor area, such as ocean detection, photomedicine and photobiology. The classical semi-analytical MC method requires the scattering events to be located in a single-layer medium. To address the multilayer problem, the energy loss mechanism of photons propagating in tissue was analyzed in this study. According to the energy contribution to the detector, only photons that contribute significantly were considered. Simulations were conducted for stochastic turbid media with different optical parameters. Temporal profiles of the echo signal were obtained with a satisfactory convergence. Compared to the classical MC method, the SMCML method can dramatically reduce the computation time by more than two orders of magnitude.     

The role of auxiliary photons in state discrimination with linear optical devices

The role of auxiliary photons in the problem of identifying a state secretly chosen from a given set of L-photon states is analysed. It is shown that auxiliary photons do not increase the ability to discriminate such states by means of a global measurement using only optical linear elements, conditional transformation and auxiliary photons.     

Optically detected X-ray absorption spectroscopy measurements as a means of monitoring corrosion layers on copper

XANES and EXAFS information is conventionally measured in transmission through the energy-dependent absorption of X-rays or by observing X-ray fluorescence, but secondary fluorescence processes, such as the emission of electrons and optical photons (e.g., 200-1000 nm), can also be used as a carrier of the XAS signatures, providing complementary information such as improved surface specificity. Where the near-visible photons have a shorter range in a material, the data will be more surface specific. Moreover, optical radiation may escape more readily than X-rays through liquid in an environmental cell. Here, we describe a first test of optically detected X-ray absorption spectroscopy (ODXAS) for monitoring electrochemical treatments on copper-based alloys, for example, heritage metals. Artificially made corrosion products deposited on a copper substrate were analyzed in air and in a 1% (w/v) sodium sesquicarbonate solution to simulate typical conservation methods for copper-based objects recovered from marine environments. The measurements were made on stations 7.1 and 9.2 MF (SRS Daresbury, UK) using the mobile luminescence end station (MoLES), supplemented by XAS measurements taken on DUBBLE (BM26 A) at the ESRF. The ODXAS spectra usually contain fine structure similar to that of XAS spectra measured in X-ray fluorescence. Importantly, for the compounds examined, the ODXAS is significantly more surface specific, and >98% characteristic of thin surface layers of 0.5-1.5-microm thickness in cases where X-ray measurements are dominated by the substrate. However, EXAFS and XANES from broadband optical measurements are superimposed on a high background due to other optical emission modes. This produces statistical fluctuations up to double what would be expected from normal counting statistics because the data retain the absolute statistical fluctuation in the original raw count, while losing up to 70% of their magnitude when background is removed. The problem may be solved in future through optical filtering to isolate the information-containing band, combined with the use of higher input X-ray fluxes available on third-generation light sources.