Characterization of phase-averaged coherent states

We present the full characterization of phase-randomized or phase-averaged coherent states, a class of states exploited in communication channels and in decoy state-based quantum key distribution protocols. In particular, we report on the suitable formalism to analytically describe the main features of this class of states and on their experimental investigation, that results in agreement with theory. We also show the results we obtained by manipulating the phase-averaged coherent states with linear optical elements and testify their good quality by employing some non-Gaussianity measures and the concept of mutual information.Comment: 15 pages, 11 figure

Effect of noisy channels on the transmission of mesoscopic twin-beam states

Quantum properties of light, which are crucial resources for quantum technologies, are quite fragile in nature and can be degraded and even concealed by the environment. We show, both theoretically and experimentally, that mesoscopic twin-beam states of light can preserve their nonclassicality even in the presence of major losses and different types of noise, thus suggesting their potential usefulness to encode information in quantum communication protocols. We develop a comprehensive general analytical model for a measurable nonclassicality criterion and find thresholds on noise and losses for the survival of entanglement in the twin beam

Reliable source of conditional non-Gaussian states from single-mode thermal fields

We address both theoretically and experimentally the generation of pulsed non-Gaussian states from classical Gaussian ones by means of conditional measurements. The setup relies on a beam splitter and a pair of linear photodetectors able to resolve up to tens of photons in the two outputs. We show the reliability of the setup and the good agreement with the theory for a single-mode thermal field entering the beam splitter and present a thorough characterization of the photon statistics of the conditional states.Comment: 18 pages, 12 figure

Self-consistent characterization of light statistics

We demonstrate the possibility of a self-consistent characterization of the photon-number statistics of a light field by using photoemissive detectors with internal gain simply endowed with linear input/output responses. The method can be applied to both microscopic and mesoscopic photon-number regimes. The detectors must operate in the linear range without need of photon-counting capabilities.Comment: To be published in "Journal of Modern Optics

Governance structure and Operating Performance of Japanese Major Banks

近年，銀行の経営破綻が相次ぎ，金融システムの不安定性が高まっている。そのため，銀行のガバナンス構造を明らかにし，銀行経営の効率性を高めることは喫緊の課題である。ところで，非金融企業と異なり預金という特殊な負債を保有する銀行の場合，株主によるガバナンスが重要である。本稿では， 1990年から1998年をサンプル期間として，日本の大手銀行18行のパネル・データを用いて，株式所有構造の変化，株式所有構造と経営効率性および労働分配率との関係を分析した。その結果，①銀行の株式所有構造は経営効率性を重視する株主の保有比率が上昇する傾向にあること，②銀行経営に対して株主は経営効率性を高める方向で影響を与えていること，③株式保有比率が高いほど労働分配率が低下すること，すなわち株主への分配が多くなる傾向があること，の3点が明らかになった。従って，少なくとも1990年代において，銀行のガバナンス構造は株主への利益分配を重視する「新古典派型企業」としての性格を示していると言える

Conditional measurements on multimode pairwise entangled states from spontaneous parametric downconversion

We address the intrinsic multimode nature of the quantum state of light obtained by pulsed spontaneous parametric downconversion and develop a theoretical model based only on experimentally accessible quantities. We exploit the pairwise entanglement as a resource for conditional multimode measurements and derive closed formulas for the detection probability and the density matrix of the conditional states. We present a set of experiments performed to validate our model in different conditions that are in excellent agreement with experimental data. Finally, we evaluate nonGaussianity of the conditional states obtained from our source with the aim of discussing the effects of the different experimental parameters on the efficacy of this type of conditional state preparation

Robust generation of entanglement in Bose-Einstein condensates by collective atomic recoil

We address the dynamics induced by collective atomic recoil in a Bose-Einstein condensate in presence of radiation losses and atomic decoherence. In particular, we focus on the linear regime of the lasing mechanism, and analyze the effects of losses and decoherence on the generation of entanglement. The dynamics is that of three bosons, two atomic modes interacting with a single-mode radiation field, coupled with a bath of oscillators. The resulting three-mode dissipative Master equation is solved analytically in terms of the Wigner function. We examine in details the two complementary limits of {\em high-Q cavity} and {\em bad-cavity}, the latter corresponding to the so-called superradiant regime, both in the quasi-classical and quantum regimes. We found that three-mode entanglement as well as two-mode atom-atom and atom-radiation entanglement is generally robust against losses and decoherence,thus making the present system a good candidate for the experimental observation of entanglement in condensate systems. In particular, steady-state entanglement may be obtained both between atoms with opposite momenta and between atoms and photons

Non-Gaussian states by conditional measurements

We address realistic schemes for the generation of non-Gaussian states of light based on conditional intensity measurements performed on correlated bipartite states. We consider both quantum and classically correlated states and different kind of detection, comparing the resulting non Gaussianity parameters upon varying the input energy and the detection efficiency. We find that quantum correlations generally lead to higher non Gaussianity, at least in the low energy regime. An experimental implementation feasible with current technology is also suggested.Comment: 8 pages, 3 figure

Special issue on basics and applications in quantum optics

Quantum technologies are advancing very rapidly and have the potential to innovate communication and computing far beyond current possibilities. Among the possible plat- forms suitable to run quantum technology protocols, in the last decades quantum optics has received a lot of attention for the handiness and versatility of optical systems. In addition to studying the fundamentals of quantum mechanics, quantum optical states have been exploited for several applications, such as quantum-state engineering, quantum communication and quantum cryptography protocols, enhanced metrology and sensing, quantum optical integrated circuits, quantum imaging, and quantum biological effects. In this Special Issue, we collect some papers and also a review on some recent research activities that show the potential of quantum optics for the advancement of quantum technologies

The balance of quantum correlations for a class of feasible tripartite continuous variable states

We address the balance of quantum correlations for continuous variable (CV) states. In particular, we consider a class of feasible tripartite CV pure states and explicitly prove two Koashi-Winter-like conservation laws involving Gaussian entanglement of formation, Gaussian quantum discord and sub-system Von Neumann entropies. We also address the class of tripartite CV mixed states resulting from the propagation in a noisy environment, and discuss how the previous equalities evolve into inequalities.Comment: Special issue "Classical Vs Quantum correlations in composite systems" edited by L. Amico, S. Bose, V. Korepin and V. Vedral, published versio