[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] 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] Extended-time measurements of the viscosity field have been performed on two Fe-base amorphous ferromagnetic alloys by means of a feedback technique. No significant saturation was observed at both limits of the explored time interval. The observed kinetics may be described by a simple law only over restricted time intervals

[en] We reply to Akhavan and Golshani's comment on our double-slit experiment addressed to test standard quantum mechanics versus de Broglie-Bohm theory. We show that their results, albeit representing a relevant contribution to the discussion on the realizability of a test of de Broglie-Bohm theory, do not allow us to draw any conclusion about our experiment since their wavefunction does not describe well the photon states produced by our set-up. On the other hand, we give arguments of why our experiment can be considered a good realization of the theoretical proposal of Ghose (1999 Foundations of Quantum Theory and Quantum Optics ed S M Roy), Ghose et al (2001 Phys. Lett. A 290 205) and Golshani and Akhavan (2001 J. Phys. A: Math. Gen. 34 5259). (reply)

[en] Quantum mechanics is a pillar of modern physics, confirmed by a huge number of experiments. Nevertheless, it presents many unintuitive properties, strongly differing from classical mechanics due to its intrinsic non-epistemic probabilistic nature. Many attempts have been devoted to building a deterministic theory reproducing all the results of standard quantum mechanics (SQM). One of the most successful attempts, in this sense, is de Broglie-Bohm (dBB) theory. This theory is built in order to reproduce almost all SQM predictions. However, it has recently been suggested that different coincidence patterns are predicted by SQM and dBB when a double-slit experiment is realized under specific conditions. In this paper we present the first realization of such a double-slit experiment with correlated photons produced in type I parametric down-conversion. We observe a perfect agreement with SQM prediction, whilst our result is almost eight standard deviations from the dBB prediction

[en] Quantum properties of light and, in particular, quantum correlations have recently disclosed the possibility of realising protocols addressed to overcome limits of classical imaging, a field collectively christened quantum imaging. In particular quantum correlations between twin beams represent a fundamental resource for these studies. Here we present three experimental applications of these properties.

[en] In counterfactual quantum key distribution (CQKD) information is transferred, in a secure way, between Alice and Bob even when no particle carrying the information is in fact transmitted between them. In this letter we fully implement the scheme for CQKD proposed in [1], demonstrating for the first time that information can be transmitted between two parties without the transmission of a carrier

[en] We report on the absolute calibration of a Charge-Coupled Device (CCD) camera by exploiting quantum correlation. This method exploits a certain number of spatial pairwise quantum correlated modes produced by spontaneous parametric-down-conversion. We develop a measurement model accounting for all the uncertainty contributions, and we reach the relative uncertainty of 0.3% in low photon flux regime. This represents a significant step forward for the characterization of (scientific) CCDs used in mesoscopic light regime.

[en] A conditional unitary transformation (90 deg. polarization rotation) is performed at single-photon level. The transformation is realized by rotating polarization for one of the photons of a polarization-entangled biphoton state (signal photon) by means of a Pockel cell triggered by the detection of the other (idler) photon after polarization selection. As a result, the state of the signal photon is losslessly changed from being completely unpolarized to being partially polarized, so that the final polarization degree is given by the idler detector quantum efficiency. This experiment can be used for developing a different method of absolute quantum efficiency calibration

[en] Two-photon entangled quantum states are a fundamental tool for quantum information and quantum cryptography. A complete description of a generic quantum state is provided by its density matrix: the technique allowing experimental reconstruction of the density matrix is called quantum state tomography. Entangled states density matrix reconstruction requires a large number of measurements on many identical copies of the quantum state. An alternative way of certifying the amount of entanglement in two-photon states is represented by the estimation of specific parameters, e.g., negativity and concurrence. If we have a priori partial knowledge of our state, it’s possible to develop several estimators for these parameters that require lower amount of measurements with respect to full density matrix reconstruction. The aim of this work is to introduce and test different estimators for negativity and concurrence for a specific class of two-photon states. (paper)