arXiv subjects

Shiqun Zhu

Publications and source records attributed to Shiqun Zhu.

30 records · Page 2Linked to original sources

arXiv2010

Fidelity and entanglement close to quantum phase transition in a two-leg $XXZ$ spin ladder

The fidelity susceptibility and entanglement entropy in a system of two-leg $XXZ$ spin ladder with rung coupling is investigated by using exact diagonalization of the system. The effects of rung coupling on fidelity susceptibility, entanglement entropy and quantum phase transition are analyzed. It is found that the quantum phase transition between two different $XY$ phases can be well characterized by the fidelity susceptibility. Though the quantum phase transition from $XY$ phase to rung singlet phase can be hardly detected by fidelity susceptibility, it can be predicted by the first derivative of the entanglement entropy of the system.

quant-ph↗

arXiv2009

Controlled multiqubit entangled states and quantum transmission in quantum molecule-spin systems

he multiqubit entangled states are coherently controlled in the quantum spin systems composed of $N+1$ interacting antiferromagnetic molecular rings. The tunable intermolecular couplings arise from the local exchange interactions between electron spins of $N$ circumjacent magnets and those of one central molecular ring. The quantum dynamics of such system is analytically deduced by the effective spin hamiltonian with anisotropic Heisenberg couplings. It is found that entangled $W$ states can be generated with a high precision under the circumstance of quantum fluctuations. The multiqubit entangled state is also transferred from some molecular rings to others by the selection of intermolecular couplings.

quant-ph↗

arXiv2008

Entanglement in a hardcore-boson Hubbard model

The entanglement in a Hubbard chain of hardcore bosons is investigated. The analytic expression of the global entanglement in ground state is derived. The divergence of the derivative of the global entanglement shows the quantum criticality of the ground state. For the thermal equilibrium state, the bipartite and the multipartite entanglement are evaluated. The entanglement decreases to zero at a certain temperature. The thermal entanglement is rapidly decreasing with the increase of the number of sites in the lattice. The bipartite thermal entanglement approaches a constant value at a certain number of sites while the multipartite entanglement eventually vanishes.

quant-ph↗

arXiv2008

Entanglement in a Dimerized Antiferromagnetic Heisenberg Chain

The entanglement properties in an antiferromagnetic dimerized Heisenberg spin-1/2 chain are investigated. The entanglement gap, which is the difference between the ground-state energy and the minimal energy that any separable state can attain, is calculated to detect the entanglement. It is found that the entanglement gap can be increased by varying the alternation parameter. Through thermal energy, the witness of the entanglement can determine a characteristic temperature below that an entangled state can be obtained. The entanglement detected by the energy can provide a lower bound for that determined by the concurrence. If the alternation parameter is smaller than a critical value, there is always no inter-dimer entanglement in the chain.

quant-ph↗

arXiv2008

Swap Action in a Solid-State Controllable Anisotropic Heisenberg Model

Correct swap action can be realized via the control of the anisotropic Heisenberg interaction in solid-state quantum systems. The conditions of performing a swap are derived by the dynamics of arbitrary bipartite pure state. It is found that swap errors can be eliminated in the presence of symmetric anisotropy. In realistic quantum computers with unavoidable fluctuations, the gate fidelity of swap action is estimated. The scheme of quantum computation via the anisotropic Heisenberg interaction is implemented in a one dimensional quantum dots. The slanting and static magnetic field can be used to adjust the anisotropy.

quant-ph↗

arXiv2008

Quantum computation in semiconductor quantum dots of electron-spin asymmetric anisotropic exchange

The universal quantum computation is obtained when there exists asymmetric anisotropic exchange between electron spins in coupled semiconductor quantum dots. The asymmetric Heisenberg model can be transformed into the isotropic model through the control of two local unitary rotations for the realization of essential quantum gates. The rotations on each qubit are symmetrical and depend on the strength and orientation of asymmetric exchange. The implementation of the axially symmetric local magnetic fields can assist the construction of quantum logic gates in anisotropic coupled quantum dots. This proposal can efficiently use each physical electron spin as a logical qubit in the universal quantum computation.

quant-ph↗

arXiv2008

Quantum information processing in a spin-bus system of coupled chains

The effective Heisenberg interaction of long distance is constructed in spin qubits connected to a bus of two strongly coupled chains. Universal quantum computation can be realized on the basis of the bus which always keeps frozen at the ground state. It is found that the effective interaction is primarily determined by the energy spectra of the bus. With the variation of the distance between two connecting nodes, the interaction alternately occurs between antiferromagnetic and ferromagnetic ones. The long range interaction can also be attained in coupled infinite chains. The quantum gate operations with the high precision are implemented in the condition of quantum fluctuations.

quant-ph↗

arXiv2006

Entanglement in a Spin-$s$ Antiferromagnetic Heisenberg Chain

The entanglement in a general Heisenberg antiferromagnetic chain of arbitrary spin-$s$ is investigated. The entanglement is witnessed by the thermal energy which equals to the minimum energy of any separable state. There is a characteristic temperature below that an entangled thermal state exists. The characteristic temperature for thermal entanglement is increased with spin $s$. When the total number of lattice is increased, the characteristic temperature decreases and then approaches a constant. This effect shows that the thermal entanglement can be detected in a real solid state system of larger number of lattices for finite temperature. The comparison of negativity and entanglement witness is obtained from the separability of the unentangled states. It is found that the thermal energy provides a sufficient condition for the existence of the thermal entanglement in a spin-$s$ antiferromagnetic Heisenberg chain.

quant-ph↗

arXiv2006

Entanglement in a Quantum Mixed-Spin Chain

The entanglement in a general mixed spin chain with arbitrary spin $S$ and 1/2 is investigated in the thermodynamical limit. The entanglement is witnessed by the magnetic susceptibility which decides a characteristic temperature for an entangled thermal state. The characteristic temperature is nearly proportional to the interaction $J$ and the mixed spin $S$. The bound of negativity is obtained on the basis of the magnetic susceptibility. It is found that the macroscopic magnetic properties are affected by the quantum entanglement in the real solids. Meanwhile, the entanglement can be quantitatively evaluated by the thermodynamical observable.

quant-ph↗

arXiv2006

Synchronization on fast and slow dynamics in drive-response systems

Two types of synchronization, Achronal Synchronization and Isochronous synchronization are investigated numerically when unidirectionally coupled laser systems are considered both on fast and slow dynamics by studying the correlation function. Although the synchronization behaviors are found to be separated on relatively fast dynamics of chaotic fluctuations while blended together on slow dynamics of dropout events, their main features revealed are in a good agreement, which most probably suggests that the observation based on detections on slow dynamics is of importance when it is the only choice as detection on fast dynamics is usually extremely hard to be done.

nlin.CD↗

arXiv2004

Combination of anticipated and isochronous synchronization in coupled semiconductor lasers system

Combination of two basic types of synchronization, anticipated and isochronous synchronization, is investigated numerically in coupled semiconductor lasers. Due to the combination, a synchronization of good quality can be obtained. We study the dependence of the lag time between two lasers and the synchronization quality on the converse coupling retardation time $τ_{c21}$. When $τ_{c21}$ is close to the difference of external cavity round trip time $τ$ and coupling retardation time $τ_{c12}$, the combination of anticipated and isochronous synchronization may produce a better synchronization, with a lag time proportional to $τ_{c21}$. When $τ_{c21}$ is largely different from $τ-τ_{c12}$, the combination is noneffective and even negative in some cases, with a lag time independent of $τ_{c21}$.

nlin.CD↗

arXiv2002

Coexistence of Anticipating Synchronization and Lag Synchronization in an Optical System

The chaotic synchronization between two bi-directionally coupled external cavity single-mode semiconductor lasers is investigated. Numerical simulation shows that anticipating synchronization and lag synchronization coexist in certain parameter regime. The anticipating time with different effects that were discussed quite differently in the previous theoretical analysis and experimental observation is determined by the involved parameters in the system.

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