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

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

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

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

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

On the structure of the Nx phase of symmetric dimers: inferences from NMR

NMR measurements on a selectively deuterated liquid crystal dimer CB-C9-CB, exhibiting two nematic phases, show that the molecules in the lower temperature nematic phase, NX, experience a chiral environment and are ordered about a uniformly oriented director throughout the macroscopic sample. The results are contrasted with previous interpretations that suggested a twist-bend spatial variation of the director. A structural picture is proposed wherein the molecules are packed into highly correlated chiral assemblies

The determinants of "domestic" original sin in emerging market economies

This paper explains why domestic debt composition in some emerging economies is risky. To this end, it carries out a systematic analysis of the determinants of the so-called domestic original sin, which refers to the inability of emerging economies to borrow domestically in local currency, at long maturities and fixed interest rates. As such, the latter is a measure of financial vulnerabilities arising from domestic debt composition, which encompasses maturity mismatches, rollover risk and interest payment contingency. The paper builds on a large dataset compiled by the authors from national sources. It finds that domestic original sin is particularly severe when inflation is lofty, the debt service-to-GDP ration high, the slope of the yield curve inverted and the investor base narrow. These results suggest that sound macroeconomic policies, attractive long-term yields and policies aimed at widening the investor base are instrumental to overcome domestic original sin, reduce domestic debt riskiness and tilt its composition towards safer, long-term, unindexed, local currency instrumentsOriginal sin, domestic debt, emerging economies

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

Uncovered interest parity at distant horizons: evidence on emerging economies & nonlinearities

This paper tests for uncovered interest parity (UIP) at distant horizons for the US and its main trading partners, including both mature and emerging market economies, also exploring the existence of nonlinearities. At long and medium horizons, it finds support in favour of the standard, linear, specification of UIP for dollar rates vis-à-vis major floating currencies, but not vis-à-vis emerging market currencies. Moreover, the paper finds evidence that, not only yield differentials widen, but that US bond yields do react in anticipation of exchange rate movements, notably when these take place vis-à-vis major floating currencies. Last, the paper detects signs of nonlinearities in UIP at the mediumterm horizon for dollar rates vis-à-vis some of the major floating currencies, albeit surrounded by some uncertainty. JEL Classification: E43, F31, F41distant horizon, emerging economies, Nonlinearities, Uncovered interest parity