New, efficient and robust, fiber-based quantum key distribution schemes

We present a new fiber based quantum key distribution (QKD) scheme which can be regarded as a modification of an idea proposed by Inoue, Waks and Yamamoto (IWY) [1]. The scheme described here uses a single phase modulator and two differential delay elements in series at the transmitter that form an interferometer when combined with a third differential delay element at the receiver. The protocol is characterized by a high efficiency, reduced exposure to an attack by an eavesdropper, and higher sensitivity to such an attack when compared to other QKD schemes. For example, the efficiency with which transmitted data contribute to the private key is 3/4 compared with 1/4 for BB84 [2]. Moreover, an eavesdropper can aquire a maximum of 1/3 of the key which leads to an error probability in the private key of 1/3. This can be compared to 1/2 and 1/4 for these same parameters in both BB84 and IWY. The combination of these considerations should lead to increased range and key distribution rate over present fiber-based QKD schemes.Comment: 4 pages, 5 figures, 1 equatio

About entanglement properties of kaons and tests of hidden variables models

In this letter we discuss entanglement properties of neutral kaons systems and their use for testing local realism. In particular we show that, as previous proposals, also a scheme recently suggested for performing a test of hidden variable theories against standard quantum mechanics cannot be conclusive

Reducing the communication complexity with quantum entanglement

We propose a probabilistic two-party communication complexity scenario with a prior nonmaximally entangled state, which results in less communication than that is required with only classical random correlations. A simple all-optical implementation of this protocol is presented and demonstrates our conclusion.Comment: 4 Pages, 2 Figure

Thermal correction to the Casimir force, radiative heat transfer, and an experiment

The low-temperature asymptotic expressions for the Casimir interaction between two real metals described by Leontovich surface impedance are obtained in the framework of thermal quantum field theory. It is shown that the Casimir entropy computed using the impedance of infrared optics vanishes in the limit of zero temperature. By contrast, the Casimir entropy computed using the impedance of the Drude model attains at zero temperature a positive value which depends on the parameters of a system, i.e., the Nernst heat theorem is violated. Thus, the impedance of infrared optics withstands the thermodynamic test, whereas the impedance of the Drude model does not. We also perform a phenomenological analysis of the thermal Casimir force and of the radiative heat transfer through a vacuum gap between real metal plates. The characterization of a metal by means of the Leontovich impedance of the Drude model is shown to be inconsistent with experiment at separations of a few hundred nanometers. A modification of the impedance of infrared optics is suggested taking into account relaxation processes. The power of radiative heat transfer predicted from this impedance is several times less than previous predictions due to different contributions from the transverse electric evanescent waves. The physical meaning of low frequencies in the Lifshitz formula is discussed. It is concluded that new measurements of radiative heat transfer are required to find out the adequate description of a metal in the theory of electromagnetic fluctuations.Comment: 19 pages, 4 figures. svjour.cls is used, to appear in Eur. Phys. J.

Entangled-Photon Generation from Parametric Down-Conversion in Media with Inhomogeneous Nonlinearity

We develop and experimentally verify a theory of Type-II spontaneous parametric down-conversion (SPDC) in media with inhomogeneous distributions of second-order nonlinearity. As a special case, we explore interference effects from SPDC generated in a cascade of two bulk crystals separated by an air gap. The polarization quantum-interference pattern is found to vary strongly with the spacing between the two crystals. This is found to be a cooperative effect due to two mechanisms: the chromatic dispersion of the medium separating the crystals and spatiotemporal effects which arise from the inclusion of transverse wave vectors. These effects provide two concomitant avenues for controlling the quantum state generated in SPDC. We expect these results to be of interest for the development of quantum technologies and the generation of SPDC in periodically varying nonlinear materials.Comment: submitted to Physical Review