[en] In this work we demonstrate the benefits and the feasibility, of different antireflection coatings made with hydrogenated amorphous silicon-nitrogen alloys for the most used superconducting film materials. Such materials, having a refractive index tunable up to 4 in the visible range, allow to realise multilayers that reduce reflectance to few percent

[en] In this paper we report a theoretical analysis of the distortion effects on the probability distribution of the photon number for a classical state of light, induced by non-ideal detectors with non-unitary quantum efficiency and dead time. We present a theory for the reconstruction of the true photon arrival-time distribution, evaluating the probability of detecting the n-th photon at a fixed time. (authors)

[en] An experimental set-up based on correlated photons has been developed at the Istituto Elettrotecnico Nazionale G. Ferraris (IEN) for the absolute measurement of the quantum efficiency and dead time of single-photon counting photodiodes. The overall accuracy is improved with respect to previously reported results, by using a thin and highly transmitting non-linear crystal. For comparison, a conventional set-up has been developed at 632.8 nm with a He-Ne laser, a trap detector and calibrated neutral-density filters. Measurements at different count levels have been performed with both techniques and results are compared for the quantum efficiency in the limit of zero count, η₀, and for the dead-time measurement, t_D. Contributions to the final uncertainty are analysed for both techniques. The results obtained are η₀ = 0.3372 (standard uncertainty u(η₀) = 0.0016) and t_D = 33.8 ns (u(t_D) = 2.0 ns) in the case of the two-photon technique and η'₀ = 0.3376 (u(η'_D 0.0036) and t'_D = 31.7 ns (u(t'_D) = 5.4 ns) in the case of the conventional technique. (authors)

[en] In this paper, the effect of rotating-wave and counter-rotating terms in the energy spectrum of a single two-level atom interacting with a quantized field is investigated within a fermionic linearization scheme involving Clifford variables. After reproducing the Jaynes-Cummings results in the rotating-wave approximation, the counter-rotating terms are shwon to determine an energy spectrum sensitive to low photon-number condictions

[en] The aim of this paper is a critical investigation of some correction factors in the quantum-efficiency calibration of photodetectors, in the photon-counting regime, by means of correlated photons generated through a parametric down-conversion process inside a non-linear crystal. An analysis is given of the main correction factors found in a real experimental situation, with different techniques for performing coincidence counts. (authors)

[en] This paper tables a list of measurement quantities along with their corresponding units of measure, measurement error limits and responsible national metrological institutions. The table is that which is contained in Italian Law No. 273/1991 governing standardized measurement units and terminology. A discussion is made of the impact on industrial practices of the adoption on a nation-wide basis by Italian institutions of the International System of Units (SI) and the legal implications of calibration techniques in specialized fields using units other than those specified in Law No. 273

[en] Quantum-cryptography key distribution (QCKD) experiments have been recently reported using polarization-entangled photons. However, in any practical realization, quantum systems suffer from either unwanted or induced interactions with the environment and the quantum measurement system, showing up as quantum and, ultimately, statistical noise. In this paper, we investigate how an ideal polarization entanglement in spontaneous parametric down-conversion (SPDC) suffers quantum noise in its practical implementation as a secure quantum system, yielding errors in the transmitted bit sequence. Since all SPDC-based QCKD schemes rely on the measurement of coincidence to assert the bit transmission between the two parties, we bundle up the overall quantum and statistical noise in an exhaustive model to calculate the accidental coincidences. This model predicts the quantum-bit error rate and the sifted key and allows comparisons between different security criteria of the hitherto proposed QCKD protocols, resulting in an objective assessment of performances and advantages of different systems

[en] In this paper, we discuss the insecurity with present implementations of the Ekert protocol for quantum-key distribution based on the Wigner inequality. We propose a modified version of this inequality which guarantees safe quantum-key distribution

[en] This paper proposes a protocol for quantum dense key distribution. This protocol embeds the benefits of a quantum dense coding and a quantum key distribution and is able to generate shared secret keys four times more efficiently than the Bennet-Brassard 1984 protocol. We hereinafter prove the security of this scheme against individual eavesdropping attacks, and we present preliminary experimental results, showing its feasibility

[en] In this Reply we propose a modified security proof of the quantum dense key distribution protocol, detecting also the eavesdropping attack proposed by Wojcik in his Comment [Wojcik, Phys. Rev. A 71, 016301 (2005)]