This PDF file contains the front matter associated with SPIE Proceedings Volume 8440, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We study the quantized electromagnetic (EM) field in cavities and cavity like structures and develop models to describe EM energy transfer. Our starting point is based on including the quantum mechanical field-matter interaction in the traveling wave (TW) formalism with appropriate boundary conditions accounting for the interference to obtain the spatially resolved quantized field operators. This allows evaluating the Poynting vector to calculate e.g. the energy fluxes through a cavity structure, the energy emitted and absorbed by an element placed in a leaky cavity and the formation of its steady state.

We calculate the Schmidt number for a two-dimensional model of the nonfactorable spatiotemporal wave-function of biphotons produced in type-I spontaneous parametric down-conversion with degenerate and collinear phase- matching taking into consideration a major part of the broad spectral and angular bandwidth of the down- converted light. We derive an analytical expression for the Schmidt number as a function of the filter bandwidth in the limit of spectrally narrow pump.

Strong coupling of single cesium atoms with a high-finesse optical micro-cavity (the finesse of our Fabry-Perot-type micro-cavity is F = 3.3 x 1⁰⁵ and the cavity length is l_c = 86 μm) has been realized for the both cases of TEM00 and TEM10 cavity modes in our experiments. The typical parameters are (g₀₀, κ, Υ) = 2π x (23.9, 2.6, 2.6) MHz and (g₁₀, κ, Υ) = 2π x (20.5, 2.6, 2.6) MHz for these two cases, respectively. Obviously our system reaches the strong coupling regime. The first application is to adopt strong coupling of free-fall individual atoms with the TEM00 cavity mode for determining the effective temperature of laser-cooled atoms prepared in a magneto-optical trap located just above the micro-cavity. The second application is to employ strong coupling of free-fall individual atoms with the tilted TEM10 cavity mode, in stead of TEM₀₀ mode, for more precisely tracking the trajectories of atoms passing through the cavity mode.

In this work we exploit two-dimensional photon echo experiments (2DPE) to observe quantum coherence dynamics in energy transfer in light-harvesting proteins isolated from marine cryptophyte algae. Previous data, recorded on two complexes (PC645 and PE545) at room temperature, revealed exceptionally long lasting oscillations with distinct correlations and anti-correlations even at ambient temperature. These observations provided compelling evidence for quantum-coherent sharing of electronic excitation across the 5-nm-wide proteins under biologically relevant conditions, suggesting that distant molecules within the photosynthetic proteins are 'wired' together by quantum coherence for more efficient light-harvesting. In this work measurements performed on a different evolutionary related complex (PE555) at two excitation wavelengths are presented. The new results highlight different lifetimes for electronic coherences. Although preliminary, these evidences can be tentatively interpreted considering the difference in the protein structures.

Quantum communications can provide almost perfect security through the use of quantum laws to detect any possible leak of information. We discuss critical issues in the implementation of quantum communication systems over installed optical fibers. We use stimulated four-wave mixing to generate single photons inside optical fibers, and by tuning the separation between the pump and the signal we adjust the average number of photons per pulse. We report measurements of the source statistics and show that it goes from a thermal to Poisson distribution with the increase of the pump power. We generate entangled photons pairs through spontaneous four-wave mixing. We report results for different type of fibers to approach the maximum value of the Bell inequality. We model the impact of polarization rotation, attenuation and Raman scattering and present optimum configurations to increase the degree of entanglement. We encode information in the photons polarization and assess the use of wavelength and time division multiplexing based control systems to compensate for the random rotation of the polarization during transmission. We show that time division multiplexing systems provide a more robust solution considering the values of PMD of nowadays installed fibers. We evaluate the impact on the quantum channel of co-propagating classical channels, and present guidelines for adding quantum channels to installed WDM optical communication systems without strongly penalizing the performance of the quantum channel. We discuss the process of retrieving information from the photons polarization. We identify the major impairments that limit the speed and distance of the quantum channel. Finally, we model theoretically the QBER and present results of an experimental performance assessment of the system quality through QBER measurements.

The ability to manipulate quantum states of light by integrated devices may open new perspectives both for fundamental tests of quantum mechanics and for novel technological applications. The technology for handling polarization-encoded qubits, the most commonly adopted approach, was still missing in quantum optical circuits until the ultrafast laser writing (ULW) technique was adopted for the first time to realize integrated devices able to support and manipulate polarization encoded qubits.1 Thanks to this method, polarization dependent and independent devices can be realized. In particular the maintenance of polarization entanglement was demonstrated in a balanced polarization independent integrated beam splitter1 and an integrated CNOT gate for polarization qubits was realized and carachterized.2 We also exploited integrated optics for quantum simulation tasks: by adopting the ULW technique an integrated quantum walk circuit was realized3 and, for the first time, we investigate how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk. Such experiment has been realized by adopting two-photon entangled states and an array of integrated symmetric directional couplers. The polarization entanglement was exploited to simulate the bunching-antibunching feature of non interacting bosons and fermions. To this scope a novel three-dimensional geometry for the waveguide circuit is introduced, which allows accurate polarization independent behaviour, maintaining a remarkable control on both phase and balancement of the directional couplers.

All of the widely used public-key encryption schemes will not remain their security in the environment of quantum computing. We present here two quantum public-key encryption protocols of classical message, and show that they can achieve information-theoretical security owing to a new type structure of public-key encryption algorithm.

The orbital angular momentum carried by single photons represents a promising resource in the quantum information field. In this paper we report some recent results regarding the adoption of higher dimensional quantum states encoded in the polarization and orbital angular momentum for quantum information and cryptographic processing.

We experimentally characterize sources of frequency degenerate down-converted photons at 826.4 nm generated in 2 mm, 5 mm and 10 mm long periodically-poled KTP crystals. The crystals are pumped by 413.2 nm laser pulses with 2 ps duration. The dispersion D=1.3 ps/mm puts a limit to the length over which phase matching is efficient for a 2 ps pulse and provides a lower limit for the angular width of SPDC in the far-field. We investigate the far-field distribution of SPDC produced by periodically-poled KTP crystals and compare this with the calculated intensity distribution and find good agreement with theory. We also discuss the performance of PPKTP in terms of nonlinearity and group velocity walk-off compared to other available materials.

The conditions for the appearance of a sharp laser transition are formulated in terms of a scaling limit, involving vanishing cavity loss and light-matter coupling, k → 0, g → 0, such that g²/k stays finite. It is shown analytically that in this asymptotic parameter domain, and for pump rates above the threshold value, the photon output becomes large in a sense that is specified, and the photon statistics becomes strictly Poissonian. Numerical examples for the case of a two-level and a three-level emitter are presented and discussed in relation to the analytic result.

We report on the study of two-photon interference in the frequency domain. Bell and Hong-Ou-Mandel experiments are investigated. These experiments involve the manipulation of photons in the frequency domain, using off-the-shelf telecommunication components such as electro-optic phase modulators and narrow-band frequency filters. In the first experiment, photon pairs entangled in frequency are created and separated. Each photon is then directed through an independent electro-optic phase modulator. Variation of the radio-frequency parameters of the modulation gives rise to a well-controlled Bessel-shape two-photon interference pattern in the frequency domain. This is efficiently measured with narrow-band frequency filters and superconducting single photon detectors. Experimental measurements exhibit high visibilities (over 99 percent both for net and raw visibilities) and allow the (theoretically proven) optimal violation of a Bell inequality for our setup (by more than 18 standard deviations). The second experiment is a Hong-Ou-Mandel experiment in the frequency domain. We show that a grating (spatial domain) or a phase modulator (temporal domain) can be seen as a frequency beam splitter. A broadband spectrum of photon pairs is divided into two interleaved frequency combs, each one used as an independent input to this acting beam splitter. A theoretical calculation shows clear photon anti-bunching behavior.

We propose two definitions of quantum one-way transformation and quantum trapdoor one-way transformation beyond computational hypothesis, and give four examples. Then, we present a general way to construct a quantum public-key encryption scheme from a quantum one-way transformation and a quantum trapdoor oneway transformation, and give a concrete example which is quantum-message-oriented. The security of this kind of encryption schemes is based on the one-way property of quantum one-way transformation and quantum trapdoor one-way transformation, and this kind of schemes is information-theoretically secure.

Standard linear optical detectors have a maximum sensitivity in the few hundreds of photons range, limited by amplifier noise. On the other hand, single photon detectors, which are the most sensitive detectors, are strongly nonlinear: One or more photons result in the same output signal. Photon number resolving (PNR) detectors, which have the ability to discriminate the number of photons in a weak optical pulse, are of great importance in the field of quantum information processing and quantum cryptography. Moreover, a PNR detector with large dynamic range can cover the gap between these two detection modes. Such detectors are greatly desirable not only in quantum information science and technology, but also in any application dealing with low light levels. In this work, we propose a novel approach to photon number resolving detectors based on spatial multiplexing of nanowire superconducting single-photon detectors. In the proposed approach, N superconducting nanowires, each connected in parallel to an integrated resistor, are connected in series. Photon absorption in a nanowire switches its bias current to the parallel resistor, forming a voltage pulse across it. The sum of these voltages, proportional to the number of absorbed photons, is measured at the output. The use of a cryogenic preamplifier with high input impedance for the read-out increases the linearity, the signal to noise ratio, and the speed. With this combination, we expect to be able to count up to few tens of photons with high fidelity, excellent timing resolution, and very high sensitivity in the telecommunication wavelength range.

In this work, we present a Jaynes-Cummings model of an indirect bandgap semiconductor engineered to confine simultaneously photons and phonons (acousto-optical cavity). From our theoretical analysis, the typical collapse-revival behavior is obtained. Finally, we get an analytical approximate expression of Rabi frequency in such a system.

Optical dipole traps (ODT) with far-off-resonance laser are important tools in more and more present cold-atom experiments, which allow confinement of laser-cooled atoms with a long storage time. Particularly, the magic wavelength ODT can cancel the position-dependent spatially inhomogeneous light shift of desired atomic transition, which is introduced by the ODT laser beam. Now it plays an important role in the state-insensitive quantum engineering and the atomic optical clock. To verify the magic wavelength or the magic wavelength combination for D₂ line transition of cesium (Cs) and rubidium (Rb) atoms, we calculated and analyzed the light shift of the ¹³³Cs 6S_1/2 - 6P_3/2 transition for a monochromatic ODT, and also the ⁸⁷Rb 5S_1/2 - 5P_3/2 transition for a dichromatic ODT with a laser frequency ratio of 2:1. Also a dichromatic magic-wavelength ODT laser system for 87Rb atoms is proposed and experimentally realized by employing the quasi-phase-matched frequency doubling technique with telecom laser and fiber amplifier.

We study the behavior of a two-level system inside a photonic cavity in which mechanical oscillations are induced. We distinguish four different regimes: a) no interaction, b) optomechanical, c) atom-cavity-field coupling and d) "competition". We observe how the revival and collapse processes become chaotic when the coupling constants of these regimes are sufficiently high. Changes of the Rabi frequency and collapse and revival times as a consequence of the injection of mechanical oscillations are discussed.

In this paper authors discuss the inverse problem for the density operator describing a bi-modal quantum mixed states of polarized optical field with the reduced probability density function. It is reduced because we assume that photons phase is known - it is represented by the Dirac delta function in the probability distribution. We ask for example if it is possible to represent an elliptically polarized plane wave in the basis of linearly polarized photons (or photons with any other arbitrary chosen phase). Our goal is to define a reversible integral transformation in order to represent the reduced probability density function by the density operator describing a mixed state and to analyze the uniqueness of the solution. This problem is similar to calculating Glauber-Sudarshan function when representing a quantum mixed state in the coherent states basis. However the integral transformation that we search is not that easy to define. It is based on convolution and cross-correlation operations. The operator that generates this transformation is defined using the Stokes operators.

We attempt to develop Shannon's concept of the unicity distance into the quantum context. Based on the definition of information-theoretic security for quantum cryptography, we present a quantum probabilistic encryption algorithm with bounded information-theoretic security, and then work out its quantum unicity distances. The result shows that quantum unicity distance is much bigger than the unicity distance of classical cryptography.

We extend the study of Kapitza-Dirac diffraction in Raman-Nath regime removing the limitations put on resonance detuning. Discussing the case of extended Raman-Nath regime we develop an approximation for the case of strong standing waves which will correspond to the well known tight-binding approximation for periodic structures. Due to internal structure of the diffracting particles the system eigenvalues and eigenfunctions show important differences from the generally known cases.