Hamiltonians for one-way quantum repeaters

Quantum information degrades over distance due to the unavoidable imperfections of the transmission channels, with loss as the leading factor. This simple fact hinders quantum communication, as it relies on propagating quantum systems. A solution to this issue is to introduce quantum repeaters at regular intervals along a lossy channel, to revive the quantum signal. In this work we study unitary one-way quantum repeaters, which do not need to perform measurements and do not require quantum memories, and are therefore considerably simpler than other schemes. We introduce and analyze two methods to construct Hamiltonians that generate a repeater interaction that can beat the fundamental repeaterless key rate bound even in the presence of an additional coupling loss, with signals that contain only a handful of photons. The natural evolution of this work will be to approximate a repeater interaction by combining simple optical elements.Comment: 8 pages, 3 figure

Academic freedom and changing focus on student assessment in Dutch higher education

The paper addresses this issue of academic freedom by exploring how the new regulation regarding student assessment procedures has been implemented in Dutch higher education institutions. We endeavour to understand how teaching staff have responded to the new rules and to what extent decision-making processes have included teaching staff. To answer our research question, we would like to concentrate on two issues: the role of the Examination Board and the design of the assessment procedures. Our working hypothesis is that new institutional arrangements of student assessments reduce academic freedom. Previous research has showed that academics largely engage in symbolic compliance with the imperatives of management requirements while keeping their academic freedom intact (Leisyte and Dee 2012). Universities in their turn, are increasingly acting as strategic actors in not only complying but also shaping their institutional environments (Meyer and Rowan 1977, Greenwood and Hinnings 1996, Krücken 2011). The responses to established norms and beliefs range from passivity to increasingly active resistance (Oliver 1991). We will discuss if the impingement on academic freedom is offset by increased assurance of quality for students, and explore how to balance the two values of freedom and quality. Methods used are document analysis and interviews with teachers, university administrators and during Fall 2011 in the frame of the Identifying Barriers in promoting European Standards and Guidelines for Quality Assurance (IBAR) project

Multi-partite entanglement speeds up quantum key distribution in networks

The laws of quantum mechanics allow for the distribution of a secret random key between two parties. Here we analyse the security of a protocol for establishing a common secret key between N parties (i.e. a conference key), using resource states with genuine N-partite entanglement. We compare this protocol to conference key distribution via bipartite entanglement, regarding the required resources, achievable secret key rates and threshold qubit error rates. Furthermore we discuss quantum networks with bottlenecks for which our multipartite entanglement-based protocol can benefit from network coding, while the bipartite protocol cannot. It is shown how this advantage leads to a higher secret key rate.Comment: 18 pages, 8 figures, longer and improved versio