Symmetric Autocompensating Quantum Key Distribution

We present quantum key distribution schemes which are autocompensating (require no alignment) and symmetric (Alice and Bob receive photons from a central source) for both polarization and time-bin qubits. The primary benefit of the symmetric configuration is that both Alice and Bob may have passive setups (neither Alice nor Bob is required to make active changes for each run of the protocol). We show that both the polarization and the time-bin schemes may be implemented with existing technology. The new schemes are related to previously described schemes by the concept of advanced waves.Comment: 4 pages, 2 figur

On Approximation of the Eigenvalues of Perturbed Periodic Schrodinger Operators

This paper addresses the problem of computing the eigenvalues lying in the gaps of the essential spectrum of a periodic Schrodinger operator perturbed by a fast decreasing potential. We use a recently developed technique, the so called quadratic projection method, in order to achieve convergence free from spectral pollution. We describe the theoretical foundations of the method in detail, and illustrate its effectiveness by several examples.Comment: 17 pages, 2 tables and 2 figure

Longitudinal NMR and Spin States in the A-like Phase of 3He in Aerogel

It was found that two different spin states of the A-like phase can be obtained in aerogel sample. In one of these states we have observed the signal of the longitudinal NMR, while in another state no trace of such a signal was found. The states also have different properties in transverse NMR experiments. Longitudinal NMR signal was also observed in the B-like phase of 3He in aerogel.Comment: 8 pages, 7 figure

Nonorthogonal Quantum States Maximize Classical Information Capacity

I demonstrate that, rather unexpectedly, there exist noisy quantum channels for which the optimal classical information transmission rate is achieved only by signaling alphabets consisting of nonorthogonal quantum states.Comment: 5 pages, REVTeX, mild extension of results, much improved presentation, to appear in Physical Review Letter

Magnetospheric convection from Cluster EDI measurements compared with the ground-based ionospheric convection model IZMEM

Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for more than seven and a half years (2001–2008) have been used to derive a statistical model of the high-latitude electric potential distribution for summer conditions. Based on potential pattern for different orientations of the interplanetary magnetic field (IMF) in the GSM y-z-plane, basic convection pattern (BCP) were derived, that represent the main characteristics of the electric potential distribution in dependence on the IMF. The BCPs comprise the IMF-independent potential distribution as well as patterns, which describe the dependence on positive and negative IMF<I>B<sub>z</sub></I> and IMF<I>B<sub>y</sub></I> variations. The full set of BCPs allows to describe the spatial and temporal variation of the high-latitude electric potential (ionospheric convection) for any solar wind IMF condition near the Earth's magnetopause within reasonable ranges. The comparison of the Cluster/EDI model with the IZMEM ionospheric convection model, which was derived from ground-based magnetometer observations, shows a good agreement of the basic patterns and its variation with the IMF. According to the statistical models, there is a two-cell antisunward convection within the polar cap for northward IMF<I>B<sub>z</sub></I>+≤2 nT, while for increasing northward IMF<I>B<sub>z</sub></I>+ there appears a region of sunward convection within the high-latitude daytime sector, which assumes the form of two additional cells with sunward convection between them for IMF<I>B<sub>z</sub></I>+≈4–5 nT. This results in a four-cell convection pattern of the high-latitude convection. In dependence of the ±IMF<I>B<sub>y</sub></I> contribution during sufficiently strong northward IMF<I>B<sub>z</sub></I> conditions, a transformation to three-cell convection patterns takes place

Practical quantum key distribution: On the security evaluation with inefficient single-photon detectors

Quantum Key Distribution with the BB84 protocol has been shown to be unconditionally secure even using weak coherent pulses instead of single-photon signals. The distances that can be covered by these methods are limited due to the loss in the quantum channel (e.g. loss in the optical fiber) and in the single-photon counters of the receivers. One can argue that the loss in the detectors cannot be changed by an eavesdropper in order to increase the covered distance. Here we show that the security analysis of this scenario is not as easy as is commonly assumed, since already two-photon processes allow eavesdropping strategies that outperform the known photon-number splitting attack. For this reason there is, so far, no satisfactory security analysis available in the framework of individual attacks.Comment: 11 pages, 6 figures; Abstract and introduction extended, Appendix added, references update

Experimental Demonstration of Post-Selection based Continuous Variable Quantum Key Distribution in the Presence of Gaussian Noise

In realistic continuous variable quantum key distribution protocols, an eavesdropper may exploit the additional Gaussian noise generated during transmission to mask her presence. We present a theoretical framework for a post-selection based protocol which explicitly takes into account excess Gaussian noise. We derive a quantitative expression of the secret key rates based on the Levitin and Holevo bounds. We experimentally demonstrate that the post-selection based scheme is still secure against both individual and collective Gaussian attacks in the presence of this excess noise.Comment: 4 pages, 4 figure

No-Switching Quantum Key Distribution using Broadband Modulated Coherent Light

We realize an end-to-end no-switching quantum key distribution protocol using continuous-wave coherent light. We encode weak broadband Gaussian modulations onto the amplitude and phase quadratures of light beams at the Shannon's information limit. Our no-switching protocol achieves high secret key rate via a post-selection protocol that utilizes both quadrature information simultaneously. We establish a secret key rate of 25 Mbits/s for a lossless channel and 1 kbit/s, per 17 MHz of detected bandwidth, for 90% channel loss. Since our scheme is truly broadband, it can potentially deliver orders of magnitude higher key rates by extending the encoding bandwidth with higher-end telecommunication technology.Comment: 5 pages, 3 figures, publishe