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Cooperative scattering in cold atoms has gained renewed interest, in particular in the context of single-photon superradiance, with the recent experimental observation of super- and subradiance in dilute atomic clouds. Numerical simulations to support experimental signatures of cooperative scattering are often limited by the number of dipoles which can be treated, well below the number of atoms in the experiments. In this paper, we provide systematic numerical studies aimed at matching the regime of dilute atomic clouds. We use a scalar coupled-dipole model in the low excitation limit and an exclusion volume to avoid density-related effects. Scaling laws for super- and subradiance are obtained and the limits of numerical studies are pointed out. We also illustrate the cooperative nature of light scattering by considering an incident laser field, where half of the beam has a phase shift. The enhanced subradiance obtained under such condition provides an additional signature of the role of coherence in the detected signal. [ABSTRACT FROM AUTHOR]

The ability to determine the joint spectral properties of photon pairs produced by the processes of spontaneous parametric downconversion (SPDC) and spontaneous four-wave mixing (SFWM) is crucial for guaranteeing the usability of heralded single photons and polarization-entangled pairs for multi-photon protocols. In this paper, we compare six different techniques that yield either a characterization of the joint spectral intensity or of the closely related purity of heralded single photons. These six techniques include: (i) scanning monochromator measurements, (ii) a variant of Fourier transform spectroscopy designed to extract the desired information exploiting a resource-optimized technique, (iii) dispersive fibre spectroscopy, (iv) stimulated-emission-based measurement, (v) measurement of the second-order correlation function for one of the two photons, and (vi) two-source Hong-Ou-Mandel interferometry. We discuss the relative performance of these techniques for the specific cases of a SPDC source designed to be factorable and SFWM sources of varying purity, and compare the techniques’ relative advantages and disadvantages. [ABSTRACT FROM AUTHOR]

A quantum dialogue protocol is proposed and proved to be secure. In our scheme, single photons are used as a quantum channel, the security of our scheme is guaranteed by BB84 QKD and the fact that controlled-not operation ensures the control qubit is unchanged and the target qubit flips only if the control qubit is [image omitted]. By using controlled-not operations, two legitimate communicators can share a private initial state and realize quantum dialogue securely. Compared with previous quantum dialogue protocols, the good merit of this work is that the insecurity, i.e. 'information leakage' is eliminated. Comparing this dialogue protocol with the recent work in another paper [Shi, G.F.; Xi, X.Q.; Hu, M.L.; Yue, R.H. Opt. Commun. 2010, 283, 1984] where two adjacent single photons must be of the same quantum state, this scheme is experimentally simple. [ABSTRACT FROM AUTHOR]

We describe a type of collective entangled dark state which exists when a group of many-level atoms interact simultaneously with both high-Q cavity modes and free space modes of the electromagnetic field. As an example we describe a collection of atoms with a level structure of the form of [image omitted] and show that there are states of this system in which the atoms remain trapped in a collective coherent superposition of ground state levels in spite of the presence of photons in resonant field modes. Efficient methods of generalization of these results to other atomic level structures and cavity fields are described. [ABSTRACT FROM AUTHOR]

A programme has been started to investigate photon properties that are not currently exploited in astronomical instruments, namely second- and higher-order coherence functions encoded in their arrival time, and the orbital angular momentum. This paper expounds the first results achieved in the study of a novel astronomical photometer capable of pushing time tagging towards the picosecond region. This conceptual device has been developed as a possible focal plane instrument for the future OverWhelmingly Large Telescope (OWL) of the European Southern Observatory. This instrument has been named QuantEYE, that is, the Quantum Eye of OWL. [ABSTRACT FROM AUTHOR]

Second-order interference effects in the spontaneous emission by two identical two-level atoms, separated a distance apart and both initially excited, are considered. On the basis of the analysis in our previous paper published in 2004, within fourth-order perturbation, detailed analysis of the second-order quantum interference in the emitted field is made. The interference arises both from single-photon states and two-photon states owing to interferences between different decaying sequences. By detailed analysis we have found some cancellations between single-photon and two-photon interference terms, and an interpretation of this is given. The contributions from the single- and the two-photon states as well as the total interference are analysed numerically for various interatomic distances and times. The contribution from a two-photon state is mostly larger than that from a single-photon state except when the atomic electric dipole is large, in which case the two contributions can be comparable. It is shown that the emission patterns from the two interacting atoms provide more information on atom–atom correlations than the decay time evolution of the atomic states population does. The number of interference fringes and the distance between them depend on the interatomic distance in a similar fashion to Young's interference fringes. [ABSTRACT FROM AUTHOR]

In an earlier paper we concluded that time-dependent parameters in the atom-mode interaction can be utilized to modify the quantum field in a cavity. When an atom shoots through the cavity field, it is expected to experience a trigonometric time dependence of its coupling constant. We investigate the possibilities this offers to modify the field. As a point of comparison we use the solvable Rosen-Zener model, which has parameter dependences roughly similar to the ones expected in a real cavity. We do confirm that by repeatedly sending atoms through the cavity, we can obtain filters on the photon states. Highly non-classical states can be obtained. We find that the Rosen-Zener model is more sensitive to the detuning than the case of a trigonometric coupling. [ABSTRACT FROM AUTHOR]

The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two di9erent methods are discussed. The first is based on the one-atom maser or micromaser operating under the conditions of the so-called trapping states. In this situation the micromaser stabilises to a photon number state. Recently, we also succeeded in determining the Wigner function of a single-photon state. The second device, recently realized in our laboratory, uses a single trapped ion in an optical cavity. [ABSTRACT FROM AUTHOR]

In this paper, we study theoretically the optomechanical interaction of an interacting condensate of photons with an oscillating mechanical membrane in a microcavity. We show that in the Bogoliubov approximation, due to the large number of photons in the condensate, there is a linear strong effective coupling between the Bogoliubov mode of the photonic Bose–Einstein condensate (BEC) and the mechanical motion of the membrane which depends on the photon–photon scattering potential. This coupling leads to the cooling of the mechanical motion, the normal mode splitting (NMS), the squeezing of the output field and the entanglement between the excited mode of the cavity and the mechanical mode. Since the photon condensation occurs at room temperature, this hybrid system can be potentially considered as a room temperature source of squeezed light as well as a suited candidate for exploring the quantum effects. We show that, on one hand, the non-linearity of the photon gas increases the degree of the squeezing of the output field of the microcavity and the efficiency of the cooling process at high temperatures. On the other hand, it reduces the NMS in the displacement spectrum of the oscillating membrane and the degree of the optomechanical entanglement. In addition, the temperature of the photonic BEC can be used to control the above-mentioned phenomena. [ABSTRACT FROM PUBLISHER]

In this paper, we study the unconventional photon blockade of symmetric and antisymmetric modes in a photonic molecule system with bilateral drive. We show that the photon blockade effect is controllable by manipulating the tunnelling strength of two cavities and the ratio of driving strengths. Under the weak driving condition, we present the exact optimal conditions for strong antibunching photons. The optimal conditions are in good agreement with the numerical results. It is also found that the optimal value of detuning frequencies is not sensitive to the value of the ratio to achieve the unconventional photon blockade effect. The feasibility of tunable unconventional photon blockade effect also has been studied based on the experimental parameters. Our studies of the photonic molecule system can be used as a single-photon source based on the symmetric and antisymmetric modes in the regime of weak nonlinearity. [ABSTRACT FROM AUTHOR]

Mathematically, the simplest state of light containing phase information is a superposition of the vacuum and the one-photon state and, as we show in this paper, such a state is reasonably simple to measure. We investigate how the information contained in a more complicated pure state of light, in particular the ratio of successive number-state coefficients, can be transferred selectively to fields in this two-state superposition for subsequent measurement. By this means the number-state representation of the more complicated state can be ascertained, provided there are no gaps in the number state distribution. We also discuss how to correct for the effect of non-unit efficiencies of the photodetectors involved in the transferral process. [ABSTRACT FROM AUTHOR]

The boson-sampling problem has triggered a lot of interest in the scientific community because of its potential of demonstrating the computational power of quantum interference without the need of non-linear processes. However, the intractability of such a problem with any classical device relies on the realization of single photons approximately identical in their spectra. In this paper, we discuss the physics of boson sampling with non-identical single-photon sources, which is strongly relevant in view of scalable experimental realizations and triggers fascinating questions in the complexity theory. [ABSTRACT FROM PUBLISHER]

Noise plays a troublesome role for entanglement generation, because the polarization entanglement of photons can be easily disturbed by the noise. In this paper, we propose a protocol to prepare Greenberger–Horne–Zeilinger state and W state of atoms in quantum nodes connected by collective-noise channels assisted by quantum nondemolition detectors (QNDs) and the controlled phase flip gates. The frequency degrees of freedom are exploited in our protocol. The successful probability of our protocol can reach 100% neglecting the photon loss and assuming that the efficiency of QNDs is 1. Moreover, the number of times of using QNDs is limited, and this makes the protocol quite effective when collective noise is small. [ABSTRACT FROM PUBLISHER]

This paper presents the dynamics of two spatially separated optical solitons in two-photon photorefractive materials. The variational formalism has been employed to derive evolution equations of different parameters which characterize the dynamics of two interacting solitons. This approach yields a system of coupled ordinary differential equations for evolution of different parameters characterizing solitons such as amplitude, spatial width, chirp, center of gravity, etc., which have been subsequently solved adopting numerical method to extract information on their dynamics. Depending on their initial separation and power, solitons are shown to either disperse or compresses individually and attract each other. Dragging and trapping of a probe soliton by another pump have been discussed. [ABSTRACT FROM AUTHOR]

In this paper we discuss the reconstruction process of one-photon mixed states of partially polarised light. To solve this issue, we obtain the Stokes parameters by means of the degree of polarisation. The density operator describing the examined state is represented with these parameters. In the proposed two-beams method the degree of polarisation is measured on the analysed beam combined with reference beams containing photons with a settled state of polarisation. Coupling these beams allows one to obtain the Stokes parameters from the intensity contrast behind the rotating polariser. We discuss the influence of the quantum uncertainty on this technique of one-photon states reconstruction and we consider it for three aspects – the possibility of reducing the number of reference beams that are needed, the optimal state of polarisation of reference beams and the accuracy of the reconstruction method. [ABSTRACT FROM PUBLISHER]

In this paper, a simple method for obtaining general analytical solutions of the time evolution equations for a fully quantized parametric oscillation process is developed. Heisenberg’s equations for the signal–idler photon annihilation operators are converted into a matrix equation equivalent to a two-state Jaynes–Cummings time evolution equation which has exact analytical solutions. The mean intensity inversion for the coupled signal–idler photon pair is found to undergo fractional revivals for pump photon in a Fock state, provided both signal and idler photons are in occupied Fock states. General collapses and revivals occur for interactions with pump photon in a coherent state, but now with both or either of signal and idler photons in occupied Fock states. An interpretation of the coupled signal–idler photon pair as a circularly polarized two-state system specified by positive and negative helicity states leads to an appropriate description of photon polarization state dynamics governed by the underlying Jaynes–Cummings interaction. [ABSTRACT FROM AUTHOR]

In this paper the authors present the idea for reconstructing one-photon states. Reconstructing a quantum state means measuring the probability distribution P that allows one to write the density operator for the analysed state. The most commonly known approach for the quantum reconstruction is the quantum tomography. Our alternative method assumes that the analysed field is coupled with the reference field which is described by the parameters settled during a measurement. In the proposed gedankenexperiment the degree of linear polarisation (DOLP) of this combined beam is measured using a rotating linear polariser. We state that it is possible to obtain the P-function by changing the parameters of reference beams and by preparing the series of DOLP measurements. This series of data leads to the system of equations. The values of the P-function at chosen points are the unknowns of this system. This article focuses on the numerical algorithm for solving these equations. [ABSTRACT FROM AUTHOR]

This paper describes how single photons of light can be used to determine the index of refraction and material thickness. The results of single-photon transmission and polarization interferometer experiments are analyzed to find the index of refraction and thickness of a microscope coverslip. The single photons were generated using non-collinear spontaneous down-conversion. The experimental results are in good agreement with derived theoretical models. This method of determining optical parameters at the quantum level may be useful in performing non-destructive measurements of light-sensitive samples with high precision. [ABSTRACT FROM PUBLISHER]

The UK New Light Source (NLS) project identified a strong scientific case for a next-generation light source to deliver continuous coverage of the photon energy range 50–1000 eV with variable polarisation, 20 fs pulse widths, a high degree of temporal coherence and 1 kHz repetition rate. Three separate seeded free-electron lasers (FELs) were proposed which in combination satisfy these requirements. It was proposed to use a high harmonic generation (HHG) seed source tuneable from 50–100 eV to give direct seeding at the fundamental FEL photon energy up to 100 eV, with one or two stages of harmonic up-conversion within the FEL to reach the higher photon energies. This paper focuses on the optimisation of a two-stage harmonic cascade FEL design operating at the highest (fundamental) photon energy of 1000 eV. Topics investigated include the configuration of the modulator undulators, the required seed power and the effects of the temporal structure of the unfiltered HHG seed on the FEL output. FEL simulations using realistic electron beam distributions tracked from the gun to the FEL are presented, illustrating the predicted coherence properties of the FEL output. [ABSTRACT FROM AUTHOR]

After a brief review of the physics of photon detection in single photon avalanche diode (SPAD) devices, in this paper we will outline the principle of operation of a model we developed with the aim of calculating both photon detection efficiency (PDE) and temporal response (TR) of these detectors. Then we will apply the model to the devices currently available in order to critically analyze some experimental results. We will show in particular how the use of the model allows us to gain a better understanding of the influence of each device parameter in determining both the PDE and the TR. Finally we will discuss some modifications that can be applied to the device structure in order to overcome such limitations. Their effectiveness in improving both the PDE and the TR will be investigated by means of the aforementioned model. The aim is to provide the reader with an insight of which performances can be expected in the next few years if a strong development of the SPAD structure is pursued. [ABSTRACT FROM AUTHOR]

Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPADs) for parallel detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible planar technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they have inherently higher photon detection efficiency than photomultiplier tubes and they are able to provide very high acquisition speeds. The development of fully-parallel multichannel systems is a challenge both for the detector (with new problems like optical and electrical crosstalk, isolation elements size reduction and the need for higher yields) and for the electronics. The development of a parallel detector must be accompanied by that of the time-to-amplitude converters and analysis electronics necessary in time-correlated single photon counting applications. In this paper we present the fundamental building blocks necessary to develop a parallel picosecond array detector: a new high-performance SPAD detector, compatible with the design of multi-detector systems with a large number of channels, and a new totally integrated system for time-to-amplitude conversion, to allow the simple integration of several channels on a single chip. [ABSTRACT FROM AUTHOR]

In this paper, we propose two ways of Dicke state generation using single photons and weak coherent pulse resorting to Kerr nonlinearities. By interference of two coherent beams, the Dicke state are realized in polarization and photon number degrees of freedom. By performing homodyne detection on the coherent states, the multi-particle entangled Dicke state can be generated with a high probability. The realization of the protocols are more flexible as the phase differences can be easily distinguished during homodyne detection. [ABSTRACT FROM AUTHOR]

High-density and ultracold atomic gases have emerged as promising media for storage of individual photons for quantum memory applications. In this paper we provide an overview of our theoretical and experimental efforts in this direction, with particular attention paid to manipulation of light storage (a) through complex recurrent optical scattering processes in very high density gases (b) by an external control field in a characteristic electromagnetically induced transparency configuration. [ABSTRACT FROM AUTHOR]

Geiger-mode avalanche photodiodes (APD) or photon counting detectors (PCD) have become the basis of a range of new detectors and applications. Such devices are sensitive to single photons and are being adapted to create different detector technologies such as the silicon photomultiplier (SPM). A silicon photomultiplier, so-called because of its similarity in performance to conventional photomultiplier tubes, is based on an array of photon counting detectors but with a single common output. While the silicon photomultiplier does not provide positional information, it does allow photon number resolution and photon counting at higher count rates than are achievable with a solitary PCD. These new detectors are enabling a range of new applications in the fields of medicine, biology, high-energy physics and space exploration. In this paper we report the performance of PCD with dimensions ranging from 20 μm × 20 μm to 200 μm × 200 μm and on 1 mm2 silicon photomultipliers and illustrate the use of an SPM in single photon counting mode. [ABSTRACT FROM AUTHOR]

This paper describes the results obtained so far with AquEYE, a single photon counting, fixed aperture photometer for the Asiago 182 cm telescope. AquEYE has been conceived as a prototype of a truly 'quantum' photometer for future Extremely Large Telescopes of 30-50 m aperture. This prototype is characterized by four independent channels equipped with single photon avalanche diodes (SPADs) as detectors. The counts from the four channels are acquired by a TDC board which has a nominal 25 ps time tagging capability. Taking into account the 35 ps jitter in the SPAD itself, the overall precision of the time tags is of the order of 50 ps. The internal oscillator is locked to an external rubidium clock; a GPS pulse per second is collected by the TDC itself to obtain a UTC reference. The maximum photon count rate which the present system can sustain is 12 MHz. [ABSTRACT FROM AUTHOR]

We describe the single-photon counting performance of a hybrid photomultiplier tube (HPMT) near-infrared (950-1300 nm) detector with a transfered electron InGaAsP photocathode and a GaAs Schottky avalanche diode anode. These devices have a lower photoelectron multiplication gain than conventional photomultiplier tubes, but offer a greater linear dynamic range and electrical bandwidth. With the use of a low-noise preamplifier, they can detect single photons with a greater than 20% quantum efficiency (QE) and a reasonably low dark-noise count rate. The avalanche diode at the anode operates in a low gain analog mode and has no afterpulsing. As a result, these HPMTs can detect single photons continuously at high count rates without gating. The relatively large photocathode active area (1 mm diameter) is also attractive to many applications including laser altimetry, ranging, and free-space communications through the atmosphere. We measured 25% photocathode QE and nearly the same single-photon detection efficiency at 1064 nm wavelength with a dark count rate of 60,000 per second at -22°C. The output pulse width in response to single-photon detection is about 0.8 ns. The maximum count rate exceeded 100 million counts per second and was limited only by the speed of the electronics. The rms timing jitter of the HPMT output was measured to be about 0.5 ns. The jitter is dominated by the electron diffusion time within the photocathode and can be improved by reducing the photocathode thickness at a small loss in photocathode QE. We evaluated several of these HPMTs and detailed measurement results are reported in this paper. [ABSTRACT FROM AUTHOR]

This paper reports our experimental setups to characterize the superconducting nanowire single photon detector (SNSPD) that is packaged in-house. Our packaging method ensures not only a satisfied optical coupling efficiency but also a wide band electrical connection. The packaging approach has been proved to meet the needs of cryogenic operations. The basic optoelectronic characterization results including quantum efficiency and dark count rate indicate a good agreement with the expected values. [ABSTRACT FROM AUTHOR]

The interference of single heralded photons from multiple parametric downconversion sources requires photon pairs in factorable states. Typically, these are selected from an ensemble of pairs by narrow filters that remove any exhibiting correlations. In order to eliminate these lossy filters, factorable photon pairs free from any spatio-temporal correlations must be created directly at each source. This requires careful engineering of the group velocity dispersion of the nonlinear crystal in which pair generation takes place. Several schemes have been proposed to do this in the plane-wave regime, but in a realistic experiment one must also take into account the effects of focusing on the two-photon state. Focusing leads to space-time coupling between the pump structure and the downconverted pairs that has the potential to reduce their factorability, but if carefully managed can actually increase it. In this paper, we consider some of the effects of focusing and their consequences for pure single photon generation. [ABSTRACT FROM AUTHOR]

This paper investigates the feasibility of using sequences of unitary pulses for achieving coherent-control of quantum dynamical phenomena, including quantum control of electron tunnelling in archetype model systems and control of decoherence in a quantum dot system coupled to a thermal bath. The proposed dynamical decoupling scenario is based on the repetitive application of 2π pulses, affecting the interference phenomena between wave-packet components. The pulses affect the overall relaxation dynamics without collapsing the coherent-quantum evolution of the system. It is shown that both bound-to-bound state tunnelling and bound-to-continuum tunnelling processes can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not affect the potential energy tunnelling barriers. Furthermore, the proposed quantum-control scenario is demonstrated as applied to manipulating the electronic quantum dynamics in a quantum dot coupled to a free standing quasi two-dimensional phonon cavity. The reported results are therefore particularly relevant to the understanding of coherent optical manipulation of electronic excitations in semiconductor devices where performance is limited by quantum tunnelling and decoherence. [ABSTRACT FROM AUTHOR]

This paper reports on the steady-state fluorescence, three-photon absorption-induced fluorescence emission and subsequently induced optical limiting behaviour of a fluorene derivative with D-π-D structural motifs. The lifetime of the steady-state fluorescence is 0.903 ns. Large optical limiting behaviour induced by 3PA has also been demonstrated, and the nonlinear absorption coefficient γ derived from 3PA fitting curves is 5.92 × 10-20 cm3/W2 and the corresponding molecular 3PA cross-section is 1.14 × 10-76 cm6 s2. [ABSTRACT FROM AUTHOR]

Phase steps are an important type of wavefront aberration generated by large telescopes with segmented mirrors. In a closed-loop correction cycle these phase steps have to be measured with the highest possible precision using natural reference stars, that is with a small number of photons. In this paper the classical Fisher information of statistics is used for calculating the Cramér–Rao minimum variance bound, which determines the limit to the precision with which the height of the steps can be estimated in an unbiased fashion with a given number of photons and a given measuring device. Four types of such measurement devices are discussed: a Shack–Hartmann sensor with one small cylindrical lenslet covering a subaperture centred on a border, a modified Mach–Zehnder interferometer, a Foucault test, and a curvature sensor. The Cramér–Rao bound is calculated for all sensors under ideal conditions, that is narrowband measurements including photon shot noise, but without other forms of noise or disturbances. This limit is compared with the ultimate quantum statistical limit for the estimate of such a step, which is independent of any measuring device. For one device, the Shack–Hartmann sensor, the effects on the Cramér–Rao bound of broadband measurements, finite sampling, and disturbances such as atmospheric seeing and detector readout noise are also investigated. The methods presented here can be used to compare the precision limits of various devices for measuring differential segment phases, and for optimizing the devices. Under ideal conditions the Shack–Hartmann and the Foucault devices nearly attain the ultimate quantum statistical limit, whereas the Mach–Zehnder and the curvature devices each require approximately twenty times as many photons in order to reach the same precision. [ABSTRACT FROM AUTHOR]

It was shown by Riedmatten et al. [J. Mod. Opt. 51 1637 (2004)] that two different four-photon states could be generated according to the relation between the duration time of the pump laser T and the coherence time t of the single photon from spontaneous parametric down-conversion (SPDC). In this paper, using four photons generated by the SPDC in a Periodically Poled Potassium Titanyl Phosphate (PPKTP), we experimentally observe the four-photon interference in a Michelson interferometer. All experimental results can be explained by the product of two independent two-photon interferences. Our experimental condition corresponds to the case of T ?≫? t . This experiment clearly confirms the theory given by Riedmatten et al . [ABSTRACT FROM AUTHOR]

Single-photon detectors have become more important with the advent of set-ups for optical communication using single-photon pulses, mainly quantum key distribution. The performance of quantum key distribution systems depends strongly on the performance of single-photon detectors. In this paper, aiming to overcome the afterpulsing that limits strongly the maximal transmission rate of quantum key distribution systems, three smart strategies for single-photon detection are discussed using analytical and numerical procedures. The three strategies are: hold-off time conditioned to avalanche presence, termed the Norwegian strategy, using one avalanche photodiode, using two raffled avalanche photodiodes and using two switched avalanche photodiodes. Finally we give examples using these strategies in a quantum key distribution set-up. [ABSTRACT FROM AUTHOR]

Theoretical analysis shows that the correlation time of a photon pair is closely related to the spectrum of two-photon light. This entails limitations on the shortest achievable two-photon correlation time. These limitations are important for quantum optical metrology and information processing applications, especially those involving quasi-monochromatic light. In this paper we present a more rigorous proof of the fact that the shortest possible two-photon correlation time is limited by approximately the inverse width of the optical spectrum. We further show that this limit can be closely approached with Type-I spontaneous parametric down-conversion at a specific pump wavelength. This study also results in the prediction of very unusual parametric down-conversion spectra, suggesting a method for engineering of frequency-entangled two-photon states. [ABSTRACT FROM AUTHOR]

Experiments with single atoms have become routine. In this paper two groups of these experiments will be reviewed with special emphasis on applications to study quantum phenomena in the atom-radiation interaction. The first one deals with the one-atom maser and the second one with another cavity quantum electrodynamic device on the basis of trapped ions. The latter device has interesting applications in quantum computing and quantum information. [ABSTRACT FROM AUTHOR]

Although it has been known for some time that quantum mechanics can be formulated in a way that treats prediction and retrodiction on an equal footing, most attention in engineering quantum states has been devoted to predictive states, that is, states associated with a preparation event. Retrodictive states, which are associated with a measurement event and propagate backwards in time, are also useful, however. In this paper it is shown how any retrodictive state of light that can be written to a good approximation as a finite superposition of photon number states can be generated by an optical multiport device. The composition of the state is adjusted by controlling predictive coherent input states. It is shown how the probability of successful state generation can be optimized by adjusting the multiport device and also a versatile configuration that is useful for generating a range of states is examined. [ABSTRACT FROM AUTHOR]

Solid state single photon detectors are receiving more and more attention in a number of areas of applied physics: optical sensors, communications, quantum cryptography, optical ranging and Lidar, time resolved spectroscopy, opaque media imaging and ballistic photon identification. This paper reports on results of research and development in the field of solid state single photon detectors at the Czech Technical University in Prague over the last 20 years. Avalanche photodiodes specifically designed for single photon counting devices have been developed based on various semiconductor materials: Si, Ge, GaP, GaAs and InGaAs. Electronic circuits for biasing, quenching and control of these detectors have been developed and optimized for different applications. The sensitivity of solid state photon counters spans from 0.1 nanometre X-rays up to 1800 nanometres in the near infrared region. Timing resolution of solid state photon counters as high as 50 picoseconds full width at a half maximum has been achieved when detecting single photon signals. Circuits permitting operation of solid state photon counters in both single and multiple photon signal regimes have been developed and applied. The compact and rugged design, radiation resistance, and low operating voltage are attractive features of solid state photon counters in various space projects. [ABSTRACT FROM AUTHOR]

The paper reports on research and development in the field of avalanche photodiodes operated as photon counters in a Geiger mode. A technique has been developed and tested that permits estimation of the photon number involved in a detection process. It can be applied in a time correlated photon counting experiment simultaneously with original required time interval estimation. A time walk compensation circuit provides uniform electrical pulses, and the time interval between them is related to the number of photons detected. Employing a picosecond event timing device, the photon number can be estimated within the dynamic range 1-1000 photons with resolution better than a factor of three. [ABSTRACT FROM AUTHOR]

Correlated photons can be used to directly measure the detection efficiency of photon counting detectors without any ties to externally calibrated standards. An overview of the history of this technique is given and the paper reviews how to implement it in a practical lab setting. Some of the sources of uncertainty in the technique and how they can be minimized and quantified are discussed. The intent is to provide the information necessary to encourage the movement of this technique from the metrology lab into the general photon-counting detector community. [ABSTRACT FROM AUTHOR]

The paper reports progress on the design and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultrafast counting of near-infrared photons for secure quantum communications. The SSPDs operate in the quantum detection mode, based on photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 3.5 nm thick NbN films and kept at cryogenic (liquid helium) temperatures inside a cryostat. The detector experimental quantum efficiency in the photon-counting mode reaches above 20% in the visible radiation range and up to 10% at the 1.3-1.55 µm infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2 GHz and is limited by readout electronics (the intrinsic response time is below 30 ps). The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 2 × 10 -18 W/Hz 1/2 at 1.3 µm. In terms of photon-counting efficiency and speed, these NbN SSPDs significantly outperform semiconductor avalanche photodiodes and photomultipliers. [ABSTRACT FROM AUTHOR]

The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory using the one-atom maser or micromaser to produce these fields on demand. In order to achieve this goal the maser is operating under the condition of the so-called trapping states. In this situation the field of the micromaser stabilises to a photon number state. Recently, we have also succeeded in determining the Wigner function of a single-photon state. Presently measurements of the phase diffusion of the one-atom maser field are underway, whereby the case will be studied in detail when the field approaches a number state. The measurements will also give information on the decoherence of the maser field. [ABSTRACT FROM AUTHOR]

Quantum communication is a new and fascinating area in telecommunications that provides new ways of communication without classical counterparts. At the centre of this theory is quantum entanglement, a powerful and enigmatic property shown by some composite quantum systems. In this paper, the entanglement produced by sending of number states through a beam splitter is studied. The property entangled is the parity of the number of photons of the number states at the beam splitter's outputs. The decoherence is taken into account through the losses. The entanglement of pure and mixed states at the output of the beam splitter is calculated and some protocols for quantum teleportation for bipartite (C 2 ⊗ C 2 ) and tripartite (C 2 ⊗ C 2 ⊗ C 2 ), systems are described. [ABSTRACT FROM AUTHOR]

The paper discusses the single-mode Jaynes-Cummings model with time-dependent parameters. Solvable models for two-level systems are utilized to consider the changes in the photon distribution effected by the passage of atoms through the cavity. It is suggested that such systems may be used as filters to modify the photon distribution. The effect can be enhanced by repeatedly sending new atoms through the cavity. We show that such filters can cut out either small or large photon numbers. It is also shown that the method can be used to narrow down photon distributions and in this way achieve highly non-classical sub-Poissonian states. Some limitations and applications of the method are presented. [ABSTRACT FROM AUTHOR]

This paper describes a completely new way to perform noise rejection using a two-photon sensitive detector and taking advantage of the properties of correlated photons to improve an optical communications link in the presence of uncorrelated noise. In particular, a detailed analysis is made of the case where a classical link would be saturated by an intense background, such as when a satellite is in front of the sun, and identifies a regime where the quantum correlating system has superior performance. [ABSTRACT FROM AUTHOR]

Type-II parametric down conversion in nonlinear optical crystals has turned out to be one of the most powerful methods to generate entangled photon pairs, and even allowed a large set of experiments on foundations of quantum mechanics and recent applications in quantum communication. In this paper, we present an experimental investigation of the emission characteristics for down-converted photons in order to investigate the possibility of high brightness sources for correlated photon pairs. Furthermore, we discuss possible ways to increase the collection efficiency for polarization entangled photon pairs. [ABSTRACT FROM AUTHOR]

In this paper a two-photon Jaynes-Cummings model interacting with a Kerr-like medium is studied. It is assumed that the electromagnetic field is in different states such as coherent, squeezed vacuum and pair coherent, and that the atom is initially in the excited state. The temporal evolution of the population of the excited level, and the second-order coherence function are studied. The results obtained show that this system has some similarities with the two-mode Stark system. Two photon entanglement are analysed at different initial conditions. [ABSTRACT FROM AUTHOR]

Fluorescent photon emission probability from a system of three atoms has been studied as a function of detection angles. The system under investigation consists of identical bi-level atoms forming an equilateral triangle. Only a single atom in the triangle is resonantly excited by a weak laser. The interaction mechanism among the atoms is the vacuum-mediated dipole–dipole interaction and the collective decay process. The focus is to analyse the photon pairs simultaneously radiated by the system in the far-field domain. The analysis has been done for various interatomic separations. It is seen that the interference between the photon emitters can lead to strong emission in certain directions. An intuitive understanding of the interesting directions for strong photon bunching is given. It is found that a reduction in the intensity of radiation leads to superradiance along specific directions. [ABSTRACT FROM AUTHOR]

We propose new methods to characterize the spectral-temporal distribution of single photons. The presented protocols take advantage of spectral filtering, frequency entanglement between two single photons, the one of interest and a reference, followed by the generalized Hong–Ou–Mandel interferometer. The measurement of the coincidence probability in these different schemes reveals the chronocyclic Wigner distribution, the pseudo-Wigner one and the spectrogram at the single photon level. [ABSTRACT FROM AUTHOR]

A correction to the article "Special Issue: Single Photon Sources, Detectors, Applications, and Measurement Methods. Including selected papers from the 3rd International Workshop" that was published in the October 2008 issue is presented.