This paper deals with soft-information based information reconciliation and data sifting for Quantum Key Distribution (QKD).We propose a novel composite channel model for QKD, which includes both a hard output quantum channel and a soft output classic channel. The metrics derived from the two channels are jointly processed at the receiver, exploiting capacity achieving soft-metric based iteratively decoded block codes. The performance of the proposed mixed-softmetric algorithms are studied via simulations as a function of the system parameters. The core ideas of the paper are: a) employing FEC coding as opposed to two-way communication for information reconciliation, b) exploiting all the available information for data processing at the receiver including information available from the quantum channel, since optimized use of this information can lead to significant performance improvement.

We carried out ghost imaging experiments using nondegenerate entangled beams with the central wavelengths at 810 nm and 1550 nm. The pulsed pump at 532 nm had the high efficiency of parametric down conversion and enabled ghost imaging although its average pump power was 10 mW. For the first time, we demonstrated ghost imaging with two disparate detectors: Si avalanche photodiode on one arm and InGaAs avalanche photodiode on the other. Objects were placed in the arm of the 1550 nm beam, whereas the imaging lens was placed in the arm of the 810 nm beam. Ghost imaging was constructed by using the quantum correlated portion of the data due to the nature of the entangled beams. Current theory for this configuration predicted that the image magnification by a degenerate source should be one and half times larger than that of this nondegenerate source; the observed magnification followed closely the value predicted by the theory.

Quantum Ghost Imaging(QGI) has proven to be both a scientifically interesting field as well as one with many potential practical applications. In this paper we will review some of the scientifically fundamental aspects of Quantum Ghost Imaging and the supporting experiments including some recent results from the Army Research Laboratory.

Up-conversion single photon detector technology has become efficient for photons in the near infrared range. However, its dark count rate is a major concern for some applications in quantum optics. We have theoretically and experimentally studied the causes of dark counts, and developed an up-conversion detector with an ultra low dark count rate. A reduced dark count rate of only 320 counts per second is achieved at the maximum overall detection efficiency of 18% and a dark count rate of less than 100 counts per second is achieved at a detection efficiency of 10%. The ultra low dark count rate enables this type of up-conversion detector to be utilized in a variety of applications where weak signals in the near IR region are only at a level of few thousand photons per second.

Controlled manipulation of quantum dots with nanometer precision is an essential capability for basic science as well as for scalable engineering of nanophotonic and quantum optical devices. The most common methods for manipulation of particles use optical or dielectric trapping forces which scale poorly with particle size, making it difficult to manipulate single quantum dots. Here we demonstrate a particle manipulation technique that achieves nanometer positioning by controlling the flow of the surrounding liquid. This approach scales much more favorable with particle size, enabling the position of colloidal quantum dots with better precision. Using this approach we demonstrate the capture, quantum optical characterization, and manipulation of individually selected single quantum dots with up to 45.5 nm precision for times exceeding one hour. This technique can be used to place pre-selected single photon sources in nanophotonic structures such as cavities and waveguides for engineering of integrated quantum optical devices.

The process of upconversion in a nonlinear crystal involves the annihilation of two photons and the generation of a single photon with frequency given by the sum of the incident photon frequencies. In this paper we discuss recent work where we exploit the upconversion of two photonic modes, each of them generated by the process of spontaneous parametric downconversion, either from the same or different crystals. On the one hand, we describe an experiment where we exploit upconversion for the temporal characterization of photon pairs. On the other hand, we describe an experimental technique, which relies on the detection of an upconverted photon from two distinct photon-pair sources, which permits two photons that never interacted to become entangled in frequency.

Realization of novel quantum technologies requires a control over their fundamental constituent- a single photon emitter. In this work we will present a comparative study of single photon emitters based on color centers in diamond which deliver performance comparable with decoy state implementation with weak coherent sources, and are most suitable for practical applications. In particular we will present the photo-physical properties of a novel family of single photon emitters, originated from Cr impurities in the diamond lattice, which exhibit a typical emission lines in the range of 740-760 nm, and a count rate in the range of 0.8-3.2 10⁶ counts/s. We will highlight future directions to optimize these sources and present simulations of their performance in practical devices.

We report an experimental implementation of quantum random number generator based on the photon-number-path entangled state. The photon-number-path entangled state is prepared by means of two-photon Hong-Ou-Mandel quantum interference at a beam splitter. The randomness in our scheme is of truly quantum mechanical origin as it comes from the projection measurement of the entangled two-photon state. The generated bit sequences satisfy the standard randomness test.

Possible error sources in an experimentally realized linear-optics controlled-Z gate[1] are analyzed by considering the deviations of the beam splitting ratios from the ideal values (δR_H,δR_V), the polarization-dependent phase shift (birefringence) of the optical components (δφ) and the mode mismatch of input photons (δξ). It is found that the error rate is linearly dependent on δR_V and δξ , while the dependence on δR_H and δφ is approximately quadratic. As a practical result, the gate is much more sensitive to small errors in R_V than in R_H. Specifically, the reflectivity error for vertical polarization must be less than 0.1% to realize a gate with an error of less than 0.1%, whereas the reflectivity error for horizontal polarization can be up to 1%. It is also shown that the effects of different error sources are not independent of each other (linear error model). Under certain conditions, the deviation from the linear error model exceeds 10% of the total error. The method of analysis used illustrates the basic features of errors in general linear optics quantum gates and circuits, and can easily be adapted to any other device of this type.

We present a general quantum-mechanical formalism to describe photon-pair generation via four-wave mixing in a Silicon-on-Insulator (SOI) microresonator. We also provide design principles for efficient photon-pair generation in an SOI microresonator through numerical simulations. Ring-cavity designs are shown to have a much wider dispersion-compensate frequency range than straight-cavity designs. Such on-chip quantum devices are highly promising for future integrated quantum information processing.

The first ghost image (GI) of an opaque object by Meyers et al.^{1, 2} demonstrated that GI has practical applications and inspired a series of variations used in reflective ghost imaging research. Here we report on some recent Ghost Imaging experiments using Compressive Imaging (CI) and Compressive Sensing (CS) conducted at the Army Research Laboratory. These experiments covered phenomena such as Compressive Turbulence-Free Ghost Imaging (CTFGI), and Compressive Ghost-Imaging-Inspired Imaging (CGIII).

We report on our efforts in integrating a source and detection system of photon pairs that have a high end-to-end system efficiency. This requires combining appropriate detectors and photon pair sources. Preliminary measurements show that an observed heralding efficiency of 65% for single photons is readily achieved.

Ghost imaging is a transverse imaging technique that relies on the correlation between a pair of light fields, one that has interacted with the object to be imaged and one that has not. Most ghost imaging experiments have been performed in transmission, and virtually all ghost imaging theory has addressed the transmissive case. Yet stand-off sensing applications require that the object be imaged in reflection. We use Gaussian-state analysis to develop expressions for the spatial resolution, image contrast, and signal-to-noise ratio for reflective ghost imaging with a pseudothermal light source and a rough-surfaced object that creates target-returns with fullydeveloped speckle. We compare our results to the corresponding behavior seen in transmissive ghost imaging, and we develop performance results for the reflective form of computational ghost imaging. We also provide a preliminary stand-off sensing performance comparison between reflective ghost imaging and a conventional direct-detection laser radar.

We experimentally investigate a highly integrated, polarization qubit based quantum key distribution system over a real-world, 12.4 km fiber link. Our system implements quantum frames, i.e. alternating sequences of high intensity optical pulses (classical data) and faint ones (qubits or quantum data). The classical data enables compensation of time-varying birefringence in the fiber link, and will facilitate synchronization, time tagging, and allow extending point-to-point quantum key distribution systems to networks. Our system can be clocked at 980 MHz, and has demonstrated good stability over 37 hours operation. We also discuss high rate error correction using a low-density parity-check code.

The security in the next generation optical network which realizes "Cloud Computing System Service with data center" is one of the most important problems. In such a network, the encryption in physical layer which provide super security and small delay is a preferable. It has to be applicable, however, to very high speed data because the basic link is operated at 2.5 Gbit/sec ~ 10 Gbit/sec. The quantum stream cipher by Yuen-2000 protocol (Y00) is a completely new type of random cipher, which can exceed the Shannon limit of the symmetric key cipher. This paper extends some theoretical results on the security for quantum stream cipher such as Y00 protocol and generalized Y00. First, the conditions to exceed the Shannon limit are summarized. We formulate a generalized secret capacity in the sense of wire tap channel supported by secret key to clarify a cipher exceeding the Shannon limit. The generalized secret capacities for space communication and fiber communication based on the generalized Y00 are given. When the relaxation of physical constraint or device limit of the eavesdropper is allowed, we point out that a cipher scheme exceeding the Shannon limit can be realized only by the conventional optical system.

We present a novel factorization algorithm which can be computed using an analogue computer based on a polychromatic source with a given wavelength bandwidth, a multi-path interferometer and a spectrometer. The core of this algorithm stands on the measurement of the periodicity of a "factoring" function given by an exponential sum at continuous argument by recording a sequence of interferograms associated with suitable units of displacement in the inteferometer. A remarking rescaling property of such interferograms allows, in principle, the prime number decomposition of several large integers. The information about factors is encoded in the location of the inteferogram maxima.

We present a formalism within which, using entangled graph states of prime-dimensional systems, a variety of different secret-sharing schemes (involving both quantum and classical secrets and quantum and classical channels shared between parties) may be unified. We review the explicit protocols we have found for three varieties of secret sharing within this formalism, including some for which the analogous formalism using qubit graph states is not sufficient.

Quantum algorithms and computational models primarily focus on processing quantum states via qubit manipulations and measurements. While this allows for hardware independent algorithm development, it does not necessarily reflect the full capabilities of even imperfect physical implementations - which typically have access to additional degrees of freedom not routinely considered in quantum algorithm development. In analogy with electrical mixed-signal (analog and digital) processing, here we investigate the prospects of incorporating the strengths of the native physical platform into the quantum information processor. Although the treatment here will be limited to optical systems the general approach should apply to other physical systems as well.

The Jaynes-Cummings-Hubbard model describes a network of interacting atom-cavity systems. In this model the photons can hop between cavities effecting a tunneling interaction, and the Jaynes-Cummings interaction gives rise to an onsite interaction. This model forms a bridge connection between condensed matter physics and quantum optics. Here we review the physics underpinning this model, properties of Jaynes-Cummings-Hubbard systems, the requirements on systems for realizing this model, and some of the progress to date.

We propose a new method to narrow the linewidth of photon pairs generated from spontaneous parametric down conversion (SPDC). The single structure device incorporates an internal Bragg grating onto a nonlinear optical waveguide. We study theoretically the spectral characteristics of SPDC under two Bragg grating structures. We show that using the Bragg grating with a midway π- phase shifter is a promising way to implement narrow-line (~GHz to sub-GHz) entangled photon sources.

Worldwide free-space quantum communications (QC) experiments over the past decade are reviewed and discussed with attention to technological QC trends. Experiments reported in the open literature have included those conducted along horizontal propagation paths of varying distances, as well as communication paths from ground-to-aircraft, ground-to-space, and demonstrations in the laboratory. Available data characterize propagation distances, transmission speeds, quantum key distribution (QKD) protocols, and quantum bit error rates (QBER). While fiber optic implementations of quantum communications technologies are currently being tested for communications infrastructure it is important to also consider that free-space quantum communications will play an important role in securing such applications as earth-to-satellite, end of line connects, and defense implementations.

We report an experimental demonstration of slowed propagation and storage and retrieval of thermal light using the effect of electromagnetically-induced transparency (EIT) in the Λ-type system of the D1 transition of ⁸⁵Rb atom. The slowed-propagation of the probe thermal light beam through an EIT medium is observed by measuring the second-order correlation function of the light field using the Hanbury-Brown-Twiss interferometer. We also demonstrate the storage and retrieval of thermal light beam in the EIT medium. The direct measurement of the photon number statistics of the retrieved light field shows that the photon number statistics is preserved during the storage and retrieval process.

We have studied experimentally the effect of a depolarizing quantum channel on polarization-encode weak pulse BB84 and SARG04 quantum cryptography. Experimental results show that, in real world conditions in which channel depolarization cannot be ignored, BB84 is more robust than SARG04 on the effect of the depolarizing quantum channel.