11,735 research outputs found

    Quantum circuits based on coded qubits encoded in chirality of electron spin complexes in triple quantum dots

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    We present a theory of quantum circuits based on logical qubits encoded in chirality of electron spin complexes in lateral gated semiconductor triple quantum dot molecules with one electron spin in each dot. Using microscopic Hamiltonian we show how to initialize, coherently control and measure the quantum state of a chirality based coded qubit using static in-plane magnetic field and voltage tuning of individual dots. The microscopic model of two interacting coded qubits is established and mapped to an Ising Hamiltonian, resulting in conditional two-qubit phase gate

    A Unified Stochastic Formulation of Dissipative Quantum Dynamics. I. Generalized Hierarchical Equations

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    We extend a standard stochastic theory to study open quantum systems coupled to generic quantum environments including the three fundamental classes of noninteracting particles: bosons, fermions and spins. In this unified stochastic approach, the generalized stochastic Liouville equation (SLE) formally captures the exact quantum dissipations when noise variables with appropriate statistics for different bath models are applied. Anharmonic effects of a non-Gaussian bath are precisely encoded in the bath multi-time correlation functions that noise variables have to satisfy. Staring from the SLE, we devise a family of generalized hierarchical equations by averaging out the noise variables and expand bath multi-time correlation functions in a complete basis of orthonormal functions. The general hiearchical equations constitute systems of linear equations that provide numerically exact simulations of quantum dynamics. For bosonic bath models, our general hierarchical equation of motion reduces exactly to an extended version of hierarchical equation of motion which allows efficient simulation for arbitrary spectral densities and temperature regimes. Similar efficiency and exibility can be achieved for the fermionic bath models within our formalism. The spin bath models can be simulated with two complementary approaches in the presetn formalism. (I) They can be viewed as an example of non-Gaussian bath models and be directly handled with the general hierarchical equation approach given their multi-time correlation functions. (II) Alterantively, each bath spin can be first mapped onto a pair of fermions and be treated as fermionic environments within the present formalism.Comment: 31 pages, 2 figure

    Did Iraq Cheat the United Nations? Underpricing, Bribes, and the Oil for Food Program

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    From 1997 through early 2003, the United Nations Oil for Food Program allowed Iraq to export oil in exchange for humanitarian supplies. We measure the extent to which this program was corrupted by Iraq's attempts to deliberately set the price of its oil below market prices in an effort to solicit bribes, both in the form of direct cash bribes and in the form of political favors, from the buyers of the underpriced oil. We infer the magnitude of the potential bribe by comparing the gap between the official selling price of Iraq's two crude oils (Basrah Light and Kirkuk) and the market price of several comparison crude oils during the Program to the gap observed prior to the Program. We find consistent evidence that underpricing of Basrah Light averaged 1perbarrelfrom1997through1999andreachesapeak(almost1 per barrel from 1997 through 1999 and reaches a peak (almost 3 per barrel) from May 2000 through September 2001. The estimated underpricing quickly declines after the UN introduced a retroactive pricing scheme that reduced Iraq's ability to set the price of its oil. The evidence on whether Kirkuk was underpriced is less clear. Notably, we find that episodes of underpricing of Basrah Light are associated with a decline in the share of major oil multinationals among the oil buyers, and an increase in the share of obscure individual traders. The observed underpricing of Iraqi oil suggests that Iraq generated 5billioninrentsthroughitsstrategicunderpricing.Ofthisamount,weestimatethatIraqcollected5 billion in rents through its strategic underpricing. Of this amount, we estimate that Iraq collected 0.7 to $2 billion in bribes (depending on Iraq's share of the rents implied by the price gap), which is roughly 1 to 3 percent of the total value of oil sales under the Program. Finally, we find little evidence that underpricing was associated with increases in the relative supply or declines in the relative demand of Iraqi oil.

    When Schools Compete, How Do They Compete? An Assessment of Chile's Nationwide School Voucher Program

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    In 1981, Chile introduced nationwide school choice by providing vouchers to any student wishing to attend private school. As a result, more than 1,000 private schools entered the market, and the private enrollment rate increased by 20 percentage points, with greater impacts in larger, more urban, and wealthier communities. We use this differential impact to measure the effects of unrestricted choice on educational outcomes. Using panel data for about 150 municipalities, we find no evidence that choice improved average educational outcomes as measured by test scores, repetition rates, and years of schooling. However, we find evidence that the voucher program led to increased sorting, as the best' public school students left for the private sector.

    When Schools Compete, How Do They Compete? An Assessment of Chile’s Nationwide School Voucher Program

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    The notion that free choice is welfare-enhancing is one of the foundations of modern, market-oriented societies. This view is prominent in the school choice debate, where there is a widespread perception that public schools are inecientlocal monopolies, and that the quality of education would improve dramatically if only parents were allowed to freely choose between schools. For example, in recent work Hoxby (2001) asks \what is the range of productivity over which choice could cause productivity to vary? Recent history suggests that school productivity could be much higher than it is now - 60 to 70 percent higher." Two arguments underlie the view that choice would improve the quality of education. First, there is a widely-held belief that private schools are better than public schools. Al- though most research on this is hampered by selection issues, recent work that exploits quasi-experiments with vouchers nd some evidence that children benet from attending private schools.1 The implication is that unrestricted choice could raise students' achieve- ment merely by facilitating their transfer to the private sector. A second, perhaps even more compelling argument for choice comes from our instinct that people and organizations respond to incentives. Therefore, by correctly aligning the incentives public schools face, choice would force their ossied bureaucracies to improve.

    A Global View of Productivity Growth in China

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    How does a country's productivity growth affect worldwide real incomes through international trade? In this paper, we take this classic question to the data by measuring the spillover effects of China's productivity growth. Our framework features traditional terms-of-trade effects and new trade home market effects as suggested by the theoretical literature and works from a reference point which perfectly matches industry-level trade. Focusing on the years 1995 to 2007, we find that the cumulative welfare effect on individual regions ranges between -1.2 percent and 3.6 percent and only 3.0 percent of the worldwide gains of China's productivity growth accrue to the rest of the world.
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