arXiv subjects

Wei Cui

Publications and source records attributed to Wei Cui.

At least 181 records · Page 10Linked to original sources

arXiv2009

Non-Markovian entanglement dynamics between two coupled qubits in the same environment

We analyze the dynamics of the entanglement in two independent non-Markovian channels. In particular, we focus on the entanglement dynamics as a function of the initial states and the channel parameters like the temperature and the ratio $r$ between $ω_0$ the characteristic frequency of the quantum system of interest, and $ω_c$ the cut-off frequency of Ohmic reservoir. We give a stationary analysis of the concurrence and find that the dynamic of non-markovian entanglement concurrence $\mathcal{C}_ρ(t)$ at temperature $k_BT=0$ is different from the $k_BT>0$ case. We find that "entanglement sudden death" (ESD) depends on the initial state when $k_BT=0$, otherwise the concurrence always disappear at finite time when $k_BT>0$, which means that ESD must happen. The main result of this paper is that the non-Markovian entanglement dynamic is fundamentally different from the Markovian one. In the Markovian channel, entanglement decays exponentially and vanishes only asymptotically, but in the non-Markovian channel the concurrence $\mathcal{C}_ρ(t)$ oscillates, especially in the high temperature case. Then an open-loop controller adjusted by the temperature is proposed to control the entanglement and prolong the ESD time.

quant-ph↗

arXiv2009

The entanglement dynamics of the bipartite quantum system: toward entanglement sudden death

We investigate the entanglement dynamics of bipartite quantum system between two qubits with the dissipative environment. We begin with the standard Markovian master equation in the Lindblad form and the initial state which is prepared in the extended Werner-like state: $ρ^Φ_{AB}(0)$. We examine the conditions for entanglement sudden death (ESD) and calculate the corresponding ESD time by the Wootters' concurrence. We observe that ESD is determined by the parameters like the mean occupation number of the environment $N$, amount of initial entanglement $α$, and the purity $r$. For N=0, we get the analytical expression of both ESD condition and ESD time. For $N>0$ we give a theoretical analysis that ESD always occurs, and simulate the concurrence as a function of $γ_0t$ and one of the parameters $N, α$, and $r$.

quant-ph↗

arXiv2009

Optimal Decoherence Control in non-Markovian Open, Dissipative Quantum Systems

We investigate the optimal control problem for non-Markovian open, dissipative quantum system. Optimal control using Pontryagin maximum principle is specifically derived. The influences of Ohmic reservoir with Lorentz-Drude regularization are numerically studied in a two-level system under the following three conditions: ω_0\llω_c, ω_0\approxω_c or ω_0\ggω_c, where ω_0 is the characteristic frequency of the quantum system of interest, and ω_c the cut-off frequency of Ohmic reservoir. The optimal control process shows its remarkable influences on the decoherence dynamics. The temperature is a key factor in the decoherence dynamics. We analyze the optimal decoherence control in high temperature, intermediate temperature, and low temperature reservoirs respectively. It implies that designing some engineered reservoirs with the controlled coupling and state of the environment can slow down the decoherence rate and delay the decoherence time. Moreover, we compare the non-Markovian optimal decoherence control with the Markovian one and find that with non-Markovian the engineered artificial reservoirs are better than with the Markovian approximation in controlling the open, dissipative quantum system's decoherence.

quant-ph↗

arXiv2009

Bounds on probability of transformations between multi-partite pure states

For a tripartite pure state of three qubits, it is well known that there are two inequivalent classes of genuine tripartite entanglement, namely the GHZ-class and the W-class. Any two states within the same class can be transformed into each other with stochastic local operations and classical communication (SLOCC) with a non-zero probability. The optimal conversion probability, however, is only known for special cases. Here, we derive new lower and upper bounds for the optimal probability of transformation from a GHZ-state to other states of the GHZ-class. A key idea in the derivation of the upper bounds is to consider the action of the LOCC protocol on a different input state, namely $1/\sqrt{2} [\ket{000} - \ket{111}]$, and demand that the probability of an outcome remains bounded by 1. We also find an upper bound for more general cases by using the constraints of the so-called interference term and 3-tangle. Moreover, we generalize some of our results to the case where each party holds a higher-dimensional system. In particular, we found that the GHZ state generalized to three qutrits, i.e., $\ket{\mathrm{GHZ}_3} = 1/\sqrt{3} [ \ket{000} + \ket{111} + \ket{222} ] $, shared among three parties can be transformed to {\it any} tripartite 3-qubit pure state with probability 1 via LOCC. Some of our results can also be generalized to the case of a multipartite state shared by more than three parties.

quant-ph↗

arXiv2009

TeV Gamma Ray Astronomy

The field of ground-based gamma ray astronomy has enjoyed rapid growth in recent years. As an increasing number of sources are detected at TeV energies, the field has matured and become a viable branch of modern astronomy. Lying at the uppermost end of the electromagnetic rainbow, TeV photons are always preciously few in number but carry essential information about the particle acceleration and radiative processes involved in extreme astronomical settings. Together with observations at longer wavelengths, TeV gamma-ray observations have drastically improved our view of the universe. In this review, we briefly describe recent progress in the field. We will conclude by providing a personal perspective on the future of the field, in particular, on the significant roles that China could play to advance this young but exciting field.

astro-ph.HE↗

arXiv2008

Origin of X-ray Emission from Transient Black Hole Candidates in Quiescence

We report results from a systematic study of X-ray emission from black hole transients in quiescence. In this state mass accretion is thought to follow the geometry of an outer optically thick, geometrically thin disc and an inner optically thin, geometrically thick radiatively inefficient accretion flow (RIAF). The inner flow is likely also coupled to the jets near the black hole that are often seen in such systems. The goal of the study is to see whether the X-ray emission in the quiescent state is mainly powered by the accretion flow or the jets. Using data from deep XMM-Newton observations of selected black hole transients, we have found that the quiescent X-ray spectra are, to a high precision, of power-law shape in the cases of GRO J1655-40 and V404 Cyg. Such spectra deviate significantly from the expected X-ray spectrum of the RIAF at very low accretion rates. On the other hand, they can naturally be explained by emission from the jets, if the emitting electrons follow a power-law spectral distribution (as is often assumed). The situation remains ambiguous in the case of XTE J1550-564, due to the relatively poorer quality of the data. We discuss the implication of the results.

astro-ph↗

arXiv2008

Search for Pulsar Wind Nebula Associations of Unidentified TeV Gamma-Ray Sources

Many of the recently discovered TeV gamma-ray sources are associated with pulsar wind nebulae (PWNe). In fact, they represent the most populous class of Galactic sources at TeV energies. In addition, HESS has also discovered, in a survey of the Galactic plane, a population of TeV gamma-ray sources that are still without definitive counterparts at longer wavelengths. For a number of these sources, a pulsar is an evident association, which is often located within an extended region of the TeV gamma-ray emission. These particular HESS sources are promising candidates for yet not resolved pulsar wind nebulae. Here we have undertaken a systematic search for X-ray counterparts of the sources, using the archival Chandra data, within the spatial bounds of the unidentified HESS sources. A number of X-ray sources have been detected in the Chandra fields. Two of them, CXOU J161729.3-505512 and CXOU J170252.4-412848, are of a special interest because of their excellent positional coincidence with the pulsars PSR J1617-5055 and PSR J1702-4128, respectively. The first source is extended, with a bright core of 2.6 arcsec (FWHM) in radius but the emission can be seen up to roughly 20 arcsec. The second one is much fainter and detected only with marginal significance (4.6 sigma). It might also be slightly extended, although the situation is quite uncertain due to very limited statistics. The analysis of the archival Chandra data for a middle-aged pulsar (PSR J1913+1011) does not reveal any statistically significant excess at and around the position of the pulsar or the center of gravity of its plausible TeV gamma-ray counterpart (HESS J1912+101). We discuss the implications of the results.

astro-ph↗

arXiv2007

Modeling the Broadband Spectral Energy Distribution of the Microquasars XTE J1550-564 and H 1743-322

We report results from a systematic study of the spectral energy distribution (SED) and spectral evolution of XTE J1550--564 and H 1743--322 in outburst. The jets of both sources have been directly imaged at both radio and X-ray frequencies, which makes it possible to constrain the spectrum of the radiating electrons in the jets. We modelled the observed SEDs of the jet `blobs' with synchrotron emission alone and with synchrotron emission plus inverse Compton scattering. The results favor a pure synchrotron origin of the observed jet emission. Moreover, we found evidence that the shape of the electron spectral distribution is similar for all jet `blobs' seen. Assuming that this is the case for the jet as a whole, we then applied the synchrotron model to the radio spectrum of the total emission and extrapolated the results to higher frequencies. In spite of significant degeneracy in the fits, it seems clear that, while the synchrotron radiation from the jets can account for nearly 100% of the measured radio fluxes, it contributes little to the observed X-ray emission, when the source is relatively bright. In this case, the X-ray emission is most likely dominated by emission from the accretion flows. When the source becomes fainter, however, the jet emission becomes more important, even dominant, at X-ray energies. We also examined the spectral properties of the sources during outbursts and the correlation between the observed radio and X-ray variabilities. The implication of the results is discussed.

astro-ph↗

arXiv2007

Modeling Accretion Disk X-ray Continuum of Black Hole Candidates

We critically examine issues associated with determining the fundamental properties of the black hole and the surrounding accretion disk in an X-ray binary based on modeling the disk X-ray continuum of the source. We base our work mainly on two XMM-Newton observations of GX 339-4, because they provided high-quality data at low energies (below 1 keV) which are critical for reliably modeling the spectrum of the accretion disk. A key issue examined is the determination of the so-called "color correction factor", which is often empirically introduced to account for the deviation of the local disk spectrum from a blackbody (due to electron scattering). This factor cannot be pre-determined theoretically because it may vary with, e.g., mass accretion rate, among a number of important factors. We follow up on an earlier suggestion to estimate the color correction observationally by modeling the disk spectrum with saturated Compton scattering. We show that the spectra can be fitted well and the approach yields reasonable values for the color correction factor. For comparison, we have also attempted to fit the spectra with other models. We show that even the high-soft-state continuum (which is dominated by the disk emission) cannot be satisfactorily fitted by state-of-the-art disk models. We discuss the implication of the results.

astro-ph↗

arXiv2007

Dramatic Variability of X-ray Absorption Lines in the Black Hole Candidate Cygnus X-1

We report results from a 30 ks observation of Cygnus X-1 with the High Energy Transmission Grating Spectrometer (HETGS) on board the {\em Chandra X-ray Observatory}. Numerous absorption lines were detected in the HETGS spectrum. The lines are associated with highly ionized Ne, Na, Mg, Al, Si, S, and Fe, some of which have been seen in earlier HETGS observations. Surprisingly, however, we discovered dramatic variability of the lines over the duration of the present observation. For instance, the flux of the Ne X line at 12.14 Å was about $5 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$ in the early part of the observation but became subsequently undetectable, with a 99% upper limit of $0.06 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$ on the flux of the line. This implies that the line weakened by nearly two orders of magnitude on a timescale of hours. The overall X-ray flux of the source did also vary during the observation but only by 20--30%. For Cyg X-1, the absorption lines are generally attributed to the absorption of X-rays by ionized stellar wind in the binary system. Therefore, they may provide valuable diagnostics on the physical condition of the wind. We discuss the implications of the results.

astro-ph↗

arXiv2007

On the Radio/X-ray Correlation in Microquasars

[Context] The broadband spectral energy distribution of microquasars has proven to be a valuable tool for assessing the roles of jets and accretion flows in microquasars, as well as the coupling between the two. The coupling might manifest itself observationally in the correlated radio and X-ray variabilities of a source. Such a radio/X-ray correlation has indeed been seen in several microquasars during the low-hard state and subsequently been claimed to be universal for all. If proven, the universal correlation would have profound implications on theoretical models. However, there is already observational evidence that suggests otherwise. [Aims] In this paper, we critically examine the radio/X-ray correlation in a sample of microquasars, in the low-hard state as well as during state transitions, with a goal of testing the claimed universality of the correlation on observational grounds. [Methods] We have assembled a comprehensive data set from the simultaneous/contemporaneous radio and X-ray observations of representative microquasars. The data have allowed us to quantify the radio/X-ray correlation on a source by source basis. [Results] We find that the radio/X-ray correlation of microquasars exhibits diverse behaviors, both in the low-hard and transitional states, ranging from being very week (or none at all) to very strong. There is even a hint of spectral dependence of the correlation in some cases. [Conclusions] Our results rule out the claimed universality of the radio/X-ray correlation of microquasars, even for the low-hard state. On the other hand, we do find that the radio and X-ray variabilities are, to varying degrees, correlated in most cases.

astro-ph↗

arXiv2006

TeV Astrophysics, A Review

In this paper I briefly review recent progress in the field of ground-based gamma ray astrophysics.

astro-ph↗

arXiv2006

Chandra View of the Unidentified TeV Gamma-ray Source HESS J1804-216

We present high-resolution X-ray images taken with the Chandra X-ray Observatory of the field that contains the unidentified TeV gamma-ray source HESS J1804-216. A total of eleven discrete sources were detected with a posteriori significance of > 5 sigma over the entire field of view. Among them, only one, designated as CXOU J180351.4-213707, is significantly extended. The source is about 40" away from the radio pulsar PSR J1803-2137, which was the target of the Chandra observation but was not detected in X-rays. A natural question is whether the two sources are physically related. While it is conceivable that CXOU J180351.4-213707 could be associated with a previously unknown supernova remnant (SNR), in which the pulsar was born, it seems equally plausible that it might be a pulsar wind nebula (PWN) that is powered by a different pulsar whose emission is beamed away from us. In either case, we argue that CXOU J180351.4-213707 is likely the X-ray counterpart of HESS J1804-216, based on the fact that the Galactic TeV gamma-ray sources are predominantly SNRs or PWNe. The X-ray spectrum of the source can be fitted well with a power law, although the model is not well constrained due to large statistical uncertainties. The spectrum seems to be very hard, with the best-fit photon index about 1.2. Under the assumption that CXOU J180351.4-213707 is the X-ray counterpart of HESS J1804-216, we attempted to model the X-ray and TeV emission as synchrotron and inverse Compton scattered radiation from relativistic electrons. We briefly discuss the results.

astro-ph↗

arXiv2006

X-ray Spectral Variability of TeV Blazars during Rapid Flares

The spectral energy distribution (SED) of TeV blazars peaks both at keV and TeV energies. The X-ray emission is generally believed to originate in the synchrotron emission from relativistic electrons (and positrons) in the jet of these sources, while the origin of the gamma-ray emission is still being debated. We report results from a systematic study of X-ray spectral variability of Mrk 421 and Mrk 501 during individual flares that last for several days, making use of some of the high-quality data that have recently become available. The X-ray spectra of the two sources fall on the opposite sides of the synchrotron peak of their respective SEDs, so they together may offer additional insights into the physical origin of X-ray variability. We modeled each of the time-resolved X-ray spectra over a {\em single} flare by adopting a homogeneous spatial distribution and an instantaneous power-law spectral distribution for the emitting particles. We focused on the variation of four key parameters: particle spectral index, maximum Lorentz factor, energy density, and magnetic field. While there is considerable degeneracy in the fits, we show that, in order to account for the X-ray spectral variability observed in Mrk 421, at least three of the parameters are required to vary in most cases, with the spectral index being one of them. The observations of Mrk 501 support the conclusion, although the quality of the data is not as good. We discuss the implications of the results.

astro-ph↗

arXiv2005

Hot One-Temperature Accretion Flows Revisited

The effectiveness of the thermal coupling of ions and electrons in the context of optically thin, hot accretion flows is investigated. In the limit of complete coupling, we focus on the one-temperature accretion flows. Based on a global analysis, the results are compared with two-temperature accretion flow models and with the observations of black hole sources. Many features are quite similar. That is, hot one-temperature solutions are found to exist for mass flow rates less than a critical value; i.e., $\dot{M}\la 10α^2\dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}= L_{\rm Edd}/c^2$ is the Eddington accretion rate. At low mass flow rates, $\dot{M}\la 10^{-3}α^2 \dot{M}_{\rm Edd}$, the solution is in the advection-dominated accretion flow (ADAF) regime. But at higher rates, radiative cooling is effective and is mainly balanced by advective {\em heating}, placing the solution in the regime of luminous hot accretion flow (LHAF). To test the viability of the one-temperature models, we have fitted the spectra of the two black hole sources, Sgr A* and XTE J1118+480, which have been examined successfully with two-temperature models. It is found that the one-temperature models do not provide acceptable fits to the multi-wavelength spectra of Sgr A* nor to XTE J1118+480 as a result of the higher temperatures characteristic of the one-temperature models. It is concluded that the thermal coupling of ions and electrons cannot be fully effective and that a two-temperature description is required in hot accretion flow solutions.

astro-ph↗

arXiv2005

Radio--X-ray Correlation and the "Quiescent State" of Black Hole Sources

Recently a correlation between the radio and X-ray luminosities is found, $L_{\rm R}\propto L_{\rm X}^{0.7}$, in black hole sources including black hole candidates in our galaxy and active galactic nuclei. We first show that the correlation can be understood in the context of an accretion-jet model developed for explaining the spectral and timing properties of XTE J1118+480. More importantly, we show that when the X-ray luminosity is below a critical value, $\la (10^{-5}$--$10^{-6}) L_{\rm Edd}$, if the jet persists, the correlation should turn and become steeper, $L_{\rm R}\propto L_{\rm X}^{1.23}$, and the X-ray radiation of the system should be dominated by the emission from the jet, rather than by the accretion flow. Possible observational evidence for our predictions is presented and future observations to further test our predictions are proposed.

astro-ph↗

arXiv2004

X-ray Flares from Markarian 501

Motivated by the recent finding of hierarchical X-ray flaring phenomenon in Mrk 421, we conducted a systematic search for X-ray flares from Mrk 501, another well-known TeV blazar, by making use of the rich {\em RXTE} archival database. We detected flares over a wide range of timescales, from months down to minutes, as in the case of Mrk 421. However, the flares do not seem to occur nearly as frequently in Mrk 501 as in Mrk 421 on any of the timescales. The flaring hierarchy also seems apparent in Mrk 501, suggesting that it might be common among TeV blazars. The results seem to imply a scale-invariant physical origin of the flares (large or small). The X-ray spectrum of the source shows a general trend of hardening toward the peak of long-duration flares, with indication of spectral hysteresis, which is often seen in TeV blazars. However, the data are not of sufficient quality to allow us to draw definitive conclusions about spectral variability associated with more rapid but weaker flares. We critically examine a reported sub-hour X-ray flare from Mrk 501, in light of intense background flaring activity at the time of the observation, and concluded that the flare is likely an artifact. On the other hand, we did identify a rapid X-ray flare that appears to be real. It lasted only for about 15 minutes, during which the flux of the source varied by about 30%. Sub-structures are apparent in its profile, implying variabilities on even shorter timescales. Such rapid variabilities of Mrk 501 place severe constraints on the physical properties of the flaring region in the jet, which have serious implications on the emission models proposed for TeV blazars.

astro-ph↗

arXiv2004

An Accretion-Jet Model for Black Hole Binaries: Interpreting the Spectral and Timing Features of XTE J1118+480

Multi-wavelength observations of the black hole X-ray binary XTE J1118+480 have offered abundant spectral and timing information about the source, and have thus provided serious challenges to theoretical models. We propose a coupled accretion-jet model to interpret the observations. We model the accretion flow as an outer standard thin accretion disk truncated at a transition radius by an inner hot accretion flow. The accretion flow accounts for the observed UV and X-ray emission, but it substantially under-predicts the radio and infrared fluxes, even after we allow for nonthermal electrons in the hot flow. We attribute the latter components to a jet. We model the jet emission by means of the internal shock scenario which is widely employed for gamma-ray bursts. In our accretion-jet model of XTE J1118+480, the jet dominates the radio and infrared emission, the thin disk dominates the UV emission, and the hot flow produces most of the X-ray emission. The optical emission has contributions from all three components: jet, thin disk, and hot flow. The model qualitatively accounts for timing features, such as the intriguing positive and negative time lags between the optical and X-ray emission, and the wavelength-dependent variability amplitude.

astro-ph↗