Passive decoy state quantum key distribution: Closing the gap to perfect sources

We propose a quantum key distribution scheme which closely matches the performance of a perfect single photon source. It nearly attains the physical upper bound in terms of key generation rate and maximally achievable distance. Our scheme relies on a practical setup based on a parametric downconversion source and present-day, non-ideal photon-number detection. Arbitrary experimental imperfections which lead to bit errors are included. We select decoy states by classical post-processing. This allows to improve the effective signal statistics and achievable distance.Comment: 4 pages, 3 figures. State preparation correcte

Photon-number-solving Decoy State Quantum Key Distribution

In this paper, a photon-number-resolving decoy state quantum key distribution scheme is presented based on recent experimental advancements. A new upper bound on the fraction of counts caused by multiphoton pulses is given. This upper bound is independent of intensity of the decoy source, so that both the signal pulses and the decoy pulses can be used to generate the raw key after verified the security of the communication. This upper bound is also the lower bound on the fraction of counts caused by multiphoton pulses as long as faint coherent sources and high lossy channels are used. We show that Eve's coherent multiphoton pulse (CMP) attack is more efficient than symmetric individual (SI) attack when quantum bit error rate is small, so that CMP attack should be considered to ensure the security of the final key. finally, optimal intensity of laser source is presented which provides 23.9 km increase in the transmission distance. 03.67.DdComment: This is a detailed and extended version of quant-ph/0504221. In this paper, a detailed discussion of photon-number-resolving QKD scheme is presented. Moreover, the detailed discussion of coherent multiphoton pulse attack (CMP) is presented. 2 figures and some discussions are added. A detailed cauculation of the "new" upper bound 'is presente

Fluid displacement by stokes flow past a spherical droplet

The concept of 'drift', which has been exploited in many high Reynolds number and inviscid flow problems, is here applied to examine transport by a spherical viscous droplet (of radius a) moving in a Stokes flow.In an unbounded flow, the velocity in the direction of translation of a spherical droplet is positive everywhere because streamlines, in the fluid frame of reference, 'close' at infinity. Fluid particles are displaced a positive distance, X, forward, which is expressed in terms of the initial distance from the stagnation streamline rho(0). Asymptotic expressions are developed for X in the limits of rho(0)/a much less than 1 and much greater than 1. The nature of the singularity of the centreline displacement changes from O(-a log(rho(0)/a)) to O(a(2)/rho(0)) as the viscosity of the droplet, compared to the ambient fluid, increases. By employing a mass-conservation argument, asymptotic expressions are calculated for the partial drift volume, D-p, associated with a circular material surface of radius rho(m) which starts far in front of a droplet that translates a finite distance. Since the velocity perturbation decays slowly with distance from the droplet, D-p tends to become unbounded as rho(m) increases, in contrast to inviscid flows.The presence of a rigid wall ensures that the velocity perturbation decays sufficiently rapidly that fluid particles, which do not lie on the stagnation streamline, are displaced a finite distance away from the wall. The distortion of a material surface lying a distance h(L) above a wall, by the droplet, starting a distance h(S) from the wall and moving away, is studied. The volume transported away from the wall, calculated using a multipolar flow approximation, is D-p = pih(L)(2)a(3lambda + 2)/(lambda + 1), and is weakly dependent on the starting position of the droplet, in accordance with numerical results. When the material surface is close to the wall (h(L)/a < 1), the volume transported away from a wall is significantly smaller than for inviscid flows because the no-slip condition on the rigid wall tends to inhibit 'scouring'. When the material surface is far from the wall (h(L)/a much greater than 1), the viscously dominated flow transports a larger volume of fluid away from the wall because the flow decays slowly with distance from the droplet.These results can be generalized to arbitrarily shaped bodies, since the transport processes are dominated by the strength of the Stokeslet. The effect of boundaries and inertia on fluid transport processes is briefly discussed

Fluid displacement by stokes flow past a spherical droplet

The concept of 'drift', which has been exploited in many high Reynolds number and inviscid flow problems, is here applied to examine transport by a spherical viscous droplet (of radius a) moving in a Stokes flow.In an unbounded flow, the velocity in the direction of translation of a spherical droplet is positive everywhere because streamlines, in the fluid frame of reference, 'close' at infinity. Fluid particles are displaced a positive distance, X, forward, which is expressed in terms of the initial distance from the stagnation streamline rho(0). Asymptotic expressions are developed for X in the limits of rho(0)/a much less than 1 and much greater than 1. The nature of the singularity of the centreline displacement changes from O(-a log(rho(0)/a)) to O(a(2)/rho(0)) as the viscosity of the droplet, compared to the ambient fluid, increases. By employing a mass-conservation argument, asymptotic expressions are calculated for the partial drift volume, D-p, associated with a circular material surface of radius rho(m) which starts far in front of a droplet that translates a finite distance. Since the velocity perturbation decays slowly with distance from the droplet, D-p tends to become unbounded as rho(m) increases, in contrast to inviscid flows.The presence of a rigid wall ensures that the velocity perturbation decays sufficiently rapidly that fluid particles, which do not lie on the stagnation streamline, are displaced a finite distance away from the wall. The distortion of a material surface lying a distance h(L) above a wall, by the droplet, starting a distance h(S) from the wall and moving away, is studied. The volume transported away from the wall, calculated using a multipolar flow approximation, is D-p = pih(L)(2)a(3lambda + 2)/(lambda + 1), and is weakly dependent on the starting position of the droplet, in accordance with numerical results. When the material surface is close to the wall (h(L)/a < 1), the volume transported away from a wall is significantly smaller than for inviscid flows because the no-slip condition on the rigid wall tends to inhibit 'scouring'. When the material surface is far from the wall (h(L)/a much greater than 1), the viscously dominated flow transports a larger volume of fluid away from the wall because the flow decays slowly with distance from the droplet.These results can be generalized to arbitrarily shaped bodies, since the transport processes are dominated by the strength of the Stokeslet. The effect of boundaries and inertia on fluid transport processes is briefly discussed

Rapid fabrication and characterization of sine wave targets

The effect of surface perturbations on Inertial Confinement Fusion target performance is currently being researched at Los Alamos National Laboratory (LANL). These perturbations can cause hydrodynamic instabilities which in turn reduce the targets` yield. To systematically measure the growth of these instabilities requires targets to be produced which have perturbations of a known amplitude and spatial frequency. The authors have recently assembled hardware onto one of their diamond turning lathes which enables them to machine and measure these sine waves in about 15 minutes. This is a significant reduction in time from the two and one half hours required by the previous method. This paper discusses the hardware, how it works, and how well the system is working for them to produce these targets

Quantification of the performance of chaotic micromixers on the basis of finite time Lyapunov exponents

Chaotic micromixers such as the staggered herringbone mixer developed by Stroock et al. allow efficient mixing of fluids even at low Reynolds number by repeated stretching and folding of the fluid interfaces. The ability of the fluid to mix well depends on the rate at which "chaotic advection" occurs in the mixer. An optimization of mixer geometries is a non trivial task which is often performed by time consuming and expensive trial and error experiments. In this paper an algorithm is presented that applies the concept of finite-time Lyapunov exponents to obtain a quantitative measure of the chaotic advection of the flow and hence the performance of micromixers. By performing lattice Boltzmann simulations of the flow inside a mixer geometry, introducing massless and non-interacting tracer particles and following their trajectories the finite time Lyapunov exponents can be calculated. The applicability of the method is demonstrated by a comparison of the improved geometrical structure of the staggered herringbone mixer with available literature data.Comment: 9 pages, 8 figure

Quantum Communication

Quantum communication, and indeed quantum information in general, has changed the way we think about quantum physics. In 1984 and 1991, the first protocol for quantum cryptography and the first application of quantum non-locality, respectively, attracted a diverse field of researchers in theoretical and experimental physics, mathematics and computer science. Since then we have seen a fundamental shift in how we understand information when it is encoded in quantum systems. We review the current state of research and future directions in this new field of science with special emphasis on quantum key distribution and quantum networks.Comment: Submitted version, 8 pg (2 cols) 5 fig

Translations of new public management: a decentred approach to school governance in four OECD countries

Despite the prevalence of corporate and performative models of school governance within and across different education systems, there are various cases of uneven, hybrid expressions of New Public Management (NPM) that reveal the contingency of global patterns of rule. Adopting a ‘decentred approach’ to governance (Bevir, M. 2010. “Rethinking Governmentality: Towards Genealogies of Governance.” European Journal of Social Theory 13 (4): 423–441), this paper compares the development of NPM in four OECD countries: Australia, England, Spain, and Switzerland. A focus of the paper is how certain policy instruments are created and sustained within highly differentiated geo-political settings and through different multi-scalar actors and authorities yet modified to reflect established traditions and practices

Optimization of chaotic micromixers using finite time Lyapunov exponents

In microfluidics mixing of different fluids is a highly non-trivial task due to the absence of turbulence. The dominant process allowing mixing at low Reynolds number is therefore diffusion, thus rendering mixing in plain channels very inefficient. Recently, passive chaotic micromixers such as the staggered herringbone mixer were developed, allowing efficient mixing of fluids by repeated stretching and folding of the fluid interfaces. The optimization of the geometrical parameters of such mixer devices is often performed by time consuming and expensive trial and error experiments. We demonstrate that the application of the lattice Boltzmann method to fluid flow in highly complex mixer geometries together with standard techniques from statistical physics and dynamical systems theory can lead to a highly efficient way to optimize micromixer geometries. The strategy applies massively parallel fluid flow simulations inside a mixer, where massless and noninteracting tracer particles are introduced. By following their trajectories we can calculate finite time Lyapunov exponents in order to quantify the degree of chaotic advection inside the mixer. The current report provides a review of our results published in [1] together with additional details on the simulation methodology