Photoelectrochemical Characterization and Durability Analysis of GaInPN Epilayers

Epilayer samples of GaInPN with a stoichiometry of Ga.962In.038P.976N.024 and Ga.95In.05P.975N.025 were grown by metallorganic chemical vapor deposition and were photoelectrochemically characterized to determine their potential to photoelectrolyze water. The materials had direct transition bandgap energies of _2.0 eV, as determined by photocurrent spectroscopy; however, the measured flatband potentials were several hundred millivolts too negative for unbiased water splitting. Stability was assessed by operating the electrodes as photocathodes for 24 h and qualitatively evaluated by probing the amount of surface etching with a stylus profilometer. Quantitative evaluation was done by measuring the concentration of gallium in the corrosion test electrolyte using inductively coupled plasma mass spectrometry. Results were compared to similarly treated GaP and GaInP2 samples for context

Parametrization and distillability of three-qubit entanglement

There is an ongoing effort to quantify entanglement of quantum pure states for systems with more than two subsystems. We consider three approaches to this problem for three-qubit states: choosing a basis which puts the state into a standard form, enumerating ``local invariants,'' and using operational quantities such as the number of maximally entangled states which can be distilled. In this paper we evaluate a particular standard form, the {\it Schmidt form}, which is a generalization of the Schmidt decomposition for bipartite pure states. We show how the coefficients in this case can be parametrized in terms of five physically meaningful local invariants; we use this form to prove the efficacy of a particular distillation technique for GHZ triplets; and we relate the yield of GHZs to classes of states with unusual entanglement properties, showing that these states represent extremes of distillability as functions of two local invariants.Comment: 17 pages RevTeX 3.0 including 2 figures (encapsulated Postscript) Final version, to appear in Physics Letters

Perfect state distinguishability and computational speedups with postselected closed timelike curves

Bennett and Schumacher's postselected quantum teleportation is a model of closed timelike curves (CTCs) that leads to results physically different from Deutsch's model. We show that even a single qubit passing through a postselected CTC (P-CTC) is sufficient to do any postselected quantum measurement, and we discuss an important difference between "Deutschian" CTCs (D-CTCs) and P-CTCs in which the future existence of a P-CTC might affect the present outcome of an experiment. Then, based on a suggestion of Bennett and Smith, we explicitly show how a party assisted by P-CTCs can distinguish a set of linearly independent quantum states, and we prove that it is not possible for such a party to distinguish a set of linearly dependent states. The power of P-CTCs is thus weaker than that of D-CTCs because the Holevo bound still applies to circuits using them regardless of their ability to conspire in violating the uncertainty principle. We then discuss how different notions of a quantum mixture that are indistinguishable in linear quantum mechanics lead to dramatically differing conclusions in a nonlinear quantum mechanics involving P-CTCs. Finally, we give explicit circuit constructions that can efficiently factor integers, efficiently solve any decision problem in the intersection of NP and coNP, and probabilistically solve any decision problem in NP. These circuits accomplish these tasks with just one qubit traveling back in time, and they exploit the ability of postselected closed timelike curves to create grandfather paradoxes for invalid answers.Comment: 15 pages, 4 figures; Foundations of Physics (2011

Entanglement purification of unknown quantum states

A concern has been expressed that ``the Jaynes principle can produce fake entanglement'' [R. Horodecki et al., Phys. Rev. A {\bf 59}, 1799 (1999)]. In this paper we discuss the general problem of distilling maximally entangled states from $N$ copies of a bipartite quantum system about which only partial information is known, for instance in the form of a given expectation value. We point out that there is indeed a problem with applying the Jaynes principle of maximum entropy to more than one copy of a system, but the nature of this problem is classical and was discussed extensively by Jaynes. Under the additional assumption that the state $\rho^{(N)}$ of the $N$ copies of the quantum system is exchangeable, one can write down a simple general expression for $\rho^{(N)}$. We show how to modify two standard entanglement purification protocols, one-way hashing and recurrence, so that they can be applied to exchangeable states. We thus give an explicit algorithm for distilling entanglement from an unknown or partially known quantum state.Comment: 20 pages RevTeX 3.0 + 1 figure (encapsulated Postscript) Submitted to Physical Review

Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry

Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from $1.60–$10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O_2 and H_2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energy's targeted threshold cost of $2.00–$4.00 per kg H_2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met

Virtual Special Issue on Catalysis at the U.S. Department of Energy’s National Laboratories

Catalysis research at the U.S. Department of Energy’s (DOE’s) National Laboratories covers a wide range of research topics in heterogeneous catalysis, homogeneous/molecular catalysis, biocatalysis, electrocatalysis, and surface science. Since much of the work at National Laboratories is funded by DOE, the research is largely focused on addressing DOE’s mission to ensure America’s security and prosperity by addressing its energy, environmental, and nuclear challenges through transformative science and technology solutions. The catalysis research carried out at the DOE National Laboratories ranges from very fundamental catalysis science, funded by DOE’s Office of Basic Energy Sciences (BES), to applied research and development (R&D) in areas such as biomass conversion to fuels and chemicals, fuel cells, and vehicle emission control with primary funding from DOE’s Office of Energy Efficiency and Renewable Energy. National Laboratories are home to many DOE Office of Science national scientific user facilities that provide researchers with the most advanced tools of modern science, including accelerators, colliders, supercomputers, light sources, and neutron sources, as well as facilities for studying the nanoworld and the terrestrial environment. National Laboratory research programs typically feature teams of researchers working closely together, often joining scientists from different disciplines to tackle scientific and technical problems using a variety of tools and techniques available at the DOE national scientific user facilities. Along with collaboration between National Laboratory scientists, interactions with university colleagues are common in National Laboratory catalysis R&D. In some cases, scientists have joint appointments at a university and a National Laboratory. This ACS Catalysis Virtual Special Issue {http://pubs.acs.org/page/accacs/vi/doe-national-labs} was motivated by Christopher Jones and Rhea Williams, who sent out the invitations to all of DOE’s National Laboratories where catalysis research is conducted. All manuscripts submitted went through the standard rigorous peer review required for publication in ACS Catalysis. A total of 29 papers are published in this virtual special issue, which features some of the recent catalysis research at 11 of DOE’s National Laboratories: Ames Laboratory (Ames), Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL), Lawrence Berkeley National Laboratory (LBNL), Lawrence Livermore National Laboratory (LLNL), National Energy Technology Laboratory (NETL), National Renewable Energy Laboratory (NREL), Oak Ridge National Laboratory (ORNL), Pacific Northwest National Laboratory (PNNL), Sandia National Laboratory (SNL), and SLAC National Accelerator Laboratory (SLAC). In this preface, we briefly discuss the history and impact of catalysis research at these particular DOE National Laboratories, where the majority of catalysis research continues to be conducted

HIV/AIDS, declining family resources and the community safety net

Families play central roles in the HIV/AIDS pandemic, caring for both orphaned children and the ill. This extra caregiving depletes two family resources essential for supporting children: time and money. We use recent data from published studies in sub-Saharan Africa to illustrate deficits and document community responses. In Botswana, parents caring for the chronically ill had less time for their preschool children (74 versus 96 hours per month) and were almost twice as likely to leave children home alone (53% versus 27%); these children experienced greater health and academic problems. Caregiving often prevented adults from working full time or earning their previous level of income; 47% of orphan caregivers and 64% of HIV/AIDS caregivers reported financial difficulties due to caregiving. Communities can play an important role in helping families provide adequate childcare and financial support. Unfortunately, while communities commonly offer informal assistance, the value of such support is not adequate to match the magnitude of need: 75% of children's families in Malawi received assistance from their social network, but averaging only US$81 annually. We suggest communities can strengthen the capacity of families by implementing affordable quality childcare for 0–6 year olds, after-school programming for older children and youth, supportive care for ill children and parents, microlending to enhance earnings, training to increase access to quality jobs, decent working conditions, social insurance for the informal sector, and income and food transfers when families are unable to make ends meet