Asymptotic adaptive bipartite entanglement distillation protocol

We present a new asymptotic bipartite entanglement distillation protocol that outperforms all existing asymptotic schemes. This protocol is based on the breeding protocol with the incorporation of two-way classical communication. Like breeding, the protocol starts with an infinite number of copies of a Bell-diagonal mixed state. Breeding can be carried out as successive stages of partial information extraction, yielding the same result: one bit of information is gained at the cost (measurement) of one pure Bell state pair (ebit). The basic principle of our protocol is at every stage to replace measurements on ebits by measurements on a finite number of copies, whenever there are two equiprobable outcomes. In that case, the entropy of the global state is reduced by more than one bit. Therefore, every such replacement results in an improvement of the protocol. We explain how our protocol is organized as to have as many replacements as possible. The yield is then calculated for Werner states.Comment: 11 pages, 5 figures, RevTeX

Why do smallholder farmers in northern-Ghana choose to plough by hoe, with bullocks or with tractors?

United States Agency for International Developmen

Hashing protocol for distilling multipartite CSS states

We present a hashing protocol for distilling multipartite CSS states by means of local Clifford operations, Pauli measurements and classical communication. It is shown that this hashing protocol outperforms previous versions by exploiting information theory to a full extent an not only applying CNOTs as local Clifford operations. Using the information-theoretical notion of a strongly typical set, we calculate the asymptotic yield of the protocol as the solution of a linear programming problem.Comment: 13 pages, 3 figures, RevTeX

Stabilizer state breeding

We present a breeding protocol that distills pure copies of any stabilizer state from noisy copies and a pool of predistilled pure copies of the same state, by means of local Clifford operations, Pauli measurements and classical communication.Comment: RevTeX4, 9 pages, 1 figur

Local invariants of stabilizer codes

In [Phys. Rev. A 58, 1833 (1998)] a family of polynomial invariants which separate the orbits of multi-qubit density operators $\rho$ under the action of the local unitary group was presented. We consider this family of invariants for the class of those $\rho$ which are the projection operators describing stabilizer codes and give a complete translation of these invariants into the binary framework in which stabilizer codes are usually described. Such an investigation of local invariants of quantum codes is of natural importance in quantum coding theory, since locally equivalent codes have the same error-correcting capabilities and local invariants are powerful tools to explore their structure. Moreover, the present result is relevant in the context of multipartite entanglement and the development of the measurement-based model of quantum computation known as the one-way quantum computer.Comment: 10 pages, 1 figure. Minor changes. Accepted in Phys. Rev.

Geo-elektrische profielen bij de geologische en hydrologische detailkartering

Geoelectric traverses have been made in several regions of the F1anders, in order to test their usefulness for mapping purposes. The three regions selected had previously been covered by a grid of resistivity soundings mostly in Wenner arrangement.In the Beernem area different arrays have been used along the same line. These multiple traverses have provided data which allowed a comparison of the different arrays as well as a detailed subdivision in geoelectrical homogeneous zones coinciding with lithological units.Along traverse 136 GP/EE axial dipole measurements have been made over a distance of 2,7 km. For every point two values were determined : one for an array 2-20-2 and another for a 5-20-5 array. The first number stands for the length in meters of the current electrode separation, the third for the length of the voltage electrode separation, and the second for the distance between the two inner electrodes of bath dipoles.On the traverse 136-212 GP / AA Wenner arrangement perpendicular to the line has been used over a distance of 9,6 km to determine at each spot Q20' Q40' and Q80. Along the same line apparent resistivities have been measured with axial dipole arrays 2-20-2, 5-20-5, 5-40-5 and 10-40-10, parallel with the line.In the Westhoek area, a part of the coastal plain with salt-water encroachment in the unconfined aquifer, 21 traverses (194 GPW to 194 GPW21) have been run in the vicinity of Veurne (Furnes). The resistivities Q10 were measured by a longitudinal Wenner array. Since the traverses had been confined to a geoelectric homogeneous zone a quantitative interpretation could be performed. A relationship between Q10 and the depth of the saltwater - fresh water interface was computed from previous soundings. This relationship has been used to convert the apparent-resistivity data along the traverses into depth to interface values.The polder area of the F1emish Valley north of Ghent also has an unconfined aquifer with brackish water at variable depths. The situation is very similar to the one in the coastal plain although the variations here may not so readily find an explanation. One single traverse 142 GPW1 of 3550 m length has been run with a Wenner array for Q10. A relationship based upon former soundings has been used to convert resistivity data into interface depths.Resistivity traversing proves to be a very accurate and speedy tool for detailed mapping. Even a qualitative interpretation of a traverse, multiple if possible, can be sufficient to outline the boundaries between lithological or hydrogeological units. Quantitative interpretation is possible when in conjunction with a sounding grid, previously established, a relationship between the fixed electrode array and the unknown factor is found.Once a relationship is established, fixed-electrode separations allow a more detailed survey than resistivity soundings. These have to be long enough to obtain a reliable curve and hence average depth or resistivity data over longer a distance than fixed-electrode measurements which tend to be as short as possible. When traversing for qualitative purpose dipole arrangements have an advantage over Wenner arrangement in being speedier