This PDF file contains the front matter associated with SPIE Proceedings Volume 6710, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

In this paper we present a scheme for ghost imaging based on partially coherent light produced when coherent light passes through a turbid medium. The light is described by a transport model incorporating Mie scattering theory. Using this model it is possible to predict the second order correlation function of the pseudo thermal source and to estimate key parameters for its application in ghost imaging experiments. Analytical and numerical simulations of the second order coherence function are performed in terms of scattered and un-scattered components of the light. The coherence length of the scattered component, responsible for ghost images formation, is determined in terms of concentration and diameters of the scattering elements. The advantage of using this type of source with respect to previous experiments is a better understanding of its coherence properties responsible for resolution and visibility of ghost imaging with thermal light.

We implement a frequency degenerate seeded downconversion process in which the seed field is a spatially multimode chaotic field. The two output fields are quantum correlated in space and intensity and maintain the same spatial and temporal structure as the seeding field and thus represent two almost twin multimode fields that can be used for ghost imaging applications.

We demonstrate, by direct measurement of the number of photons in signal and idler, that the twin-beam of light produced by ps-pulsed spontaneous parametric downconversion is endowed with sub-shot-noise photon-number correlations in a mesoscopic intensity regime (more than 1000 detected photons). The noise reduction, calculated from the variance of the difference in the numbers of detected-photons, resulted to be 3.25 dB below the shot-noise level. From experimental data we can recover joint photon-number distribution and a negative-valued joint signal-idler quasi-distributions of integrated intensities, which demonstrates the nonclassical character of the generated field.

We review and compare the results of quantum spatial correlation measurements in parametric down-conversion in the high-gain pulsed regime, with preliminary measurements performed in the low-gain cw-pumped regime. The diagnostics is based on a high quantum efficiency CCD camera, and in the second case the radiation pattern is recorded after temporal integration of the "single-photon" spatial distribution. The effect of the detected number of temporal modes on the accessibility of the sub-shot noise regime is discussed, together with the identification of suitable regimes for weak image detection.

Distinct from previous principle-demonstrations, this practical quantum ghost imaging experiment presents the first set of two-photon images captured by a photon counting CCD camera by means of jointly counting 'reflected' photons from the object. In fact, the CCD camera was not 'looking' at the object at all. Rather, the CCD camera was facing the chaotic light source. The output of the CCD camera was used for coincidence registration of the two-photon joint-detection events with another photon counting detector which simply collects all randomly reflected photons from the surface of the object. It is also interesting to find that the observed two-photon images are 'distortion-free', i.e., any disturbances made along the light path has no effect on the quality of the image. These experimental observations are not only useful for practical field-applications, but also important from fundamental point of view. The experiment evidences a rejection of the 'projection shadow' idea in a non-deniable way and further explores the two-photon interference nature of thermal light ghost imaging.

We describe a fiber-coupled parametric down-conversion (PDC) source designed for entanglement-enhanced twophoton absorption experiments. The key feature of the source is a narrowband (~1 MHz) UV diode pumping laser which can be tuned to match the energy of the 5S_1/2 to 5D_5/2 two-photon transition in Rubidium. The weak narrowband pumping beam is delivered to the PDC crystal through a single-mode fiber, which allows the source to be pre-aligned with a much stronger broadband auxiliary pump laser. The motivation for this PDC source lies within the context of Linear Optics Quantum Computing and Quantum Zeno Gates.

Experimental observation of the conservation of orbital angular momentum in spontaneous parametric down-conversion has been theoretically attributed to phase-matching, transfer of plane-wave spectrum from pump beam to down-converted beams. However, according to quantum mechanics, the conservation of angular momentum arises from rotational symmetry of the Hamiltonian describing the studied physical process. Recently, experimental evidence has been found which shows that non-conservation of orbital angular momentum can occur in spontaneous parametric down-conversion due to rotational asymmetry of the Hamiltonian. In this paper, we theoretically show that all reported experimental results of conservation of orbital angular momentum in spontaneous parametric down-conversion are determined only by the Hamiltonian symmetry, and not by phase matching, transfer of plane-wave spectrum.

The generation of paired photons entangled in orbital angular momentum (OAM), provides a new degree of freedom that is increasingly being used as a source of quantum states. Elucidation of the OAM spectrum of OAM spectrum of the generated photons is of paramount importance, since quantum information applications require the ability to generate arbitrary entangled states with the appropriate OAM correlations. Here we discuss the OAM spectrum of the photon generated via SPDC in two complementary scenarios. On one hand, we should consider the whole geometry of the nonlinear process, taking into account azimuthal variations of the nonlinear coefficient or the phase matching conditions. On the other hand, all relevant experiments reported to date detect only a small section of the full down-conversion cone. In this scenario, the measured OAM correlations depend of the emission angle of the photons and the strength of the Poynting vector walk-off. We will present experiments in SPDC where this dependence is clearly revealed.

Although two-photon absorption (TPA) is a nonlinear optical process, it is not typically considered a fundamental resource for optical quantum information processing (QIP). We have recently shown that TPA and the quantum Zeno effect can be used to make deterministic quantum logic devices (Zeno gates) from an otherwise linear optical system. In a Zeno gate, TPA is used to suppress the failure events that would normally occur in a linear optics device when multiple photons exit the device in the same optical mode. We have also recently shown that additional two-photon absorbing media can be used in a more conventional manner, along with a Zeno gate, to convert weak laser pulses into heralded single photon pulses. Sources of this kind could have many potential benefits; however, the use of detectors and classical switches could limit their scalability, and therefore their usefulness in larger QIP systems. Here we describe how TPA and Zeno gates alone could be used to make more efficient, and possibly scalable, single-photon sources. Because the Zeno gates also rely on TPA, we show that the only critical enabling resource for this approach is an efficient TPA medium.

One of the goals of quantum optics is to implement new sources of quantum light with tunable control of the relevant photonic properties. Here, we add to the toolkit of available techniques in quantum optics for the full control of the properties of quantum light, new strategies to manage the spectrum of photons, namely, type of frequency correlations, bandwidth and waveform. As a source of quantum light, spontaneous parametric downconversion (SPDC) is considered. Interestingly, the techniques presented might be used in any nonlinear medium and frequency band of interest. One of the schemes to control the frequency correlations makes use of light pulses with pulse-front tilt. The method is based on the proper tailoring of the group velocities of all the waves that interact in the nonlinear process, through the use of beams with angular dispersion. Noncollinear SPDC is the other strategy that is considered, since it allows mapping the spatial characteristics of the pump beam into the frequency properties of the downconverted photons.

We report the demonstration of strong coupling between single Cesium atoms and a high-Q chip-based microresonator. Our toroidal microresonators are compact, Si chip-based whispering gallery mode resonators that confine light to small volumes with extremely low losses, and are manufactured in large numbers by standard lithographic techniques. Combined with the capability to couple efficiently light to and from these microresonators by a tapered optical fiber, toroidal microresonators offer a promising avenue towards scalable quantum networks. Experimentally, laser cooled Cs atoms are dropped onto a toroidal microresonator while a probe beam is critically coupled to the cavity mode. When an atom interacts with the cavity, it modifies the resonance spectrum of the cavity, leading to rejection of some of the probe light from the cavity, and thus to an increase in the output power. By observing such transit events while systematically detuning the cavity from the atomic resonance, we determine the maximal accessible single-photon Rabi frequency of Ω₀/2π ≈ (100 ± 24) MHz. This value puts our system in the regime of strong coupling, being significantly larger than the dissipation rates in our system.

Quantum key distribution (QKD) can, in principle, provide unconditional security based on the fundamental laws of physics. Unfortunately, a practical QKD system may contain overlooked imperfections and violate some of the assumptions in a security proof. Here, we report two types of eavesdropping attacks against a practical QKD system. The first one is "time-shift" attack, which is applicable to QKD systems with gated single photon detectors (SPDs). In this attack, the eavesdropper, Eve, exploits the time mismatch between the open windows of the two SPDs. She can acquire a significant amount of information on the final key by simply shifting the quantum signals forwards or backwards in time domain. Our experimental results in [9] with a commercial QKD system demonstrate that, under this attack, the original QKD system is breakable. This is the first experimental demonstration of a feasible attack against a commercial QKD system. This is a surprising result. The second one is "phase-remapping" attack [10]. Here, Eve exploits the fact that a practical phase modulator has a finite response time. In principle, Eve could change the encoded phase value by time-shifting the signal pulse relative to the reference pulse.

This paper proposes an application of quantum stream cipher by Yuen-2000 protocol (Y-00) to transmission of High Definition Television(HDTV) and super HDTV signals. First, we summarize several adhoc strengthening methods of security which may provide provable security under individual quantum measurement. Y-00 with the modifications has a provable practical security for HDTV transmission system in which one never needs the change of the seed key forever. Second, we give a brief report on a demonstration of quantum stream cipher by the basic Y-00 of 2.4 Gbps in a real commercial optical network of 196 km by Hitachi Inform.and Commun.Eng. And how to improve it to 10 Gbps system with provable security is discussed, which is a national project of NICT. Finally, design parameters for its application to HDTV(1.48 Gbps) and super HDTV(24 Gbps) of 4320 scanning lines and 60 frame/sec are clarified.

We describe an effect called Dipole Induced Transparency which enables a dipole emitter to strongly modify the cavity spectrum, even in the weak coupling regime. We then describe a method for generating entanglement and performing a full Bell measurement between two QDs using Dipole Induced Transparency. Finally, we show how DIT enables entanglement between QDs with vastly different radiative properties. The proposal is shown to be robust to cavity resonance mismatch.

The photon density operator function is used to describe the propagation of single-photon pulses through a turbulent atmosphere. The effects of statistical properties of photon source and the effects of a random phase screen on the variance of photon counting are studied. A procedure for reducing the total noise is discussed. The physical mechanisms responsible for this reduction are explained.

We present two experiments geared toward the realization of a robust and intense source of polarization-entangled photons. First, we describe a novel source of polarization-entangled pairs that uses periodically-poled potassium titanyl phosphate (PPKTP) and an interferometer based on polarization beam displacers. The source emits an intense flux of high-quality single-mode entangled photons and is stable, robust, and easy to align. Second, we report on sources of correlated photons generated in PPKTP waveguides. Waveguide sources of correlated pairs have been shown to generate high fluxes of pairs: we theoretically and experimentally investigate spontaneous parametric down-conversion generation of photon pairs in waveguides at different wavelengths.

In this paper we review our recent works on the generation of different Bell states within the lineshape of parametric down-conversion (SPDC) and their possible applications. Indeed, for polarization-entangled two-photon states produced via SPDC, the frequency-angular lineshape allowed by phase matching is considered. It is shown that there are always different Bell states generated for different mismatch values within the natural bandwidth. Consideration is made for two different methods of polarization entanglement preparation, based on type-II SPDC and on SPDC in two type-I crystals producing orthogonally polarized photon pairs. Different Bell states can be filtered out by either frequency selection or angular selection, or both. Our theoretical calculations are confirmed by a series of experiments, performed for the two above-mentioned ways of producing polarization-entangled photon pairs and with two kinds of measurements: frequency-selective and angular-selective. Finally, we mention possible application to quantum communication with fibers.

We propose an experiment in which an entangled pair of optical pulses follow different paths through a gravitational field. We use a non-standard technique based on localized operators to analyze this situation. The calculation predicts decorrelation of the optical entanglement under experimentally realistic conditions.

The so called NOON states are the main ingredient of many quantum optic schemes. The reliability of NOON-state production protocols thus plays an important role in view of practical applications. In realistic situations, the reliability of NOON-state sources strongly depends on the non-unitary photodetection efficiency of the single photon detectors involved in the protocol. We discuss and compare the reliability of NOON-state schemes based on both single-photon detection and non detection. Our result may be of great interest for practical implementation of NOON-state schemes.

Quantum communications is an emerging field with many promising applications. Its usefulness and range of applicability in optical fiber will depend strongly on the extent to which quantum channels can be reliably transported over transparent reconfigurable optical networks, rather than being limited to dedicated point-to-point links. This presents a number of challenges, particularly when single-photon quantum and much higher power classical optical signals are combined onto a single physical infrastructure to take advantage of telecom networks built to carry conventional traffic. In this paper, we report on experimental demonstrations of successful quantum key distribution (QKD) in this complex environment, and on measurements of physical-layer impairments, including Raman scattering from classical optical channels, which can limit QKD performance. We then extend the analysis using analytical models incorporating impairments, to investigate QKD performance while multiplexed with conventional data channels at other wavelengths. Finally, we discuss the implications of these results for evaluating the most promising domains of use for QKD in real-world optical networks.

A novel system for ultra-long-distance quantum key distribution in optical fiber, incorporating ultra-low-noise transition-edge sensor (TES) photodetectors, is described. Integration of the TES detectors into the system was facilitated with a unique optically switched interferometer design. The performance of the system over 101 km of dark, single-mode fiber at 1550 nm and a clock rate of 1 MHz is described. Secret-key bits were produced after error correction and privacy amplification when using mean photon numbers of 0.01, 0.0148, 0.02, 0.0304, and 0.2 photons/pulse at the output of the transmitter. At a mean photon number of 0.1 photons per pulse at the transmitter, a transmission line loss of 29.92 dB, roughly equivalent to 150 km of optical fiber, could be tolerated and secret bits extracted from the transmitted key.

This paper considers two issues on the quantum stream cipher by Yuen-2000 (Y-00) protocol. In the fist part of this paper we investigate the optimal modulation scheme for the basic model of the quantum stream cipher by Y-00 protocol, and in the remaining part we study the deliberate signal randomization. For the problem on the optimal modulation scheme, several modulation schemes are investigated for the cipher text-only attacks and the known plaintext attack under the error probability criterion and the information criterion to find the preferable modulation scheme. As a result, it will be shown that the phase shift keying signal yields the best performance among the modulation schemes investigated in our consideration by numerical simulations. After that, the roperty of the randomization technique called the deliberate signal randomization is considered for the cipher text-only attacks and the known plaintext attack in the information criterion. From this, it will be shown by numerical simulations that the amount of leakage of information from the legitimate user to the eavesdropper is reduced by the deliberate signal randomization. At the last section we will mention about the implementation issues of the deliberate signal randomization, taking account of the numerical results.

When eavesdropping on a quantum channel, two distinct properties of quantum physics arise. First, if the quantum system is prepared in one of several non-orthogonal states, then no measurement can discriminate these states with certainty. Second, any measurement that acquires information about a system must necessarily disturb it. We investigate a simple and optimal eavesdropping scheme that minimizes the disturbance caused by a given amount of information gain, and show that an in principle demonstration of such a scheme could be performed using existing experimental techniques in single-photon quantum technologies.

Optical clock-transitions such as the ones in Ytterbium are prime candidates for encoding qubits for quantum information processing applications due to very low decoherence rates. In this work, we investigate the challenges involved in using these prime candidates for fundamental tests of quantum mechanics. We design entangling operations for pairs of indistinguishable atoms trapped in optical tweezers, as well as determine the feasibility of rapid qubit rotation and measurement of qubits encoded in these desirable low-decoherence clock transitions. In particular, we propose multi-photon transitions for fast rotation of qubits, followed by ultrafast readout via resonant multiphoton ionization. The rapid measurement of atomic qubits is crucial for high-speed synchronization of quantum information processors, but is also of interest for tests of Bell inequalities. We investigate a Bell inequality test that avoids the detection loophole in entangled qubits, which are spacelike separated over only a few meters.

Here we show that the photon echo equivalent of an NMR gradient echo is completely efficient if the sample is optically thick, the detunings of the atoms vary linearly along the direction of propagation and the storage time is short compared to the decay rate of the atoms. In this process the only light that interacts with the sample of atoms during the storage and retrieval process is the light that is to be stored and then retrieved, their are no auxiliary beams. The stored and recalled light travel in the same direction and their is no need for the phase matching operation that is present in previous quantum memory proposals using controlled inhomogeneous broadening. This greatly simplifies various possible implementations. We present analytical, numerical and experimental results of this scheme. We report experimental efficiencies of up to 15% and suggest simple realizable improvements to significantly increase the efficiency.

We investigate the multiphoton states generated by high-gain optical parametric amplification of a single injected photon, polarization encoded as a "qubit". The experiment configuration exploits the optimal phase-covariant cloning in the high gain regime. The interference fringe pattern showing the non local transfer of coherence between the injected qubit and the mesoscopic amplified output field involving up to 4000 photons has been investigated. A probabilistic new method to extract full information about the multiparticle output wavefunction has been implemented. This technique can be adopted to test the entanglement between a microscopic system and a macro one.

In this paper, some devices were reviewed to be used in quantum communications. We presented a low density of Quantum Dots, which could be used to get single quantum dot as light emitting source for generating single photons. An analytical model to study the thermal behavior of a solid media in interaction with one, two or three laser beams was developed using the classical heat equation. Integrated optic micro-ring resonators and its simulated result also are presented. Development of active micro-ring in silicon is at an early stage, where both vertical and horizontal techniques are feasible. With the epitaxy growth techniques, a possibility for achieving controllable QD density, size and good uniformity are proposed. A low density of QDs in range of 10⁸ cm^-2 has demonstrated through successive adjustment of the growth parameters. Details among the devices are presented and discussed.

We analyze a new method for single-photon frequency upconversion. This technique uses a byproduct of the avalanche process - electroluminescence resulting from hot-carrier recombination - as a means of upconversion. Because the spectrum of the emitted photons peaks near the bandgap of the multiplying material and has a significant tail at higher energies, it is possible to generate secondary photons at significantly higher energies than the primary absorbed photon. The secondary photons can then be detected by a coupled CMOS silicon single-photon avalanche diode (SPAD), where the information can also be processes. This upconversion scheme does not require any electrical connections between the detecting device and the silicon SPAD, so glass-to-glass bonding can be used, resulting in inexpensive, high-density arrays of detectors. We calculate the internal and system upconversion efficiencies, and show that the proposed scheme is feasible and highly efficient for application such as quantum key distribution and near infrared low-light-level imaging.

The performance of a practical quantum key distribution (QKD) system is often limited by the multi-photon state emission of its source and the dark counts of its detectors. Here, we present two methods to improve its performance. The first method is decoy state QKD: the sender randomly sends out weak coherent states with various average photon numbers (which are named as signal state and decoy states). In [14,15], we have performed the first experimental implementation of decoy state QKD over 15km and 60km respectively, thus dramatically increasing the distance and secure key generation rate of practical QKD systems. Our work has been followed up by many research groups worldwide [16-18]. The second scheme is QKD with "dual detectors" [19]: the legitimate receiver randomly uses either a fast (but noisy) detector or a quiet (but slow) detector to measure the incoming quantum signals. The measurement results from the quiet detector can be used to upper bound the eavesdropper's information, while the measurement results from the fast detector are used to generate a secure key. We applied this idea to various QKD protocols. Simulation results demonstrated significant improvements in both BB84 protocol with ideal single photon source and Gaussian-modulated coherent states protocol.

Bose-Einstein condensates have been, by now, observed in as many as nine different atomic assemblies of bosons. Such a condensate is quantum mechanical interacting system whose ground state properties can be studied theoretically by solving the appropriate non-linear Gross-Pitaevskii-Ginzburg GPG equation. One can now study the change in the behavior of Bose-Einstein condensate by introducing a localized impurity which interacts with the condensate as a function of position of impurity in the condensate. The introduction of such an impurity can be mimicked by simply allowing an intensely focused laser beam to interact with the condensate. This would lead to alteration of ground state properties of the condensate as it would now interact with a potential of type V Sech²(r/w) where, V and w are amplitude and width of the impurity potential, respectively. The modified GPG equation in the presence of localized impurity potential as function of position in the condensate, has been numerically solved to obtain its various ground state properties as function of position, such as total energy per particle, chemical potential, kinetic, harmonic trap potential and two-body interaction energies per particle in addition to energy associated with impurity potential, correlation length, healing length etc. We have studied the behavior of the above-mentioned ground state properties as the position of localized impurity is changed in the condensate from core to peripheral position. While the total, harmonic oscillator potential and impurity energies decrease as the position of localized impurity is displaced from core of the condensate to its periphery, the value of two-body inter-particle interaction energy increases. Further, the values of chemical potential and total energy per particle shows decrease by ~ 9% and ~ 17% respectively, leading to the inference that the stability of condensate increases as the localized impurity is moved away from the core of the condensate.