162 research outputs found
Analysis and mitigation of residual exchange coupling in linear spin qubit arrays
In recent advancements of quantum computing utilizing spin qubits, it has
been demonstrated that this platform possesses the potential for implementing
two-qubit gates with fidelities exceeding 99.5%. However, as with other qubit
platforms, it is not feasible to completely turn qubit couplings off. This
study aims to investigate the impact of coherent error matrices in gate set
tomography by employing a double quantum dot. We evaluate the infidelity caused
by residual exchange between spins and compare various mitigation approaches,
including the use of adjusted timing through simple drives, considering
different parameter settings in the presence of charge noise. Furthermore, we
extend our analysis to larger arrays of exchange-coupled spin qubits to provide
an estimation of the expected fidelity. In particular, we demonstrate the
influence of residual exchange on a single-qubit gate and the native
two-qubit SWAP gate in a linear chain. Our findings emphasize the significance
of accounting for residual exchange when scaling up spin qubit devices and
highlight the tradeoff between the effects of charge noise and residual
exchange in mitigation techniques.Comment: 15 pages, 8 figure
Second Quantization: Gating a Quantum Dot Through the Sequential Removal of Single Electrons from a Nanoscale Floating Gate
We use the tip of an atomic force microscope (AFM) to charge floating
metallic gates defined on the surface of a Si/SiGe heterostructure. The AFM tip
serves as an ideal and movable cryogenic switch, allowing us to bias a floating
gate to a specific voltage and then lock the charge on the gate by withdrawing
the tip. Biasing with an AFM tip allows us to reduce the size of a quantum dot
floating gate electrode down to . Measurements of the
conductance through a quantum dot formed beneath the floating gate indicate
that its charge changes in discrete steps. From the statistics of the
single-electron leakage events, we determine the floating gate leakage
resistance - a value immeasurable by
conventional means
Dispersive readout of a silicon quantum device using an atomic force microscope-based rf gate sensor
We demonstrate dispersive charge sensing of Si/SiGe single and double quantum
dots (DQD) by coupling sub-micron floating gates to a radio frequency
reflectometry (rf-reflectometry) circuit using the tip of an atomic force
microscope (AFM). Charge stability diagrams are obtained in the phase response
of the reflected rf signal. We demonstrate single-electron dot-to-lead and
dot-to-dot charge transitions with a signal-to-noise ratio (SNR) of 2 and
integration time of and ,
respectively. The charge sensing SNR compares favorably with results obtained
on conventional devices. Moreover, the small size of the floating gates largely
eliminates the coupling to parasitic charge traps that can complicate the
interpretation of the dispersive charge sensing data
Microwave-frequency scanning gate microscopy of a Si/SiGe double quantum dot
Conventional quantum transport methods can provide quantitative information
on spin, orbital, and valley states in quantum dots, but often lack spatial
resolution. Scanning tunneling microscopy, on the other hand, provides
exquisite spatial resolution of the local electronic density of states, but
often at the expense of speed. Working to combine the spatial resolution and
energy sensitivity of scanning probe microscopy with the speed of microwave
measurements, we couple a metallic probe tip to a Si/SiGe double quantum dot
that is integrated with a local charge detector. We first demonstrate that a
dc-biased tip can be used to change the charge occupancy of the double dot. We
then apply microwave excitation through the scanning tip to drive
photon-assisted tunneling transitions in the double dot. We infer the double
dot energy level diagram from the frequency and detuning dependence of the
photon-assisted tunneling resonance condition. These measurements allow us to
resolve 65 eV excited states, an energy scale consistent with
typical valley splittings in Si/SiGe. Future extensions of this approach may
allow spatial mapping of the valley splitting in Si devices, which is of
fundamental importance for spin-based quantum processors
Obeticholic acid for the treatment of non-alcoholic steatohepatitis: interim analysis from a multicentre, randomised, placebo-controlled phase 3 trial
Background Non-alcoholic steatohepatitis (NASH) is a common type of chronic liver disease that can lead to cirrhosis. Obeticholic acid, a farnesoid X receptor agonist, has been shown to improve the histological features of NASH. Here we report results from a planned interim analysis of an ongoing, phase 3 study of obeticholic acid for NASH. Methods In this multicentre, randomised, double-blind, placebo-controlled study, adult patients with definite NASH,non-alcoholic fatty liver disease (NAFLD) activity score of at least 4, and fibrosis stages F2–F3, or F1 with at least oneaccompanying comorbidity, were randomly assigned using an interactive web response system in a 1:1:1 ratio to receive oral placebo, obeticholic acid 10 mg, or obeticholic acid 25 mg daily. Patients were excluded if cirrhosis, other chronic liver disease, elevated alcohol consumption, or confounding conditions were present. The primary endpointsfor the month-18 interim analysis were fibrosis improvement (≥1 stage) with no worsening of NASH, or NASH resolution with no worsening of fibrosis, with the study considered successful if either primary endpoint was met. Primary analyses were done by intention to treat, in patients with fibrosis stage F2–F3 who received at least one dose of treatment and reached, or would have reached, the month 18 visit by the prespecified interim analysis cutoff date. The study also evaluated other histological and biochemical markers of NASH and fibrosis, and safety. This study is ongoing, and registered with ClinicalTrials.gov, NCT02548351, and EudraCT, 20150-025601-6. Findings Between Dec 9, 2015, and Oct 26, 2018, 1968 patients with stage F1–F3 fibrosis were enrolled and received at least one dose of study treatment; 931 patients with stage F2–F3 fibrosis were included in the primary analysis (311 in the placebo group, 312 in the obeticholic acid 10 mg group, and 308 in the obeticholic acid 25 mg group). The fibrosis improvement endpoint was achieved by 37 (12%) patients in the placebo group, 55 (18%) in the obeticholic acid 10 mg group (p=0·045), and 71 (23%) in the obeticholic acid 25 mg group (p=0·0002). The NASH resolution endpoint was not met (25 [8%] patients in the placebo group, 35 [11%] in the obeticholic acid 10 mg group [p=0·18], and 36 [12%] in the obeticholic acid 25 mg group [p=0·13]). In the safety population (1968 patients with fibrosis stages F1–F3), the most common adverse event was pruritus (123 [19%] in the placebo group, 183 [28%] in the obeticholic acid 10 mg group, and 336 [51%] in the obeticholic acid 25 mg group); incidence was generally mild to moderate in severity. The overall safety profile was similar to that in previous studies, and incidence of serious adverse events was similar across treatment groups (75 [11%] patients in the placebo group, 72 [11%] in the obeticholic acid 10 mg group, and 93 [14%] in the obeticholic acid 25 mg group). Interpretation Obeticholic acid 25 mg significantly improved fibrosis and key components of NASH disease activity among patients with NASH. The results from this planned interim analysis show clinically significant histological improvement that is reasonably likely to predict clinical benefit. This study is ongoing to assess clinical outcomes
Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions
We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe
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