This PDF file contains the front matter associated with SPIE Proceedings Volume 7092, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Spectral correlations between photon pairs generated by spontaneous parametric down conversion (SPDC) in bulk non-linear optical crystals remain a hindrance to the implementation of efficient quantum communication architectures. It has been demonstrated that SPDC within a distributed micro-cavity can result in little or no correlation between photon pairs. We present results on modeling three different cavity configurations based on integrated Bragg gratings. Output from the SPDC process can be tailored by altering the periodicity and geometry of such nanostructures. We will discuss the merits of each cavity configuration from the standpoint of degenerate type-II SPDC.

Optical parametric oscillators emit light with non-classical correlations between opposite spatial modes (twin beams). We consider these devices in presence of an intracavity photonic crystal, modeled by a spatial modulation of the refractive index. The introduction of photonic crystals allows to control not only the macroscopic transverse profile of the emitted light beam but also its quantum fluctuations. We employ the Q representation to study pump and signal spatial correlations.

Detection-time-bin-shift (DTBS) is a scheme that projects the measurement bases or measured photon values into detection time-bins and then time division multiplexes a single photon detector in a quantum key distribution (QKD) system. This scheme can simplify the structure of a QKD system, reduce its cost and overcome the security problems caused by the dead-time introduced self-correlation and the unbalanced characteristics of detectors. In this paper, we present several DTBS schemes for QKD systems based on attenuated laser pulses and entangled photon sources. We study the security issues of these DTBS schemes, especially the time-bin-shift intercept-resend attack and its countermeasures. A fiber-based DTBS QKD system has been developed and its results are presented in this paper.

We describe the generation of entangled and hyperentangled photon pairs using a microstructure-fiber Sagnac interferometer, which is formed by a polarizing beam splitter and a highly nonlinear microstructure fiber twisted by 90° from end to end. This interferometer allows two identical four-wave mixing processes to occur on the same fiber principal axis, ensuring perfect spatial and temporal mode matching of the two four-wave mixing outputs on the polarizing beam splitter to create entanglement over the entire four-wave mixing phase-matching spectral range. With an average pump power of 220 μW, we measure a two-photon coincidence rate of 1 kHz with ▵λ = 0.9 nm. Two-photon interference visibilities exceed 91% for polarization-entangled photon pairs generated from this source, and are > 84% for both time-bin and polarization degrees of freedom for hyperentangled photons, all without subtracting accidental coincidences.

Speckle structure¹ of Parametric Down Conversion (PDC) has recently received a large attention due to the relevance in view of applications to quantum imaging. The possibility of tailoring the speckle size by acting on the pump intensity and dimensions is an interesting tool for the applications to quantum imaging and in particular to the detection of weak objects under shot-noise limit. In this paper we report a systematic detailed study of the speckle structure, in particular of the one in type II PDC, with attention to its variation with pump beam characteristics (power and radius).

We propose a technique to obtain sub-wavelength resolution using photons from uncorrelated single photon sources. The method employs N photons of wavelength λ spontaneously emitted from N atoms and subsequently detected by N detectors. We demonstrate that for certain detector positions the N-th order correlation function as a function of the first detector position is a pure sinusoidal oscillation with a fringe spacing of λ/N and a contrast of 100%. The result corresponds to an N-fold increase in resolution compared to classical microscopy. Our technique is also capable of imaging a distinct physical object with sub-Rayleigh resolution.

Quantum imaging of an obscured object is achieved by the gated coincidence measurement of first the thermal source transverse intensity and second the measured count of scattered and reflected photons that have traversed the obscurant. Interestingly, the CCD camera is looking at the pseudo thermal source of photons not the object. The image does not exist in the mean, but only in the coincidence and has a higher image quality metric than that achieved by a camera viewing the object through the obscurant.

Entangled multi-spatial-mode fields have interesting applications in quantum information, such as parallel quantum information protocols, quantum computing, and quantum imaging. We study the use of a nondegenerate four-wave mixing process in rubidium vapor at 795 nm to demonstrate generation of quantum-entangled images. Owing to the lack of an optical resonator cavity, the four-wave mixing scheme generates inherently multi-spatialmode output fields. We have verified the presence of entanglement between the multi-mode beams by analyzing the amplitude difference and the phase sum noise using a dual homodyne detection scheme, measuring more than 4 dB of squeezing in both cases. This paper will discuss the quantum properties of amplifiers based on four-wave-mixing, along with the multi mode properties of such devices.

A quantum stream cipher by Yuen 2000 (Y-00) protocol with a feedback shift register --- a linear feedback shift register (LFSR) or a nonlinear feedback shift register --- is very attractive in implementing a secure high-speed optical data transmission system for next-generation optical networking. So far, a LFSR has been used in a quantum stream cipher by Y-00 as a running key generator, rather than a nonlinear feedback shift register. But, it is well-known that an appropriately designed nonlinear feedback shift register has larger period and linear complexity than the corresponding quantities of a LFSR driven by a secret key of the same length.Although large linear complexity of a key generator does not immediately guarantee the security of the key generator itself, it forces the eavesdropper at least to collect more measurement data to carry out the attacks. This motivates us to use a nonlinear feedback shift register as a running key generator in a quantum stream cipher by Y-00. The purpose of this study is to make a quantum stream cipher more costly in terms of cryptoanalysis, enhancing the advantages of using a nonlinear feedback shift register. For this purpose, we propose a new randomization technique for a running key generator in this paper.

Complete high-speed quantum key distribution (QKD) systems over fiber networks for campus and metro areas have been developed at NIST. The systems include an 850-nm QKD system for a campus network, a 1310-nm QKD system for metro networks, and a 3-user QKD network and network manager. In this paper we describe the key techniques used to implement these systems, including polarization recovery, noise reduction, frequency up-conversion detection based on PPLN waveguide, custom high-speed data handling and network management. A QKD-secured video surveillance system has been used to experimentally demonstrate these systems.

In most implementations of quantum-key-distribution (QKD) protocols the secure keys originate from single-photon signals. However, due to the unavoidable channel losses and the low efficiencies of single photon detectors, the key generation rate of a single-photon QKD system is low. Recently, there has been a growing interest in the Gaussianmodulated coherent state (GMCS) QKD protocol because it can be implemented with conventional laser sources and high efficiency homodyne-detectors. Here, we present our experimental results with a fully fiber-based one-way GMCS QKD system. Our system employed a double Mach-Zehnder interferometer (MZI) configuration in which the weak quantum signal and the strong local-oscillator (LO) go through the same fiber between Alice and Bob. We employed two novel techniques to suppress system excess noise. First, to suppress the LO's leakage, an important contribution to the excess noise, we implemented a scheme combining polarization and frequency multiplexing, achieving an extinction ratio of 70dB. Second, to further minimize the system excess noise due to phase drift of the double MZI, the sender simply remaps her data by performing a rotation operation. Under a "realistic model", the secure key rates determined with a 5km and a 20km fiber link are 0.3bit/pulse and 0.05bit/pulse, respectively. These key rates are significantly higher than that of a practical BB84 QKD system.

Unconditionally secure multiuser quantum key distribution (QKD) over long distance switched telecom fiber networks will have a revolutionary impact on people's, companies', and governmental exchange of confidential information. We report five-user QKD over switched fiber networks in both star and tree configurations, using the BB84-protocol with phase encoding. Both setups implement polarization insensitive phase modulators, necessary for birefringent single mode fiber networks. In both configurations, for the mean photon number μ=0.1, we have achieved raw rates between 50Hz and 300Hz at distances between 25km to 50km with quantum bit error rates between 1.24% to 5.56%. The measurements have showed feasibility of multiuser QKD over switched telecom fiber networks, using standard fiber telecom components.

Quantum entanglement is known to enable otherwise impossible feats in various communication protocols, such as quantum key distribution and super-dense coding. Here we describe efforts to further enhance the usual benefits, by incorporating quantum states that are simultaneously entangled in multiple degrees of freedom - "hyperentangled". Via the process of spontaneous parametric down conversion, we have demonstrated photon pairs simultaneously entangled in polarization and spatial mode, and have used these to realize remote entangled state preparation, full polarization Bell-state analysis, and the highest reported capacity quantum dense coding.

Photons produced in the process of spontaneous parametric down-conversion (SPDC) are typically entangled in multiple degrees of freedom. Although polarization is often the parameter of interest, the presence of spectral or spatial entanglement can complicate experiments, either by reducing the visibility of certain interference effects or by reducing collection efficiency. Several recent works have shown that the collection efficiency can be improved by focusing the pump beam. This approach has the added benefit of increasing the bandwidths of the emitted photons. We show that, in the case of type II SPDC, a focused pump can result in different spatial profiles for the signal and idler. If the crystal is configured for emission of polarization-entangled photon pairs, this effect will reduce the fidelity of that entanglement. Moreover, this spatial asymmetry leads to different spectral profiles for the two photons, even when the pump is monochromatic. The spectral and spatial asymmetries can be attributed to the difference in the angular dispersion (walkoff) of the two polarizations, along with a strong correlation between wavelength and emission direction. We also examine the link between spatial entanglement and single-mode coupling efficiency. We find that efficiency is maximized when spatial entanglement can be eliminated. For the case in which walk-off does not play a role, this can be accomplished by properly focusing the pump.

In the present work we propose to realize a macroscopic light-matter entangled state, obtained by the interaction of a multiphoton quantum superposition with a BEC system. The multiphoton quantum state is generated by a quantum-injected optical parametric amplifier (QI-OPA) seeded by a single-photon belonging to an EPR entangled pair and interacts with a Mirror-BEC shaped as a Bragg interference structure. The overall process will realize an entangled macroscopic quantum superposition involving a "microscopic" single-photon state of polarization and the coherent "macroscopic" displacement of the BEC structure acting in space-like separated distant places. This hybrid photonic-atomic system could open new perspectives on the possibility of coupling the amplified radiation with an atomic ensemble, a Bose-Einstein condensate, in order to implement innovative quantum interface between light and matter.

We demonstrate a linear optical device which performs optimal quantum measurement or minimum disturbance measurement on a single-photon polarization qubit with the help of an ancillary path qubit introduced to the same photon. We show theoretically and experimentally that this device satisfies the minimum disturbance measurement condition by investigating the relation between the information gain (estimation fidelity) and the state disturbance due to measurement (operation fidelity). Our implementation of minimal disturbance measurement is postselection-free in the sense that all detection events are counted toward evaluation of the estimation fidelity and the operation fidelity, i.e., there is no need for coincidence postselection of the detection events.

Recent demonstrations of teleportation have transferred quantum information encoded into either polarization or fieldquadrature degrees of freedom (DOFs), but an outstanding question is how to simultaneously teleport quantum information encoded into multiple DOFs. We describe how the transverse-spatial, spectral and polarization states of a single photon can be simultaneously teleported using a pair of multimode, polarization-entangled photons derived from spontaneous parametric down-conversion. Furthermore, when the initial photon pair is maximally entangled in the spatial, spectral, and polarization DOFs then the photon's full quantum state can be reliably teleported using a Bell-state measurement based on sum-frequency generation.

We present an unconditionally secure Oblivious Transfer protocol relying on two rounds of entanglement-free quantum communication. When played honestly, the protocol only requires the ability to measure a single qubit in a fixed basis, and to perform a coherent bit-flip (Pauli X) operation. We present a generalization to a "Private Data Sampling" protocol, where a player (Bob) can obtain a random sample of fixed size from a classical database of size N, while the database owner (Alice) remains oblivious as to which bits were accessed. The protocol is efficient in the sense that the communication complexity per query scales at most linearly with the size of the database. It does not violate Lo's "no-go" theorem for one-sided two-party secure computation, since a given joint input by Alice and Bob can result in randomly different protocol outcomes. Finally it could be used to implement a practical bit string commitment protocol, among other applications.

Recently, we have shown the advantages of two-way quantum communications in continuous variable quantum cryptography. Thanks to this new approach, two honest users can achieve a non-trivial security enhancement as long as the Gaussian interactions of an eavesdropper are independent and identical. In this work, we consider asymmetric strategies where the Gaussian interactions can be different and classically correlated. For several attacks of this kind, we prove that the enhancement of security still holds when the two-way protocols are used in direct reconciliation.

It is well known that totally incoherent light cannot exhibit first-order interference with photons that are uncorrelated, i.e., the normalized first-order correlation function is g⁽¹⁾(0) = 0, whereas the second-order correlation function is g⁽²⁾(0) = 1. Less familiar is the fact that both chaotic and coherent sources can exhibit first-order interference, so that merely using the term "interference" is ambiguous. If fact, some previous QIQC presentations have centered around whether or not two-photon correlations are actually a form of two-photon interference.¹ Another area of ambiguity concerns the detection of quantum state coherence using interference.² In an attempt to disambiguate the concept of interference, we examine associated photon states using chaotic sources and the Hanbury Brown and Twiss (HBT) detection of bunched photons. The unambiguous determination of coherent quantum states has important applications for: (1) Atomic Bose-Einstein condensate (BEC) determined using scattered laser interference³ (2) Exciton-Polariton BEC determined using emitted photon interference⁴ (3) Coherent light states. (4) Characterizing photon statistics. (5) Characterization of extended sources. In this paper, we present imaging results for topics 3-5. The difficulties of HBT data acquisition are generally underappreciated. An advantage of our approach is super-linear speedup through the development of a new imaging device consisting of a 2-dimensional array of single-photon avalanche detectors.^{5, 6} A 4 × 4 array enables 120 HBT coincidence experiments to be run in parallel to generate the 2-dimensional distribution of g(2)(x) spatial correlations, thus making plausible the term "g² camera" for this quantum imaging device.

The goal of presented study is to recognize the dependences of avalanche breakdown pulses on number of detected photons and also on operational and quenching conditions of avalanche photodiode. The study of avalanche breakdown initiated by one or more photons simultaneously is presented. This research is based on several years experience with operation of high dynamical range photon counter based on silicon single photon avalanche photodiode. The existing photon counters use the difference in avalanche breakdown inherently, only as a initiating signal for time delay compensation. Using obtained information we are intended to build a new version of photon counter providing more precise information about number of detecting photons or filtering detection event depending up selected photon number. For the first experiments has been selected silicon single photon sensitive avalanche photodiode with diameter up to 200 µm and very high homogeneity of active area. The detection chip was operated in a passive quenching circuit with active gating. This setup enabled us to monitor both the diode reverse current using an electrometer and avalanche current build up using fast digitizing oscilloscope operating at the sample rate 40 Gsamples per second. As the optical signal source with near-Poisson photon number distribution the stabilized diode laser with pulse width of 42 ps at the wavelength 778 nm. The recorded avalanche buildup waveforms enable to resolve the photon number in the dynamical range 1 to 10 000 photons per pulse. The first experimental results are presented.

Quantum imagers have been demonstrated in the laboratory by several groups. However, there are many practical concerns that must be considered in order to make such a system as successful as classical imagers in field applications. Consequently, we develop a model for signal-to-noise ratio (SNR) to estimate the performance of a quantum imager in comparison with that of the classical case. We assume simple architectures for both systems with components in the two as common to each other as possible. Comparisons between the imagers are made under conditions of solar background for ranges up to 2 km. The performance of quantum imager is shown to be superior to that of the classical case under conditions of narrow joint (or coincidence) detection windows and very strong pumping of the spontaneous parametric downconverter illumination source, for which the degree of photon entanglement may be severely degraded.