arXiv subjects

Hanyu Zhang

Publications and source records attributed to Hanyu Zhang.

65 records · Page 4Linked to original sources

arXiv2021

Transferring quantum entangled states between multiple single-photon-state qubits and coherent-state qubits in circuit QED

We present a way to transfer maximally- or partially-entangled states of n single-photon-state (SPS) qubits onto n coherent-state (CS) qubits, by employing 2n microwave cavities coupled to a superconducting flux qutrit. The two logic states of a SPS qubit here are represented by the vacuum state and the single-photon state of a cavity, while the two logic states of a CS qubit are encoded with two coherent states of a cavity. Because of using only one superconducting qutrit as the coupler, the circuit architecture is significantly simplified. The operation time for the state transfer does not increase with the increasing of the number of qubits. When the dissipation of the system is negligible, the quantum state can be transferred in a deterministic way since no measurement is required. Furthermore, the higher-energy intermediate level of the coupler qutrit is not excited during the entire operation and thus decoherence from the qutrit is greatly suppressed. As a specific example, we numerically demonstrate that the high-fidelity transfer of a Bell state of two SPS qubits onto two CS qubits is achievable within the present-day circuit QED technology. Finally, it is worthy to note that when the dissipation is negligible, entangled states of n CS qubits can be transferred back onto n SPS qubits by performing reverse operations. This proposal is quite general and can be extended to accomplish the same task, by employing a natural or artificial atom to couple 2n microwave or optical cavities.

quant-ph↗

arXiv2021

Public Transit for Special Events: Ridership Prediction and Train Optimization

Many special events, including sport games and concerts, often cause surges in demand and congestion for transit systems. Therefore, it is important for transit providers to understand their impact on disruptions, delays, and fare revenues. This paper proposes a suite of data-driven techniques that exploit Automated Fare Collection (AFC) data for evaluating, anticipating, and managing the performance of transit systems during recurring congestion peaks due to special events. This includes an extensive analysis of ridership of the two major stadiums in downtown Atlanta using rail data from the Metropolitan Atlanta Rapid Transit Authority (MARTA). The paper first highlights the ridership predictability at the aggregate level for each station on both event and non-event days. It then presents an unsupervised machine-learning model to cluster passengers and identify which train they are boarding. The model makes it possible to evaluate system performance in terms of fundamental metrics such as the passenger load per train and the wait times of riders. The paper also presents linear regression and random forest models for predicting ridership that are used in combination with historical throughput analysis to forecast demand. Finally, simulations are performed that showcase the potential improvements to wait times and demand matching by leveraging proposed techniques to optimize train frequencies based on forecasted demand.

math.OC↗

arXiv2020

Comparison of phase change process in Si-GST hybrid integrated waveguide and MMI devices

In the past decades, silicon photonic integrated circuits (PICs) have been considered as a promising approach to solve the bandwidth bottleneck in optical communications and interconnections. Despite significant advances, large-scale PICs still face a series of technical challenges, such as footprint, power consumption, and routing state storage, resulting from the active tuning methods used to control the optical waves. These challenges can be partially addressed by combining chalcogenide phase change materials (PCMs) such as Ge2Sb2Te5 (GST) with silicon photonics, especially applicable in switching applications due to the nonvolatile nature of the GST. Although GST phase transitions between amorphous and crystalline states actuated by optical and electrical pulses heating have been experimentally demonstrated, there is no direct comparison between them. We carried out simulations and experiments to systematically investigate the difference in the phase change process induced by optical and electrical pulses for two types of Si-GST hybrid waveguides. For the phase transition induced by optical pulses, the device has a clear advantage in terms of power consumption and operation speed. For the phase transition induced by electrical pulses, the device is suitable for large-scale integration because it does not require complex light routing. It helps us better understand the phase change process and push forward the further development of Si-GST hybrid integration platform, bringing in new potential applications.

physics.optics↗

arXiv2020

Spatially Resolved Persistent Photoconductivity in MoS$_2$-WS$_2$ Lateral Heterostructures

The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS$_2$--WS$_2$ lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days, and informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS$_2$ domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state.

cond-mat.mes-hall↗

arXiv2020

Wide band gap chalcogenide semiconductors

Wide band gap semiconductors are essential for today's electronic devices and energy applications due to their high optical transparency, as well as controllable carrier concentration and electrical conductivity. There are many categories of materials that can be defined as wide band gap semiconductors. The most intensively investigated are transparent conductive oxides (TCOs) such as ITO and IGZO used in displays, carbides and nitrides used in power electronics, as well as emerging halides (e.g. CuI) and 2D electronic materials used in various optoelectronic devices. Chalcogen-based (S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out due to their propensity for p-type doping, high mobilities, high valence band positions (i.e. low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent conductors. We proceed to summarize progress in wide band gap (Eg > 2 eV) chalcogenide materials, such as II-VI MCh binaries, CuMCh2 chalcopyrites, Cu3MCh4 sulvanites, mixed anion layered CuMCh(O,F), and 2D materials, among others, and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications of chalcogenide wide band gap semiconductors, e.g. photovoltaic and photoelectrochemical solar cells, transparent transistors, and diodes, that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this review aims to inspire continued research on this emerging class of transparent conductors and to enable future innovations for optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Learning High-order Structural and Attribute information by Knowledge Graph Attention Networks for Enhancing Knowledge Graph Embedding

The goal of representation learning of knowledge graph is to encode both entities and relations into a low-dimensional embedding spaces. Many recent works have demonstrated the benefits of knowledge graph embedding on knowledge graph completion task, such as relation extraction. However, we observe that: 1) existing method just take direct relations between entities into consideration and fails to express high-order structural relationship between entities; 2) these methods just leverage relation triples of KGs while ignoring a large number of attribute triples that encoding rich semantic information. To overcome these limitations, this paper propose a novel knowledge graph embedding method, named KANE, which is inspired by the recent developments of graph convolutional networks (GCN). KANE can capture both high-order structural and attribute information of KGs in an efficient, explicit and unified manner under the graph convolutional networks framework. Empirical results on three datasets show that KANE significantly outperforms seven state-of-arts methods. Further analysis verify the efficiency of our method and the benefits brought by the attention mechanism.

cs.AI↗

arXiv2019

Miniature Multi-Level Optical Memristive Switch Using Phase Change Material

The optical memristive switches are electrically activated optical switches that can memorize the current state. They can be used as optical latching switches in which the switching state is changed only by applying an electrical Write/Erase pulse and maintained without external power supply. We demonstrate an optical memristive switch based on a silicon MMI structure covered with nanoscale-size Ge2Sb2Te5 (GST) material on top. The phase change of GST is triggered by resistive heating of the silicon layer beneath GST with an electrical pulse. Experimental results reveal that the optical transmissivity can be tuned in a controllable and repeatable manner with the maximum transmission contrast exceeding 20 dB. Partial crystallization of GST is obtained by controlling the width and amplitude of the electric pulses. Crucially, we also demonstrate that both Erase and Write operations, to and from any intermediate level, are possible with accurate control of the electrical pulses. Our work marks a significant step forward towards realizing photonic memristive switches without static power consumption, which are highly demanded in high-density large-scale integrated photonics.

physics.optics↗

arXiv2019

A model independent constraint on the temporal evolution of the speed of light

We present a new, model-independent method to reconstruct the temporal evolution of the speed of light $c(z)$ using astronomical observations. After validating our pipeline using mock datasets, we apply our method to the latest BAO and supernovae observations, and reconstruct $c(z)$ in the redshift range of $z\in[0,1.5]$. We find no evidence of a varying speed of light, although we see some interesting features of $Δc(z)$, the fractional difference between $c(z)$ and $c_0$ (the speed of light in SI), \eg, $Δc(z)<0$ and $Δc(z)>0$ at $0.2\lesssim z\lesssim0.5$ and $0.8\lesssim z\lesssim1.3$, respectively, although the significance of these features is currently far below statistical importance.

astro-ph.CO↗

arXiv2017

Dynamical dark energy in light of the latest observations

A flat Friedman-Roberson-Walker universe dominated by a cosmological constant ($Λ$) and cold dark matter (CDM) has been the working model preferred by cosmologists since the discovery of cosmic acceleration. However, tensions of various degrees of significance are known to be present among existing datasets within the $Λ$CDM framework. In particular, the Lyman-$α$ forest measurement of the Baryon Acoustic Oscillations (BAO) by the Baryon Oscillation Spectroscopic Survey (BOSS) prefers a smaller value of the matter density fraction $Ω_{\rm M}$ compared to the value preferred by cosmic microwave background (CMB). Also, the recently measured value of the Hubble constant, $H_0=73.24\pm1.74 \ {\rm km}\ {\rm s}^{-1} \ {\rm Mpc}^{-1}$, is $3.4σ$ higher than $66.93\pm0.62 \ {\rm km}\ {\rm s}^{-1} \ {\rm Mpc}^{-1}$ inferred from the Planck CMB data. In this work, we investigate if these tensions can be interpreted as evidence for a non-constant dynamical dark energy (DE). Using the Kullback-Leibler (KL) divergence to quantify the tension between datasets, we find that the tensions are relieved by an evolving DE, with the dynamical DE model preferred at a $3.5σ$ significance level based on the improvement in the fit alone. While, at present, the Bayesian evidence for the dynamical DE is insufficient to favour it over $Λ$CDM, we show that, if the current best fit DE happened to be the true model, it would be decisively detected by the upcoming DESI survey.

astro-ph.CO↗

arXiv2017

Probing dynamics of dark energy with latest observations

We examine the validity of the $Λ$CDM model, and probe for the dynamics of dark energy using latest astronomical observations. Using the $Om(z)$ diagnosis, we find that different kinds of observational data are in tension within the $Λ$CDM framework. We then allow for dynamics of dark energy and investigate the constraint on dark energy parameters. We find that for two different kinds of parametrisations of the equation of state parameter $w$, a combination of current data mildly favours an evolving $w$, although the significance is not sufficient for it to be supported by the Bayesian evidence. A forecast of the DESI survey shows that the dynamics of dark energy could be detected at $7σ$ confidence level, and will be decisively supported by the Bayesian evidence, if the best fit model of $w$ derived from current data is the true model.

astro-ph.CO↗

arXiv2016

The extended Baryon Oscillation Spectroscopic Survey (eBOSS): a cosmological forecast

We present a science forecast for the eBOSS survey, part of the SDSS-IV project, which is a spectroscopic survey using multiple tracers of large-scale structure, including luminous red galaxies (LRGs), emission line galaxies (ELGs) and quasars (both as a direct probe of structure and through the Ly-$α$ forest). Focusing on discrete tracers, we forecast the expected accuracy of the baryonic acoustic oscillation (BAO), the redshift-space distortion (RSD) measurements, the $f_{\rm NL}$ parameter quantifying the primordial non-Gaussianity, the dark energy and modified gravity parameters. We also use the line-of-sight clustering in the Ly-$α$ forest to constrain the total neutrino mass. We find that eBOSS LRGs ($0.6 0.6$), ELGs ($0.6<z<1.2$) and Clustering Quasars (CQs) ($0.6<z<2.2$) can achieve a precision of 1%, 2.2% and 1.6% precisions, respectively, for spherically averaged BAO distance measurements. Using the same samples, the constraint on $fσ_8$ is expected to be 2.5%, 3.3% and 2.8% respectively. For primordial non-Gaussianity, eBOSS alone can reach an accuracy of $σ(f_{\rm NL})\sim10-15$, depending on the external measurement of the galaxy bias and our ability to model large-scale systematic errors. eBOSS can at most improve the dark energy Figure of Merit (FoM) by a factor of $3$ for the Chevallier-Polarski-Linder (CPL) parametrisation, and can well constrain three eigenmodes for the general equation-of-state parameter (Abridged).

astro-ph.CO↗