Detecting entanglement of two electron spin qubits with witness operators

We propose a scheme for detecting entanglement between two electron spin qubits in a double quantum dot using an entanglement witness operator. We first calculate the optimal configuration of the two electron spins, defined as the position in the energy level spectrum where, averaged over the nuclear spin distribution, 1) the probability to have two separated electrons, and 2) the degree of entanglement of the quantum state quantified by the concurrence are both large. Using a density matrix approach, we then calculate the evolution of the expectation value of the witness operator for the two-spin singlet state, taking into account the effect of decoherence due to quantum charge fluctuations modeled as a boson bath. We find that, for large interdot coupling, it is possible to obtain a highly entangled and robust ground state.Comment: 4 pages, 3 figure

Entanglement and the Quantum Brachistochrone Problem

Entanglement is closely related to some fundamental features of the dynamics of composite quantum systems: quantum entanglement enhances the "speed" of evolution of certain quantum states, as measured by the time required to reach an orthogonal state. The concept of "speed" of quantum evolution constitutes an important ingredient in any attempt to determine the fundamental limits that basic physical laws impose on how fast a physical system can process or transmit information. Here we explore the relationship between entanglement and the speed of quantum evolution in the context of the quantum brachistochrone problem. Given an initial and a final state of a composite system we consider the amount of entanglement associated with the brachistochrone evolution between those states, showing that entanglement is an essential resource to achieve the alluded time-optimal quantum evolution.Comment: 6 pages, 3 figures. Corrected typos in Eqs. 1 and

Towards Understanding the Politics of Flex Crops and Commodities: Implications for Research and Policy Advocacy

This discussion paper offers a preliminary exploration of the concept and phenomenon of “flex crops and commodities”, building on an earlier and initial analysis and abbreviated idea put forward by some of the authors of this paper. We discuss the dual concepts of the “multiple-ness” and “flexible-ness” of crops and commodities as two distinct but intertwined dimensions of some key crops and commodities. These key crops and commodities are shaped by the changing global context that is itself (re)moulded in large part by the convergence of multiple crises and the various responses to those crises. Building on these dual concepts, we will identify and explain the minimum requirements for crop and commodity flexing. We will also try to typologize the various types of crop and commodity flexing, namely, “real flexing”, “anticipated/speculated flexing”, and “imagined flexing”—to allow for a deeper examination of these interrelated processes. The boundaries between these categories (multiple/flexible, real, anticipated and imagined) are not always clearly demarcated, requiring us to examine the issue of flex crops and commodities in a more interlinked manner. We will focus our initial exploration on the political dynamics of such interactions and intersections, looking into the factors that encourage or discourage, facilitate or hinder maximization of the “multiple-ness” and/or “flexible-ness” of particular crops and commodities. Finally, and as a way of closing, we will outline the implications of these dynamics for how we think of engaged research, public actions and policy advocacy, including a brief discussion of what we call “flex policy narratives” by governments and corporations.

Scholar-Activism and Land Struggles:Agrarian Change & Peasant Studies

This book is about scholar-activism and political struggles for land. Scholar-activism is a way of working that tries to change society by combining the best features of radical academic and political activist traditions, despite the many contradictions and challenges that this entails. The role played by scholar-activists in land struggles is important, but is not straightforward. This book unapologetically celebrates the contributions of scholar-activism in land struggles and scholarship, but more than this, it is about exploring the contradictions and challenges facing scholar-activism. It is neither a glorification of the achievements of scholar-activism, nor a set of prescriptive propositions on how to ‘do’ scholar-activism. Rather, it addresses contentious issues in scholar-activism, many of which are rarely discussed, or are discussed only gingerly and awkwardly when they cannot be avoided. It is a book written by two scholar-activists who have focused their individual and collaborative research and activist works on the politics of land and the role played by radical agrarian movements. Insights in this book are drawn on the experiences of the authors working in the three main sites of global knowledge circuits: academic institutions, independent research institutions oriented to practical politics, and left-wing agrarian movements

Plantationocene and Contemporary Agrarian Struggles

Wolford’s (2021) article on the Plantationocene compels us to reexamine the state of agrarian struggles today in relation to struggles within and against capitalism. Although contemporary agrarian movements are relatively vibrant overall, their movement organizations and alliances tend to be sectoral and localized, and plantation workers remain weakly organized. This commentary argues that agrarian struggles can become more relevant if they are better embedded within broader anticapitalist struggles; conversely, broad anticapitalist struggles are better grounded if they are linked to contemporary agrarian struggles. The Plantationocene scholarship validates this point; moreover, scholarship on the Plantationocene can beenriched by engagement with studies on agrarian struggles

Fundamental Limits on the Speed of Evolution of Quantum States

This paper reports on some new inequalities of Margolus-Levitin-Mandelstam-Tamm-type involving the speed of quantum evolution between two orthogonal pure states. The clear determinant of the qualitative behavior of this time scale is the statistics of the energy spectrum. An often-overlooked correspondence between the real-time behavior of a quantum system and the statistical mechanics of a transformed (imaginary-time) thermodynamic system appears promising as a source of qualitative insights into the quantum dynamics.Comment: 6 pages, 1 eps figur

Efficient generation of random multipartite entangled states using time optimal unitary operations

We review the generation of random pure states using a protocol of repeated two qubit gates. We study the dependence of the convergence to states with Haar multipartite entanglement distribution. We investigate the optimal generation of such states in terms of the physical (real) time needed to apply the protocol, instead of the gate complexity point of view used in other works. This physical time can be obtained, for a given Hamiltonian, within the theoretical framework offered by the quantum brachistochrone formalism. Using an anisotropic Heisenberg Hamiltonian as an example, we find that different optimal quantum gates arise according to the optimality point of view used in each case. We also study how the convergence to random entangled states depends on different entanglement measures.Comment: 5 pages, 2 figures. New title, improved explanation of the algorithm. To appear in Phys. Rev.