Efficient quantum key distribution secure against no-signalling eavesdroppers

By carrying out measurements on entangled states, two parties can generate a secret key which is secure not only against an eavesdropper bound by the laws of quantum mechanics, but also against a hypothetical "post-quantum" eavesdroppers limited by the no-signalling principle only. We introduce a family of quantum key distribution protocols of this type, which are more efficient than previous ones, both in terms of key rate and noise resistance. Interestingly, the best protocols involve large number of measurements. We show that in the absence of noise, these protocols can yield one secret bit per entanglement bit, implying that the key rates in the no-signalling post-quantum scenario are comparable to the key rates in usual quantum key distribution.Comment: 11 pages, 2 color figures. v2: minor modifications, added references, added note on the relation to quant-ph/060604

Getting to the point? Rethinking arrows on maps

Maps help to form public opinion and build public morale. When the war is over, they will contribute to shaping the thought and action of those responsible for the reconstruction of a shattered world. Hence it is important in these times that the nature of the information they set forth should be well understood (Wright, Citation1942: 527). Maps form an essential part of the language used to communicate conflict. They are used to plan military operations and to indicate their consequences to a wider audience by providing the visual basis for articulating the movement of troops, displaced populations and shifting territories between competing powers. Consequently, mapmakers draw from a well-used toolbox of symbols and techniques to describe the dynamic nature of war in familiar ways. For those maps designed for public consumption, which is our focus here, these methods often include broad, swooping arrows for representing movements of troops and displaced people, starburst or explosion symbols for battles and the use of strong colours for changes in territory. In many respects, mapmakers working in the news media conformed to these established cartographic norms when they first responded to Russia’s invasion of Ukraine on 24th February 2022. However, unlike previous conflicts that have attracted global attention, their maps were subject to wider critique – particularly on social media – which brought into sharp focus the way that maps can communicate the consequences of war and prompted some cartographic innovation. The critique coalesced around two themes: first, how territory was being represented in terms of Russian gains (and therefore Ukrainian losses) during the first weeks of the conflict (see Fafinski, Citation2022); and second, how the huge exodus of Ukrainians from their country should be captured cartographically (see Cheshire, Citation2022). The latter, especially, prompted questions about the suitability of using arrows to indicate the flow of Ukrainian refugees to surrounding countries when arrows were being used simultaneously to indicate invading Russian troops. In this short paper, we aim to provide a critical examination of whether the arrow is fit for purpose in communicating the displacement of people as a direct result of conflict. Since arrows have become the go-to cartographic symbol for portraying movement, particularly as a result of war and other geopolitical events, we explore their origins and evolution on maps before discussing how arrows and their alternatives have been used in news media coverage of the war in Ukraine. We contend that critique and innovation is necessary for the development and use of more effective, and ultimately more appropriate, cartographic symbology

Bipolar High Field Excitations in Co/Cu/Co Nanopillars

Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars ($\sim$50 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At sufficiently high current densities excitations, which lead to a decrease in differential resistance, are observed for both current polarities. Such bipolar excitations are not expected in a single domain model of spin-transfer. We propose that at high current densities strong asymmetries in the longitudinal spin accumulation cause spin-wave instabilities transverse to the current direction in bilayer samples, similar to those we have reported for single magnetic layer junctions.Comment: 4 pages, 4 figures+ 2 additional jpg figures (Fig. 2d and Fig. 3) high resolution figures and recent related articles are available at: http://www.physics.nyu.edu/kentlab/news.htm

Causal Quantum Theory and the Collapse Locality Loophole

Causal quantum theory is an umbrella term for ordinary quantum theory modified by two hypotheses: state vector reduction is a well-defined process, and strict local causality applies. The first of these holds in some versions of Copenhagen quantum theory and need not necessarily imply practically testable deviations from ordinary quantum theory. The second implies that measurement events which are spacelike separated have no non-local correlations. To test this prediction, which sharply differs from standard quantum theory, requires a precise theory of state vector reduction. Formally speaking, any precise version of causal quantum theory defines a local hidden variable theory. However, causal quantum theory is most naturally seen as a variant of standard quantum theory. For that reason it seems a more serious rival to standard quantum theory than local hidden variable models relying on the locality or detector efficiency loopholes. Some plausible versions of causal quantum theory are not refuted by any Bell experiments to date, nor is it obvious that they are inconsistent with other experiments. They evade refutation via a neglected loophole in Bell experiments -- the {\it collapse locality loophole} -- which exists because of the possible time lag between a particle entering a measuring device and a collapse taking place. Fairly definitive tests of causal versus standard quantum theory could be made by observing entangled particles separated by $\approx 0.1$ light seconds.Comment: Discussion expanded; typos corrected; references adde

Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films

Ferromagnetic resonance has been used to study the magnetic properties and magnetization dynamics of polycrystalline Fe$_{1-x}$V$_{x}$ alloy films with $0\leq x < 0.7$. Films were produced by co-sputtering from separate Fe and V targets, leading to a composition gradient across a Si substrate. FMR studies were conducted at room temperature with a broadband coplanar waveguide at frequencies up to 50 GHz using the flip-chip method. The effective demagnetization field $4 \pi M_{\mathrm{eff}}$ and the Gilbert damping parameter $\alpha$ have been determined as a function of V concentration. The results are compared to those of epitaxial FeV films

No Signalling and Quantum Key Distribution

Standard quantum key distribution protocols are provably secure against eavesdropping attacks, if quantum theory is correct. It is theoretically interesting to know if we need to assume the validity of quantum theory to prove the security of quantum key distribution, or whether its security can be based on other physical principles. The question would also be of practical interest if quantum mechanics were ever to fail in some regime, because a scientifically and technologically advanced eavesdropper could perhaps use post-quantum physics to extract information from quantum communications without necessarily causing the quantum state disturbances on which existing security proofs rely. Here we describe a key distribution scheme provably secure against general attacks by a post-quantum eavesdropper who is limited only by the impossibility of superluminal signalling. The security of the scheme stems from violation of a Bell inequality.Comment: Clarifications and minor revisions in response to comments. Final version; to appear in Phys. Rev. Let

Current Induced Excitations in Cu/Co/Cu Single Ferromagnetic Layer Nanopillars

Current-induced magnetic excitations in Cu/Co/Cu single layer nanopillars (~50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let