160 research outputs found
A spectroscopic study of the cycling transition 4s[3/2]_2-4p[5/2]_3 at 811.8 nm in Ar-39: Hyperfine structure and isotope shift
Doppler-free saturated absorption spectroscopy is performed on an enriched
radioactive Ar-39 sample. The spectrum of the 3s^2 3p^5 4s [3/2]_2 - 3s^2 3p^5
4p [5/2]_3 cycling transition at 811.8 nm is recorded, and its isotope shift
between Ar-39 and Ar-40 is derived. The hyperfine coupling constants A and B
for both the 4s [3/2]_2 and 4p [5/2]_3 energy levels in Ar-39 are also
determined. The results partially disagree with a recently published
measurement of the same transition. Based on earlier measurements as well as
the current work, the isotope shift and hyperfine structure of the
corresponding transition in Ar-37 are also calculated. These spectroscopic data
are essential for the realization of laser trapping and cooling of Ar-37 and
Ar-39
Spectroscopic study of the cycling transition 4s[3/2]2-4p[5/2] 3 at 811.8 nm in Ar39: Hyperfine structure and isotope shift
Doppler-free saturated absorption spectroscopy is performed on an enriched radioactive Ar39 sample. The spectrum of the 3s23p54s[3/2]2- 3s23p54p[5/2]3 cycling transition at 811.8 nm is recorded, and its isotope shift between Ar39 and Ar40 is derived. The hyperfine coupling constants A and B for both the 4s[3/2]2 and 4p[5/2]3 energy levels in Ar39 are also determined. The results partially disagree with a recently published measurement of the same transition. Based on earlier measurements as well as the current work, the isotope shift and hyperfine structure of the corresponding transition in Ar37 are also calculated. These spectroscopic data are essential for the realization of laser trapping and cooling of Ar37,39. © 2011 American Physical Society
Colloidal particles at a nematic-isotropic interface: effects of confinement
When captured by a flat nematic-isotropic interface, colloidal particles can
be dragged by it. As a result spatially periodic structures may appear, with
the period depending on a particle mass, size, and interface
velocity~\cite{west.jl:2002}. If liquid crystal is sandwiched between two
substrates, the interface takes a wedge-like shape, accommodating the
interface-substrate contact angle and minimizing the director distortions on
its nematic side. Correspondingly, particles move along complex trajectories:
they are first captured by the interface and then `glide' towards its vertex
point. Our experiments quantify this scenario, and numerical minimization of
the Landau-de Gennes free energy allow for a qualitative description of the
interfacial structure and the drag force.Comment: 7 pages, 9 figure
Two-Qubit Gate Set Tomography with Fewer Circuits
Gate set tomography (GST) is a self-consistent and highly accurate method for
the tomographic reconstruction of a quantum information processor's quantum
logic operations, including gates, state preparations, and measurements.
However, GST's experimental cost grows exponentially with qubit number. For
characterizing even just two qubits, a standard GST experiment may have tens of
thousands of circuits, making it prohibitively expensive for platforms. We show
that, because GST experiments are massively overcomplete, many circuits can be
discarded. This dramatically reduces GST's experimental cost while still
maintaining GST's Heisenberg-like scaling in accuracy. We show how to exploit
the structure of GST circuits to determine which ones are superfluous. We
confirm the efficacy of the resulting experiment designs both through numerical
simulations and via the Fisher information for said designs. We also explore
the impact of these techniques on the prospects of three-qubit GST.Comment: 46 pages, 13 figures. V2: Minor edits to acknowledgment
Benchmarking quantum logic operations relative to thresholds for fault tolerance
Contemporary methods for benchmarking noisy quantum processors typically
measure average error rates or process infidelities. However, thresholds for
fault-tolerant quantum error correction are given in terms of worst-case error
rates -- defined via the diamond norm -- which can differ from average error
rates by orders of magnitude. One method for resolving this discrepancy is to
randomize the physical implementation of quantum gates, using techniques like
randomized compiling (RC). In this work, we use gate set tomography to perform
precision characterization of a set of two-qubit logic gates to study RC on a
superconducting quantum processor. We find that, under RC, gate errors are
accurately described by a stochastic Pauli noise model without coherent errors,
and that spatially-correlated coherent errors and non-Markovian errors are
strongly suppressed. We further show that the average and worst-case error
rates are equal for randomly compiled gates, and measure a maximum worst-case
error of 0.0197(3) for our gate set. Our results show that randomized
benchmarks are a viable route to both verifying that a quantum processor's
error rates are below a fault-tolerance threshold, and to bounding the failure
rates of near-term algorithms, if -- and only if -- gates are implemented via
randomization methods which tailor noise
Consistency of high-fidelity two-qubit operations in silicon
The consistency of entangling operations between qubits is essential for the
performance of multi-qubit systems, and is a crucial factor in achieving
fault-tolerant quantum processors. Solid-state platforms are particularly
exposed to inconsistency due to the materials-induced variability of
performance between qubits and the instability of gate fidelities over time.
Here we quantify this consistency for spin qubits, tying it to its physical
origins, while demonstrating sustained and repeatable operation of two-qubit
gates with fidelities above 99% in the technologically important silicon
metal-oxide-semiconductor (SiMOS) quantum dot platform. We undertake a detailed
study of the stability of these operations by analysing errors and fidelities
in multiple devices through numerous trials and extended periods of operation.
Adopting three different characterisation methods, we measure entangling gate
fidelities ranging from 96.8% to 99.8%. Our analysis tools also identify
physical causes of qubit degradation and offer ways to maintain performance
within tolerance. Furthermore, we investigate the impact of qubit design,
feedback systems, and robust gates on implementing scalable, high-fidelity
control strategies. These results highlight both the capabilities and
challenges for the scaling up of spin-based qubits into full-scale quantum
processors
Amino acids and peptides. LVIII. Cyclisation of peptides with 2-ethyl-5-phenylisoxazolium-3'-sulphonate
Amino acids and peptides. XLVII. Rates of fission of some substituted benzyloxycarbonylglycines and two heterocyclic analogues with hydrogen bromide in acetic acid
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