This PDF file contains the front matter associated with SPIE Proceedings Volume 8375, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We present the latest results on two kinds of photon detectors: single photon detectors (SPDs) and photon number resolving detector (PNRD). We developed high speed and low noise SPDs using superconducting nano-wire (abbreviated by SNSPD) and semiconductor (InGaAs) avalanche photodiode (APD). The SNSPD system has totally four channels all of which have the detection eciency higher than 16% at 100Hz dark count rate. The InGaAs APD system also has four channels and the best performance is represented by the after-pulse probability of 0.61%, the dark count probability of 0.71×10^-6 (~1kHz), and the detection eciency of 10.9%. Both systems were applied to wavelength division multiplexing quantum key distribution (WDM-QKD) operated at 1.2GHz repetition rate in a eld environment. The PNRD is made of superconducting transition edge sensor. It was applied to the implementation of quantum receiver which could beat the homodyne limit of the bit error rate of binary coherent states. We discuss future perspective of quantum communications with those photon detection technologies, including multi-user QKD networks and low-power high capacity communications.

Random number generators (RNG) are an important resource in many areas: cryptography (both quantum and classical), probabilistic computation (Monte Carlo methods), numerical simulations, industrial testing and labeling, hazard games, scientific research etc. Because today's computers are deterministic, they can not create random numbers unless complemented with a physical RNG. Randomness of a RNG can be defined and scientifically characterized and measured. Especially valuable is the information-theoretic provable RNG which, at state of the art, seem to be possible only by harvest of randomness inherent to certain (simple) quantum systems and such a generator we call Quantum RNG (QRNG). On the other hand, current industry standards dictate use of RNGs based on free running oscillators (FRO) whose randomness is derived from electronics noise present in logic circuits and which, although quantum in nature, cannot be strictly proven. This approach is currently used in FPGA and ASIC chips. We compare weak and strong aspects of the two approaches for use in cryptography and in general. We also give an alternative definition of randomness, discuss usage of single photon detectors in realization of QRNGs and give several examples where QRNG can significantly improve security of a cryptographic system.

By using hybrid photodetectors we exploited the photon-number correlations existing in bipartite optical states to demonstrate the effect of multiple-photon subtraction on the generation of conditional states in the pulsed regime. We operated on both classical and quantum Gaussian bipartite states in the mesoscopic regime without background subtraction and corrections. The obtained conditional states are non-Gaussian in nature, thus particularly useful for applications to Quantum Information. All the experimental results are in excellent agreement with theoretical models.

Currently, the applications of fiber optic sensors on the automobile industry are gaining importance due to their potential for implementation in data acquisition and signal transmission. This paper covers from quantum mechanics, the photon counting in optical fibers using coherent states and generalized intelligent states, described by hyper-geometric functions and Bessel functions. Different fiber optic configurations will be showed, in order to show some representative factors that influence the probability of coherent and intelligent photons detected and transmitted by optical fibers. Finally, from the automotive industry, some applications are presented, from which the quantum-optical approach here proposed makes sense.

We present a novel image sensor for high dynamic range imaging. The sensor performs an adaptive one-bit quantization at each pixel, with the pixel output switched from 0 to 1 only if the number of photons reaching that pixel is greater than or equal to a quantization threshold. With an oracle knowledge of the incident light intensity, one can pick an optimal threshold (for that light intensity) and the corresponding Fisher information contained in the output sequence follows closely that of an ideal unquantized sensor over a wide range of intensity values. This observation suggests the potential gains one may achieve by adaptively updating the quantization thresholds. As the main contribution of this work, we propose a time-sequential threshold-updating rule that asymptotically approaches the performance of the oracle scheme. With every threshold mapped to a number of ordered states, the dynamics of the proposed scheme can be modeled as a parametric Markov chain. We show that the frequencies of different thresholds converge to a steady-state distribution that is concentrated around the optimal choice. Moreover, numerical experiments show that the theoretical performance measures (Fisher information and Cram´er-Rao bounds) can be achieved by a maximum likelihood estimator, which is guaranteed to find globally optimal solution due to the concavity of the log-likelihood functions. Compared with conventional image sensors and the strategy that utilizes a constant single-photon threshold considered in previous work, the proposed scheme attains orders of magnitude improvement in terms of sensor dynamic ranges.

Photon counting techniques using direct detection has recently gained considerable interest within the laser radar community. The high sensitivity is of special importance to achieve high area coverage in surveillance and mapping applications and long range with compact systems for imaging, profiling and ranging. New short pulse lasers including the super continuum laser is of interest for active spectral imaging. A special technique in photon counting is the "time correlated single photon counting" (TCSPC). This can be utilized together with short pulse (ps) lasers to achieve very high range resolution and accuracy (mm level). Low average power lasers in the mW range enables covert operation with respect to present laser warning technology. By analyzing the return waveform range and shape information from the target can be extracted. By scanning the beam high resolution 3D images are obtained. At FOI we have studied the TCSPC with respect to range profiling and imaging. Limitations due to low SNR and dwell times are studied in conjunction with varying daylight background and atmospheric turbulence. Examples of measurements will be presented and discussed with respect to some system applications.

The unparalleled sensitivity of 3D LADAR imaging sensors based on single photon detection provides substantial benefits for imaging at long stand-off distances and minimizing laser pulse energy requirements. To obtain 3D LADAR images with single photon sensitivity, we have demonstrated focal plane arrays (FPAs) based on InGaAsP Geiger-mode avalanche photodiodes (GmAPDs) optimized for use at either 1.06 μm or 1.55 μm. These state-of-the-art FPAs exhibit excellent pixel-level performance and the capability for 100% pixel yield on a 32 x 32 format. To realize the full potential of these FPAs, we have recently developed an integrated camera system providing turnkey operation based on FPGA control. This system implementation enables the extremely high frame-rate capability of the GmAPD FPA, and frame rates in excess of 250 kHz (for 0.4 μs range gates) can be accommodated using an industry-standard CameraLink interface in full configuration. Real-time data streaming for continuous acquisition of 2 μs range gate point cloud data with 13-bit time-stamp resolution at 186 kHz frame rates has been established using multiple solid-state storage drives. Range gate durations spanning 4 ns to 10 μs provide broad operational flexibility. The camera also provides real-time signal processing in the form of multi-frame gray-scale contrast images and single-frame time-stamp histograms, and automated bias control has been implemented to maintain a constant photon detection efficiency in the presence of ambient temperature changes. A comprehensive graphical user interface has been developed to provide complete camera control using a simple serial command set, and this command set supports highly flexible end-user customization.

Three dimensional (3D) image acquisitions is the enabling technology of a great number of applications; culture heritage morphology study, industrial robotics, automotive active safety and security access control are example of applications. The most important feature is the high frame-rate, to detect very fast events within the acquired scenes. In order to reduce the computational complexity, Time-of-Flight algorithms for single sensor cameras are used. To achieve high-frame rate and high distance measurement accuracy it is important to collect the most part of the reflected light using sensor with very high sensitivity, allowing the implementation of a low-power light source. We designed and developed a single-photon detection based 3D ranging camera, capable to acquire distance image up to 22.5 m, with a resolution down to one centimeter. The light source used in this prototype employs 8 laser diodes sinusoidally modulated. The imager used in the application is based on Single-Photon Avalanche Diodes (SPADs) fabricated in a standard CMOS 0.35 μm technology. The sensor has 1024 pixels arranged in a 32x32 squared layout, with overall dimensions of 3.5mm x 3.5mm. The camera acquires 3D images through the continuous-wave indirect Time of Flight (cw-iTOF) technique. The typical frame-rate is 20 fps while the theoretical maximum frame-rate is 5 kfps. The precision is better than 5 cm within 22.5 m range, and can be effectively used in indoor applications, e.g. in industrial environment.

We present a multi-channel coincidence-counting module for use in quantum optics experiments. The circuit takes up to four TTL pulse inputs and counts either 2-, 3-, or 4-fold coincidences, within a user-selected coincidence time window as short as 12 ns. The module can accurately count eight sets of multi-channel coincidences, for input rates of up to 84 MHz. Due to their low cost and small size, multiple modules can easily be combined to count arbitrary M-order coincidences among N inputs.

Over the past years an always growing interest has arisen about the measurement technique of time-correlated single photon counting (TCSPC) and many applications exploiting TCSPC have been developing in several fields, such as medicine and chemistry. The use of multianode PMTs and of single photon avalanche diode arrays led to the development of acquisition systems with several parallel channels, to employ the TCSPC technique in even more applications. Since TCSPC basically consists in the photons arrival time measurement, the most important part of an acquisition chain is the time measurement block, which must have high resolution and low differential nonlinearity and, in order to implement multidimensional systems, it has to be integrated to reduce both cost and area. To accomplish all these specifications, we have designed and fabricated a 4 channel fully integrated time-to-amplitude converter (TAC), built in 0.35 μm Si-Ge technology, characterized by a variable full scale range from 11 ns to 89 ns, very good time resolution (better than 20 ps FWHM), low differential nonlinearity (better than 0.04 LSB peak-peak and less than 0.2% LSB rms), high counting rate (16 MHz), low and constant power dissipation (50 mW) and low area occupation (340 × 390 μm² per channel). Our measurements also show a very little crosstalk between converters integrated on the same chip; this feature together with low power and low area make the fabricated converter suitable for parallelization, so that it can be the starting point for future acquisition chains with a high number of parallel channels.

We present a novel configuration for a photon number resolving detector based on a series array of shunted superconducting nanowires, which has the potential to provide a high dynamic range. The first prototype of the detector consisting of four series elements is demonstrated with the ability to resolve up to four photons in an incident optical pulse at the telecommunication wavelength window.

Thanks to the steady improvement in the detectors' performance, single-photon techniques are nowadays employed in a large number of applications ranging from single molecule dynamics to astronomy. In particular, silicon Single Photon Avalanche Diodes (SPAD) play a crucial role in this field thanks to their remarkable performance in terms of Photon Detection Efficiency (PDE), temporal response and Dark Count Rate (DCR). While CMOS technology allows the fabrication of large arrays of SPAD with built-in electronics, it is only resorting to custom fabrication processes that is possible to attain detectors with high-end performance required by most demanding applications. However, the fabrication of arrays for timing applications, even with a small number of pixels, is quite challenging with custom processes owing to electrical coupling between pixels. In this paper we will discuss technological solutions for the fabrication of arrays of high-performance SPAD for parallel photon timing.

Motivated by the need for correct characterization and operation of single-photon avalanche diodes (SPADs), three methods for discriminating between single- and multi-photon triggering in a single CMOS SPAD are compared. The first method, utilizing a measurement of the avalanche's quench time, correctly distinguishes between avalanches initiated by one photon or 100 incident photons with p > 0.80 in single-shot measurements. The second method, which modulates the detector efficiency, correctly distinguished streams of pulses with single photons or 100 photons with p > 0.99, but is unable to provide a single-shot measurement. The third method, which examines distortions to the timing jitter's diffusion tail, requires knowledge of the incident timing jitter a priori and is unlikely to be useful in most systems. All compared methods are independent of one another, and show promise for distinguishing how many photons seed an avalanche.

In the last years many progresses have been made in the field of silicon Single Photon Avalanche Diodes (SPAD) thanks to the improvements both in device design and in fabrication technology. Particularly, the Dipartimento di Elettronica e Informazione of Politecnico di Milano and the CNR-IMM of Bologna have been in the forefront of this research activity by designing and fabricating a new device structure enabling the fabrication of SPADs with red enhanced photon detection efficiency. In this paper we present a compact photon counting and timing module that fills the gap between the high temporal resolution and the high detection efficiency systems. The module exploits Red-Enhanced SPAD technology to attain a Photon Detection Efficiency (PDE) as high as 37% at 800 nm (peak of 58% at 600 nm) while maintaining a temporal resolution of about 100 ps FWHM, even with light diffused across the whole active area. A thermo-electric cooling system guarantees a noise as low as few counts per second for a 50 μm diameter SPAD while a low threshold avalanche pick-up circuit assures a limited shift in the temporal response.

The Ge APD detectors are fabricated on Si by using a selective chemical-vapor deposition (CVD) epitaxial growth technique. A novel processing procedure was developed for the p⁺ Ge surface doping by a sequence of pure-Ga and pure-B depositions (PureGaB). Then, PVD Al is used to contact the n-type Si and the anode of p⁺n Ge diode. Arrays of diodes with different areas, as large as 40×40 μm², were fabricated. The resulting p⁺n diodes have exceptionally good IV characteristics with ideality factor of ~1.1 and low saturation currents. The devices can be fabricated with a range of breakdown voltages from a minimum of 9 V to a maximum of 13 V. They can be operated both in proportional and in Geiger mode, and exhibit relatively low dark counts, as low as 10 kHz at 1 V excess reverse bias. The dark current at 1 V reverse bias are as low as 2 pA and 20 pA for a 2×2 μm² and 2×20 μm² devices, respectively. Higher IR-induced current than that induced by visible light confirms the sensitivity of Ge photodiodes at room temperature. The 25% peak in I_d/I_ref at an IR-wavelength of 1100 nm in Geiger mode is measured for excess bias voltages of 3 V and 4 V, where I_d refers to the photocurrent of the 2×20 μm² device at different wavelengths, and I_ref is the reference photodiode current. The timing response (Jitter) for the APD when exposed to a pulsed laser at 637 nm and 1 V excess bias is measured as 900 ps at full width of half maximum (FWHM).

High resolution imaging in the UV band has a lot of applications in defense and commercial systems. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPD's have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss model that can predict sensor performance in the UV band using APD's with various gain and other parameters for a desired UV band of interest. SNR's can be modeled from illuminated targets at various distances with high resolution under standard atmospheres in the UV band and the solar-blind region using detector arrays with unity gain and with high-gain APD's. We will present recent data on the GaN based APD's for their gain, detector response, dark current noise and the 1/f noise. We will discuss various approaches and device designs that are being evaluated for developing APD's in wide band gap semiconductors. The paper will also discuss state-of-the-art in UV APDs and the future directions for small unit cell size and gain in the APD's.

New generation of photon detectors - so-called Solid State (Silicon) Photomultipliers (SSPM, SiPM), Negative Feedback APDs (NFAD), and a few more names - is widely recognized to be competitive with PMTs and conventional APDs in various low light level applications. SSPM designs are mostly associated with multi-pixel Geiger mode APD with built-in negative feedback elements. Strong negative feedback applied to Geiger avalanche breakdown enables near ideal single electron multiplication with very high gain and ultra-low excess noise. Multi-pixel architecture provides capability of multi-photon pulse detection with remarkable photon number resolution starting from single photons at room temperature. On the other hand, the SSPM design concept results in low dynamic range due to limited number of pixels with some recovery time and in considerable excess noises of crosstalk and afterpulsing. These specific drawbacks affect signal-tonoise ratio and complicate estimation of the SSPM applicability and competitiveness with other detectors and within the SSPM generation. This study presents probabilistic analysis of the SSPM and analytical results on probability distributions of the output signals with crosstalk and afterpulsing, accounting of the saturation and non-linearity effects, representation of the key photodetection processes in terms of excess noise factors (ENF). Results of the study seem to be useful for the improvements in designing, characterization, and application-specific optimization of the SSPM.

We present the characterization of a multi-pixel detector (SiPM, Hamamatsu) in the presence of dark-count and cross-talk effects. Our description yields a self-consistent calibration of the device, based on the light under investigation, which is used to evaluate shot-by-shot detected-photon numbers including dark-counts and cross-talk. The analysis allows us to reliably reconstruct the detected-photons statistics of different light states by taking into account the modifications introduced by detector features. Finally we quantify photon-number correlations in bipartite states and use the data to produce conditional states: only if dark-count and cross-talk effects can be neglected, the experimental results match theory.

Due to the inherent positive feedback mechanism involved in the impact ionization avalanche process, InGaAs/InP single photon avalanche photodiodes (SPADs) have historically exhibited certain shortcomings such as low counting rate and inability to resolve photon number. To overcome some of the performance limitations of regular SPADs, we have developed negative feedback avalanche diodes (NFADs) which employ a negative feedback mechanism to regulate the avalanche process. The fabrication process of NFADs is flexible and is based on our design platform used to provide industry-leading SPAD performance. The operation of NFAD devices is also very simple, with only a DC bias is required. Various discrete devices and matrices composed of different elements have been designed, fabricated and characterized. For discrete devices, ~10% photon detection efficiency has been realized consistent with acceptable afterpulsing probability, providing a convenient photon-counting solution for certain applications. The negative feedback mechanism significantly improves the uniformity of the output pulse heights and avalanche charge per detection event, resulting in a low "charge excess noise" factor. We demonstrate that when NFAD devices are configured in a matrix format, they have the ability to resolve photon number and work effectively as solid state photomultipliers (SSPMs) in the short wave infrared (SWIR) region. The InGaAs/InP NFAD SSPMs will have the potential to replace photomultiplier tubes and silicon photomultiplier (SiPMs) in applications where single photon sensitivity in the SWIR region beyond ~0.9 μm is critical.

An asynchronous readout integrated circuit (ROIC) has been developed for hybridization to a 32x32 array of single-photon sensitive avalanche photodiodes (APDs). The asynchronous ROIC is capable of simultaneous detection and readout of photon times of arrival, with no array blind time. Each pixel in the array is independently operated by a finite state machine that actively quenches an APD upon a photon detection event, and re-biases the device into Geiger mode after a programmable hold-off time. While an individual APD is in hold-off mode, other elements in the array are biased and available to detect photons. This approach enables high pixel refresh frequency (PRF), making the device suitable for applications including optical communications and frequency-agile ladar. A built-in electronic shutter that de-biases the whole array allows the detector to operate in a gated mode or allows for detection to be temporarily disabled. On-chip data reduction reduces the high bandwidth requirements of simultaneous detection and readout. Additional features include programmable single-pixel disable, region of interest processing, and programmable output data rates. State-based on-chip clock gating reduces overall power draw. ROIC operation has been demonstrated with hybridized InP APDs sensitive to 1.06-μm and 1.55-μm wavelength, and fully packaged focal plane arrays (FPAs) have been assembled and characterized.

Proportional photon detection has been demonstrated using linear mode HgCdTe avalanche photodiodes (APDs) hybridized on a specially designed read-out integrated circuit (ROIC). The ROIC was designed to detect photons at a moderate bandwidth (10 MHz) with a low noise of 10 electrons per characteristics time of the ROIC and to be compatible with large area-small pixel focal plane array (FPA) applications. Proportional photon counting was demonstrated by reproducing the Poisson statics for average photon number states ranging between m=0.8 to 8 photons, at low to moderate avalanche gains M=40-200, using both mid-wave infrared (MWIR) and (short-wave infrared) SWIR HgCdTe APDs. The probability distribution function of the gain was estimated from the analysis of the amplitude of detected residual thermal photons in the MWIR APDs. The corresponding probability distribution functions was characterized by a low excess noise factor F and high asymmetry which favours a high photon detection efficiency (PDE), even at high threshold values. An internal PDE of 90 % was estimated at a threshold level of 40 % of the average signal for a single photon. The dark count rate (DCR) was limited by residual thermal photons in the MWIR APD to about 1 MHz. A geometrical and spectral filtering of this contribution is important to achieve the ultimate performance with MWIR detectors. In this case, the DCR was estimated by interpolation to about 8 kHz. The SWIR HgCdTe APD device had a lower residual photon flux (60 kHz), but was found to be limited by tunnelling dark current noise at high gains at a rate of 100 kHz.

We discuss avalanche discrimination in a periodically-gated InGaAs/InP single-photon avalanche diode. We investigate the interrelation between the minimum detectable avalanche charge and the detection efficiency, and we show that the technical solutions we implement can improve performance. Gating the detector at 1.25 GHz, single-photon count rates above 250x10⁶ s^-1 can be obtained while maintaining low afterpulse probability with detection efficiencies larger than 0.10.

We report sinusoidal gating of InGaAs/InP single photon avalanche diodes (SPAD) operated at wavelength of 1310 nm with high photon detection efficiency (PDE) and low dark count rate (DCR). At a gating frequency of 80 MHz and temperature of 240 K the DCR and PDE were 15.5 kHz and 55%, respectively. The slope of DCR versus PDE increases with higher laser repetition rate. There are two mechanisms that contribute to this trend. The first is due to the lower afterpulse probability associated with a lower laser repetition rate. The other is due to the RC effect, which is illustrated by an equivalent circuit that includes a model of the SPAD. We also show that relative to gated passive quenching with active reset (PQAR) for fixed PDE, sinusoidal gating yields lower afterpulsing rates for the same hold-off time. This is explained in terms of the integrated pulse shape and the resultant charge flow. The afterpulse probability, P_a, is related to the hold off time, T, through the power law, P_a∝T^-α where α is a measure of the detrapping time in the multiplication region.

By tens-of-picosecond resolved fluorescence detection we study Förster resonance energy transfer between a donor and a black-hole-quencher bound at the 5'- and 3'-positions of an oligonucleotide probe matching the highly polymorphic region between codons 51 and 58 of the human leukocyte antigen DQB1 0201 allele, conferring susceptibility to type-1 diabetes. The probe is annealed with non-amplified genomic DNAs carrying either the 0201 sequence or other DQB1 allelic variants. We detect the longest-lived donor fluorescence in the case of hybridization with the 0201 allele and definitely faster and distinct decays for the other allelic variants, some of which are single-nucleotide polymorphic.