Time-correlated single photon counting (TCSPC) is based on the detection of single photons of a periodic light signal, measurement of the detection time of the photons, and the build-up of the photon distribution versus the time in the signal period. TCSPC achieves a near ideal counting efficiency and transit-time-spread-limited time resolution for a given detector. The drawback of traditional TCSPC is the low count rate, long acquisition time, and the fact that the technique is one-dimensional, i.e. limited to the recording of the pulse shape of light signals. We present an advanced TCSPC technique featuring multi-dimensional photon acquisition and a count rate close to the capability of currently available detectors. The technique is able to acquire photon distributions versus wavelength, spatial coordinates, and the time on the ps scale, and to record fast changes in the fluorescence lifetime and fluorescence intensity of a sample. Biomedical applications of advanced TCSPC techniques are time-domain optical tomography, recording of transient phenomena in biological systems, spectrally resolved fluorescence lifetime imaging, FRET experiments in living cells, and the investigation of dye-protein complexes by fluorescence correlation spectroscopy. We demonstrate the potential of the technique for selected applications.

Ocular fundus autofluorescence imaging has been introduced into clinical diagnostics recently. It is in use for the observation of the age pigment lipofuscin, a precursor of age - related macular degeneration (AMD). But other fluorophores may be of interest too: The redox pair FAD - FADH₂ provides information on the retinal energy metabolism, advanced glycation end products (AGE) indicate protein glycation associated with pathologic processes in diabetes as well as AMD, and alterations in the fluorescence of collagen and elastin in connective tissue give us the opportunity to observe fibrosis by fluorescence imaging. This, however, needs techniques able to differentiate particular fluorophores despite limited permissible ocular exposure as well as excitation wavelength (limited by the transmission of the human ocular lens to >400 nm). We present an ophthalmic laser scanning system (SLO), equipped with picosecond laser diodes (FWHM 100 ps, 446 nm or 468 nm respectively) and time correlated single photon counting (TCSPC) in two emission bands (500 - 560 nm and 560 - 700 nm). The decays were fitted by a bi-exponential model. Fluorescence spectra were measured by a fluorescence spectrometer fluorolog. Upon excitation at 446 nm, the fluorescence of AGE, FAD, and lipofuscin were found to peak at 503 nm, 525 nm, and 600 nm respectively. Accordingly, the statistical distribution of the fluorescence decay times was found to depend on the different excitation wavelengths and emission bands used. The use of multiple excitation and emission wavelengths in conjunction with fluorescence lifetime imaging allows us to discriminate between intrinsic fluorophores of the ocular fundus. Taken together with our knowledge on the anatomical structure of the fundus, these findings suggest an association of the short, middle and long fluorescence decay time to the retinal pigment epithelium, the retina, and connective tissue respectively.

The time-correlated single photon counting (TCSPC) technique combined with clock oscillator set by the pulsed laser provides a precise measurement of the arrival time of the detected photons with picosecond resolution for a time-scale of hours. If TCSPC is combined with other experimental techniques such as optical spectroscopy and mechanical manipulation, it is possible to coincide the detected fluorescence signal with the changes of the sample properties. High temporal resolution achieved in TCSPC (down to ps) allows us to monitor fast mechanical processes in single molecules. Here we present recent developments in fluorescence correlation spectroscopy (FCS) as well as the combination of TCSPC with optical scanning microscopy and mechanical manipulation by means of an atomic-force microscope (AFM).

We present an approach which monitors both time- and spectral information of the fluorescence in order to receive the full information content of the light emitted from a sample. Our instrumentation covers a combination of the TCSPC technique extended by a multi-wavelength detection scheme. Based on the spectral properties of the participating chromophores their relative contributions to the fluorescence within each wavelength channel can be derived. This information serves as an additional parameter for the decay curve analysis which allows us to identify emission patterns for both lifetime and wavelength. Potential applications of the multi-wavelength TCSPC technique are FRET experiments, new marker techniques based on environment-dependent lifetime, and the separation of fluorophores in autofluorescence images of tissue.

We develop a novel approach to infer depth information about a small fluorophore-filled inclusion immersed in a scattering medium. It relies on time-resolved measurements of the time of flight distribution of emitted fluorescent photons after short pulse laser excitation. The approach uses a novel numerical constant fraction discrimination technique to assign a stable arrival time to the distribution's early photons. Our experimental results show a linear relationship between these arrival times and the position of the inclusion. This approach will serve as a useful technique in fluorescence diffuse optical tomography.

Picosecond fluorescence lifetime imaging microscopy (FLIM) provides a most valuable tool to analyze the primary processes of photosynthesis in individual cells and chloroplasts of living cells. In order to obtain correct lifetimes of the excited states, the peak intensity of the exciting laser pulses as well as the average intensity has to be sufficiently low to avoid distortions of the kinetics by processes such as singlet-singlet annihilation, closing of the reaction centers or photoinhibition. In the present study this requirement is achieved by non-scanning wide-field FLIM based on time- and space-correlated single-photon counting (TSCSPC) using a novel microchannel plate photomultiplier with quadrant anode (QA-MCP) that allows parallel acquisition of time-resolved images under minimally invasive low-excitation conditions. The potential of the wide-field TCSPC method is demonstrated by presenting results obtained from measurements of the fluorescence dynamics in individual chloroplasts of moss leaves and living cells of the chlorophyll d-containing cyanobacterium Acaryochloris marina.

A new plate reader (Nanotaurus) has been developed by Edinburgh Instruments that has the principle design features of a confocal microscope and utilises the technique of Time Correlated Single Photon Counting for data acquisition. The advantages of Fluorescence Lifetime Measurements in the nanosecond time scale and analysis methods to recover lifetime parameters are discussed based on experimental data. First working assays using changes of lifetime parameters are presented that clearly demonstrate the advantages of the new instrument for biochemical assays and show strong promise for cell-based assays, by utilising the independence of lifetime parameters from sample volume and concentration.

Advanced time-correlated single-photon counting (TCSPC) devices are able to record several 106 photons per second and deliver an instrument response function down to 25 ps FWHM. Under these conditions the accuracy of fluorescence decay or photon migration times is limited by systematic timing errors rather than by the photon statistics. The experiments described below determined the variation of the instrument response function (IRF) with the count rate and the timing drift for an SPC-140 TCSPC module and a number of commonly used detectors. For count rates from 3×10⁴ to 4×10⁶ s-1 a shift of the first moment of the IRF smaller than 2 ps was obtained. The drift over 16 minutes was within ±0.7 ps.

A multichannel fluorescence detection system for multi-lane DNA sequencer based on single photon counting is presented. The problem of elimination of the lane cross-talk caused by both optical and electronic cross-talk between the detection lanes is discussed. A novel method for elimination of the channel cross-talk is proposed. The calibration procedure for a DNA sequencer with lane cross-talk cancellation is described. Improvement in sensitivity of a DNA sequencer setup by introducing wide collection angle optical system is discussed.

We continue our work on the design and implementation of multi-channel single photon detection systems for highly sensitive detection of ultra-weak fluorescence signals, for high-performance, multi-lane DNA sequencing instruments. A fiberized, 32-channel single photon detection (SPD) module based on single photon avalanche diode (SPAD), model C30902S-DTC, from Perkin Elmer Optoelectronics (PKI) has been designed and implemented. Unavailability of high performance, large area SPAD arrays and our desire to design high performance photon counting systems drives us to use individual diodes. Slight modifications in our quenching circuit has doubled the linear range of our system from 1MHz to 2MHz, which is the upper limit for these devices and the maximum saturation count rate has increased to 14 MHz. The detector module comprises of a single board computer PC-104 that enables data visualization, recording, processing, and transfer. Very low dark count (300-1000 counts/s), robust, efficient, simple data collection and processing, ease of connectivity to any other application demanding similar requirements and similar performance results to the best commercially available single photon counting module (SPCM from PKI) are some of the features of this system.

Endothelial cells (ECs) convert mechanical stimuli into chemical signaling pathways to regulate their functions and properties. It is hypothesized that perturbation of cellular structures by force is accompanied by changes in molecular dynamics. In order to address these fundamental issues in mechanosensation and transduction, we have developed a hybrid multimodal microscopy - time-correlated single photon counting (TCSPC) spectroscopy system intended to determine time- and position dependent mechanically-induced changes in the dynamics of molecules in live cells as determined from fluorescence lifetimes and autocorrelation analysis (fluorescence correlation spectroscopy). Colocalization of cell-structures and mechanically-induced changes in molecular dynamics can be done in post-processing by comparing TCSPC data with 3-D models generated from total internal reflection fluorescence (TIRF), differential interference contrast (DIC), epifluorescence, and deconvolution. We present control experiments in which the precise location of the apical cell membrane with respect to a confocal probe is assessed using information obtainable only from TCSPC. Such positional accuracy of TCSPC measurements is essential to understanding the role of the membrane in mechanotransduction. We predict that TCSPC will become a useful method to obtain high temporal and spatial resolution information on localized mechanical phenomena in living endothelial cells. Such insight into mechanotransduction phenomenon may uncover the origins of mechanically-related diseases such as atherosclerosis.

We have recently developed a wide-field photon-counting detector (the H33D detector) having high-temporal and highspatial resolutions and capable of recording up to 500,000 photons per sec. Its temporal performance has been previously characterized using solutions of fluorescent materials with different lifetimes, and its spatial resolution using sub-diffraction objects (beads and quantum dots). Here we show its application to fluorescence lifetime imaging of live cells and compare its performance to a scanning confocal TCSPC approach. With the expected improvements in photocathode sensitivity and increase in detector throughput, this technology appears as a promising alternative to the current lifetime imaging solutions.

The method of Time Correlated Single Photon Counting requires high repetitive light sources (>100kHz) with pulse widths of ideally less than approximately 20ps. While these light sources have been available for some time now in the form of Ti:Sapphire lasers, picosecond pulsed diode lasers (<90ps) and light emitting diodes (<700ps), they all have the drawback of either having no spectral tunability, or tunability over a very narrow spectral range (10nm-100nm). While this is often sufficient for specific laboratory setups for measurements of fluorescence lifetimes, commercial Fluorescence Lifetime Spectrometers have suffered for a long time from the lack of the availability of simple, compact and relatively inexpensive broad spectral band light sources that can be employed for Time Correlated Single Photon Counting. A new light source as an integral part of a commercial fluorescence lifetime spectrometer will be discussed that allows tunability over a wide spectral band of more than 500nm.

Silicon Single-Photon Avalanche-Diodes (SPAD) are nowadays considered a solid-state alternative to Photomultiplier Tubes (PMT) in single photon counting (SPC) and time-correlated single photon-counting (TCSPC) over the visible spectral range up to 1 micron wavelength. SPADs implemented in planar epitaxial technology compatible with CMOS circuits offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage and low power, etc.). Furthermore, they have inherently higher photon detection efficiency, since they do not rely on electron emission in vacuum from a photocathode as PMT, but instead on the internal photoelectric effect. However, PMTs offer much wider sensitive area, which greatly simplifies the design of optical systems; they provide position-sensitive photon detection and imaging capability; they attain remarkable performance at high counting rate and offer picosecond timing resolution with Micro-Channel Plate (MCP) models. In order to make SPADs more competitive in a broader range of SPC and TCPC applications it is necessary to face both semiconductor technology issues and circuit design issues, which will be here dealt with. Technology issues will be discussed in the context of two possible approaches: employing a standard industrial high-voltage compatible CMOS technology or developing a dedicated CMOS-compatible technology. Circuit design issues will be discussed taking into account problems arising from conflicting requirements set by various required features, such as fast and efficient avalanche quenching and reset, high resolution photon timing, etc.

We discuss a practical implementation of a photon-counting detector calibration using correlated photon pairs produced by parametric down-conversion. In this calibration scheme, the detection of a first photon triggers the measurement sequence aimed at detection of a second photon by a detector under test (DUT). We also describe measurements of radiant power with a photon-counting detector, which is important for implementation of a conventional calibration technique based on detector substitution. In the experiment, we obtain a time-delay histogram of DUT detection events consisting of a correlated signal and a background. We present a method for separating the correlated signal from the background signal that appropriately handles complex properties of typical avalanche photodiode (APD) detectors. Also discussed are measurements of relevant APD properties, including count-rate-dependent afterpulsing, delayed (by up to 10 ns) electronic detections and deadtime effects. We show that understanding of these is essential to perform an accurate calibration.

Avalanche photodiodes optimized for Geiger-mode single photon counting at 1.06 microns have been fabricated using a quaternary InGaAsP absorber to reduce the dark count rate without sacrificing high photon detection probability. The dark count rate at a given detection probability is more than an order of magnitude lower than that of comparable Geiger-mode APDs fabricated with ternary InGaAs absorbers. Some devices show anomalous afterpulsing behavior that is reduced in severity at lower temperatures, the inverse of typical behavior. This unusual afterpulsing behavior allows for lower temperature operation without sacrificing maximum count rate, and may also offer new clues to the physical origin of afterpulsing in general.

We are reporting our results in research and development in the field of avalanche semiconductor single photon detectors and their application in high precision laser ranging during the last 20 years. Our objectives where: avalanche detector structure, sensitive area diameters exceeding 50 microns, active quenching and gating electronic circuit, high timing resolution and rugged design. Avalanche photodiodes specifically designed for photon counting devices have been developed on the basis of various semiconductor materials: Si, Ge, SiGe, GaP, GaAs and InGaAs. All the semiconductor detectors operate at a room temperature or at thermoelectrically achievable temperatures except of the germanium based detector, which requires liquid nitrogen cooling. Electronic circuits for these detectors biasing, quenching and control have been developed and optimised for different applications. Circuits permitting operation of solid state photon counters in both single and multiple photon signal regimes have been developed and applied. Additionally, these circuits provide the estimate of the photon number involved in the detection process. The timing resolution of the order of units to several tens of picoseconds enables millimeter precision laser ranging. The different photon counting detectors for applications in ground-ground, ground-air, air-ground and ground to space high precision laser ranging have been developed and operated in the field on 11 different wavelengths in the range of 355-1548 nanometres.

A compact single photon detector designed for the detection in the visible spectral range is presented. A fast active quenching circuit is integrated on the chip in order to operate the APD in single photon counting mode. The sensor consists of a 0.8x0.8mm² silicon chip mounted on a thermo-electric cooler and packaged in a standard TO5 header, bringing the degree of miniaturization to a level never reached. Reliability, compactness, low power and low cost make the detector essential for portable devices, implantable sensors, fluorescence lifetime spectrometers or laser scanning microscopes. In addition, the sensor exhibits best-in-class timing resolution of 50ps. The photon detection probability peaks in the blue/green at almost 35% and is limited to a few percents in the red and near-infrared regions. When cooled down to -10°C, the 50μm diameter diode achieves a dark count rate lower than 10Hz. The afterpulsing is maintained to a low level, below 1%. The fast active quenching circuit leads to a dead time of 50ns allowing a measurement frequency of up to 20MHz. The detector, unlike legacy photon counters, does not suffer from memory effects and is not damaged by ambient light.

The detection of light at ultraviolet (UV) wavelengths is important for many military, medical and environmental applications. Applications such as biological agent detection and non-line-of-sight communications require the detection of scattered UV light. Currently, photomultiplier tubes operated as single photon counters are used to detect these low light levels, but they have many unfavorable characteristics for such applications. SiC based avalanche photodiodes (APDs) operated in Geiger mode could potentially meet the needs of these applications. Our first results, using SiC Geiger mode single photon counting avalanche photodiodes (SPADs), showed prohibitively high dark counts, due to a large tunneling current component in the multiplied dark current. Here we show the results of two p-i-n structures with 260μm and 480μm i-regions, which reduced the primary dark current by two orders of magnitude, operated under gated quenching conditions at 325nm. The lower dark current resulted in a dark count rate of 28kHz at 3.6% single photon detection efficiency (SPDE) in a 100μm diameter device. This is a three order reduction in the dark count rate over our previous results using a p-n junction SPAD.

Si avalanche photodiode (APD) single photon counting modules (SPCMs) are used in the Geoscience Laser Altimeter System (GLAS) on Ice, Cloud, and land Elevation Satellite (ICESat), currently in orbit measuring Earth surface elevation and atmosphere backscattering. These SPCMs are used to measure cloud and aerosol backscattering to the GLAS laser light at 532-nm wavelength, with quantum efficiencies of 60 to 70% and maximum count rates greater than 13 millions/s. The performance of the SPCMs has been monitored since ICESat launch on January 12, 2003. There has been no measurable change in the quantum efficiency when comparing the average photon count rates in response to the background light from the sunlit Earth. The linearity and the afterpulsing, seen from the cloud and surface backscattering profiles have been the same as those during ground testing. The detector dark counts rates monitored while the spacecraft was in the dark side of the Earth have increased linearly at about 55.5 counts/s per day due to space radiation damage, which is a little lower than what we expected based on the ground testing and sufficiently low to provide useful atmosphere measurements through the end of the ICESat mission. The radiation damage appeared to be slightly dependent of the device temperature. There was also a distinct increase in the dark counts during the solar storm in 28-31 October 2003. These SPCMs have been in orbit for almost four years to date. The accumulated operating time has reached to over 380 days (9150 hours). These SPCMs have provided unprecedented receiver sensitivity and clarity in atmosphere backscattering measurements from space.

InGaAs and Germanium devices employed as Single-Photon Avalanche-Diodes (SPAD) for the infrared spectral range must be cooled to low temperature for reducing the dark-counting rate due to thermal generation and are plagued by strong avalanche carrier trapping. Released trapped carriers re-trigger the avalanche and generate correlated afterpulses. This effect can be counteracted by reducing the avalanche pulse charge and by covering the trapped carrier release transient with a hold-off time after quenching. Gated operation is employed, but simple gated passive circuits are suitable only for short gate intervals (few nanoseconds). For longer gate times, we investigated gated operation of a SPAD under the control of an active quenching circuit, which yielded accurate timing performance. We designed an integrated active quenching circuit (iAQC) suitable for gated mode for operating the SPAD down to cryogenic temperature. The iAQC senses the avalanche and swiftly quenches it, without waiting the end of the gate interval. Operation with longer gate times (100 ns and more) is achieved with remarkably reduced afterpulsing with respect to passive gated circuits. We expressly designed fast circuits for processing the avalanche pulse, cancelling spurious spikes due to gate transients and accurately extracting the photon timing information, with less than 50 ps jitter.

Time-correlated single photon counting (TCSPC) is exploited in emerging scientific applications in life sciences, such as single molecule spectroscopy, DNA sequencing, fluorescent lifetime imaging. Detectors with wide active area (diameter > 100 μm) are desirable for attaining good photon collection efficiency without requiring complex and time-consuming optical alignment and focusing procedures. Fiber pigtailing of the detector, often employed for having a more flexible optical system, is also obtained more simply and with greater coupling efficiency for wide-area detectors. TCSPC, however, demands to detectors also high photon-timing resolution besides low noise and high quantum efficiency. Particularly tight requirements are set for single-molecule fluorescence analysis, where components with lifetimes of tens of picoseconds are often met. Small photon timing jitter and wide area are considered conflicting requirements for the detector. We developed an improved planar silicon technology for overcoming the problem and providing a solid-state alternative to MCP-PMTs in demanding TCSPC applications. We fabricated Single Photon Avalanche Diodes (SPADs) with 200 μm active area diameter and fairly low dark counting rate (DCR). At moderately low temperature (-25 °C with Peltier cooler) the typical DCR is 1500 c/s and it is not difficult to select devices with less than 1000 c/s. The photon detection efficiency peaks at 48% around 530 nm and stays above 30% over all the visible range. A photon timing resolution of 35 ps FWHM (full width at half maximum) is obtained by using our patented pulse pick-up for processing the avalanche current.

The design and characterization of an imaging sensor based on single photon avalanche diodes is presented. The sensor was fully integrated in a 0.35μm CMOS technology. The core of the imager is an array of 4x112 pixels that independently and simultaneously detect the arrival time of photons with picosecond accuracy. A novel event-driven readout scheme allows parallel column-wise and non-sequential, on-demand row-wise operation. Both time-correlated and time-uncorrelated measurements are supported in the sensor. The readout scheme is scalable and requires only 11 transistors per pixel with a pitch of 25μm. A number of standard performance measurements for the imager are presented in the paper. An average dark count rate of 6Hz and 750Hz are reported at room temperature respectively for an active area diameter of 4μm and 10μm, while the dead time is 40ns with negligible crosstalk. A timing resolution better than 80ps over the entire integrated array makes this technique ideal for a fully integrated high resolution streak camera, thus enabling fast TCSPC experiments. Applications requiring low noise, picosecond timing accuracies, and measurement parallelism are prime candidates for this technology. Examples of such applications include bioimaging at cellular and molecular level based on fluorescence lifetime imaging and/or, fluorescence correlation spectroscopy, as well as fast optical imaging, optical rangefinders, LIDAR, and low light level imagers.

Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) are high-efficiency single-photon detectors which have multi-photon counting capability. While both the VLPCs and the SSPMs have inferred internal quantum efficiencies above 93%, the actual measured values for both the detectors were in fact limited to less than 88%, attributed to in-coupling losses. We are currently improving this overall detection efficiency via a) custom anti-reflection coating the detectors and the in-coupling fibers, b) implementing a novel cryogenic design to reduce transmission losses and, c) using low-noise electronics to obtain a better signal-to-noise ratio.

We report use of a niobium nitride superconducting single-photon detector in a time-correlated single-photon counting experiment. The detector has a timing jitter of 68 ± 3 ps full width at half maximum with a Gaussian temporal profile. The detector's dark count rate and detection efficiency can be tuned by adjusting the bias current applied to the device. Typical values include a detection efficiency of ~1-2% and a dark count rate below 100 Hz. We use this detector to measure time-resolved photoluminescence at wavelengths up to 1650 nm, well beyond the range of conventional silicon detectors. We also use this superconducting detector to measure the emission of a quantum dot single-photon source.

We have fabricated and tested single photon detectors based on a current biased superconducting niobium nanowire patterned into a meander. The detectors are fabricated from high quality, ultra high vacuum sputtered niobium thin films on a sapphire substrate. For detection of single optical photons, we show that the superconductor's intrinsic kinetic inductance does not limit the reset time of the detector, which is ≈ 2 nanoseconds, in contrast to the longer reset times seen in niobium nitride detectors of similar size and geometry. We also describe a readout scheme for photon counting that is unique to Nb due to its lower resistivity. These detectors have applications in imaging of infrared photoemission from CMOS logic circuits as well as in optical communications and quantum information processing.

We demonstrate a new single-photon avalanche diode (SPAD) device, which utilizes the silicon-dioxide shallow-trench isolation (STI) structure common to all deep-submicron CMOS technologies, both for junction planarization and as an area-efficient guard-ring. This makes it possible to achieve an order-of-magnitude improvement in fill factor and a significant reduction in pixel area compared with existing CMOS SPADs, and results in improved SPAD performance. We present numerical simulations as well preliminary experimental results from a test chip, which was manufactured in an IBM 0.18 μm CMOS technology, and which incorporates the devices. With these new and efficient structures, 12 μm-pitch pixels with sub-10ns dead times are achievable without requiring active recharge, creating the opportunity to integrate large arrays of these ultra-fast SPADs for use in biological imaging systems.

As low light detection technologies are advancing, novel experiments like single molecule spectroscopy, quantum computation, quantum encryption are proliferating. Quantum mechanical detectors can produce only discrete "clicks" at different rates based on the propagating field energy through them, irrespective of whether the photons are divisible or indivisible packets of energy. This is because electrons are quantized elementary particles and they are always bound in quantized energy levels in different quantum systems. Highly successful quantum formalism is not capable of providing the microscopic picture of the processes undergoing during QM interactions; that is left to human imaginations allowing for sustained controversies and mis-interpretations. This paper underscores the paradoxes that arise with the assumption that photons are indivisible elementary particles based on the obvious but generally ignored fact that EM fields do not operate on (interfere with) each other. Then we propose that atomic or molecular emissions emerge and propagate out as space and time finite classical wave packets. We also suggest experiments to validate that the amplitude of a photon wave packet can be split and combined by classical optical components using the specific example of an N-slit grating.

We provide a direct comparison between the InGaAs avalanche photodiode (APD) and the NbN superconducting single photon detector (SSPD) for applications in fiber-based quantum cryptography. The quantum efficiency and dark count rate were measured for each detector, and used to calculate the quantum bit error rate (QBER) and shared key rate for a QKD link. The results indicate that, despite low quantum efficiency, the speed of the SSPD makes it a superior detector for quantum information applications. Finally, we present results of an initial integration of an SSPD into a receiver node of the DARPA quantum network to perform quantum key distribution.

We present a fiber-based Quantum Cryptography (QC) system in which data is acquired by utilizing a new Time-Correlated Single Photon Counting (TCSPC) instrument. This device captures single photon events on two synchronized channels with picosecond resolution over virtually unlimited time spans and with extremely short dead-times (<95ns). The QC system operates at a wavelength of 1550nm and employs an interferometric approach in which quantum-level information is encoded in the relative phase shift between pairs of faint optical pulses generated by a strongly attenuated semiconductor laser. The QC channel and three additional conventional data channels are carried over a single transmission fiber using a coarse wavelength division-multiplexing (CWDM) scheme with a 20nm channel separation. We assess the impact of the various sources of errors in the system, such as imperfect interference visibility, detector dark counts and Raman scattering in the transmission fiber. Secure key distributions with mean photon numbers of 0.1 and 0.2 per pulse pair were demonstrated for transmission distances up to 25km and 38km respectively.

Quantum Cryptography has demonstrated the potential for ultra-secure communications. However, with quantumchannel transmission rates in the MHz range, typical link losses and signal-to-noise ratios have resulted in keyproduction rates that are impractical for continuous one-time-pad encryption of high-bandwidth communications. We have developed high-speed data handling electronics that support quantum-channel transmission rates up to 1.25 GHz. This system has demonstrated error-corrected and privacy-amplified key rates above 1 Mbps over a free-space link. While the transmission rate is ultimately limited by timing jitter in the single-photon avalanche photodiodes (SPADs), we find the timing resolution of silicon SPADs sufficient to operate efficiently with temporal gates as short as 100 ps. We have developed systems to implement such high-resolution gating in our system, and anticipate the attendant reduction in noise to produce significantly higher secret-key bitrates.

The sensitivity of a high-rate photon-counting optical communications link depends on the performance of the photon counter used to detect the optical signal. In this paper, we focus on ways to reduce the effect of blocking, which is loss due to time periods in which the photon counter is inactive following a preceding detection event. This blocking loss can be reduced by using an array of photon counting detectors or by using photon counters with a shorter inactive period. Both of these techniques for reducing the blocking loss can be employed by using a multi-element superconducting nanowire single-photon detector. Two-element superconducting nanowire single-photon detectors are used to demonstrate error-free photon counting optical communication at data rates of 781 Mbit/s and 1.25 Gbit/s.

We have developed our pervious experimental setup using correlated photon pairs (to the calibration of photo detectors) to realize a controllable photon source. For the generation of such photon pairs we use the non-linear process of parametric down conversion. When a photon of the pump beam is incident to a nonlinear crystal with phase matching condition, a pair of photons (signal and idler) is created at the same time with certain probability. We detect the photons in the signal beam with a single photon counting module (SPCM), while delaying those in the idler beam. Recently we have developed a fast electronic unit to control an optical shutter (a Pockels cell) placed to the optical output of the idler beam. When we detect a signal photon with the controlling electronic unit we are also able to open or close the fast optical shutter. Thus we can control which idler photons can propagate through the Pockels cell. So with this photon source we are able to program the number of photons in a certain time window. This controllable photon source that is able to generate a known number of photons with specified wavelength, direction, and polarization could be useful for applications in high-accuracy optical characterisation of photometric devices at the ultra-low intensities. This light source can also serve as a standard in testing of optical image intensifiers, night vision devices, and in the accurate measurement of spectral distribution of transmission and absorption in optical materials.