Role of hyperfine interaction for cavity-mediated coupling between spin qubits

We consider two qubits interacting by means of an optical cavity, where each qubit is represented by a single electron spin confined to a quantum dot. It is known that electron spins in III-V semiconductor quantum dots are affected by the decoherence due to the hyperfine interaction with nuclear spins. Here we show that the interaction between two qubits is influenced by the Overhauser field as well. Starting from an unpolarizied nuclear ensemble, we investigate the dependance of the fidelities for two-qubit gates on the Overhauser field. We include the hyperfine interaction perturbatively to second order in our analytical results, and to arbitrary precision numerically.Comment: 7 pages, 7 figure

Grid Integration Costs of Fluctuating Renewable Energy Sources

The grid integration of intermittent Renewable Energy Sources (RES) causes costs for grid operators due to forecast uncertainty and the resulting production schedule mismatches. These so-called profile service costs are marginal cost components and can be understood as an insurance fee against RES production schedule uncertainty that the system operator incurs due to the obligation to always provide sufficient control reserve capacity for power imbalance mitigation. This paper studies the situation for the German power system and the existing German RES support schemes. The profile service costs incurred by German Transmission System Operators (TSOs) are quantified and means for cost reduction are discussed. In general, profile service costs are dependent on the RES prediction error and the specific workings of the power markets via which the prediction error is balanced. This paper shows both how the prediction error can be reduced in daily operation as well as how profile service costs can be reduced via optimization against power markets and/or active curtailment of RES generation.Comment: Accepted for SUSTECH 2014, Portland, Oregon, USA, July 201

Die Christenheit oder Europa oder von Novalis lernen? : zur Relevanz eines romantischen Referenztextes im aktuellen Europadiskurs

Was Novalis seinen Lesern in dieser wie ein Fanal der Sehnsucht und der Rückkehr klingenden Exposition vor Augen führt, ist das Bild eines Kontinents, dessen Homogenität durch die gemeinsame Religion und die Weisheit der geistlichen Führer gestiftet wird. Eine wahre Steilvorlage für heutige Verfechter eines Vereinigten Europa auf der gemeinsamen Basis genuin christlicher Werte. Doch hält dieser geschichtsphilosophische Essay wirklich, wofür er im aktuellen Europadiskurs vereinnahmt wird? Lassen sich diesem frühromantischen Zeugnis aus der Zeit an der Schwelle zum 19. Jahrhundert Impulse abgewinnen, die am Beginn des 21. Jahrhunderts zu einer Präzisierung der strittigen Unbestimmtheit einer europäischen Identität beitragen können? Gibt es triftige Gründe dafür, weshalb Europapolitiker noch heute Novalis lesen sollten? Oder müssen wir nach eingehender Prüfung doch von einem „unnützen“ Aufsatz sprechen, wie dies der ungekrönte König der Romantik, Ludwig Tieck, bereits 1837 unverblümt getan hat? Solche Fragen zu erwidern, soll in einem ersten Schritt zunächst die Struktur des Textes nachgezeichnet werden, um zu verdeutlichen, wie Novalis darin die abend-ländische Geschichte triadisch als Heilsgeschichte entwickelt. Dem τέλος dieser Heilsgeschichte widmet sich dann der darauffolgende Abschnitt, ehe abschließend der von Novalis genannte „Zauberstab der Analogie“ auch mit Blick auf unsere Gegenwart angewandt werden soll, um nach der Relevanz dieses romantischen Refe-renztextes für den aktuellen Europadiskurs zu fragen

Revisiting the Merit-Order Effect of Renewable Energy Sources

An on-going debate in the energy economics and power market community has raised the question if energy-only power markets are increasingly failing due to growing feed-in shares from subsidized renewable energy sources (RES). The short answer to this is: No, they are not failing. Energy-based power markets are, however, facing several market distortions, namely from the gap between the electricity volume traded at day-ahead markets versus the overall electricity consumption as well as the (wrong) regulatory assumption that variable RES generation, i.e., wind and photovoltaic (PV), truly have zero marginal operation costs. In this paper we show that both effects over-amplify the well-known merit-order effect of RES power feed-in beyond a level that is explainable by underlying physical realities, i.e., thermal power plants being willing to accept negative electricity prices to be able to stay online due to considerations of wear & tear and start-stop constraints. We analyze the impacts of wind and PV power feed-in on the day-ahead market for a region that is already today experiencing significant feed-in tariff (FIT)-subsidized RES power feed-in, the EPEX German-Austrian market zone ($\approx\,$20% FIT share). Our analysis shows that, if the necessary regulatory adaptations are taken, i.e., increasing the day-ahead market's share of overall load demand and using the true marginal costs of RES units in the merit-order, energy-based power markets can remain functional despite high RES power feed-in.Comment: Working Paper (9 pages, 11 figures, 5 tables) - Some revisions since last version (10 February 2014). (Under 2nd review for IEEE Transactions on Power Systems