5,385 research outputs found
From Forbidden Coronal Lines to Meaningful Coronal Magnetic Fields
We review methods to measure magnetic fields within the corona using the
polarized light in magnetic-dipole (M1) lines. We are particularly interested
in both the global magnetic-field evolution over a solar cycle, and the local
storage of magnetic free energy within coronal plasmas. We address commonly
held skepticisms concerning angular ambiguities and line-of-sight confusion. We
argue that ambiguities are in principle no worse than more familiar remotely
sensed photospheric vector-fields, and that the diagnosis of M1 line data would
benefit from simultaneous observations of EUV lines. Based on calculations and
data from eclipses, we discuss the most promising lines and different
approaches that might be used. We point to the S-like [Fe {\sc XI}] line (J=2
to J=1) at 789.2nm as a prime target line (for ATST for example) to augment the
hotter 1074.7 and 1079.8 nm Si-like lines of [Fe {\sc XIII}] currently observed
by the Coronal Multi-channel Polarimeter (CoMP). Significant breakthroughs will
be made possible with the new generation of coronagraphs, in three distinct
ways: (i) through single point inversions (which encompasses also the analysis
of MHD wave modes), (ii) using direct comparisons of synthetic MHD or
force-free models with polarization data, and (iii) using tomographic
techniques.Comment: Accepted by Solar Physics, April 201
Bypassing the energy-time uncertainty in time-resolved photoemission
The energy-time uncertainty is an intrinsic limit for time-resolved
experiments imposing a tradeoff between the duration of the light pulses used
in experiments and their frequency content. In standard time-resolved
photoemission, this limitation maps directly onto a tradeoff between the time
resolution of the experiment and the energy resolution that can be achieved on
the electronic spectral function. Here we propose a protocol to disentangle the
energy and time resolutions in photoemission. We demonstrate that dynamical
information on all time scales can be retrieved from time-resolved
photoemission experiments using suitably shaped light pulses of quantum or
classical nature. As a paradigmatic example, we study the dynamical buildup of
the Kondo peak, a narrow feature in the electronic response function arising
from the screening of a magnetic impurity by the conduction electrons. After a
quench, the electronic screening builds up on timescales shorter than the
inverse width of the Kondo peak and we demonstrate that the proposed
experimental scheme could be used to measure the intrinsic time scales of such
electronic screening. The proposed approach provides an experimental framework
to access the nonequilibrium response of collective electronic properties
beyond the spectral uncertainty limit and will enable the direct measurement of
phenomena such as excited Higgs modes and, possibly, the retarded interactions
in superconducting systems.Comment: Extended introduction, added references to section IIB, improved
wording in section II
Maximum information photoelectron metrology
Photoelectron interferograms, manifested in photoelectron angular
distributions (PADs), are a high-information, coherent observable. In order to
obtain the maximum information from angle-resolved photoionization experiments
it is desirable to record the full, 3D, photoelectron momentum distribution.
Here we apply tomographic reconstruction techniques to obtain such 3D
distributions from multiphoton ionization of potassium atoms, and fully analyse
the energy and angular content of the 3D data. The PADs obtained as a function
of energy indicate good agreement with previous 2D data and detailed analysis
[Hockett et. al., Phys. Rev. Lett. 112, 223001 (2014)] over the main spectral
features, but also indicate unexpected symmetry-breaking in certain regions of
momentum space, thus revealing additional continuum interferences which cannot
otherwise be observed. These observations reflect the presence of additional
ionization pathways and, most generally, illustrate the power of maximum
information measurements of this coherent observable
Characterization of a two-transmon processor with individual single-shot qubit readout
We report the characterization of a two-qubit processor implemented with two
capacitively coupled tunable superconducting qubits of the transmon type, each
qubit having its own non-destructive single-shot readout. The fixed capacitive
coupling yields the \sqrt{iSWAP} two-qubit gate for a suitable interaction
time. We reconstruct by state tomography the coherent dynamics of the two-bit
register as a function of the interaction time, observe a violation of the Bell
inequality by 22 standard deviations after correcting readout errors, and
measure by quantum process tomography a gate fidelity of 90%
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