Two-Qubit Pulse Gate for the Three-Electron Double Quantum Dot Qubit

The three-electron configuration of gate-defined double quantum dots encodes a promising qubit for quantum information processing. I propose a two-qubit entangling gate using a pulse-gated manipulation procedure. The requirements for high-fidelity entangling operations are equivalent to the requirements for the pulse-gated single-qubit manipulations that have been successfully realized for Si QDs. This two-qubit gate completes the universal set of all-pulse-gated operations for the three-electron double-dot qubit and paves the way for a scalable setup to achieve quantum computation.Comment: 8 pages, 4 figure

Inverted Singlet-Triplet Qubit Coded on a Two-Electron Double Quantum Dot

The $s_z=0$ spin configuration of two electrons confined at a double quantum dot (DQD) encodes the singlet-triplet qubit (STQ). We introduce the inverted STQ (ISTQ) that emerges from the setup of two quantum dots (QDs) differing significantly in size and out-of-plane magnetic fields. The strongly confined QD has a two-electron singlet ground state, but the weakly confined QD has a two-electron triplet ground state in the $s_z=0$ subspace. Spin-orbit interactions act nontrivially on the $s_z=0$ subspace and provide universal control of the ISTQ together with electrostatic manipulations of the charge configuration. GaAs and InAs DQDs can be operated as ISTQs under realistic noise conditions.Comment: 10 pages, 4 figure

Simple operation sequences to couple and interchange quantum information between spin qubits of different kinds

Efficient operation sequences to couple and interchange quantum information between quantum dot spin qubits of different kinds are derived using exchange interactions. In the qubit encoding of a single-spin qubit, a singlet-triplet qubit, and an exchange-only (triple-dot) qubit, some of the single-qubit interactions remain on during the entangling operation; this greatly simplifies the operation sequences that construct entangling operations. In the ideal setup, the gate operations use the intra-qubit exchange interactions only once. The limitations of the entangling sequences are discussed, and it is shown how quantum information can be converted between different kinds of quantum dot spin qubits.Comment: 9 pages, 4 figure

Domestic Debt Structures in Emerging Markets : New Empirical Evidence

This paper explains why public domestic debt composition in emerging economies can be risky, namely in foreign currency, with a short maturity or indexed. It analyses empirically the determinants of these risk sources separately, developing a new large dataset compiled from national sources for 33 emerging economies over 1994-2006. The paper finds that economic size, the breadth of the domestic investor base, inflation and fiscal soundness are all associated with risky public domestic debt compositions, yet to an extent that varies considerably in terms of magnitude and significance across sources of risk. Only inflation impacts all types of risky debt, underscoring the overarching importance of monetary credibility to make domestic debt compositions in emerging economies safer. Given local bond markets' rapid development, monitoring risky public domestic debt compositions in emerging economies becomes increasingly relevant to global financial stability.Public domestic debt, composition, risk, emerging economies.

Noise Analysis of Qubits Implemented in Triple Quantum Dot Systems in a Davies Master Equation Approach

We analyze the influence of noise for qubits implemented using a triple quantum dot spin system. We give a detailed description of the physical realization and develop error models for the dominant external noise sources. We use a Davies master equation approach to describe their influence on the qubit. The triple dot system contains two meaningful realizations of a qubit: We consider a subspace and a subsystem of the full Hilbert space to implement the qubit. We test the robustness of these two implementations with respect to the qubit stability. When performing the noise analysis, we extract the initial time evolution of the qubit using a Nakajima-Zwanzig approach. We find that the initial time evolution, which is essential for qubit applications, decouples from the long time dynamics of the system. We extract probabilities for the qubit errors of dephasing, relaxation and leakage. Using the Davies model to describe the environment simplifies the noise analysis. It allows us to construct simple toy models, which closely describe the error probabilities.Comment: 30 pages, 18 figure

Noise-Protected Gate for Six-Electron Double-Dot Qubits

Singlet-triplet spin qubits in six-electron double quantum dots, in moderate magnetic fields, can show superior immunity to charge noise. This immunity results from the symmetry of orbitals in the second energy shell of circular quantum dots: singlet and triplet states in this shell have identical charge distributions. Our phase-gate simulations, which include $1/f$ charge noise from fluctuating traps, show that this symmetry is most effectively exploited if the gate operation switches rapidly between sweet spots deep in the (3,3) and (4,2) charge stability regions; fidelities very close to one are predicted if subnanosecond switching can be performed.Comment: 7 pages, 3 figure

Do China and oil exporters influence major currency configurations?

This paper analyses the impact of the shift away from a U.S. dollar focus of systemically important emerging market economies (EMEs) on configurations between the U.S. dollar, the euro and the yen. Given the difficulty that fixed or managed U.S. dollar exchange rate regimes remain pervasive and reserve compositions mostly kept secret, the identification strategy of the paper is to analyse the market impact on major currency pairs of official statements made by EME policy-makers about their exchange rate regime and reserve composition. Developing a novel database for 18 EMEs, we find that such statements not only have a statistically but also an economically significant impact on the euro, and to a lesser extent the yen against the U.S. dollar. The findings suggest that communication hinting at a weakening of EMEs' U.S. dollar focus contributed substantially to the appreciation of the euro against the U.S. dollar in recent years. Interestingly, EME policy-makers appear to have become more cautious in their communication more recently. Overall, the results underscore the growing systemic importance of EMEs for global exchange rate configurations.Foreign exchange rates ; Monetary policy ; International finance ; Financial markets

Applications of a tight-binding total energy method for transition and noble metals: Elastic Constants, Vacancies, and Surfaces of Monatomic Metals

A recent tight-binding scheme provides a method for extending the results of first principles calculations to regimes involving $10^2 - 10^3$ atoms in a unit cell. The method uses an analytic set of two-center, non-orthogonal tight-binding parameters, on-site terms which change with the local environment, and no pair potential. The free parameters in this method are chosen to simultaneously fit band structures and total energies from a set of first-principles calculations for monatomic fcc and bcc crystals. To check the accuracy of this method we evaluate structural energy differences, elastic constants, vacancy formation energies, and surface energies, comparing to first-principles calculations and experiment. In most cases there is good agreement between this theory and experiment. We present a detailed account of the method, a complete set of tight-binding parameters, and results for twenty-nine of the alkaline earth, transition and noble metals.Comment: 24 pages (REVTEX), 6 figures (epsf.tex required). Several new results have been added. Re-submitted to Physical Review