Internal bores and gravity currents in a two-fluid system

In this paper, a unified theory of internal bores and gravity currents is presented within the framework of the one-dimensional two-layer shallow-water equations. The equations represent four basic physical laws: the theory is developed on the basis of these laws. Though the first three of the four basic laws are apparent, the forth basic law has been uncertain. This paper shows first that this forth basic law can be deduced from the law which is called in this paper the conservation law of circulation. It is then demonstrated that, within the framework of the equations, an internal bore is represented by a shock satisfying the shock conditions that follow from the four basic laws. A gravity current can also be treated within the framework of the equations if the front conditions, i.e. the boundary conditions to be imposed at the front of the current, are known. Basically, the front conditions for a gravity current also follow from the four basic laws. When the gravity current is advancing along a no-slip boundary, however, it is necessary to take into account the influence of the thin boundary layer formed on the boundary; this paper describes how this influence can be evaluated. It is verified that the theory can satisfactorily explain the behaviour of internal bores advancing into two stationary layers of fluid. The theory also provides a formula for the rate of advance of a gravity current along a no-slip lower boundary; this formula proves to be consistent with some empirical formulae. In addition, some well-known theoretical formulae on gravity currents turn out to be obtainable on the basis of the theory.Comment: 47 pages, 5 figure

Unconditionally secure quantum key-distribution with relatively strong signal pulse

We propose an unconditionally secure quantum key distribution (QKD) protocol, which uses a relatively strong signal pulse. While our protocol shares similar security bases as the Bennett 1992 protocol with a strong reference pulse (B92), our scheme uses a smaller number of detectors and it is robust against Rayleigh scattering in an optical fibre. We derive a lower bound of secret key generation rate of our protocol and show that our protocol can cover relatively long distances, assuming precise phase modulations and stable interferometers.Comment: 4 page

Wilson Loops in Open String Theory

Wilson loop elements on torus are introduced into the partition function of open strings as Polyakov's path integral at one-loop level. Mass spectra from compactification and expected symmetry breaking are illustrated by choosing the correct weight for the contributions from annulus and M\"obius strip. We show that Jacobi's imaginary transformation connects the mass spectra with the Wilson loops. The application to thermopartition function and cosmological implications are briefly discussed.Comment: 5 pages, no figur