283 research outputs found
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
Two-Qubit Couplings of Singlet-Triplet Qubits Mediated by One Quantum State
We describe high-fidelity entangling gates between singlet-triplet qubits
(STQs) which are coupled via one quantum state (QS). The QS can be provided by
a quantum dot itself or by another confined system. The orbital energies of the
QS are tunable using an electric gate close to the QS, which changes the
interactions between the STQs independent of their single-qubit parameters.
Short gating sequences exist for the controlled NOT (CNOT) operations. We show
that realistic quantum dot setups permit excellent entangling operations with
gate infidelities below , which is lower than the quantum error
correction threshold of the surface code. We consider limitations from
fabrication errors, hyperfine interactions, spin-orbit interactions, and charge
noise in GaAs and Si heterostructures.Comment: 12 pages, 6 figure
Validity of the single-particle description and charge noise resilience for multielectron quantum dots
We construct an optimal set of single-particle states for few-electron
quantum dots (QDs) using the method of natural orbitals (NOs). The NOs include
also the effects of the Coulomb repulsion between electrons. We find that they
agree well with the noniteracting orbitals for GaAs QDs of realistic
parameters, while the Coulomb interactions only rescale the radius of the NOs
compared to the noninteracting case. We use NOs to show that four-electron QDs
are less susceptible to charge noise than their two-electron counterparts.Comment: 11+ pages, 5 figure
Housing and Services for Homeless and At-Risk People: Newport\u27s Experiment
Large and small communities alike have diverse groups of people in need of housing and services: mental health patients, single homeless adults, individuals with substance abuse problems, the elderly, the mentally retarded, the unemployed, and people with low or fixed incomes. Even with unlimited resources to create necessary solutions, most communities would subscribe to the conventional wisdom of segregating people according to their major needs and managing the resulting environment with a combination of behavioral regulation and casework. In Newport, Rhode Island, an ambitious nonprofit housing organization decided to break those rules and provide a continuum of secure housing choices and support services under one roof
The Scalability-Efficiency/Maintainability-Portability Trade-off in Simulation Software Engineering: Examples and a Preliminary Systematic Literature Review
Large-scale simulations play a central role in science and the industry.
Several challenges occur when building simulation software, because simulations
require complex software developed in a dynamic construction process. That is
why simulation software engineering (SSE) is emerging lately as a research
focus. The dichotomous trade-off between scalability and efficiency (SE) on the
one hand and maintainability and portability (MP) on the other hand is one of
the core challenges. We report on the SE/MP trade-off in the context of an
ongoing systematic literature review (SLR). After characterizing the issue of
the SE/MP trade-off using two examples from our own research, we (1) review the
33 identified articles that assess the trade-off, (2) summarize the proposed
solutions for the trade-off, and (3) discuss the findings for SSE and future
work. Overall, we see evidence for the SE/MP trade-off and first solution
approaches. However, a strong empirical foundation has yet to be established;
general quantitative metrics and methods supporting software developers in
addressing the trade-off have to be developed. We foresee considerable future
work in SSE across scientific communities.Comment: 9 pages, 2 figures. Accepted for presentation at the Fourth
International Workshop on Software Engineering for High Performance Computing
in Computational Science and Engineering (SEHPCCSE 2016
Fault-Tolerant Quantum Computation for Singlet-Triplet Qubits with Leakage Errors
We describe and analyze leakage errors of singlet-triplet qubits. Even though
leakage errors are a natural problem for spin qubits encoded using quantum dot
arrays, they have obtained little attention in previous studies. We describe
the realization of leakage correction protocols that can be implemented
together with the quantum error correction protocol of the surface code.
Furthermore we construct explicit leakage reduction units that need, in the
ideal setup, as few as three manipulation steps. Our study shows that leakage
errors can be corrected without the need of measurements and at the cost of
only a few additional ancilla qubits and gate operations compared to standard
quantum error correction codes.Comment: 7+ pages, 5 figure
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