Mean field dynamo action in renovating shearing flows

We study mean field dynamo action in renovating flows with finite and non zero correlation time ($\tau$) in the presence of shear. Previous results obtained when shear was absent are generalized to the case with shear. The question of whether the mean magnetic field can grow in the presence of shear and non helical turbulence, as seen in numerical simulations, is examined. We show in a general manner that, if the motions are strictly non helical, then such mean field dynamo action is not possible. This result is not limited to low (fluid or magnetic) Reynolds numbers nor does it use any closure approximation; it only assumes that the flow renovates itself after each time interval $\tau$. Specifying to a particular form of the renovating flow with helicity, we recover the standard dispersion relation of the $\alpha^2 \Omega$ dynamo, in the small $\tau$ or large wavelength limit. Thus mean fields grow even in the presence of rapidly growing fluctuations, surprisingly, in a manner predicted by the standard quasilinear closure, even though such a closure is not strictly justified. Our work also suggests the possibility of obtaining mean field dynamo growth in the presence of helicity fluctuations, although having a coherent helicity will be more efficient.Comment: 16 pages, 1 figur

The quantum cryptographic switch

We illustrate using a quantum system the principle of a cryptographic switch, in which a third party (Charlie) can control to a continuously varying degree the amount of information the receiver (Bob) receives, after the sender (Alice) has sent her information. Suppose Charlie transmits a Bell state to Alice and Bob. Alice uses dense coding to transmit two bits to Bob. Only if the 2-bit information corresponding to choice of Bell state is made available by Charlie to Bob can the latter recover Alice's information. By varying the information he gives, Charlie can continuously vary the information recovered by Bob. The performance of the protocol subjected to the squeezed generalized amplitude damping channel is considered. We also present a number of practical situations where a cryptographic switch would be of use.Comment: 7 pages, 4 Figure

Continuous loading of an electrostatic trap for polar molecules

A continuously operated electrostatic trap for polar molecules is demonstrated. The trap has a volume of ~0.6 cm^3 and holds molecules with a positive Stark shift. With deuterated ammonia from a quadrupole velocity filter, a trap density of ~10^8/cm^3 is achieved with an average lifetime of 130 ms and a motional temperature of ~300 mK. The trap offers good starting conditions for high-precision measurements, and can be used as a first stage in cooling schemes for molecules and as a "reaction vessel" in cold chemistry.Comment: 4 pages, 3 figures v2: several small improvements, new intr

Quantum Bit Commitment with a Composite Evidence

Entanglement-based attacks, which are subtle and powerful, are usually believed to render quantum bit commitment insecure. We point out that the no-go argument leading to this view implicitly assumes the evidence-of-commitment to be a monolithic quantum system. We argue that more general evidence structures, allowing for a composite, hybrid (classical-quantum) evidence, conduce to improved security. In particular, we present and prove the security of the following protocol: Bob sends Alice an anonymous state. She inscribes her commitment $b$ by measuring part of it in the + (for $b = 0$) or $\times$ (for $b=1$) basis. She then communicates to him the (classical) measurement outcome $R_x$ and the part-measured anonymous state interpolated into other, randomly prepared qubits as her evidence-of-commitment.Comment: 6 pages, minor changes, journal reference adde

A flattening in the Optical Light Curve of SN 2002ap

We present the $UBVR_cI_c$ broad band optical photometry of the Type Ic supernova SN 2002ap obtained during 2002 February 06 -- March 23 in the early decline phases and also later on 2002 15 August. Combining these data with the published ones, the general light curve development is studied. The time and luminosity of the peak brightness and the peak width are estimated. There is a flattening in the optical light curve about 30 days after the $B$ maximum. The flux decline rates before flattening are 0.127$\pm$0.005, 0.082$\pm$0.001, 0.074$\pm$0.001, 0.062$\pm$0.001 and 0.040$\pm$0.001 mag day$^{-1}$ in $U$, $B$, $V$, $R_c$ and $I_c$ passbands respectively, while the corresponding values after flattening are about 0.02 mag day$^{-1}$ in all the passbands. The maximum brightness of SN 2002ap $M_V = - 17.2$ mag, is comparable to that of the type Ic 1997ef, but fainter than that of the type Ic hypernova SN 1998bw. The peak luminosity indicates an ejection of $\sim$ 0.06 M$_{\odot}$ ${}^{56}$Ni mass. We also present low-resolution optical spectra obtained during the early phases. The SiII absorption minimum indicates that the photospheric velocity decreased from $\sim$ 21,360 km s$^{-1}$ to $\sim$ 10,740 km s$^{-1}$ during a period of $\sim$ 6 days.Comment: 7 pages, 5 figures, Submitted to MNRA

Observing the CMB with the AMiBA

I discuss the capabilities and limitations of the AMiBA for imaging CMB anisotropies. Michael Kesteven (ATNF-CSIRO) has proposed drift-scanning as an observing strategy for measuring and rejecting any instrumental response that the close-packed interferometers may have to the local environment. The advantages of mosaic imaging CMB anisotropies using a co-mounted interferometric array in a drift-scanning observing mode are discussed. A particular case of mosaic imaging a sky strip using a two-element AMiBA prototype interferometer is considered and the signal-to-noise ratio in the measurement of sky anisotropy using this observing strategy is analysed.Comment: 6 pages, includes 2 figures, to appear in the ASP Conf Ser. proceedings of AMiBA 2001: High-z clusters, Missing baryons, and CMB polarizatio

Suppression of thermal conduction in non-cooling flow clusters

Recent X-ray observations have revealed a universal temperature profile of the intracluster gas of non-cooling flow clusters which is flat for r \le 0.2 r_{180}. Numerical simulations, however, obtain a steeper temperature profile in the inner region. We study the effect of thermal conduction on the intracluster gas in non-cooling flow clusters in light of these observations, using the steep temperature profiles obtained by authors of numerical simulations. We find that given 10^{10} yr for the intracluster gas to evolve, thermal conduction should be suppressed from the classical value by a factor \sim 10^{-3} in order to explain the observations.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS (pink pages

HI Density Distribution Driven by Supernovae: A Simulation Study

We model the complex distribution of atomic hydrogen (HI) in the interstellar medium (ISM) assuming that it is driven entirely by supernovae (SN). We develop and assess two different models. In the first approach, the simulated volume is randomly populated with non-overlapping voids of a range of sizes. This may relate to a snapshot distribution of supernova-remnant voids, although somewhat artificially constrained by the non-overlap criterion. In the second approach, a simplified time evolution (considering momentum conservation as the only governing constraint during interactions) is followed as SN populate the space with the associated input mass and energy. We describe these simulations and present our results in the form of images of the mass and velocity distributions and the associated power spectra. The latter are compared with trends indicated by available observations. In both approaches, we find remarkable correspondence with the observed statistical description of well-studied components of the ISM, wherein the spatial spectra have been found to show significant deviations from the Kolmogorov spectrum. One of the key indications from this study, regardless of whether or not the SN-induced turbulence is the dominant process in the ISM, is that the apparent non-Kolmogorov spectral characteristics (of HI and/or electron column density across thick or thin screens) needed to explain related observations may not at all be in conflict with the underlying turbulence (i.e. the velocity structure) being of Kolmogorov nature. We briefly discuss the limitations of our simulations and the various implications of our results.Comment: To appear in Astrophysical Journal. 21 pages, 6 figure

Three dimensional, axisymmetric cusps without chaos

We construct three dimensional axisymmetric, cuspy density distributions, whose potentials are of St\"ackel form in parabolic coordinates. As in Sridhar and Touma (1997), a black hole of arbitrary mass may be added at the centre, without destroying the St\"ackel form of the potentials. The construction uses a classic method, originally due to Kuzmin (1956), which is here extended to parabolic coordinates. The models are highly oblate, and the cusps are "weak", with the density, $\rho \propto 1/r^k$, where $0<k<1$.Comment: 5 pages, 2 figures, submitted to MNRA

Dirac Quantization of Parametrized Field Theory

Parametrized field theory (PFT) is free field theory on flat spacetime in a diffeomorphism invariant disguise. It describes field evolution on arbitrary foliations of the flat spacetime instead of only the usual flat ones, by treating the `embedding variables' which describe the foliation as dynamical variables to be varied in the action in addition to the scalar field. A formal Dirac quantization turns the constraints of PFT into functional Schrodinger equations which describe evolution of quantum states from an arbitrary Cauchy slice to an infinitesimally nearby one.This formal Schrodinger picture- based quantization is unitarily equivalent to the standard Heisenberg picture based Fock quantization of the free scalar field if scalar field evolution along arbitrary foliations is unitarily implemented on the Fock space. Torre and Varadarajan (TV) showed that for generic foliations emanating from a flat initial slice in spacetimes of dimension greater than 2, evolution is not unitarily implemented, thus implying an obstruction to Dirac quantization. We construct a Dirac quantization of PFT,unitarily equivalent to the standard Fock quantization, using techniques from Loop Quantum Gravity (LQG) which are powerful enough to super-cede the no- go implications of the TV results. The key features of our quantization include an LQG type representation for the embedding variables, embedding dependent Fock spaces for the scalar field, an anomaly free representation of (a generalization of) the finite transformations generated by the constraints and group averaging techniques. The difference between 2 and higher dimensions is that in the latter, only finite gauge transformations are defined in the quantum theory, not the infinitesimal ones.Comment: 33 page