This PDF file contains the front matter associated with SPIE Proceedings Volume 6780, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

In this paper we demonstrate the application of multi-user quantum key distribution (QKD) to typical broadband fibrebased passive optical access links to metropolitan area networks. We propose a technique to utilize the currently unused 850nm waveband in standard telecommunications fiber for QKD in two network architectures. Net bit rates of up to 100's of kilobits^-1 were achieved for each receiver, depending on the network topology. The transmission distances between sender and receivers were compatible with the typical span of optical access links ( ≤ 10km).

We study various non-Gaussian states generated by photon subtrastion from a squeezed light source. The source is a cw beam generated by optical parametric oscillator. The photon subtraction is made by tapping a small fraction of the squeezed light source and by guiding it into two Si-APDs, which enable the subtraction of one to two photons. Trigger photon clicks specify a certain temporally localized mode in the remaining squeezed beam. By filtering the remaining squeezed beam through an appropriate mode function, one can generate a variety of non-Gaussian states. This includes single and two photon states, the NOON state (N = 2), Schrödinger kitten states of both odd and even parities, and their arbitrarily desired superposition.

We describe a design to build a highly efficient parametric downconversion source for single spatial mode polarization-entangled photons based on a periodically poled lithium niobate crystal in a polarization Sagnac interferometer. We have investigated pump focusing to improve the coupling of the output light into a single-mode fiber and obtained a single-mode generation and fiber coupling efficiency of 49%.

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.

Entanglement-based quantum cryptography has the appealing advantage of intuitively more evident security. While originally, weak laser pulse schemes were implemented earlier as technologically simpler, it is now possible to build entanglement-based quantum key distribution systems on a technically equally advanced level. The existing polarization-based systems as developed in Vienna now cover distances of the order of 100 km in fiber and of 144 km in free space. In a recent fiber experiment, an asymmetric source is used such that one photon at the 1.550 nm telecom wavelength is transmitted to Bob, while the other photon at 810 nm is locally measured by Alice. It turns out that polarization entanglement is rather robust, certainly over distances of 100 km in fibers. In a recent long-distance free-space experiment, one photon was sent over 144 km from the Canary Island of La Palma to the island of Tenerife, while again the other photon was measured locally. The receiving station uses the OGS telescope operated by the European Space Agency ESA. This experiment opens up the possibility for future quantum key distribution using satellites.

The desire for quantum-generated cryptographic key for broadband encryption services has motivated the development of high-transmission-rate single-photon quantum key distribution (QKD) systems. The maximum operational transmission rate of a QKD system is ultimately limited by the timing resolution of the single-photon detectors and recent advances have enabled the demonstration of QKD systems operating at transmission rates well in to the GHz regime. We have demonstrated quantum generated one-time-pad encryption of a streaming video signal with high transmission rate QKD systems in both free-space and fiber. We present an overview of our high-speed QKD architecture that allows continuous operation of the QKD link, including error correction and privacy amplification, and increases the key-production rate by maximizing the transmission rate and minimizing the temporal gating on the single-photon channel. We also address count-rate concerns that arise at transmission rates that are orders of magnitude higher than the maximum count rate of the single-photon detectors.

We present strategies to obtain different classes of three and four photon entangled symmetric states from a single experimental setup. The basic idea originates from the property of the symmetric Dicke state with two excitations to connect the two inequivalent types of genuine tripartite entanglement. We experimentally confirm the distinct types of entanglement of the observed states. We further propose an extension of the applied scheme that allows one to obtain different classes of four-photon entanglement by adding a fifth photon. The requirement of a single fifth photon is currently a technical challenge, and thus we consider the approach of using a strongly attenuated weak coherent beam instead.

Quantum states of two photons simultaneously entangled in polarization and linear momentum, namely hyper-entangled or cluster states, allow to operate in a larger Hilbert space, since we can associate four qubits to two photons. We describe how these states are generated, characterized and manipulated by linear optics technique. Some recent results verifying that the ratio between the quantum and classical prediction grows with the size of the Hilbert space are also presented in this work. Finally, we show the efficient realization of a C-NOT gate by using two photon cluster states operating in the one-way model of quantum computation.

The distance in quantum communication is limited for direct transmission, but can be increased by the means of entanglement swapping implemented in a quantum relay¹ or quantum repeater configuration.² Apart from this, entanglement swapping is an intriguing phenomenon in quantum physics and deserves thus by itself to be demonstrated in an experiment. Here we present an entanglement swapping experiment realized for the first time with autonomous CW-sources.

We theoretically investigate the feasibility of using spectral hole burning in Pr³⁺:Y₂SiO₅ to prepare an ensemble of Pr³⁺ ions with a spectral distribution optimized for use as a quantum memory for single-photon states. We introduce figures of merit for the spectral distribution of the Pr³⁺ ions when used as a quantum-memory node in a Duan-Lukin-Cirac-Zoller-type quantum-repeater scheme. Finally, we describe progress toward optimizing the hole-burning sequence by using a computational model of the hole-burning process to calculate these figures of merit over a wide range of parameters.

Two "optical lattice clocks", which based on spin-polarized fermionic ⁸⁷Sr trapped in a one-dimensional optical lattice and on bosonic ⁸⁸Sr in a three-dimensional lattice, were operated simultaneously. From a beat note between the two optical clocks, stability as well as accuracy of "optical lattice clocks" were evaluated. We report the progress of the measurements.

We report on the evaluation of an optical lattice clock using fermionic ⁸⁷Sr. The measured frequency of the ¹S₀ → ³P₀ clock transition is 429 228 004 229 873.7Hz with a fractional acuracy of 2.6 × 10^-15. This evaluation is performed on m_F = ±9/2 spin-polarized atoms. This technique also enables to evaluate the value of the differential Landé factor, 110.6Hz/G. by probing symmetrical σ-transitions.

We describe the preparation of a high spectral brightness, broad wavelength coverage, single-spatial mode source of polarization-entangled photon pairs operated at room temperature. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 μW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm.

We describe a fiber-based telecom-band polarization-entangled photon-pair source. Preliminary experimental results of tomographic reconstruction of all four Bell states generated by this source are presented. They show that a fiber source is an excellent producer of entangled photon-pairs for practical quantum communications.

This paper describes the detection of single photons, which have been transmitted through standard fiber at the telecom wavelength of 1310 nm. Following transmission, the 1310-nm photon is up-converted to 710 nm in a periodical-poled LiNbO₃ (PPLN) waveguide and then detected by a silicon-based avalanche photodiode (Si-APD). The overall detection efficiency of the detector is 20%. We have also characterized the sensitivity of the PPLN's efficiency to temperature and wavelength changes. We focused on the noise property of the up-conversion detector. Without classical channel co-propagation, the dark count rate is 2.2 kHz, which is lower than current up-conversion detectors by more than one order of magnitude. The up-conversion detector is then applied to a QKD system, which is characterized and is shown to have a very strong performance.

We report on both theoretical and experimental aspects of a fully implemented quantum key distribution device with coherent states. This system features a final key rate of more than 2 kb/s over 25 km of optical fiber. It comprises all required elements for field operation: a compact optical setup, a fast secret bit extraction using efficient LDPC codes, privacy amplification algorithms and a classical channel software. Both hardware and software are operated in real time.

A "Schrödinger cat" state of free-propagating light can be defined as a quantum superposition of well separated coherent states. 1, 2 We demonstrated, theoretically and experimentally, a protocol which allows to generate arbitrarily large squeezed Schrödinger cat states, using a homodyne detection and photon number states as resources. We implemented this protocol experimentally with light pulses containing two photons, producing a squeezed Schrödinger cat state with a negative Wigner function. This state clearly presents several quantum phase-space interference fringes between the "dead" and "alive" components, and it is large enough to become useful for experimental tests of quantum theory and quantum information processing.

Requirements and specifications of quantum key distribution (QKD) systems are examined particularly for metropolitanarea networks. A design of QKD system is considered to satisfy the specifications. BB84 protocol with decoy method is chosen. The unidirectional system based on planer light wave circuit for high speed operation is proposed for quantum transmission. Issues on system control such as clock synchronization and frame synchronization are discussed. Finally, decoy method is analyzed and demonstrated to guarantee the quantitative security of the final key in real systems.

We have constructed an entanglement based quantum key distribution system that links three buildings, covering a largest distance of 1575 m. The photons are transmitted via telescopes through free space. In this paper, we give a detailed description of our system and the protocol that we implemented. We analyze system components and design considerations. Some preliminary results of a one-link experiment are presented.

The National Institute of Standards and Technology (NIST) high-speed quantum key distribution (QKD) system was designed to include custom hardware to support the generation and management of gigabit data streams. As our photonics improved our software sifting algorithm couldn't keep up with the amount of data generated. To eliminate this problem we implemented the sifting algorithm into our programmable chip (FPGA) hardware, gaining a factor of 50x improvement in the sifting capacity rate. As we increased the distance and speed of our QKD systems, we discovered a number of other performance bottlenecks in our custom hardware. We discuss those bottlenecks along with a new custom hardware design that will alleviate them, resulting in an order of magnitude increase in capacity of secret key generation rate.

A novel type of quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, was proposed several years ago. A sender transmits a highly-attenuated coherent pulse train with {0, π} phase, and a receiver receives it with a one-bit delay Mach-Zehnder interferometer followed by photon detectors. A secret key is created from photon detection events, whose security is based on the fact that an eavesdropper cannot perfectly measure the phase information of a highly-attenuated coherent pulse train. This protocol has some features of simple setup, potential for a high key creation rate, and robustness against photon-number-splitting attack. This paper overviews DPS-QKD. The operation mechanism is described, and then some experimental efforts are introduced, featuring use of a glass waveguide Mach-Zehnder interferometer and advanced single-photon detectors. The highest key rate and the longest distance have been achieved with the DPS-QKD protocol. Some modified schemes are also presented, including that utilizing quantum entanglement, that using decoy pulses, and that using macroscopic coherent light.