985 research outputs found
Engineering Dynamical Sweet Spots to Protect Qubits from 1/ Noise
Protecting superconducting qubits from low-frequency noise is essential for
advancing superconducting quantum computation. Based on the application of a
periodic drive field, we develop a protocol for engineering dynamical sweet
spots which reduce the susceptibility of a qubit to low-frequency noise. Using
the framework of Floquet theory, we prove rigorously that there are manifolds
of dynamical sweet spots marked by extrema in the quasi-energy differences of
the driven qubit. In particular, for the example of fluxonium biased slightly
away from half a flux quantum, we predict an enhancement of pure-dephasing by
three orders of magnitude. Employing the Floquet eigenstates as the
computational basis, we show that high-fidelity single- and two-qubit gates can
be implemented while maintaining dynamical sweet-spot operation. We further
confirm that qubit readout can be performed by adiabatically mapping the
Floquet states back to the static qubit states, and subsequently applying
standard measurement techniques. Our work provides an intuitive tool to encode
quantum information in robust, time-dependent states, and may be extended to
alternative architectures for quantum information processing
Arginine interactions with anatase TiO2 (100) surface and the perturbation of 49Ti NMR chemical shifts – a DFT investigation: relevance to Renu-Seeram bio solar cell
Density functional theoretical calculations have been utilized to investigate the interaction of the amino acid arginine with the (100) surface of anatase and the reproduction of experimentally measured 49Ti NMR chemical shifts of anatase. Significant binding of arginine through electrostatic interaction and hydrogen bonds of the arginine guanidinium protons to the TiO2 surface oxygen atoms is observed, allowing attachment of proteins to titania surfaces in the construction of bio-sensitized solar cells. GIAO-B3LYP/6-31G(d) NMR calculation of a three-layer model based on the experimental structure of this TiO2 modification gives an excellent reproduction of the experimental value (-927 ppm) within +/- 7 ppm, however, the change in relative chemical shifts, EFGs and CSA suggest that the effect of the electrostatic arginine binding might be too small for experimental detection
Variable Resolution Sampling and Deep Learning-Based Image Recovery for Faster Multi-Spectral Imaging Near Metal Implants
Purpose: In multi-spectral imaging (MSI), several fast spin echo volumes with
discrete Larmor frequency offsets are acquired in an interleaved fashion with
multiple concatenations. Here, a variable resolution (VR) method to nearly
halve scan time is proposed by only acquiring low resolution autocalibrating
signal in half of the concatenations.
Methods: Knee MSI datasets were retrospectively undersampled with the
proposed variable resolution sampling scheme. A U-Net model was trained to
predict the full-resolution images from the VR input. Image quality was
assessed in 10 test subjects.
Results: Spectral bin-combined images produced with the proposed variable
resolution sampling with deep learning reconstruction appear to be of high
quality and exhibited a median structural image similarity of 0.984 across test
subjects and slices.
Conclusion: The proposed variable resolution sampling method shows promise
for drastically reducing the time it takes to collect multi-spectral imaging
data near metallic implants. Further studies will rigorously examine its
clinical utility across multiple implant scenarios
On Functional Activations in Deep Neural Networks
Background: Deep neural networks have proven to be powerful computational
tools for modeling, prediction, and generation. However, the workings of these
models have generally been opaque. Recent work has shown that the performance
of some models are modulated by overlapping functional networks of connections
within the models. Here the techniques of functional neuroimaging are applied
to an exemplary large language model to probe its functional structure.
Methods: A series of block-designed task-based prompt sequences were generated
to probe the Facebook Galactica-125M model. Tasks included prompts relating to
political science, medical imaging, paleontology, archeology, pathology, and
random strings presented in an off/on/off pattern with prompts about other
random topics. For the generation of each output token, all layer output values
were saved to create an effective time series. General linear models were fit
to the data to identify layer output values which were active with the tasks.
Results: Distinct, overlapping networks were identified with each task. Most
overlap was observed between medical imaging and pathology networks. These
networks were repeatable across repeated performance of related tasks, and
correspondence of identified functional networks and activation in tasks not
used to define the functional networks was shown to accurately identify the
presented task. Conclusion: The techniques of functional neuroimaging can be
applied to deep neural networks as a means to probe their workings. Identified
functional networks hold the potential for use in model alignment, modulation
of model output, and identifying weights to target in fine-tuning
Floquet-engineered enhancement of coherence times in a driven fluxonium qubit
We use the quasienergy structure that emerges when a fluxonium
superconducting circuit is driven periodically to encode quantum information
with dynamically induced flux-insensitive sweet spots. The framework of Floquet
theory provides an intuitive description of these high-coherence working points
located away from the half-flux symmetry point of the undriven qubit. This
approach offers flexibility in choosing the flux bias point and the energy of
the logical qubit states as shown in [\textit{Huang et al., 2020}]. We
characterize the response of the system to noise in the modulation amplitude
and DC flux bias, and experimentally demonstrate an optimal working point which
is simultaneously insensitive against fluctuations in both. We observe a
40-fold enhancement of the qubit coherence times measured with Ramsey-type
interferometry at the dynamical sweet spot compared with static operation at
the same bias point.Comment: 12 pages, 7 figure
Time-reversal symmetry breaking in circuit-QED based photon lattices
Breaking time-reversal symmetry is a prerequisite for accessing certain
interesting many-body states such as fractional quantum Hall states. For
polaritons, charge neutrality prevents magnetic fields from providing a direct
symmetry breaking mechanism and similar to the situation in ultracold atomic
gases, an effective magnetic field has to be synthesized. We show that in the
circuit QED architecture, this can be achieved by inserting simple
superconducting circuits into the resonator junctions. In the presence of such
coupling elements, constant parallel magnetic and electric fields suffice to
break time-reversal symmetry. We support these theoretical predictions with
numerical simulations for realistic sample parameters, specify general
conditions under which time-reversal is broken, and discuss the application to
chiral Fock state transfer, an on-chip circulator, and tunable band structure
for the Kagome lattice.Comment: minor revisions, version published in PRA; 19 pages, 13 figures, 2
table
The Association Between Persistent White-Matter Abnormalities and Repeat Injury After Sport-Related Concussion
Objective: A recent systematic review determined that the physiological effects of concussion may persist beyond clinical recovery. Preclinical models suggest that ongoing physiological effects are accompanied by increased cerebral vulnerability that is associated with risk for subsequent, more severe injury. This study examined the association between signal alterations on diffusion tensor imaging following clinical recovery of sport-related concussion in athletes with and without a subsequent second concussion. Methods: Average mean diffusivity (MD) was calculated in a region of interest (ROI) in which concussed athletes (n = 82) showed significantly elevated MD acutely after injury (<48 h), at an asymptomatic time point, 7 days post-return to play (RTP), and 6 months relative to controls (n = 69). The relationship between MD in the identified ROI and likelihood of sustaining a subsequent concussion over a 1-year period was examined with a binary logistic regression (re-injured, yes/no). Results: Eleven of 82 concussed athletes (13.4%) sustained a second concussion within 12 months of initial injury. Mean MD at 7 days post-RTP was significantly higher in those athletes who went on to sustain a repeat concussion within 1 year of initial injury than those who did not (p = 0.048; d = 0.75). In this underpowered sample, the relationship between MD at 7 days post-RTP and likelihood of sustaining a secondary injury approached significance [χ2 (1) = 4.17, p = 0.057; B = 0.03, SE = 0.017; OR = 1.03, CI = 0.99, 1.07]. Conclusions: These preliminary findings raise the hypothesis that persistent signal abnormalities in diffusion imaging metrics at RTP following concussion may be predictive of a repeat concussion. This may reflect a window of cerebral vulnerability or increased susceptibility following concussion, though understanding the clinical significance of these findings requires further study
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