8,185 research outputs found
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 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 subspace. Spin-orbit
interactions act nontrivially on the 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 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-Ă -vis major floating currencies, but not vis-Ă -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-Ă -vis major floating currencies. Last, the paper detects signs of nonlinearities in UIP at the mediumterm horizon for dollar rates vis-Ă -vis some of the major floating currencies, albeit surrounded by some uncertainty. JEL Classification: E43, F31, F41distant horizon, emerging economies, Nonlinearities, Uncovered interest parity
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