Two Party Non-Local Games

In this work we have introduced two party games with respective winning conditions. One cannot win these games deterministically in the classical world if they are not allowed to communicate at any stage of the game. Interestingly we find out that in quantum world, these winning conditions can be achieved if the players share an entangled state. We also introduced a game which is impossible to win if the players are not allowed to communicate in classical world (both probabilistically and deterministically), yet there exists a perfect quantum strategy by following which, one can attain the winning condition of the game.Comment: Accepted in International Journal of Theoretical Physic

The Complexity of Separating Points in the Plane

We study the following separation problem: given n connected curves and two points s and t in the plane, compute the minimum number of curves one needs to retain so that any path connecting s to t intersects some of the retained curves. We give the first polynomial (O(n3)) time algorithm for the problem, assuming that the curves have reasonable computational properties. The algorithm is based on considering the intersection graph of the curves, defining an appropriate family of closed walks in the intersection graph that satisfies the 3-path-condition, and arguing that a shortest cycle in the family gives an optimal solution. The 3-path-condition has been used mainly in topological graph theory, and thus its use here makes the connection to topology clear. We also show that the generalized version, where several input points are to be separated, is NP-hard for natural families of curves, like segments in two directions or unit circles

Experimental observation of impossible-to-beat quantum advantage on a hybrid photonic system

Quantum resources outperform classical ones for certain communication and computational tasks. Remarkably, in some cases, the quantum advantage cannot be improved using hypothetical postquantum resources. A class of tasks with this property can be singled out using graph theory. Here we report the experimental observation of an impossible-to-beat quantum advantage on a four-dimensional quantum system defined by the polarization and orbital angular momentum of a single photon. The results show pristine evidence of the quantum advantage and are compatible with the maximum advantage allowed using postquantum resources.Comment: REVTeX4, 5 pages, 2 figure

No-hidden-variables proof for two spin-1/2 particles preselected and postselected in unentangled states

It is a well-known fact that all the statistical predictions of quantum mechanics on the state of any physical system represented by a two-dimensional Hilbert space can always be duplicated by a noncontextual hidden-variables model. In this paper, I show that, in some cases, when we consider an additional independent (unentangled) two-dimensional system, the quantum description of the resulting composite system cannot be reproduced using noncontextual hidden variables. In particular, a no-hidden-variables proof is presented for two individual spin-1/2 particles preselected in an uncorrelated state AB and postselected in another uncorrelated state aB, B being the same state for the second particle in both preselection and postselection.Comment: LaTeX, 8 page

Multi-level, multi-party singlets as ground states and their role in entanglement distribution

We show that a singlet of many multi-level quantum systems arises naturally as the ground state of a physically-motivated Hamiltonian. The Hamiltonian simply exchanges the states of nearest-neighbours in some network of qudits (d-level systems); the results are independent of the strength of the couplings or the network's topology. We show that local measurements on some of these qudits project the unmeasured qudits onto a smaller singlet, regardless of the choice of measurement basis at each measurement. It follows that the entanglement is highly persistent, and that through local measurements, a large amount of entanglement may be established between spatially-separated parties for subsequent use in distributed quantum computation.Comment: Corrected method for physical preparatio

Alternative scheme to generate a supersinglet state of three-level atoms

In this paper we propose an alternative scheme to generate a supersinglet state of three three-level atoms via a single-mode of a cavity QED based on the two-photon transitions described by the 'full microscopical Hamiltonian approach'. In it, three three-level atoms prepared in suitable initial states are sequentially sent through the cavity originally prepared in its vacuum state. After an appropriate choice of the atom-cavity interaction times plus a field detection the state that describes the whole atom-field system is projected in the desired supersinglet state. The fidelity and success probability of the state as well as the practical feasibility of the scheme are discussed.Comment: 10 pages, 3 figures, 4 table

Proposed experiment to test the bounds of quantum correlations

Clauser-Horne-Shimony-Holt inequality can give values between the classical bound, 2, and Tsirelson's bound, 2 \sqrt 2. However, for a given set of local observables, there are values in this range which no quantum state can attain. We provide the analytical expression for the corresponding bound for a parametrization of the local observables introduced by Filipp and Svozil, and describe how to experimentally trace it using a source of singlet states. Such an experiment will be useful to identify the origin of the experimental errors in Bell's inequality-type experiments and could be modified to detect hypothetical correlations beyond those predicted by quantum mechanics.Comment: REVTeX4, 4 pages, 2 figure

Does evidence-based practice improve patient outcomes

Background Evidence based practice (EBP) is widely promoted, but does EBP practice produce better patient outcomes? We report a natural experiment when part of the internal medicine service in a hospital was reorganized in 2003 to form an EBP unit, the rest of the service remaining unchanged. The units attended similar patients until 2012 permitting comparisons of outcomes and activity. Methods We used routinely collected statistics (2004 11) to compare the two different methods of practice and test whether patients being seen by the EBP unit differed from standard practice (SP) patients. Data were available by doctor and year. To check for differences between the EBP and SP doctors prior to reorganization, we used statistics from 2000 2003. We looked for changes in patient outcomes or activity following reorganization and whether the EBP unit was achieving significantly different results from SP. Data across the periods were combined and tested using Mann-Whitney. Results No statistically significant differences in outcomes were detected between the EBP and the SP doctors prior to reorganization. Following the unit’s establishment, the mortality of patients being treated by EBP doctors compared to their previous performance dropped from 7.4% to 6.3% (P<0.02) and length of stay from 9.15 to 6.01 days (P=0.002). No statistically significant improvements were seen in SP physicians’ performance. No differences in the proportion of patients admitted or their complexity between the services were detected. Despite this, EBP patients had a clinically significantly lower risk of death 6.27% vs 7.75% (P<0.001) and a shorter length of stay 6.01 vs 8.46 days (P<0.001) than SP patients. Readmission rates were similar: 14.4% (EBP); 14.5% (SP). EBP doctors attended twice as many patients/doctor as SP doctors. Conclusion The EBP unit was associated with better patient outcomes and more efficient performance than achieved by the same physicians previously or by SP concurrently