This PDF file contains the front matter associated with SPIE Proceedings Volume 7236, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Cluster states of two photons and four qubits, built on the double entanglement of two photons in the degrees of freedom of polarization and linear momentum, have been used in the realization of a complete set of basic operations of one-way quantum computation. Basic computation algorithms, namely, the Grover's search and the Deutsch's algorithm, have been realized by using these states. Hyperentangled states of increasing size are of paramount importance for the realization of even more complex algorithms and can be extended to a lager number of degrees of freedom of the photons. Some recent results obtained with entangled states of two photons and six qubits are presented.

Quantum repeaters enable us to distribute entanglement between remote parties by relying on a network of quantum memory units that exhibit efficient coupling to light, scalability, and long coherence times. Entanglement is initially distributed between nearest neighbors and then extended to the far-end nodes using entanglement swapping techniques. For real-time applications, such as quantum key distribution, the above tasks must be repeated successively, according to a proper protocol, to generate entangled states at a certain rate. This paper studies a number of such protocols and the interplay between the rate of entanglement generation, the number of employed memories, and the coherence time of memory units.

We have witnessed proliferate growth in theoretical and experimental efforts to understand and control physical systems in a quantum level. In a quantum optics laboratory, Gaussian states, whose phase properties are described by Gaussian probability-like functions, were generated but there was some limitation to use them for various tasks of quantum information processing. There have been suggestions and realisations to engineer the quantum state by subtracting or adding single photons from/to a Gaussian field. It is also possible to test fundamental quantum theories using these techniques. We discuss various issues in subtraction and addition of single photons.

We investigate the interaction between a single atom and a light field in the strong focusing regime. Such a configuration is subject to recent experimental work not only with atoms but also molecules and other atom-like systems such as quantum dots. We derive the scattering probability for photons by such a microscopic object modeled by a two-level system, starting with a Gaussian beam as the spatial mode of the light field. The focusing by an ideal lens is modeled by adopting a field with spherical wave fronts compatible with Maxwell equations. Using a semi-classical approach for the atom-field interaction, we predict a scattering probability of photons by a single atom of up to 98% for realistic focusing parameters. Experimental results for different focusing strengths are compared with our theoretical model.

Fundamental quantum optics test as well as quantum cryptography and quantum teleportation are based on the distribution of single quantum states and quantum entanglement respectively. We will discuss recent experimental achievements in the field of long-distance quantum communication via optical fiber as well as in free-space over a record breaking distance of 144 km. The European Space Agency (ESA) has supported a range of studies in the field of quantum physics and quantuminformation science in space for several years, and consequently a mission proposal Space-QUEST Quantum Entanglement for Space Experiments was submitted to the European Life and Physical Sciences in Space Program. This proposal envisions to perform space-to-ground quantum communication tests from the International Space Station (ISS) and will presented in this article.

A transition edge sensor (TES) is one of superconducting photon detectors, which has a photon number resolving ability in light pulses. The TES device is a kind of calorimeters operated at an extremely low temperature, and the energy of the photons is measured as a resistance change in a superconducting transition region of the TES. The advantages of the TESs are an excellent energy resolution and a high quantum efficiency. However a response speed is limited due to slow thermal recovery time. To overcome this, we fabricated new TES devices which are based on a titanium superconductor. The critical temperature of our titanium films is around 410 mK, which greatly improves the thermal recovery time. The observed decay time constant of response signals to the light pulses is around several hundreds of ns, that make it possible to operate the devices at a counting rate over 1 MHz. The photon number resolving power is 0.35 eV(FWHM) for a 5 μm size device even at the high operating temperature. The system quantum efficiency is 65 % by embedding the TES films in an optical structure with a high reflection dielectric mirror and an anti-reflection coatings fabricated by an ion beam assisted sputtering method. These features are very promising for high speed photon number resolving applications in the quantum information field.

In this paper we present a review of the state-of-the-art superconducting single-photon detector (SSPD), its characterization and applications. We also present here the next step in the development of SSPD, i.e. photon-number resolving SSPD which simultaneously features GHz counting rate. We have demonstrated resolution up to 4 photons with quantum efficiency of 2.5% and 300~ps response pulse duration providing very short dead time.

We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10 μm x 10 μm). Using a solid immersion lens we achieve a spot size of 320 nm full-width half maximum onto the device at 470 nm wavelength. We compare the response maps of two devices: the higher detection efficiency device gives a uniform response whereas the lower detection efficiency device is limited by a single defect or constriction. A second optical setup is used to simultaneously image and measure the photoresponse of the lower detection efficiency device, allowing the constriction location to be pinpointed.

A unique quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, has been proposed and developed at NTT and Osaka University, which utilizes a coherent pulse train instead of individual photons as in traditional QKD protocols such as BB84. Its security is based on the fact that every phase difference of a highlyattenuated coherent pulse train cannot be fully measured. This protocol has features of simple setup, potential for a high key creation rate, and robustness against photon-number-splitting attack. This paper presents recent research activities on DPS-QKD.

We describe a one-chip generator able to provide streams of random sequences of "1" and "0" bits at 500 kbit/s, that can be grouped in bytes or longer numeric sequences, depending on the user's application. The random "heart" of the generator is a quantum device that gets spontaneously ignited with a Poissonian probability distribution. The methodology differs from others so far reported not only for the presence of the quantum device but also for the way we use its ignitions to provide random bits. Beside the principle of operation, we developed and fabricated a CMOS chip that exploits the method. Herewith we report the design and the experimental results.

As in conventional communication systems, test and measurement play important roles in quantum key distribution (QKD) systems. Besides the observation that QKD protocols estimate the bound of information leakage from the measurement results on the transmission channel, test of quantum apparatus is necessary to ensure that the assumptions behind the security proof are satisfied in practice. Moreover, precise characterization of the device imperfection improves the final key rate, because one can specify the effect of the errors originated from the devices and sbtract it. However, careful consideration is required to guarantee that the test and measurement procedure will not open a loop-hole to the eavesdroppers.

Quantum key distribution (QKD) channels are typically realized by transmitting and detecting single photons, and therefore suffer from dramatic reductions in throughput due to both channel loss and noise. These shortcomings can be mitigated by applying telecommunications clock-recovery techniques to maximize the bandwidth of the single-photon channel and minimize the system's exposure to noise. We demonstrate a QKD system operating continuously at a quantum-channel transmission rate of 1.25 GHz, with dedicated data-handling hardware and error-correction/privacy amplification. We discuss the design and performance of our system and highlight issues which limit our maximum transmission and key production rates.

We present the first Sagnac quantum secret sharing (in three- and four-party implementations) as well as Sagnac two-user quantum key distribution (QKD) over 1550 nm single mode fiber (SMF) networks, using the BB84- protocol with phase encoding. The secret sharing experiment has implemented a single qubit protocol, which allows for a practical secret sharing implementation over fiber telecom channels and in free-space. The previous quantum secret sharing proposals were based on multiparticle entangled states, not scalable and diffcult in the experimental implementations. Our experimental data show stable, in regards to birefringence drift, quantum secret sharing transmissions at the total Sagnac transmission loop distances of 45-55 km with the quantum bit error rates (QBER) of 3.0-3.7 % for the mean photon number μ = 0.1. In the QKD experiment we have achieved the total Sagnac transmission loop distances of 100-150 km with quantum bit error rates (QBER) of 5.84-9.79 % for μ = 0.1. The distances were much longer and rates much higher than in any other published Sagnac QKD experiments. The stability of quantum transmission in both secret sharing and QKD experiments has been achieved thanks to our new concept for compensation of SMF birefringence effects in Sagnac, based on a polarization control system and a polarization insensitive phase modulator. The measurement results have showed feasibility of quantum secret sharing and QKD over telecom fiber networks in Sagnac confi;guration, using standard fiber telecom components. Our birefringence compensation in SMF Sagnac open the door to other Sagnac-based applications over SMF links such as precise optical sensing, dispersion characteristics of optical fibers, acoustic and strain sensing, and generally sensing of any time varying phenomenon.