arXiv subjects

Qi Gao

Publications and source records attributed to Qi Gao.

At least 91 records · Page 5Linked to original sources

arXiv2021

Quantum-Classical Computational Molecular Design of Deuterated High-Efficiency OLED Emitters

This study describes a hybrid quantum-classical computational approach for designing synthesizable deuterated $Alq_3$ emitters possessing desirable emission quantum efficiencies (QEs). This design process has been performed on the tris(8-hydroxyquinolinato) ligands typically bound to aluminum in $Alq_3$. It involves a multi-pronged approach which first utilizes classical quantum chemistry to predict the emission QEs of the $Alq_3$ ligands. These initial results were then used as a machine learning dataset for a factorization machine-based model which was applied to construct an Ising Hamiltonian to predict emission quantum efficiencies on a classical computer. We show that such a factorization machine-based approach can yield accurate property predictions for all 64 deuterated $Alq_3$ emitters with 13 training values. Moreover, another Ising Hamiltonian could be constructed by including synthetic constraints which could be used to perform optimizations on a quantum simulator and device using the variational quantum eigensolver (VQE) and quantum approximate optimization algorithm (QAOA) to discover a molecule possessing the optimal QE and synthetic cost. We observe that both VQE and QAOA calculations can predict the optimal molecule with greater than 0.95 probability on quantum simulators. These probabilities decrease to 0.83 and 0.075 for simulations with VQE and QAOA, respectively, on a quantum device, but these can be improved to 0.90 and 0.084 by mitigating readout error. Application of a binary search routine on quantum devices improves these results to a probability of 0.97 for simulations involving VQE and QAOA.

quant-ph↗

arXiv2021

The complete control of scattering waves in multi-channel structures

The issue of photon spin Hall effect was generalized as a universal question of how to control all the scattering waves in a multi-channel structure (complete control). A general theory was proposed, which provides a simple way to achieve the complete control. This theory shows also that the necessary condition for complete control is that the structure must contain a complete set of sources. To demonstrate the application of the theory, the typical scattering patterns in the two-channel and four-channel structures are achieved theoretically. Previous this research, one could only artificially control the scattering waves in two channels out of a four-channel structure.

physics.optics↗

arXiv2021

Prospects for the detection of the prompt very-high-energy emission from $\rmγ$-ray bursts with the High Altitude Detection of Astronomical Radiation experiment

The observation of very-high-energy (VHE, $\rm >10~GeV$) $γ$-ray emission from $\rm γ$-ray bursts (GRBs), especially in the prompt phase, will provide critical information for understanding many aspects of their nature including the physical environment, the relativistic bulk motion, the mechanisms of particle acceleration of GRBs and for studying Lorentz invariance violation, etc. For the afterglow phase, the highest energy photons detected to date by the imaging atmospheric Cherenkov telescopes extend to the TeV regime. However, for the prompt phase, years of efforts in searching for the VHE emission has yielded no statistically significant detections. A wide field-of-view (FOV) and large effective area above tens of GeV are essential for detecting the VHE emissions from GRBs in the prompt phase. The High Altitude Detection of Astronomical Radiation (HADAR) experiment has such merits. In this paper, we report the estimates of its expected annual GRB detection rate, which are obtained by combining the performance of the HADAR instrument with the theoretical calculations based on a phenomenological model to generate the pseudo-GRB population. The expected detectable gamma-ray signal from GRBs above the background is then obtained to give the detection rate. In the spectral model, an extra component is assigned to every GRB event in addition to the Band function. The results indicate that if the energy of the cutoff due to internal absorption is higher than 50 GeV, the detection rate for GRBs for the HADAR experiment is approximately two or three GRBs per year, which varies slightly depending upon the characteristics of the extra component.

astro-ph.HE↗

arXiv2021

Exploring More When It Needs in Deep Reinforcement Learning

We propose a exploration mechanism of policy in Deep Reinforcement Learning, which is exploring more when agent needs, called Add Noise to Noise (AN2N). The core idea is: when the Deep Reinforcement Learning agent is in a state of poor performance in history, it needs to explore more. So we use cumulative rewards to evaluate which past states the agents have not performed well, and use cosine distance to measure whether the current state needs to be explored more. This method shows that the exploration mechanism of the agent's policy is conducive to efficient exploration. We combining the proposed exploration mechanism AN2N with Deep Deterministic Policy Gradient (DDPG), Soft Actor-Critic (SAC) algorithms, and apply it to the field of continuous control tasks, such as halfCheetah, Hopper, and Swimmer, achieving considerable improvement in performance and convergence speed.

cs.LG↗

arXiv2021

Particle reconstruction of volumetric particle image velocimetry with strategy of machine learning

Three-dimensional particle reconstruction with limited two-dimensional projections is an under-determined inverse problem that the exact solution is often difficult to be obtained. In general, approximate solutions can be obtained by iterative optimization methods. In the current work, a practical particle reconstruction method based on a convolutional neural network (CNN) with geometry-informed features is proposed. The proposed technique can refine the particle reconstruction from a very coarse initial guess of particle distribution generated by any traditional algebraic reconstruction technique (ART) based methods. Compared with available ART-based algorithms, the novel technique makes significant improvements in terms of reconstruction quality, {robustness to noises}, and at least an order of magnitude faster in the offline stage.

eess.IV↗

arXiv2021

Potential PeVatron supernova remnant G106.3+2.7 seen in the highest-energy gamma rays

Cosmic rays (protons and other atomic nuclei) are believed to gain energies of petaelectronvolts (PeV) and beyond at astrophysical particle accelerators called 'PeVatrons' inside our Galaxy. Although a characteristic feature of a PeVatron is expected to be a hard gamma-ray energy spectrum that extends beyond 100 teraelectronvolts (TeV) without a cutoff, none of the currently known sources exhibits such a spectrum due to the low maximum energy of accelerated cosmic rays or insufficient detector sensitivity around 100 TeV. Here we report the observation of gamma-ray emission from the supernova remnant G106.3+2.7 above 10 TeV. This work provides flux data points up to and above 100 TeV and indicates that the very-high-energy gamma-ray emission above 10 TeV is well correlated with a molecular cloud rather than the pulsar PSR J2229+6114. Regarding the gamma-ray emission mechanism of G106.3+2.7, this morphological feature appears to favor a hadronic origin via the π0 decay caused by accelerated relativistic protons over a leptonic one via the inverse-Compton scattering by relativistic electrons. Furthermore, we point out that an X-ray flux upper limit on the synchrotron spectrum would provide important information to firmly establish the hadronic scenario as the mechanism of particle acceleration at the source.

astro-ph.HE↗

arXiv2021

Lithium Evolution of Giant Stars Observed by LAMOST and Kepler

Mapping lithium evolution for evolved stars will provide restrictions and constraints on the fundamental stellar interior physical processes, which further shed light on our understanding of the theory of stellar structure and evolution. Based on a sample of 1,848 giants with known evolutionary phases and lithium abundances from the LAMOST-\kepler{} and LAMOST-\emph{K}2 fields, we construct mass-radius diagrams to characterize the evolutionary features of lithium. The stars at red giant branch (RGB) phase show natural depletion along with their stellar evolution, particularly, there is no obvious crowd stars with anomalously high Li abundances near the bump. Most of the low-mass stars reaching their zero-age sequence of core-helium-burning (ZAHeB) have Li abundances around $\sim1.0$\,dex, which show an increase of Li abundance by $\sim0.6$\,dex compared to the stars above the bump of RGB. This suggests the helium flash can be responsible for moderate Li production. While for super Li-rich stars, some special mechanisms should be considered during helium flash. Other scenarios, such as merger, could also be interpretations given the Li-rich stars can be found at anytime during the steady state phase of core He-burning. During the core He-burning (HeB) phase, there is no indication of obvious lithium depletion.

astro-ph.SR↗

arXiv2021

Chemical abundances of three new Ba stars from the Keck/HIRES spectra

Based on high resolution, high signal-to-noise (S/N) ratio spectra from Keck/HIRES, we have determined abundances of 20 elements for 18 Ba candidates. The parameter space of these stars are in the range of 4880 $\leq$ $\rm{T_{eff}}$ $\leq$ 6050 K, 2.56 $\leq$ log $g$ $\leq$ 4.53 dex and -0.27 $\leq$ [Fe/H] $\leq$ 0.09 dex. It is found that four of them can be identified as Ba stars with [s/Fe] $>$ 0.25 dex (s: Sr, Y, Zr, Ba, La, Ce and Nd), and three of them are newly discovered, which includes two Ba giants (HD 16178 and HD 22233) and one Ba subgiant (HD 2946). Our results show that the abundances of $α$, odd and iron-peak elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, Mn, Ni and Cu) for our program stars are similar to those of the thin disk, while the distribution of [hs/ls] (hs: Ba, La, Ce and Nd, ls: Sr, Y and Zr) ratios of our Ba stars is similar to those of the known Ba objects. None of the four Ba stars show clear enhancement in carbon including the known CH subgiant HD 4395. It is found that three of the Ba stars present clear evidences of hosting stellar or sub-stellar companions from the radial velocity data.

astro-ph.SR↗

arXiv2021

Protein-protein interactions enhance the thermal resilience of SpyRing enzymes: a molecular dynamic simulation study

Recently a technique based on the interaction between adhesion proteins extracted from Streptococcus pyogenes, known as SpyRing, has been widely used to improve the thermal resilience of enzymes, the assembly of biostructures, cancer cell recognition and other fields. In SpyRing, the two termini of the target enzyme are respectively linked to the peptide SpyTag and its protein partner SpyCatcher. SpyTag spontaneously reacts with SpyCatcher to form an isopeptide bond, with which the target enzyme forms a close ring structure. It was believed that the covalent cyclization of protein skeleton caused by SpyRing reduces the conformational entropy of biological structure and improves its rigidity, thus improving the thermal resilience of the target enzyme. However, the effects of SpyTag/ SpyCatcher interaction with this enzyme are poorly understood, and their regulation of enzyme properties remains unclear. Here, for simplicity, we took the single domain enzyme lichenase from Bacillus subtilis 168 as an example, studied the interface interactions in the SpyRing system by molecular dynamics simulations, and examined the effects of the changes of electrostatic interaction and van der Waals interaction on the thermal resilience of target enzyme. The simulations showed that the interface between SpyTag/SpyCatcher and lichenase is different from that found by geometric matching method and highlighted key mutations that affect the intensity of interactions at the interface and might have effect on the thermal resilience of the enzyme. Our calculations provided new insights into the rational designs in the SpyRing.

q-bio.BM↗

arXiv2021

Gamma-ray Observation of the Cygnus Region in the 100 TeV Energy Region

We report observations of gamma-ray emissions with energies in the 100 TeV energy region from the Cygnus region in our Galaxy. Two sources are significantly detected in the directions of the Cygnus OB1 and OB2 associations. Based on their positional coincidences, we associate one with a pulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN G75.2+0.1 with the pulsar moving away from its original birthplace situated around the centroid of the observed gamma-ray emission. This work would stimulate further studies of particle acceleration mechanisms at these gamma-ray sources.

astro-ph.HE↗

arXiv2021

The Lithium Abundances from the Large Sky Area Multi-object Fiber Spectroscopic Telescope Medium-resolution Survey. I. The Method

Standard stellar evolution model predicts a severe depletion of lithium (Li) abundance during the first dredge-up process (FDU). Yet a small fraction of giant stars are still found to preserve a considerable amount of Li in their atmospheres after FDU. Those giants are usually identified as Li-rich by a widely used criterion, A(Li) $ > 1.5$\,{\it dex}. A large number of works dedicated to search for and investigate this minority of the giant family, and the amount of Li-rich giants has been largely expanded, especially in the era of big data. In this paper, we present a catalog of Li-rich giants found from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey with Li abundances derived from a template matching method developed for LAMOST low-resolution spectra. The catalog contains $10,535$ Li-rich giants with Li abundances from $\sim 1.5$\,{\it dex} to $\sim 4.9$\,{\it dex}. We also confirm that the ratio of Li-rich phenomenon among giant stars is about one percent, or for a more expression, $1.29\%$ from our statistically important sample. This is the largest Li-rich giant sample ever reported to date, which significantly exceeds amount of all the reported Li-rich giants combined. The catalog will help the community to better understand the Li-rich phenomenon in giant stars.

astro-ph.SR↗

arXiv2021

First Detection of sub-PeV Diffuse Gamma Rays from the Galactic Disk: Evidence for Ubiquitous Galactic Cosmic Rays beyond PeV Energies

We report, for the first time, the long-awaited detection of diffuse gamma rays with energies between 100 TeV and 1 PeV in the Galactic disk. Particularly, all gamma rays above 398 TeV are observed apart from known TeV gamma-ray sources and compatible with expectations from the hadronic emission scenario in which gamma rays originate from the decay of $π^0$'s produced through the interaction of protons with the interstellar medium in the Galaxy. This is strong evidence that cosmic rays are accelerated beyond PeV energies in our Galaxy and spread over the Galactic disk.

astro-ph.HE↗

arXiv2021

Calculating transition amplitudes by variational quantum deflation

Variational quantum eigensolver (VQE) is an appealing candidate for the application of near-term quantum computers. A technique introduced in [Higgot et al., Quantum 3, 156 (2019)], which is named variational quantum deflation (VQD), has extended the ability of the VQE framework for finding excited states of a Hamiltonian. However, no method to evaluate transition amplitudes between the eigenstates found by the VQD without using any costly Hadamard-test-like circuit has been proposed despite its importance for computing properties of the system such as oscillator strengths of molecules. Here we propose a method to evaluate transition amplitudes between the eigenstates obtained by the VQD avoiding any Hadamard-test-like circuit. Our method relies only on the ability to estimate overlap between two states, so it does not restrict to the VQD eigenstates and applies for general situations. To support the significance of our method, we provide a comprehensive comparison of three previously proposed methods to find excited states with numerical simulation of three molecules (lithium hydride, diazene, and azobenzene) in a noiseless situation and find that the VQD method exhibits the best performance among the three methods. Finally, we demonstrate the validity of our method by calculating the oscillator strength of lithium hydride, comparing results from numerical simulations and real-hardware experiments on the cloud enabled quantum computer IBMQ Rome. Our results illustrate the superiority of the VQD to find excited states and widen its applicability to various quantum systems.

quant-ph↗

arXiv2020

LAMOST/HRS Spectroscopic Analysis of Two New Li-rich Giants

Two Li-rich candidates, TYC 1338-1410-1 and TYC 2825-596-1, were observed with the new high-resolution echelle spectrograph, LAMOST/HRS. Based on the high-resolution and high-signal-to-noise ratio (SNR) spectra, we derived stellar parameters and abundances of 14 important elements for the two candidates. The stellar parameters and lithium abundances indicate that they are Li-rich K-type giants, and they have A(Li)$_\mathrm{NLTE}$ of 1.77 and 2.91 dex, respectively. Our analysis suggests that TYC 1338-1410-1 is likely a red giant branch (RGB) star at the bump stage, while TYC 2825-596-1 is most likely a core helium-burning red clump (RC) star. The line profiles of both spectra indicate that the two Li-rich giants are slow rotators and do not show infrared (IR) excess. We conclude that engulfment is not the lithium enrichment mechanism for either star. The enriched lithium of TYC 1338-1410-1 could be created via Cameron-Fowler mechanism, while the lithium excess in TYC 2825-596-1 could be associated with either non-canonical mixing processes or He-flash.

astro-ph.SR↗

arXiv2020

Most Lithium-rich Low-mass Evolved Stars Revealed as Red Clump stars by Asteroseismology and Spectroscopy

Lithium has confused scientists for decades at almost each scale of the universe. Lithium-rich giants are peculiar stars with lithium abundances over model prediction. A large fraction of lithium-rich low-mass evolved stars are traditionally supposed to be red giant branch (RGB) stars. Recent studies, however, report that red clump (RC) stars are more frequent than RGB. Here, we present a uniquely large systematic study combining the direct asteroseismic analysis with the spectroscopy on the lithium-rich stars. The majority of lithium-rich stars are confirmed to be RCs, whereas RGBs are minor. We reveal that the distribution of lithium-rich RGBs steeply decline with the increasing lithium abundance, showing an upper limit around 2.6 dex, whereas the Li abundances of RCs extend to much higher values. We also find that the distributions of mass and nitrogen abundance are notably different between RC and RGB stars. These findings indicate that there is still unknown process that significantly affects surface chemical composition in low-mass stellar evolution.

astro-ph.SR↗

arXiv2020

Searching for r-process-enhanced stars in the LAMOST Survey I: The Method

The abundance patterns of $r$-process-enhanced stars contain key information required to constrain the astrophysical site(s) of $r$-process nucleosynthesis, and to deepen our understanding of the chemical evolution of our Galaxy. In order to expand the sample of known $r$-process-enhanced stars, we have developed a method to search for candidates in the LAMOST medium-resolution ($R \sim 7500$) spectroscopic survey by matching the observed spectra to synthetic templates around the Eu II line at 6645.1 AA. We obtain a sample of 13 metal-poor ($-2.35<{\rm [Fe/H]}<-0.91$) candidates from 12,209 unique stars with 32,774 medium-resolution spectra. These candidates will be further studied by high-resolution follow-up observations in the near future. Extensions of this effort to include larger samples of stars, in particular at lower metallicity, using the strength of the Ba II line at 6496.9 AA, are described.

astro-ph.SR↗

arXiv2020

Analysis and synthesis of feature map for kernel-based quantum classifier

A method for analyzing the feature map for the kernel-based quantum classifier is developed; that is, we give a general formula for computing a lower bound of the exact training accuracy, which helps us to see whether the selected feature map is suitable for linearly separating the dataset. We show a proof of concept demonstration of this method for a class of 2-qubit classifier, with several 2-dimensional dataset. Also, a synthesis method, that combines different kernels to construct a better-performing feature map in a lager feature space, is presented.

quant-ph↗

arXiv2020

Applications of Quantum Computing for Investigations of Electronic Transitions in Phenylsulfonyl-carbazole TADF Emitters

A quantum chemistry study of the first singlet (S1) and triplet (T1) excited states of phenylsulfonyl-carbazole compounds, proposed as useful thermally activated delayed fluorescence (TADF) emitters for organic light emitting diode (OLED) applications, was performed with the quantum Equation-Of-Motion Variational Quantum Eigensolver (qEOM-VQE) and Variational Quantum Deflation (VQD) algorithms on quantum simulators and devices. These quantum simulations were performed with double zeta quality basis sets on an active space comprising the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the TADF molecules. The differences in energy separations between S1 and T1 ($ΔE_{st}$) predicted by calculations on quantum simulators were found to be in excellent agreement with experimental data. Differences of 16 and 88 mHa with respect to exact energies were found for excited states by using the qEOM-VQE and VQD algorithms, respectively, to perform simulations on quantum devices without error mitigation. By utilizing error mitigation by state tomography to purify the quantum states and correct energy values, the large errors found for unmitigated results could be improved to differences of, at most, 3 mHa with respect to exact values. Consequently, excellent agreement could be found between values of $ΔE_{st}$ predicted by quantum simulations and those found in experiments.

quant-ph↗