Low error measurement-free phase gates for qubus computation

We discuss the desired criteria for a two-qubit phase gate and present a method for realising such a gate for quantum computation that is measurement-free and low error. The gate is implemented between qubits via an intermediate bus mode. We take a coherent state as the bus and use cross-Kerr type interactions between the bus and the qubits. This new method is robust against parameter variations and is thus low error. It fundamentally improves on previous methods due its deterministic nature and the lack of approximations used in the geometry of the phase rotations. This interaction is applicable both to solid state and photonic qubit systems.Comment: 6 pages, 4 figures. Published versio

Mean flow instabilities of two-dimensional convection in strong magnetic fields

The interaction of magnetic fields with convection is of great importance in astrophysics. Two well-known aspects of the interaction are the tendency of convection cells to become narrow in the perpendicular direction when the imposed field is strong, and the occurrence of streaming instabilities involving horizontal shears. Previous studies have found that the latter instability mechanism operates only when the cells are narrow, and so we investigate the occurrence of the streaming instability for large imposed fields, when the cells are naturally narrow near onset. The basic cellular solution can be treated in the asymptotic limit as a nonlinear eigenvalue problem. In the limit of large imposed field, the instability occurs for asymptotically small Prandtl number. The determination of the stability boundary turns out to be surprisingly complicated. At leading order, the linear stability problem is the linearisation of the same nonlinear eigenvalue problem, and as a result, it is necessary to go to higher order to obtain a stability criterion. We establish that the flow can only be unstable to a horizontal mean flow if the Prandtl number is smaller than order , where B0 is the imposed magnetic field, and that the mean flow is concentrated in a horizontal jet of width in the middle of the layer. The result applies to stress-free or no-slip boundary conditions at the top and bottom of the layer

Effect of multimode entanglement on lossy optical quantum metrology

In optical interferometry multimode entanglement is often assumed to be the driving force behind quantum enhanced measurements. Recent work has shown this assumption to be false: single-mode quantum states perform just as well as their multimode entangled counterparts. We go beyond this to show that when photon losses occur, an inevitability in any realistic system, multimode entanglement is actually detrimental to obtaining quantum enhanced measurements. We specifically apply this idea to a superposition of coherent states, demonstrating that these states show a robustness to loss that allows them to significantly outperform their competitors in realistic systems. A practically viable measurement scheme is then presented that allows measurements close to the theoretical bound, even with loss. These results promote an alternate way of approaching optical quantum metrology using single-mode states that we expect to have great implications for the future

Charge Stripping Reactions in Mass Spectrometry: A Study of Diatomic and Triatomic Inorganic and Organic Ions

Charge stripping reactions of the type m• + N--+ m2• + N + ehave been studied for a variety of diatomic and triatomic inorganic and organic ions. Ionisation energies of the m• ions, IE (m+--+ m2• ), have been determined, most of them for the first time. The method is fast and straightforward; it is applicable to both molecular and fragment .ions. The relative cross-sections for the charge stripping processes have been also determined; they show llrge variations from one species to another. Cases of possible interferences, which are fairly infrequent, are described and discussed

Generation of Magnetic Field by Combined Action of Turbulence and Shear

The feasibility of a mean-field dynamo in nonhelical turbulence with superimposed linear shear is studied numerically in elongated shearing boxes. Exponential growth of magnetic field at scales much larger than the outer scale of the turbulence is found. The charateristic scale of the field is l_B ~ S^{-1/2} and growth rate is gamma ~ S, where S is the shearing rate. This newly discovered shear dynamo effect potentially represents a very generic mechanism for generating large-scale magnetic fields in a broad class of astrophysical systems with spatially coherent mean flows.Comment: 4 pages, 5 figures; replaced with revised version that matches the published PR

Local versus global strategies in multi-parameter estimation

We consider the problem of estimating multiple phases using a multi-mode interferometer. In this setting we show that while global strategies that estimate all the phases simultaneously can lead to high precision gains, the same enhancements can be obtained with local strategies where each phase is estimated individually. A key resource for the enhancement is shown to be a large particle-number variance in the probe state, and for states where the total particle number is not fixed, this can be obtained for mode-separable states and the phases can be read out with local measurements. This has important practical implications because local strategies are generally preferred to global ones for their robustness to local estimation failure, flexibility in the distribution of resources, and comparatively easier state preparation. We obtain our results by analyzing two different schemes: the first uses a set of interferometers, which can be used as a model for a network of quantum sensors, and the second looks at measuring a number of phases relative to a reference, which is concerned primarily with quantum imaging

Vicious walkers, friendly walkers and Young tableaux II: With a wall

We derive new results for the number of star and watermelon configurations of vicious walkers in the presence of an impenetrable wall by showing that these follow from standard results in the theory of Young tableaux, and combinatorial descriptions of symmetric functions. For the problem of $n$-friendly walkers, we derive exact asymptotics for the number of stars and watermelons both in the absence of a wall and in the presence of a wall.Comment: 35 pages, AmS-LaTeX; Definitions of n-friendly walkers clarified; the statement of Theorem 4 and its proof were correcte

Wide band observations of the new X-ray burster SAX J1747.0-2853 during the March 1998 outburst

We report on our discovery and follow-up observations of the X-ray source SAX J1747.0-2853 detected in outburst on 1998, March 10 with the BeppoSAX Wide Field Cameras in the energy range 2-28 keV. The source is located about half degree off the Galactic Nucleus. A total of 14 type-I X-ray bursts were detected in Spring 1998, thus identifying the object as a likely low-mass X-ray binary harboring a weakly magnetized neutron star. Evidence for photospheric radius expansion is present in at least one of the observed bursts, leading to an estimate of the source distance of about 9 kpc. We performed a follow-up target of opportunity observation with the BeppoSAX Narrow Field Instruments on March 23 for a total elapsed time of 72 ks. The source persistent luminosity was 2.6x10^36 erg/s in the 2-10 keV energy range. The wide band spectral data (1-200 keV) are consistent with a remarkable hard X-ray spectrum detected up to 150 keV, highly absorbed at low energies (Nh of the order of 10^23 cm^-2) and with clear evidence for an absorption edge near 7 keV. A soft thermal component is also observed, which can be described by single temperature blackbody emission at about 0.6 keV.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter

Practical quantum metrology with large precision gains in the low photon number regime

Quantum metrology exploits quantum correlations to make precise measurements with limited particle numbers. By utilizing inter- and intra- mode correlations in an optical interferometer, we find a state that combines entanglement and squeezing to give a 7-fold enhancement in the quantum Fisher information (QFI) - a metric related to the precision - over the shot noise limit, for low photon numbers. Motivated by practicality we then look at the squeezed cat-state, which has recently been made experimentally, and shows further precision gains over the shot noise limit and a 3-fold improvement in the QFI over the optimal Gaussian state. We present a conceptually simple measurement scheme that saturates the QFI, and we demonstrate a robustness to loss for small photon numbers. The squeezed cat-state can therefore give a significant precision enhancement in optical quantum metrology in practical and realistic conditions