Diffuse versus square-well confining potentials in modelling AA@C60_{60} atoms

Attention: this version-$2$ of the manuscript differs from its previously uploaded version-$1$ (arXiv:1112.6158v1) and subsequently published in 2012 J. Phys. B \textbf{45} 105102 only by a removed typo in Eq.(2) of version-$1$; there was the erroneous factor "2" in both terms in the right-hand-side of the Eq.(2) of version-$1$. Now that the typo is removed, Eq.(2) is correct. A perceived advantage for the replacement of a discontinuous square-well pseudo-potential, which is often used by various researchers as an approximation to the actual C$_{60}$ cage potential in calculations of endohedral atoms $A$@C$_{60}$, by a more realistic diffuse potential is explored. The photoionization of endohedral H@C$_{60}$ and Xe@C$_{60}$ is chosen as the case study. The diffuse potential is modelled by a combination of two Woods-Saxon potentials. It is demonstrated that photoionization spectra of $A$@C$_{60}$ atoms are largely insensitive to the degree $\eta$ of diffuseness of the potential borders, in a reasonably broad range of $\eta$'s. Alternatively, these spectra are found to be insensitive to discontinuity of the square-well potential either. Both potentials result in practically identical calculated spectra. New numerical values for the set of square-well parameters, which lead to a better agreement between experimental and theoretical data for $A$@C$_{60}$ spectra, are recommended for future studies.Comment: 11 pages, 4 figure

The Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks

Observations of H$_2$O masers from circumnuclear disks in active galaxies for the Megamaser Cosmology Project allow accurate measurement of the mass of supermassive black holes (BH) in these galaxies. We present the Very Long Baseline Interferometry (VLBI) images and kinematics of water maser emission in six active galaxies: NGC~1194, NGC~2273, NGC~2960 (Mrk~1419), NGC~4388, NGC~6264 and NGC~6323. We use the Keplerian rotation curves of these six megamaser galaxies, plus a seventh previously published, to determine accurate enclosed masses within the central $\sim0.3$ pc of these galaxies, smaller than the radius of the sphere of influence of the central mass in all cases. We also set lower limits to the central mass densities of between 0.12 and 60 $\times 10^{10} M_{\odot}$~pc$^{-3}$. For six of the seven disks, the high central densities rule out clusters of stars or stellar remnants as the central objects, and this result further supports our assumption that the enclosed mass can be attributed predominantly to a supermassive black hole. The seven BHs have masses ranging between 0.76 and 6.5$\times$10$^7 M_{\odot}$. The BH mass errors are $\approx11$\%, dominated by the uncertainty of the Hubble constant. We compare the megamaser BH mass determination with other BH mass measurement techniques. The BH mass based on virial estimation in four galaxies is consistent with the megamaser BH mass given the latest empirical value of $\langle f \rangle$, but the virial mass uncertainty is much greater. MCP observations continue and we expect to obtain more maser BH masses in the future.Comment: 18 pages, 4 figures. This paper has been submitted to ApJ. An updated version of this paper will be posted when it gets accepte

Implementation of two-party protocols in the noisy-storage model

The noisy-storage model allows the implementation of secure two-party protocols under the sole assumption that no large-scale reliable quantum storage is available to the cheating party. No quantum storage is thereby required for the honest parties. Examples of such protocols include bit commitment, oblivious transfer and secure identification. Here, we provide a guideline for the practical implementation of such protocols. In particular, we analyze security in a practical setting where the honest parties themselves are unable to perform perfect operations and need to deal with practical problems such as errors during transmission and detector inefficiencies. We provide explicit security parameters for two different experimental setups using weak coherent, and parametric down conversion sources. In addition, we analyze a modification of the protocols based on decoy states.Comment: 41 pages, 33 figures, this is a companion paper to arXiv:0906.1030 considering practical aspects, v2: published version, title changed in accordance with PRA guideline

Radio continuum of galaxies with H2_{2}O megamaser disks: 33 GHz VLA data

We investigate the nuclear environment of galaxies with observed 22 GHz water megamaser in their subparsec edge-on accretion disks, using 33 GHz (9mm) radio continuum data from VLA, with a resolution of ~ 0.2-0.5 arcsecs, and relate the maser and host galaxy properties to those of its radio continuum emission. Eighty-seven percent (21 out of 24) galaxies in our sample show 33 GHz radio continuum emission at levels of 4.5-240 $\sigma$. Five sources show extended emission, including one source with two main components and one with three main components. The remaining detected 16 sources exhibit compact cores within the sensitivity limits. Little evidence is found for extended jets (>300 pc) in most sources. Either they do not exist, or our chosen frequency of 33 GHz is too high for a detection of these supposedly steep spectrum features. In only one source among those with known maser disk orientation, NGC4388, we found an extended jet-like feature that appears to be oriented perpendicular to the water megamaser disk. Smaller 100-300 pc sized jets might also be present, as is suggested by the beam-deconvolved morphology of our sources. Whenever possible, central positions with accuracies of 20-280 mas are provided. A correlation analysis shows that the 33 GHz luminosity weakly correlates with the infrared luminosity. The 33 GHz luminosity is anticorrelated with the circular velocity of the galaxy. The black hole masses show stronger correlations with water maser luminosity than with 1.4 GHz, 33 GHz, or hard X-ray luminosities. Furthermore, the inner radii of the disks show stronger correlations with 1.4 GHz, 33 GHz, and hard X-ray luminosities than their outer radii, suggesting that the outer radii may be affected by disk warping, star formation, or peculiar density distributions.Comment: 18 pages, 10 figures, Accepted for publication in A&

The Megamaser Cosmology Project. X. High Resolution Maps and Mass Constraint for SMBHs

We present high resolution (sub-mas) VLBI maps of nuclear H2O megamasers for seven galaxies. In UGC6093, the well-aligned systemic masers and high-velocity masers originate in an edge-on, flat disk and we determine the mass of the central SMBH to be M_SMBH = 2.58*10^7Msun(+-7%). For J1346+5228, the distribution of masers is consistent with a disk, but the faint high-velocity masers are only marginally detected, and we constrain the mass of the SMBH to be in the range 1.5-2.0*10^7Msun. The origin of the masers in Mrk1210 is less clear, as the systemic and high-velocity masers are misaligned and show a disorganized velocity structure. We present one possible model in which the masers originate in a tilted, warped disk, but we do not rule out the possibility of other explanations including outflow masers. In NGC6926, we detect a set of redshifted masers, clustered within a pc of each other, and a single blueshifted maser about 4.4pc away, an offset that would be unusually large for a maser disk system. Nevertheless, if it is a disk system, we estimate the enclosed mass to be M_SMBH<4.8*10^7 Msun . For NGC5793, we detect redshifted masers spaced about 1.4pc from a clustered set of blueshifted features. The orientation of the structure supports a disk scenario as suggested by Hagiwara et al.(2001). We estimate the enclosed mass to be M SMBH<1.3*10^7 Msun. For NGC2824 and J0350-0127, the masers may be associated with pc or sub-pc scale jets or outflows.Comment: Accepted by Ap

Quantum Hacking: Experimental demonstration of time-shift attack against practical quantum key distribution systems

Quantum key distribution (QKD) systems can send signals over more than 100 km standard optical fiber and are widely believed to be secure. Here, we show experimentally for the first time a technologically feasible attack, namely the time-shift attack, against a commercial QKD system. Our result shows that, contrary to popular belief, an eavesdropper, Eve, has a non-negligible probability (~4%) to break the security of the system. Eve's success is due to the well-known detection efficiency loophole in the experimental testing of Bell inequalities. Therefore, the detection efficiency loophole plays a key role not only in fundamental physics, but also in technological applications such as QKD.Comment: 5 pages, 3 figures. Substantially revised versio

On the Origin of the Wide HI Absorption Line Toward Sgr A*

We have imaged a region of about 5' extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within about 2' (approx. 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 +/- 0.12 centered at a mean velocity of V(lsr) = -4 +/- 15 km/s. The mean full width at half maximum is 119 +/- 42 km/s. Such a wide line is absent in the spectra at positions beyond about 2' from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius approx. 1.3') surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.Comment: LaTeX, 12 pages and 9 figures, accepted for publication in J. Astrophys. Ast