A novel, user-friendly quantum key distribution (QKD) system operating at a wavelength of 1550nm and at a clock rate of 10MHz was constructed to explore the compatibility of this emerging technology with the optical fiber network environment. Custom circuit boards providing the low-level control and sensing functions for both the transmitter and receiver were developed, allowing software-based system reconfiguration via USB interface to personal computers. The computer control allowed the user to change operating parameters such as detector bias voltages and pulse delays and also allowed for self-tuning of the system. Epitaxx avalanche photodiodes, operated in Geiger mode, were used to detect the single photons. A complete QKD protocol stack incorporating the "sifting", reconciliation, privacy amplification, authentication and key confirmation functions was implemented in software. The system was tested over twenty five kilometers of dark underground fiber, producing 18.6 million sifted bits, with a sifted bit error rate of 4.9% at an average number of photons per pulse of 0.2, during a continuous 12-hour period of self-sustaining operation: a small portion of the secret bits distilled from each session's sifted bits were used to authenticate the next session. A total of 6.8 million shared secret bits were produced.

Quantum key distribution (QKD) systems are being developed for possible use in future free space optical communication links to provide high security transmissions. However, these systems restrict the communication to a low bit-rate. To increase the QKD bit rate we suggest implementing a communication system which includes both optical detector and transmitter arrays. The mathematical description of this scheme involves a multi-input multi-output (MIMO) model. The optical detector and transmitter arrays enable the receiver to communicate simultaneously with a number of transmitter elements, which consequently increases the QKD bit rate. However, this method may introduce interference effects when a photon transmitted from one transmitter element reaches the wrong detector element in the receiver, due to scattering and turbulence in the atmospheric channel. This crosstalk and interference reduce the overall system bit rate and increase the quantum bit error rate (QBER). Analyzing the effect of crosstalk and interference on the system performance is accomplished by using the optical transfer function (OTF) theory.

The quadrature amplitude modulation (QAM) signal of coherent state of light is applied to the quantum stream cipher by Y-00 protocol. We first discuss on the performance of the square-root measurement (SRM) for the QAM signals in comparison with the optimum receiver. It is shown that the quantum stream cipher with the QAM signals is designed by using the SRM, taking account of the ciphertext-only attack and the known/chosen plain attack. Furthermore, the modification of the quantum stream cipher with the QAM signals is considered.

In this talk, we present a design of keyed randomization, and an experiment for quantum stream cipher by Yuen protocol which can realize a randomized stream cipher with Gbps for long distance. A simple explanation of why Y-00 is more secure than any type of the conventional stream cipher is given, showing Y-00 with keyed randomization, for example, overlapping selection keying:OSK. That is, the noiseless Y-00 with OSK is equivalent of the conventional stream cipher, but the noise is inevitable. So one can easily see that the security is enhanced by the noise. In addition, we give a design for the implementation of the intensity modulation scheme and report the experimental demonstration of 1 Gbps quantum stream cipher with OSK randomization through 20~40 km long transmission line. This may provide a way to go toward the broad market applications which require security, high speed, compact size, maintenance free, and low price.

We present a multi-photon scheme to improve the efficiency of free-space quantum key distribution system. Two essential features of free-space quantum key distribution system are its classical timing pulses and frequency tracking devices, we make use of these two features to design a new type of quantum key distribution system with multi-photon quantum signals and synchronously delayed classical signals. This scheme is much more efficient then the usual one, and in the same time, more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time.

We have exploited pattern function quantum homodyne tomography (QHT) to enlight deviations from the Gaussian state for a squeezed vacuum field generated by a type-I below threshold OPO. This tomographic method allows to fully characterize the state without any a--priori assumption on its statistics. Applying pattern function QHT to the radiation outing a type-I below threshold OPO, we have measured photon number distributions different from those expected for a Gaussian field. Being the Wigner function and the quadrature marginal distribution Gaussian for a Gaussian field, the actual state has been analysed by looking at data variance and Kurtosis (0 for a Gaussian distribution) as a function of the quadrature angle θ. A clear deviation of the Kurtosis from zero has been found for different OPO cavities. This deviation increases as the threshold is approached.

By using a partial polarizer to apply a generalized polarization measurement to one photon of a polarization entangled pair, we remotely prepare single photons in arbitrary polarization qubits. Specifically, we are able to produce a range of states of any desired degree of mixedness or purity, over (and within) the entire Poincare sphere, with a typical fidelity exceeding 99.5%. Moreover, by using non-degenerate entangled pairs as a resource, we can prepare states in multiple wavelengths. Finally, we discuss the states remotely preparable given a particular two-qubit resource state.

We present a systematic experimental study of generating correlated photon pairs using a reversed degenerate four-wave mixing process. By overlapping a pair of parallel- (or cross-) polarized laser pump pulses at conjugate frequencies in a microstructure fiber, parallel- or (cross-) polarized correlated photon pairs are generated at the middle frequency. The generation rate of correlated photons by four-wave mixing with parallel-polarized laser pumps in a 1.8 meter microstructure fiber is comparable or higher than that of a parametric down conversion process in a bulk-crystal at similar pump power levels.

We show an experimental study of two-photon imaging using thermal light sources. It is interesting to see that the thermal source behaves like a mirror producing an equal size reproduction of a chosen object. We refer to such mirror as "magic" because the ghost image produced by the mirror is real in the sense that it lies in the real space and can be further imaged by a camera or an equivalent optical system. We also show that it is possible to overcome the main limitation towards the actual implementation of thermal light two-photon imaging, i.e. the poor contrast of the imaging pattern, by designing an appropriate correlation measurement scheme that is insensitive to the uncorrelated background noise.

High-resolution ghost image and ghost diffraction are performed by using a single source of pseudo-thermal speckle light divided by a beam splitter. By only operating on the optical setup of the light in the reference arm, that never interacted with the object, we are able to pass from the image to the diffraction pattern. The product of spatial resolutions of the ghost image and ghost diffraction experiments is shown to overcome a limit which was formerly thought to be achievable only with entangled photons. A complementarity between the spatial coherence of the beams and their mutual correlation is demonstrated by showing a complementarity between ghost diffraction and ordinary diffraction patterns.

We review our progress toward implementing the proposal of Boto et al. (Phys. Rev. Lett. 85, 2733-2736, 2000) for using entangled photons to increase the spatial resolution of the process of interferometric photolithography beyond that of the standard Rayleigh limit.

This paper develops the Quantum Probability Density Function (QPDF) method to describe the evolution of quantum probability density functions in space and time. The QPDF method has been applied to derive QPDF equations for quantum scalar particles. The formulations relate probability theory and quantum uncertainty and thereby provide insight into the quantum theory and the quantum fluid approach of quantum mechanics. The QPDF approach can be applied to problems in quantum communications, quantum imaging, and quantum computing.

Processing information quantum mechanically is known to enable new communication and computational scenarios that cannot be accessed with conventional information technology (IT). It is known that such quantum processing can be performed with linear optical techniques, measurement and feed-forward. However, here the gates are intrinsically probabilistic and so scaling up such an approach requires considerable qubit resources. Here we present an alternative approach to optical QIP, based on the use of weak cross-Kerr non-linearities and highly efficient homodyne measurements. This approach enables a parity gate between optical qubits, mediated by an additional optical probe mode, enabling near-deterministic Bell state measurement and entangling gates. Our approach is therefore the basis for very efficient, scalable optical QIP, and provides a natural method for distributing such processing, combining it with quantum communication.

In this proceeding we present the most recent studies performed at IEN "Galileo Ferraris" on quantum information by using quantum optical states. After a general summary of the most recent studies, among them we will present in some details the results of two recent experiments. The first was addressed to tomographic reconstruction of a quantum state by using an innovative theoretical scheme based on a variable quantum efficiency of the detector. This scheme has been applied to Fock (PDC heralded photons), coherent (attenuated laser beam) and thermal states, for which experimental results will be presented. The second was pointed to experimentally investigate the effects of fibre propagation of PDC light produced in a type II crystal and, in particular, to the restoration of entanglement due to wave packet dispersion. Also in this case we will present and discuss our most recent data. Finally, we will shortly acknowledge of realisation of a heralded photon source with strong spectral selection.

The nonlinear optical response obtained from a single two level atom in a one-sided cavity is studied using a model system, where a infinite atomic layer sits in front of a reflecting mirror. When the atomic layer is placed at the antinode of input field, the result given by finite difference time domain method coupled with the optical Bloch equations is consistent with previous analytical result [ H F. Hofmann, K. Kojima, S. Takeuchi, and K. Sasaki, J. Opt. B 5, 218 (2003) ] based on one-dimensional atom model.

Spontaneous parametric down-conversion (SPDC) is the most often used source of entangled photon states in quantum optics. Recently, it has been suggested in theory that the reduction of polarization entanglement between the photon pair generated via the type-II SPDC process pumped by a ultrafast laser is due to the asymmetry in the joint spectrum function, which is the indicative of the frequency entanglement between the two photons. In this paper, we report the measurement of the joint spectrum function (i.e., direct observation of the degree of frequency entanglement) of the two-photon state of type-II SPDC in both cw- and ultrafast-laser pumping conditions. We observed that, in agreement with the theory, the degree of frequency entanglement (the shape of the joint spectrum function) is indeed different for cw- and ultrafast-pumped type-II SPDC. Our results are of importance in "engineering" the two-photon state for specific quantum applications, such as quantum cryptography.

We study the advantage of pure-state quantum computation without entanglement over classical computation. For the Deutsch-Jozsa algorithm we present the maximal subproblem that can be solved without entanglement, and show that the algorithm still has an advantage over the classical ones. We further show that this subproblem is of greater significance, by proving that it contains all the Boolean functions whose quantum phase-oracle is non-entangling. For Simon's and Grover's algorithms we provide simple proofs that no non-trivial subproblems can be solved by these algorithms without entanglement.

We report the experimental manipulation of five-photon entanglement. In the experiment, we first generate a high intensity and ultra-stable source of entangled photons and then used two pairs of entangled photons to produce a four-photon entangled state. After combining it with a single-photon state, we are able to engineer a five-photon entangled state. Meanwhile, by manipulating these entangled states, we have also successfully demonstrated the open-destination quantum teleportation, and a distant nondestructive Controlled-NOT gate for quantum teleportation.

Several examples that show the relationship between atomic coherence and entanglement productions have been discussed in this paper. The first example shows how atomic coherence can lead to entanglement between two thermal fields at a temperature T ; In the second example, we demonstrate how a cascade correlated spontaneous emission laser can be used as an entanglement amplifier; The last example illustrates how an arbitrary entangled state of two cavity fields can be prepared through atomic coherence. All these examples seem indicate that the two different quantum effects-coherence and entanglement-are closely related.

We describe our work on development of high efficiency single photon sources based on the interaction of InAs quantum dots with a photonic crystal micro-cavity. Sub-poisson statistics and lifetime modifications are experimentally demonstrated. We then investigate improvement of the source using coupling to a photonic crystal waveguide for easy collection. We analyze the system using coupled mode theory, and infer the parameters needed to create good coupling efficiency.

This paper extends the Decomposition Method to analyze photon and electromagnetic radiation transport through random and nonlinear media. The Decomposition Method is extended to solve the Schrodinger equation describing propagation through a nonlinear random media. The Decomposition Method is also applied to solve Maxwell's equations applied to random scattering. This approach can be applied to mathematical problems with general boundary and initial conditions. The Decomoposition Method as developed here can find quickly converging analytic solutions to a wide range of nonlinear and stochastic mathematical problems in quantum communications, quantum imaging, and quantum computing.

When we look through ordinary window pane glass, we see the world through a 2-dimensional optical delay line of approximately 27 picoseconds duration. Were the window made of Diamond, this delay would increase to about 42 picoseconds. Suppose it was possible to increase the effective index of refraction by 18 orders of magnitude. This is the basis of Slow Glass. On-going advances in Electromagnetically Induced Transparency, Bose-Einstein Condensates, Self Induced Transparency, and "Frozen" Light provide engineering pathways toward the realization of Slow Glass. These various approaches to group velocity delay will be compared in various media such as cold gas, hot gas, solid state, fiber optics, and drifting medium in terms of complexity, stability, and magnitude of delay. Design examples will be presented and potential applications discussed.

A new class of non-classical light states has been experimentally generated and their complete phase-space characterization has been achieved by quantum homodyne tomography. Such states are produced by the action of the photon creation operator on a coherent light field and are thus the result of the elementary excitation process of a classical field by a single quantum. Being intermediate between a single-photon Fock state and a coherent one, they offer the unique opportunity to closely follow the smooth evolution between the particle-like and the wave-like behavior of the light field.

A common syndrome in much of the current quantum optics and quantum computing literature is the casual switching between classical concepts (e.g., geometric rays, electromagnetic waves) and quantum concepts (e.g., state vectors, projection operators). Such ambiguous language can confuse designers not well versed in the deeper subtleties of quantum mechanics, or worse, it can lead to a flawed analysis of new designs for quantum devices. To validate that a quantum device can be constructed with the expected characteristics and that its quantum effects are correctly interpreted, a set of unambiguous design rules would be useful. In this paper we enumerate such a set of easily applied quantum rules in the hope that they might facilitate clearer communication between researchers and system developers in the field. In part, we are motivated by recently reported interferometer results that have not only led to flawed claims about disproving fundamental quantum principles, but have elicited equally flawed counter aruments from supposedly knowledgeable respondents. After one hundred years of testing Einstein's photon, it is alarming that such widespread confusion still persists. Our proposed quantum design rules are presented in a practical diagrammatic style, demonstrating their effectiveness by analyzing several interferometers that have appeared in the recent literature. Application to other quantum devices e.g., quantum ghost imaging, are also discussed. We stress that these rules are entirely quantum in prescription, being particularly appropriate for single-photon devices. Classical optics concepts e.g., refractive index, are not required since they are subsumed by our quantum rules.

We prove that continuous variable quantum information processing via Gaussian preserving operations, with Gaussian input states plus a (highly nonclassical) Fock number state, and subject to homodyne measurements and feedforward, can be efficiently simulated on a classical computer. This result reinforces the importance of the cubic phase state as an offline resource for universal continuous variable quantum information processing. We study the practical realization of this state by determining the Wigner function for an approximate cubic phase state prepared via two-mode squeezing with postselection on the signal field by displacing and counting photons in the idler field.

Two-qubit entangling gates allow the realization of new types of generalized quantum measurement. We discuss, and use photonic systems to demonstrate, two instances of this: two-qubit entangling measurements realizing superior discrimination of locally prepared two-qubit quantum states relative to what is achievable with local measurements and classical communication; and nondestructive weak measurements with postselection, leading to quantum weak values.

It is well known that classical states of light exhibit shot noise, characteristic of independent or uncorrelated particles. For phase estimation problems, this leads to a shot-noise limited uncertainty of 1/sqrt[N], where N is the number of particles detected. It is also well known that the shot-noise limit is not fundamental: squeezed states and entangled states can be used for sub-shot-noise phase measurements. The uncertainty principle sets a fundamental limit of 1/N, known as the "Heisenberg" limit. We have recently demonstrated a method, using parametric downconversion and post-selection, to generate entangled "NOON" states suitable for sub-shot-noise phase measurements [M.W. Mitchell et al, Nature 429, 161 (2004)]. We generated a three-photon NOON state and demonstrated three-fold improvement in phase resolution with this state. The relationship between phase resolution and phase uncertainty depends on prior information about the phase being estimated. As in the case of phase measurements with squeezed states, extra precision in one dimension is gained at the cost of reduced precision in other dimensions. Only when prior information is incorporated can entangled-state metrology be applied to beat the shot-noise limit. We illustrate this relationship and discuss adaptive strategies for phase estimation and the possibility of reaching the Heisenberg limit.

The gains of Four-wave mixing and Raman scattering in an optical fiber vary with wavelength. We use this wavelength variation to experimentally optimize the four-wave mixing relative to the Raman scattering, resulting in high photon coincidence rates and high coincidence/accidental contrast ratios. We obtain a photon coincidence rate of 53.7 kHz/mW/nm with a coincidence/accidental contrast of 10:1 in a 1.8 m microstructure fiber. We discuss the use of this fiber as a source for photon-counting detector efficiency calibrations.

We present the design and construction of a high-speed telecom-band (1.5 μm) single-photon counting system based on an InGaAs/InP avalanche photodiode (APD) operating in the gated Geiger mode. The detector can be gated at high speeds (we examine its performance up to 25 MHz) to maximize the counting rate in long-distance, telecom-band, fiber-optic quantum communication applications. Narrow gate pulses (250 ps full width at half maximum) are used to reduce the dark-count and the after-pulse probability. In order to count the avalanche events, we employ a high-speed comparator to sample the unfiltered and unamplified avalanche photocurrent. The APD and all the associated electronics are integrated onto a printed circuit board with a computer interface. In addition, we cool the APD to -27°C to reduce the dark-count probability.

We describe two experiments in which photon pairs from a parametric down-conversion source are efficiently coupled into single-mode fibers, and estimation of the heralding efficiency is attempted. One experiment is based on a pulsed pump configuration in a bulk crystal and the other experiment on a CW periodically poled downconverter. The heralding and heralded photons were analyzed spectrally by a monochromator to determine the effect of the correlation between spectral and spatial mode-matching in the heralding efficiency. Moreover, we theoretically model the heralding efficiency of both bulk and periodically poled crystals. Analysis of spatial and spectral mode selection, and their mutual correlation, allows controlled engineering of the spectral and spatial emission modes of PDC single photons.

Quantum teleportation should be able to teleport states or parts of states in arbitrarily large Hilbert spaces, which will require concatenation of quantum teleportation processes that operate for a fixed dimension. We introduce the concept of concatenated quantum teleportation and obtain exact expression for teleportation fidelities for two simple cases. We also derive the conditions when concatenated teleportation can be considered as "quantum".

Four signalling states are sufficient to achieve the Holevo capacity for qubit channels, but in many cases are not necessary. There are examples known where the capacity is achieved for two orthogonal input states, two nonorthogonal states, and examples where three states are necessary and sufficient. Many previous results were obtained for a class of channel for which three states are sufficient. In this study, a systematic theory for this class of channel was developed. Simple criteria are presented that, when satisfied, mean that two states are sufficient for the ensemble. When these criteria are satisfied, there is a simple method to determine whether the states in the ensemble should be orthogonal or nonorthogonal. When the criteria are not satisfied, it is still possible that two states are sufficient, though it is possible that three states are necessary. In the case where three states are necessary, the form of the optimal ensemble is predicted. These results provide an efficient method for calculating the Holevo capacity for all channels in this class.

We have studied quantum entanglement generation using nonlinear interaction of slowly propagating quantum field in a solid medium. Due to enhanced nonlinearity based on electromagnetically induced transparency and standing light phenomenon, a reversible nondegenerate four-wave mixing processing results in quantum entanglement generation. Our attention is to determine nonlinear perspectives of the entanglement with single photon fields. To accomplish the quantum entanglement based on the slow light involved standing light scheme, non-Doppler broadening has been required, where multiple atomic coherences have been the basis in a time-reversed manner. This study is potential for nonlinear optics application to quantum information science such as quantum computing and quantum memories using macroscopic optical medium.

We have implemented a quantum key distribution (QKD) system with polarization encoding at 850 nm over 1 km of optical fiber. The high-speed management of the bit-stream, generation of random numbers and processing of the sifting algorithm are all handled by a pair of custom data handling circuit boards. As a complete system using a clock rate of 1.25 Gbit/s, it produces sifted keys at a rate of 1.1 Mb/s with an error rate lower than 1.3% while operating at a transmission rate of 312.5 Mbit/s and a mean photon number μ = 0.1. With a number of proposed improvements this system has a potential for a higher key rate without an elevated error rate.