54 research outputs found
High-impedance surface acoustic wave resonators
Because of their small size, low loss, and compatibility with magnetic fields
and elevated temperatures, surface acoustic wave resonators hold significant
potential as future quantum interconnects. Here, we design, fabricate, and
characterize GHz-frequency surface acoustic wave resonators with the potential
for strong capacitive coupling to nanoscale solid-state quantum systems,
including semiconductor quantum dots. Strong capacitive coupling to such
systems requires a large characteristic impedance, and the resonators we
fabricate have impedance values above 100 . We achieve such high
impedance values by tightly confining a Gaussian acoustic mode. At the same
time, the resonators also have low loss, with quality factors of several
thousand at millikelvin temperatures. These high-impedance resonators are
expected to exhibit large vacuum electric-field fluctuations and have the
potential for strong coupling to a variety of solid-state quantum systems
Coherent multi-spin exchange coupling in a quantum-dot spin chain
Heisenberg exchange coupling between neighboring electron spins in
semiconductor quantum dots provides a powerful tool for quantum information
processing and simulation. Although so far unrealized, extended Heisenberg spin
chains can enable long-distance quantum information transfer and the generation
of non-equilibrium quantum states. In this work, we implement simultaneous,
coherent exchange coupling between all nearest-neighbor pairs of spins in a
quadruple quantum dot. The main challenge in implementing simultaneous exchange
couplings is the nonlinear and nonlocal dependence of the exchange couplings on
gate voltages. Through a combination of electrostatic simulation and
theoretical modeling, we show that this challenge arises primarily due to
lateral shifts of the quantum dots during gate pulses. Building on this
insight, we develop two models, which can be used to predict the confinement
gate voltages for a desired set of exchange couplings. Although the model
parameters depend on the number of exchange couplings desired (suggesting that
effects in addition to lateral wavefunction shifts are important), the models
are sufficient to enable simultaneous and independent control of all three
exchange couplings in a quadruple quantum dot. We demonstrate two-, three-, and
four-spin exchange oscillations, and our data agree with simulations.Comment: 9+10 pages, 5+5 figure
Conditional Teleportation of Quantum-Dot Spin States
Among the different platforms for quantum information processing, individual
electron spins in semiconductor quantum dots stand out for their long coherence
times and potential for scalable fabrication. The past years have witnessed
substantial progress in the capabilities of spin qubits. However, coupling
between distant electron spins, which is required for quantum error correction,
presents a challenge, and this goal remains the focus of intense research.
Quantum teleportation is a canonical method to transmit qubit states, but it
has not been implemented in quantum-dot spin qubits. Here, we present evidence
for quantum teleportation of electron spin qubits in semiconductor quantum
dots. Although we have not performed quantum state tomography to definitively
assess the teleportation fidelity, our data are consistent with conditional
teleportation of spin eigenstates, entanglement swapping, and gate
teleportation. Such evidence for all-matter spin-state teleportation
underscores the capabilities of exchange-coupled spin qubits for
quantum-information transfer.Comment: 12+27 pages, 4+15 figure
Global, regional, and national sex-specific burden and control of the HIV epidemic, 1990–2019, for 204 countries and territories: the Global Burden of Diseases Study 2019
Background: The sustainable development goals (SDGs) aim to end HIV/AIDS as a public health threat by 2030. Understanding the current state of the HIV epidemic and its change over time is essential to this effort. This study assesses the current sex-specific HIV burden in 204 countries and territories and measures progress in the control of the epidemic. Methods: To estimate age-specific and sex-specific trends in 48 of 204 countries, we extended the Estimation and Projection Package Age-Sex Model to also implement the spectrum paediatric model. We used this model in cases where age and sex specific HIV-seroprevalence surveys and antenatal care-clinic sentinel surveillance data were available. For the remaining 156 of 204 locations, we developed a cohort-incidence bias adjustment to derive incidence as a function of cause-of-death data from vital registration systems. The incidence was input to a custom Spectrum model. To assess progress, we measured the percentage change in incident cases and deaths between 2010 and 2019 (threshold >75% decline), the ratio of incident cases to number of people living with HIV (incidence-to-prevalence ratio threshold <0·03), and the ratio of incident cases to deaths (incidence-to-mortality ratio threshold <1·0). Findings: In 2019, there were 36·8 million (95% uncertainty interval [UI] 35·1–38·9) people living with HIV worldwide. There were 0·84 males (95% UI 0·78–0·91) per female living with HIV in 2019, 0·99 male infections (0·91–1·10) for every female infection, and 1·02 male deaths (0·95–1·10) per female death. Global progress in incident cases and deaths between 2010 and 2019 was driven by sub-Saharan Africa (with a 28·52% decrease in incident cases, 95% UI 19·58–35·43, and a 39·66% decrease in deaths, 36·49–42·36). Elsewhere, the incidence remained stable or increased, whereas deaths generally decreased. In 2019, the global incidence-to-prevalence ratio was 0·05 (95% UI 0·05–0·06) and the global incidence-to-mortality ratio was 1·94 (1·76–2·12). No regions met suggested thresholds for progress. Interpretation: Sub-Saharan Africa had both the highest HIV burden and the greatest progress between 1990 and 2019. The number of incident cases and deaths in males and females approached parity in 2019, although there remained more females with HIV than males with HIV. Globally, the HIV epidemic is far from the UNAIDS benchmarks on progress metrics. Funding: The Bill & Melinda Gates Foundation, the National Institute of Mental Health of the US National Institutes of Health (NIH), and the National Institute on Aging of the NIH
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