Multipartite fully-nonlocal quantum states

We present a general method to characterize the quantum correlations obtained after local measurements on multipartite systems. Sufficient conditions for a quantum system to be fully-nonlocal according to a given partition, as well as being (genuinely) multipartite fully-nonlocal, are derived. These conditions allow us to identify all completely-connected graph states as multipartite fully-nonlocal quantum states. Moreover, we show that this feature can also be observed in mixed states: the tensor product of five copies of the Smolin state, a biseparable and bound entangled state, is multipartite fully-nonlocal.Comment: 5 pages, 1 figure. Version published in PRA. Note that it does not contain all the results from the previous version; these will be included in a later, more general, pape

Relay I trapped radiation measurements

Radiation measurements of geomagnetically trapped electrons by instruments on Relay I satellite in 196

Radar sounding using the Cassini altimeter waveform modeling and Monte Carlo approach for data inversion observations of Titan's seas

Recently, the Cassini RADAR has been used as a sounder to probe the depth and constrain the composition of hydrocarbon seas on Saturn's largest moon, Titan. Altimetry waveforms from observations over the seas are generally composed of two main reflections: the first from the surface of the liquid and the second from the seafloor. The time interval between these two peaks is a measure of sea depth, and the attenuation from the propagation through the liquid is a measure of the dielectric properties, which is a sensitive property of liquid composition. Radar measurements are affected by uncertainties that can include saturation effects, possible receiver distortion, and processing artifacts, in addition to thermal noise and speckle. To rigorously treat these problems, we simulate the Ku-band altimetry echo received from Titan's seas using a two-layer model, where the surface is represented by a specular reflection and the seafloor is modeled using a facet-based synthetic surface. The simulation accounts for the thermal noise, speckle, analog-to-digital conversion, and block adaptive quantization and allows for possible receiver saturation. We use a Monte Carlo method to compare simulated and observed waveforms and retrieve the probability distributions of depth, surface/subsurface intensity ratio, and subsurface roughness for the individual double-peaked waveform of Ligeia Mare acquired by the Cassini spacecraft in May 2013. This new analysis provides an update to the Ku-band attenuation and results in a new estimate for its loss tangent and composition. We also demonstrate the ability to retrieve bathymetric information from saturated altimetry echoes acquired over Ontario Lacus in December 2008

Multiple imaging by gravitational waves

Gravitational waves act like lenses for the light propagating through them. This phenomenon is described using the vector formalism employed for ordinary gravitational lenses, which was proved to be applicable also to a non-stationary spacetime, with the appropriate modifications. In order to have multiple imaging, an approximate condition analogous to that for ordinary gravitational lenses must be satisfied. Certain astrophysical sources of gravitational waves satisfy this condition, while the gravitational wave background, on average, does not. Multiple imaging by gravitational waves is, in principle, possible, but the probability of observing such a phenomenon is extremely low.Comment: 23 pages, LaTeX, no figures, to appear in Int. J. Mod. Phys.

Device-independent bounds for Hardy's experiment

In this Letter we compute an analogue of Tsirelson's bound for Hardy's test of nonlocality, that is, the maximum violation of locality constraints allowed by the quantum formalism, irrespective of the dimension of the system. The value is found to be the same as the one achievable already with two-qubit systems, and we show that only a very specific class of states can lead to such maximal value, thus highlighting Hardy's test as a device-independent self-test protocol for such states. By considering realistic constraints in Hardy's test, we also compute device-independent upper bounds on this violation and show that these bounds are saturated by two-qubit systems, thus showing that there is no advantage in using higher-dimensional systems in experimental implementations of such test.Comment: 4 pages, 2 figure

Theory of gravitation theories: a no-progress report

Already in the 1970s there where attempts to present a set of ground rules, sometimes referred to as a theory of gravitation theories, which theories of gravity should satisfy in order to be considered viable in principle and, therefore, interesting enough to deserve further investigation. From this perspective, an alternative title of the present paper could be ``why are we still unable to write a guide on how to propose viable alternatives to general relativity?''. Attempting to answer this question, it is argued here that earlier efforts to turn qualitative statements, such as the Einstein Equivalence Principle, into quantitative ones, such as the metric postulates, stand on rather shaky grounds -- probably contrary to popular belief -- as they appear to depend strongly on particular representations of the theory. This includes ambiguities in the identification of matter and gravitational fields, dependence of frequently used definitions, such as those of the stress-energy tensor or classical vacuum, on the choice of variables, etc. Various examples are discussed and possible approaches to this problem are pointed out. In the course of this study, several common misconceptions related to the various forms of the Equivalence Principle, the use of conformal frames and equivalence between theories are clarified.Comment: Invited paper in the Gravity Research Foundation 2007 special issue to be published by Int. J. Mod. Phys.

Liquid filled canyons on Titan

In May 2013 the Cassini RADAR altimeter observed channels in Vid Flumina, a drainage network connected to Titan’s second largest hydrocarbon sea, Ligeia Mare. Analysis of these altimeter echoes shows that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface reflections that indicate they are currently liquid filled. Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of drowned river valleys. Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare, consistent with drainage feeding into the main channel system

Incremental peritoneal dialysis: a 10 year single-centre experience

INTRODUCTION: Incremental dialysis consists in prescribing a dialysis dose aimed towards maintaining total solute clearance (renal + dialysis) near the targets set by guidelines. Incremental peritoneal dialysis (incrPD) is defined as one or two dwell-times per day on CAPD, whereas standard peritoneal dialysis (stPD) consists in three-four dwell-times per day. PATIENTS AND METHODS: Single-centre cohort study. Enrollement period: January 2002-December 2007; end of follow up (FU): December 2012. INCLUSION CRITERIA: incident patients with FU ≥6 months, initial residual renal function (RRF) 3-10 ml/min/1.73 sqm BSA, renal indication for PD. RESULTS: Median incrPD duration was 17 months (I-III Q: 10; 30). There were no statistically significant differences between 29 patients on incrPD and 76 on stPD regarding: clinical, demographic and anthropometric characteristics at the beginning of treatment, adequacy indices, peritonitis-free survival (peritonitis incidence: 1/135 months-patients in incrPD vs. 1/52 months-patients in stPD) and patient survival. During the first 6 months, RRF remained stable in incrPD (6.20 ± 2.02 vs. 6.08 ± 1.47 ml/min/1.73 sqm BSA; p = 0.792) whereas it decreased in stPD (4.48 ± 2.12 vs. 5.61 ± 1.49; p < 0.001). Patient survival was affected negatively by ischemic cardiopathy (HR: 4.269; p < 0.001), peripheral and cerebral vascular disease (H2.842; p = 0.006) and cirrhosis (2.982; p = 0.032) and positively by urine output (0.392; p = 0.034). Hospitalization rates were significantly lower in incrPD (p = 0.021). Eight of 29 incrPD patients were transplanted before reaching full dose treatment. CONCLUSIONS: IncrPD is a safe modality to start PD; compared to stPD, it shows similar survival rates, significantly less hospitalization, a trend towards lower peritonitis incidence and slower reduction of renal function

Are stealth scalar fields stable?

Non-gravitating (stealth) scalar fields associated with Minkowski space in scalar-tensor gravity are examined. Analytical solutions for both non-minimally coupled scalar field theory and for Brans-Dicke gravity are studied and their stability with respect to tensor perturbations is assessed using a covariant and gauge-invariant formalism developed for alternative gravity. For Brans-Dicke solutions, the stability with respect to homogeneous perturbations is also studied. There are regions of parameter space corresponding to stability and other regions corresponding to instability.Comment: 10 pages, 1 table, no figures, to appear in Phys. Rev,

Security of two quantum cryptography protocols using the same four qubit states

The first quantum cryptography protocol, proposed by Bennett and Brassard in 1984 (BB84), has been widely studied in the last years. This protocol uses four states (more precisely, two complementary bases) for the encoding of the classical bit. Recently, it has been noticed that by using the same four states, but a different encoding of information, one can define a new protocol which is more robust in practical implementations, specifically when attenuated laser pulses are used instead of single-photon sources [V. Scarani et al., Phys. Rev. Lett. {\bf 92}, 057901 (2004); referred to as SARG04]. We present a detailed study of SARG04 in two different regimes. In the first part, we consider an implementation with a single-photon source: we derive bounds on the error rate $Q$ for security against all possible attacks by the eavesdropper. The lower and the upper bound obtained for SARG04 ($Q\lesssim 10.95$ and $Q\gtrsim 14.9$ respectively) are close to those obtained for BB84 ($Q\lesssim 12.4$ and $Q\gtrsim 14.6$ respectively). In the second part, we consider the realistic source consisting of an attenuated laser and improve on previous analysis by allowing Alice to optimize the mean number of photons as a function of the distance. SARG04 is found to perform better than BB84, both in secret key rate and in maximal achievable distance, for a wide class of Eve's attacks.Comment: 19 pages, 7 figures, published versio