Photon counting was introduced and developed during four decades relying on Photomultiplier Tubes (PMT), but interesting alternatives are nowadays provided by solid-state single-photon microdetectors. In particular, Silicon Single-Photon Avalanche-Diodes (SPAD) attain remarkable basic performance, such as high photon detection efficiency over a broad spectral range up to 1 micron wavelength, low dark counting rate and photon timing jitter of a few tens of picoseconds. In recent years SPADs have emerged from the laboratory research phase and they are now commercially available from various manufactures. However, PMTs have much wider sensitive area, which greatly simplifies the design of optical systems; they attain remarkable performance at high counting rate and can provide position-sensitive photon detection and imaging capability. In order to make SPADs more competitive in a broader range of applications it is necessary to face issues in semiconductor device technology. The present state of the art, the prospect and main issues will be discussed.

We are presenting the design, technology development and tuning of the Single Photon Avalanche Diode fabricated on the germanium - silicon epitaxial layer. The ultimate goal is to develop a solid state photon detector with picosecond timing resolution and stability and an increased spectral sensitivity beyond 1100 nanometres in comparison to detectors based on silicon. The technology development steps on the Ge-Si epitaxial layer are presented together with the first results of the preparation of the shallow junction and its parameters. The diffusion and annealing models have been tuned for GeSi epitaxial layer and implantation. The resulting concentration profiles have been verified by two independent diagnostics methods. The first avalanche diode structures on the basis of the Ge0.4Si0.6, epitaxial layer on Silicon have been prepared and tested. The ability of the avalanche structure to operate in a Geiger mode has been demonstrated for the first time, the dark count rate has been measured. The serial resistance of the structure above its breakdown voltage has been measured. The detection sensitivity in the wavelength range of 500 to 1600 nanometres has been measured.

We propose a novel concept for a semiconductor-based single-photon detector for quantum information processing, which is capable of discriminating the number of photons in a light pulse. The detector exploits the charge transport by a surface acoustic wave (SAW) in order to combine a large photon absorption area (thus providing high photon collection efficiency) with a microscopic charge detection area, where the photo generated charge is detected with resolution at the single electron level using single electron transistors (SETs). We present preliminary results on acoustic transport measured in a prototype for the detector as well as on the fabrication of radio-frequency single-electron transistors (RFSETs) for charge detection. The photon detector is a particular example of acousto-electric nanocircuits that are expected to be able to control both the spatial and the spin degrees of freedom of single electrons. If realized, these circuits will contribute substantially to a scalable quantum information technology.

We present the design and fabrication of 2x2 Quad-Cells and related electronics for ultra-sensitive detection and spatial tracking of single photons in the visible wavelength range. Though four pixels do not offer the imaging capabilities of CCDs, CMOS sensors and the like, their single-photon sensitivity enables most detection equipments demanding higher detection efficiency, faster gated operation, and sharper time response. In fact Quad-Cells are aimed to both photon-counting and photon-timing applications, i.e. whenever it is important to reconstruct both continuous or slow-varying very-faint light signals (with time-slots down to 50&mgr;s) and very fast luminescence waveforms (down to 150-ps), by having at the same time either the spatial information of the photon absorption position or the possibility to track the luminous source. In this paper we present the design and fabrication of 2x2 Quad-Cell, with pixel diameters from 50&mgr;m to 100 &mgr;m, developed by means of a planar microelectronic processing. They reach 55%-detection efficiency at 520-nm-wavelength and show a broad peak, higher than 40% in the 420-660-nm range, with still more than 2% at 1,000 nm.

Time-Correlated Single Photon Counting (TCSPC) is a very powerful method for sensitive time-resolved optical measurements. Its main application was historically the measurement of fluorescence lifetime. This application is still important, however, improvements over the early designs allow the recovery of much more information from the collected photons and enable entirely new applications. In conventional TCSPC instruments the timing signals are processed by a Time to Amplitude Converter (TAC) and subsequent Analog to Digital Converter (ADC) which provides digital values to address a histogrammer. TAC and ADC must guarantee a very good linearity and short dead time. These criteria are difficult to meet simultaneously, particularly at high resolution. Even with highest technically feasible ADC resolution, the time span such a system can measure at high temporal resolution is very limited. Here we present a new TCSPC system overcoming these limitations, based on Time to Digital Converters (TDC). This allows not only picosecond timing, but can also extend the measurable time span to virtually any length by means of digital counters. Our new design uses two such circuits that work independently on each input channel but with a common crystal clock. The timing circuits are therefore precisely synchronized and provide picosecond arrival times that can be processed further in any conceivable manner. Due to the symmetrical design without any ab initio assignment of dedicated start and stop inputs, the processing provides significantly more options than in conventional TCSPC systems, while still embracing the classical case. We present measurement data and results from applications in general quantum physics as well as analytical applications including confocal time resolved fluorescence microscopy at the single molecule level.

In Time Correlated Single Photon Counting (TCSPC) the maximum signal throughput is limited by the occurrence of classical pile-up and dead-time effects: At a given photon rate characteristic distortions become visible in the TCSPC histogram. How to describe these distortions in mathematical terms is well known^1,2. While the approach of correcting these distortions directly by operations on the raw data has drawbacks, e.g. with respect to calculation effort as well as numerical stability, it is comparably straightforward to include corrections in the models describing the data. We demonstrate the applicability of a model based approach on decay data which are heavily distorted by dead-time and pile-up effects.

Fluorescence lifetime correlation spectroscopy (FLCS) is a recently developed method which combines the conventional fluorescence correlation spectroscopy (FCS) and time correlated single photon counting (TCSPC). It enables to perform a signal separation of the species which possess different lifetime. Particular diffusion components of a mixture of more fluorescent species can be thus separated. Moreover, the detector afterpulsing can be suppressed by FLCS.

We introduce a portable, inexpensive and reliable scheme for calibration of photon-counting detectors based on our recent comparison of two independent high accuracy primary standard calibration methods. We have verified the calibration method based on two-photon correlations and its uncertainty by comparing it to a substitution method using a conventionally calibrated transfer detector tied to a national primary standard detector scale. We have reported a relative standard uncertainty for the correlated-photon method of 0.18 % (k=1) and for the substitution method of 0.17 % (k=1). Further we established, by direct comparison in a series of measurements, that the two methods agree to within the uncertainty of the comparison, with the difference being 0.14(14) %. While this experiment can be repeated in the laboratory setting, the calibration methods are not appropriate for everyday use. However, using photon-counting detector properties brought to light by our comparison of the two methods we introduce a relatively low cost calibration technique that will allow for 0.5% (k=1) uncertainty.

In the great majority of the cases, present astronomical observations are realized analyzing only first order spatial or temporal coherence properties of the collected photon stream. However, a lot of information is "hidden" in the second and higher order coherence terms, as details about a possible stimulated emission mechanism or about photon scattering along the travel from the emitter to the telescope. The Extremely Large Telescopes of the future could provide the high photon flux needed to extract this information. To this aim we have recently studied a possible focal plane instrument, named QuantEYE, for the 100 m OverWhelmingly Large Telescope of the European Southern Observatory. This instrument is the fastest photon counting photometer ever conceived, with an array of 100 parallel channels operating simultaneously, to push the time tagging capabilities toward the pico-second region. To acquire some experience with this novel type of instrumentation, we are now in the process of realizing a small instrument prototype (AquEYE) for the Asiago 182 cm telescope, for then building a larger instrument for one of the existing 8-10 m class telescopes. We hope that the results we will obtain by these instruments will open a new frontier in the astronomical observations.

We implement an OTDR with photon-counting modules at 1550nm based on sum frequency generation in a PPLN waveguide. The narrow temporal response of those detectors allows achieving a 2-points resolution of few centimetres.

We report the test results of a hybrid photomultiplier tube (HPMT) with a transfer electron (TE) InGaAsP photocathode and GaAs Schottky avalanche photodiode (APD) anode. Unlike Geiger mode InGaAsP APDs, these HPMTs (also known as intensified photodiode (IPD), vacuum APD, or hybrid photodetector) operate in linear mode without the need for quenching and gating. Their greatest advantages are wide dynamic range, high speed, large photosensitive area, and potential for photon counting and analog detection dual mode operation. The photon detection efficiency we measured was 25% at 1064 nm wavelength with a dark count rate of 60,000/s at -22 degrees Celsius. The output pulse width in response to a single photon detection is about 0.9 ns. The maximum count rate was 90 Mcts/s and was limited solely by the speed of the discriminator used in the measurement (10 ns dead time). The spectral response of these devices extended from 900 to 1300 nm. We also measured the HPMT response to 60 ps laser pulses. The average output pulse amplitude increased monotonically with the input pulse energy, which suggested that we can resolve photon number in an incident pulse. The jitter of the HPMT output was found to be about 0.5 ns standard deviation and depended on bias voltage applied to the TE photocathode. To our knowledge, these HPMTs are the most sensitive non gating photon detectors at 1064 nm wavelength, and they will have many applications in laser altimeters, atmospheric lidars, and free space laser communication systems.

We demonstrate that III-V photodetectors operated with dedicated front-end electronics and cooled at sufficiently low temperature (220 K or lower) can be exploited as Single-Photon Avalanche-Diodes (SPAD) for near-infrared photon counting and timing. Low dark-count rate can be achieved in gated-mode operation, though InGaAs/InP SPADs are plagued by strong avalanche carrier trapping that leads to afterpulsing. In order to reach the best performance, we designed fast circuits for gating SPADs and properly sensing the photoninduced avalanche pulse, cancelling spurious spikes due to gate transients thus accurately extracting photon timing information, and reducing avalanche charge thus minimizing afterpulsing. We report the results obtained with In_0.53Ga_0.47As/InP SPADs employing an integrated Active Quenching Circuit, designed for gated-mode operation at cryogenic temperature, and a fast signal pick-up network for extracting the best timing resolution. The joint use of a good InGaAs/InP detector and the presented electronics allows to reach low dark count rate (below 20 kHz), low time jitter (about 40 ps_FWHM), high operation frequency (up to 100 kHz) with limited afterpulsing even when the photodetector is enabled for long gate-on times (even longer than 100 ns).

Single photon detection at telecom wavelengths is of importance in many industrial applications ranging from quantum cryptography, quantum optics, optical time domain reflectometry, non-invasive testing of VLSI circuits, eye-safe LIDAR to laser ranging. In practical applications, the combination of an InGaAs/InP APD with an appropriate electronic circuit still stands as the best solution in comparison with emerging technologies such as superconducting single photon detectors, MCP-PMTs for the near IR or up-conversion technique. An ASIC dedicated to the operation of InGaAs/InP APDs in both gated mode and free-running mode is presented. The 1.6mm² chip is fabricated in a CMOS technology. It combines a gate generator, a voltage limiter, a fast comparator, a precise timing circuit for the gate signal processing and an output stage. A pulse amplitude of up to +7V can be achieved, which allows the operation of commercially available APDs at a single photon detection probability larger than 25% at 1.55&mgr;m. The avalanche quenching process is extremely fast, thus reducing the afterpulsing effects. The packaging of the diode in close proximity with the quenching circuit enables high speed gating at frequencies larger than 10MHz. The reduced connection lengths combined with impedance adaptation technique provide excellent gate quality, free of oscillations or bumps. The excess bias voltage is thus constant over the gate width leading to a stable single photon detection probability and timing resolution. The CMOS integration guarantees long-term stability, reliability and compactness.

Low photon flux measurements are widely used in the fields of biology, nuclear physics, medical physics and astrophysics. This paper will highlight the key requirements and considerations needed for accurate, traceable measurement at these low light levels. A new driver for these techniques is the rapidly advancing field of optical quantum information processing¹ which requires the development of single photon counting detectors, in addition to the wider use of optical technologies in the photon counting regime. The paper will present the results of the measurement of the quantum efficiency of a channel photomultiplier detector using an absolute radiometric technique based on correlated photons produced in non-linear crystals. Case studies will also be presented to illustrate this work.

We present our latest generation of superconducting single-photon detectors (SSPDs) patterned from 4-nm-thick NbN films, as meander-shaped ~0.5-mm-long and ~100-nm-wide stripes. The SSPDs exhibit excellent performance parameters in the visible-to-near-infrared radiation wavelengths: quantum efficiency (QE) of our best devices approaches a saturation level of ~30% even at 4.2 K (limited by the NbN film optical absorption) and dark counts as low as 2x10^-4 Hz. The presented SSPDs were designed to maintain the QE of large-active-area devices, but, unless our earlier SSPDs, hampered by a significant kinetic inductance and a nanosecond response time, they are characterized by a low inductance and GHz counting rates. We have designed, simulated, and tested the structures consisting of several, connected in parallel, meander sections, each having a resistor connected in series. Such new, multi-element geometry led to a significant decrease of the device kinetic inductance without the decrease of its active area and QE. The presented improvement in the SSPD performance makes our detectors most attractive for high-speed quantum communications and quantum cryptography applications.

We have fabricated fiber-coupled superconducting single-photon detectors (SSPDs), designed for quantum-correlationtype experiments. The SSPDs are nanostructured (~100-nm wide and 4-nm thick) NbN superconducting meandering stripes, operated in the 2 to 4.2 K temperature range, and known for ultrafast and efficient detection of visible to nearinfrared photons with almost negligible dark counts. Our latest devices are pigtailed structures with coupling between the SSPD structure and a single-mode optical fiber achieved using a micromechanical photoresist ring placed directly over the meander. The above arrangement withstands repetitive thermal cycling between liquid helium and room temperature, and we can reach the coupling efficiency of up to ~33%. The system quantum efficiency, measured as the ratio of the photons counted by SSPD to the total number of photons coupled into the fiber, in our early devices was found to be around 0.3 % and 1% for 1.55 &mgr;m and 0.9 &mgr;m photon wavelengths, respectively. The photon counting rate exceeded 250 MHz. The receiver with two SSPDs, each individually biased, was placed inside a transport, 60-liter liquid helium Dewar, assuring uninterrupted operation for over 2 months. Since the receiver's optical and electrical connections are at room temperature, the set-up is suitable for any applications, where single-photon counting capability and fast count rates are desired. In our case, it was implemented for photon correlation experiments. The receiver response time, measured as a second-order photon cross-correlation function, was found to be below 400 ps, with timing jitter of less than 40 ps.

We present an experimental setup for generation of entangled-photon pairs via spontaneous parametric down-conversion, based on the femtosecond-pulsed laser. Our entangled-photon source utilizes a 76-MHz-repetition-rate, 100-fs-pulsewidth, mode-locked, ultrafast femtosecond laser, which can produce, on average, more photon pairs than a cw laser of an equal pump power. The output infrared pump photons (λ = 810 nm) are first up-converted to blue light (λ = 405 nm) and, subsequently, down-converted in a 1.5-mm-thick, type-II BBO crystal via spontaneous down-conversion. The resulting entangled pairs are counted by a pair of high-quantum-efficiency, single-photon, silicon avalanche photodiodes. The total down-conversion efficiency of our system, corresponding criterion of the pump power for real entangled coincident events, has been calculated to be 0.86 × 10^-9. Our apparatus is intended as an efficient source/receiver system for the quantum communications and quantum cryptography applications.

Quantum Key Distribution (QKD) consists in the exchange of a secrete key between two distant points [1]. Even if quantum key distribution systems exist and commercial systems are reaching the market [2], there are still improvements to be made: simplify the construction of the system; increase the secret key rate. To this end, we present a new protocol for QKD tailored to work with weak coherent pulses and at high bit rates [3]. The advantages of this system are that the setup is experimentally simple and it is tolerant to reduced interference visibility and to photon number splitting attacks, thus resulting in a high efficiency in terms of distilled secret bits per qubit. After having successfully tested the feasibility of the system [3], we are currently developing a fully integrated and automated prototype within the SECOQC project [4]. We present the latest results using the prototype. We also discuss the issue of the photon detection, which still remains the bottleneck for QKD.

We propose an imaging scheme based on the quantum spatial correlation of twin beams generated by PDC, and we show that it provides a substantial enhancement of the signal-to-noise ratio with respect to classical schemes.

We have demonstrated the coherent control of photonic bandgap (PBG) devices composed of a three-level atom embedded in three-dimensional PBG structures, which enables us to strongly suppress the loss of quantum information not only during its storage but also during its processing. We find that this ultralow-loss optical quantum information processing can be realized by generating the upper atomic level splitting inside the PBG and by tuning one of the split levels to a dark line in the emission spectrum (zeros in the spectrum at certain values of the emitted photon frequency). Furthermore, we show that the coherent control may provide a basis for the universal quantum logic gate operation in quantum computing.

CV entangled CW bright beams are experimentally generated by a Non-degenerate "triply resonant" Optical Parametric Amplifier (NOPA) based on PPKTP type-II crystal below threshold. Operating the OPA at frequency degeneracy makes the down-converted entangled beams able to be optically manipulated for generating different combination of the entangled system. Particular care has been required by the triply resonance condition essential to obtain a suffcient degree of entanglement. The triply resonance condition is pursued by combining temperature phase-matching and crystal tilting in an optical cavity so to optimize the triple resonance condition acting on different independent parameters. Exploiting the dependence of the bipartite system covariance matrix on different combination of this continuous variable two-mode states it can be directly measured using a single homodyne detector and few linear optical elements. The system is actually operating at the University of Napoli-Quantum Optics Laboratory. Preliminary measurements show that CV entanglement is present at the OPO output. This source of CV entanglement can be used for quantum communication purposes. The technical tricks herein implemented are interesting also for realizing triply resonant CW OPOs whose spectral properties and conversion efficiencies are better compared to single and double resonant devices. The quality of entanglement is also improved thanks to the use of a single cavity instead of dual cavities often employed in triply resonant devices. The measurement method can be applied to generic bipartite states made of frequency degenerate but orthogonally polarized modes.

Current research in optical computing and all-optical communications requires functionality that employs photons instead of electrons. Because of optical communications, there is a substantial research in developing elemental optical logic functions such as AND, OR, INVERT, and XOR. The latter logic function is one of the most important because of its versatility in encoding optical ciphertexts or decoding them, and also in designing optical macro-functions such as parity calculators. In this paper, we present an all-optical XOR gate that requires neither synchronization nor multi-wavelength control signals, and thus it is simpler to implement than other designs, it is cascadable and it is monolithically integrable. We simulate the all-optical XOR gate, we show its use in generating optical ciphertexts, and we demonstrate its cascadability towards an all-optical parallel parity generator.

The commutation relations between the generalized Pauli operators of N-qudits (i. e., N p-level quantum systems), and the structure of their maximal sets of commuting bases, follow a nice graph theoretical/geometrical pattern. One may identify vertices/points with the operators so that edges/lines join commuting pairs of them to form the so-called Pauli graph P_pN . As per two-qubits (p = 2, N = 2) all basic properties and partitionings of this graph are embodied in the geometry of the symplectic generalized quadrangle of order two, W(2). The structure of the two-qutrit (p = 3, N = 2) graph is more involved; here it turns out more convenient to deal with its dual in order to see all the parallels with the two-qubit case and its surmised relation with the geometry of generalized quadrangle Q(4, 3), the dual of W(3). Finally, the generalized adjacency graph for multiple (N > 3) qubits/qutrits is shown to follow from symplectic polar spaces of order two/three. The relevance of these mathematical concepts to mutually unbiased bases and to quantum entanglement is also highlighted in some detail.

We propose to use a linear array of singly trapped electrons to implement a spin chain for quantum communication. The effective spin-spin interaction is realized by means of a magnetic field gradient, which couples the electron spin to the motional degrees of freedom. Then the Coulomb repulsion between the particles transmits this coupling throughout the array. The resulting system can be described in terms of a Heisenberg model with long-range interactions showing a dipolar decay. We estimate the fidelity of the system in reproducing an ideal spin chain by taking into account the influence of the electron spatial motion.

In recent years time-resolved fluorescence measurement and analysis techniques became a standard in single molecule microscopy. However, considering the equipment and experimental implementation they are typically still an add-on and offer only limited possibilities to study the mutual dependencies with common intensity and spectral information. In contrast, we are using a specially designed instrument with an unrestricted photon data acquisition approach which allows to store spatial, temporal, spectral and intensity information in a generalized format preserving the full experimental information. This format allows us not only to easily study dependencies between various fluorescence parameters but also to use, for example, the photon arrival time for sorting and weighting the detected photons to improve the significance in common FCS and FRET analysis schemes. The power of this approach will be demonstrated for different techniques: In FCS experiments the concentration determination accuracy can be easily improved by a simple time-gated photon analysis to suppress the fast decaying background signal. A more detailed analysis of the arrival times allows even to separate FCS curves for species which differ in their fluorescence lifetime but, for example, cannot be distinguished spectrally. In multichromophoric systems like a photonic wire which undergoes unidirectional multistep FRET the lifetime information complements significantly the intensity based analysis and helps to assign the respective FRET partners. Moreover, together with pulsed excitation the time-correlated analysis enables directly to take advantage of alternating multi-colour laser excitation. This pulsed interleaved excitation (PIE) can be used to identify and rule out inactive FRET molecules which cause interfering artefacts in standard FRET efficiency analysis. We used a piezo scanner based confocal microscope with compact picosecond diode lasers as excitation sources. The timing performance can be significantly increased by using new SPAD detectors which enable, in conjunction with new TCSPC electronics, an overall IRF width of less than 120 ps maintaining single molecule sensitivity.

We are presenting the design, construction and performance of the compact control circuit of the InGaAs photon counter. The main design goals were to construct a detector control circuit, which enables the InGaAs photon counters to operate in gated mode, the detection window width adjustable 3-10 ns with repetition rate reaching 2 MHz. The short detecting window requires the nanosecond rise/fall edges of gating pulse. The gating pulse minimum amplitude of 5 Volts is needed. The control circuit consists of the gate signal receiver, level converter, gate driver, avalanche breakdown sensor and a detector bias control circuit with fast current limiting option. The entire circuit has been tested in connection with different InGaAs detection chips operated at room temperature and thermoelectrically cooled with gate width in the range of 3 to 10 nanoseconds and gate repetition rate up to 5 MHz. The entire circuit is quite compact 60 x 60 mm in size, it is powered with external stabilized supply +5 V and -6 V. Although the main foreseen application was quantum cryptography, the control circuit can be used in connection with various avalanche photodiodes and in different applications.

Single photon avalanche diodes (SPADs) based on various semiconductors have been developed at the Czech Technical University in Prague during the last 20 years. Much attention has been also paid to development of high-speed active quenching circuits for these detectors. Recently, we have performed a series of experiments to characterize our silicon-based photon counters and their capability of operation in a gated mode with the gate duration of single nanoseconds and the detector sensitivity rise time of hundreds of picoseconds. This performance has been achieved by optimizing the active quenching circuit and its components. The fast gating is needed in cases, when the photons of interest are generated short time after a strong optical signal, which cannot be suppressed in optical domain. The time dependence of detection sensitivity, detection delay and timing resolution within the nanosecond gates has been measured.

Optical and crystal properties of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition (MOCVD) were characterized using room-temperature photoluminescence (PL) and high-resolution Xray diffraction (HRXRD), respectively. The near bandgap excitonic peak decreased from 2.77 eV to 2.68 eV while there was a 10 Å increase in the well thickness, probably caused by variations of quantized energy levels. In addition, higher growth temperature of MQW structures had a small influence on the pair thickness, but the emission wavelength showed a blueshift attributed to the decrease in average of indium mole fraction. However, the near bandgap excitonic peak remained constant for the thicker quantum barriers. For the PL emission intensity of InGaN/GaN MQW structures, it was enhanced with a thinner quantum well width and a thicker quantum barrier, which could be resulted from the improvement of optical confinement in the quantum well. Moreover, by using the higher growth temperature, enhanced PL intensity was achieved due to the improvement of structure quality for the InGaN/GaN heterostructure. Therefore, these results suggest that the emission wavelength and intensity of the InGaN/GaN MQW-based optical device could be modulated by designing thicknesses of quantum wells as well as growth temperatures of MQW structures.