arXiv subjects

Wei-Min Zhang

Publications and source records attributed to Wei-Min Zhang.

At least 91 records · Page 5Linked to original sources

arXiv2006

Decoherence induced by electron accumulation in quantum measurement of charge qubits

In this paper, we study the quantum decoherence induced by accumulation of electron tunnelings during the quantum measurement of a charge qubit. The charge qubit is a single electron confined in coupled quantum dots. The measurement of the qubit states is performed using a quantum point contact. A set of master equations for qubit states is derived within a nonequilibrium perturbation to the equilibrium reservoir due to the electron accumulation between the source and drain of the quantum point contact. The quantum decoherence of the qubit states arose from the electron accumulation during the measurement is explored in this framework, and several interesting results on charge qubit decoherence are obtained.

cond-mat.mes-hall↗

arXiv2005

Quantum Nonlinear Sigma Model for Arbitrary Spin Heisenberg Antiferromagnets

In this Letter, we derive a quantum nonlinear sigma model (QNLSM) for quantum Heisenberg antiferromagnets (QHA) with arbitrary S (spin) values. A upper limit of the low temperature is naturally carried out for the reliability of the QNLSM. The S dependence of the effective coupling constant and the spin wave velocity in the QNLSM are also obtained explicitly. The resulting spin wave velocity for 2-dim spin-1/2 QHA highly concurs with the experimental data of high $T_c$ compound La2CuO4. The predicted correlation lengths for 2d QHA and spin-gap magnitudes for 1d QHA also agrees with the accurate numerical results.

cond-mat.str-el↗

arXiv2005

Free spin quantum computation with semiconductor nanostructures

Taking the excess electron spin in a unit cell of semiconductor multiple quantum-dot structure as a qubit, we can implement scalable quantum computation without resorting to spin-spin interactions. The technique of single electron tunnelings and the structure of quantum-dot cellular automata (QCA) are used to create a charge entangled state of two electrons which is then converted into spin entanglement states by using single spin rotations. Deterministic two-qubit quantum gates can also be manipulated using only single spin rotations with help of QCA. A single-short read-out of spin states can be realized by coupling the unit cell to a quantum point contact.

quant-ph↗

arXiv2004

Universal quantum computation with electron spins in quantum dots based on superpositions of spacetime paths and Coulomb blockade

Using electrostatic gates to control the electron positions, we present a new controlled-NOT gate based on quantum dots. The qubit states are chosen to be the spin states of an excess conductor electron in the quantum dot; and the main ingredients of our scheme are the superpositions of space-time paths of electrons and the effect of Coulomb blockade. All operations are performed only on individual quantum dots and are based on fundamental interactions. Without resorting to spin-spin terms or other assumed interactions, the scheme can be realized with a dedicated circuit and a necessary number of quantum dots. Gate fidelity of the quantum computation is also presented.

quant-ph↗

arXiv2004

Chiral Symmetry in Light-front QCD

The definition of chiral transformations in light-front field theory is very different from the conventional form in equal-time formalism. We study the consistency of chiral transformations and chiral symmetry in light-front QCD and derive a complete new light-front axial-vector current for QCD. The breaking of chiral symmetry in light-front QCD is only associated with helicity flip interaction between quarks and gluons. Remarkably, the new axial-vector current does not contain the pion pole part so that the associate chiral charge smoothly describes pion transitions for various hadronic processes.

hep-ph↗

arXiv2004

The Effects of the interfacial Pseudo-spin Coupling Fluctuation on the Dielectric Property of a Ferroelectric Superlattice

Using effective-field theory with correlations, we investigate the effects of interfacial pseudo-spin coupling fluctuations on the susceptibility and polarization of ferroelectric superlattices within the framework of transverse Ising model. It is found that the interfacial coupling fluctuations increase the susceptibility in the low temperature region. For a strong interfacial coupling, the phase transition temperature decreases with the strength of fluctuations of the interfacial coupling. The dependence of the susceptibility on the superlattice period of $BaTiO_{3}/SrTiO_{3}$ are plotted for different interfacial coupling fluctuations strength. At room temperature, when the interfacial coupling fluctuation increases, the peak position of the susceptibility will shift to a large superlattice period.

cond-mat.mtrl-sci↗

arXiv2001

Quantum Heisenberg Antiferromagnets versus Nonlinear $σ$ Model Without the Large S Limit

In this letter, I develop a new topologically invariant coherent state path integral for spin systems, and apply it to the quantum Heisenberg model on a square lattice. As a result, the quantum nonlinear $σ$ model for arbitrary values of spin can be directly obtained. The effective coupling constant and spin wave velocity are modified by $g_s= {2\over S}\sqrt{d+{T_Λ\over 2SJ}}$ and $c_s=2JSa \sqrt{d+{T_Λ\over 2SJ}}$, where $T_Λ$ is a natural temperature scale for the reliability of the theory. The formulation can also be extended to other generalized coherent state path integrals.

cond-mat.str-el↗

arXiv2001

SU$_M$(2)$\times$U$_C$(1) Gauge Symmetry in High $T_c$ Superconductivity

The square lattice structure of $CuO_2$ layers and the strongly correlated property of electrons indicate that the high $T_c$ superconductivity in cuprates can be described by a SO$_M$(5) coherent pairing state in which a SU$_M$(2)$\times$U$_C$(1) gauge symmetry is embedded. The spin and charge fluctuations that characterize the low energy magnetic excitations in cuprates are controlled by this intrinsic SU$_M$(2)$\times$U$_C$(1) gauge symmetry.

cond-mat.str-el↗

arXiv2000

Study of Radiative Leptonic D Meson Decays

We study the radiative leptonic $D$ meson decays of $D^+_{(s)}\to ł^+ν_łγ$ ($ł=e,μ,τ$), $D^0\to ν\barνγ$ and $D^0\to ł^+ł^-γ$ ($l=e,μ$) within the light front quark model. In the standard model, we find that the decay branching ratios of $D^+_{(s)}\to e^+ν_eγ$, $D^+_{(s)}\toμ^+ν_μγ$ and $D^+_{(s)}\toτ^+ν_τγ$ are $6.9\times 10^{-6}$ ($7.7\times 10^{-5}$), $2.5\times 10^{-5}$ ($2.6\times 10^{-4}$), and $6.0\times 10^{-6}$ ($3.2\times 10^{-4}$), and that of $D^0\toł^+ł^-γ$ ($ł=e,μ$) and $D^0\toν\barνγ$ are $6.3\times 10^{-11}$ and $2.7\times 10^{-16}$, respectively.

hep-ph↗

arXiv2000

Study of $B_{s,d}\to l^+l^-γ$ Decays

We study the decays of $B_{s,d} \to l^+ l^- γ(l=e, μ,τ)$ within the light-front model. We calculate the tensor type form factors and use these form factors to evaluate the decay branching ratios. We find that, in the standard model, the branching ratios of $B_{s(d)} \to ł^+ ł^- γ$ ($ł=e, μ,τ$) are $7.1\times 10^{-9}$ ($1.5\times 10^{-10}$), $8.3\times 10^{-9}$ ($1.8\times 10^{-10}$), $1.6\times 10^{-8}$ ($6.2\times 10^{-10}$), respectively.

hep-ph↗

arXiv1999

Underlying Pairing States of High $T_c$ Superconductivity

I present a microscopic theory I proposed recently to describe high-T_c superconductivity in cuprates. I show that coherent pairing states consisting of extended singlet Cooper pairs and triplet $π$ pairs can manifest both the Mott insulating antiferromagnetic order and the d-wave superconducting order. From this configuration of coherent pairing states, I can describe both the single electron properties and the low energy collective excitations in high T_c superconductivity in the same framework. The quasiparticle can be derived directly with respect to the coherent pairing states. While, the low-lying quantum fluctuations associated with spin-wave and charge excitation can be investigated from the path integral of the coherent pairing states.

cond-mat.supr-con↗

arXiv1999

Coherent States in Field Theory

Coherent states have three main properties: coherence, overcompleteness and intrinsic geometrization. These unique properties play fundamental roles in field theory, especially, in the description of classical domains and quantum fluctuations of physical fields, in the calculations of physical processes involving infinite number of virtual particles, in the derivation of functional integrals and various effective field theories, also in the determination of long-range orders and collective excitations, and finally in the exploration of origins of topologically nontrivial gauge fields and associated gauge degrees of freedom.

hep-th↗

arXiv1999

Underlying Pairing States in Cuprate Superconductors

In this Letter, we develop a microscopic theory to describe the close proximity between the insulating antiferromagnetic (AF) order and the d-wave superconducting (dSC) order in cuprates. We show that the cuprate ground states form a configuration of coherent pairing states consisting of extended singlet Cooper pairs and triplet $π$ pairs, which can simultaneously describe AF and dSC orders.

cond-mat.str-el↗

arXiv1998

Nonperturbative Determination of Heavy Meson Bound States

In this paper we obtain a heavy meson bound state equation from the heavy quark equation of motion in heavy quark effective theory (HQET) and the heavy meson effective field theory we developed very recently. The bound state equation is a covariant extention of the light-front bound state equation for heavy mesons derived from light-front QCD and HQET. We determine the covariant heavy meson wave function variationally by minimizing the binding energy $\barΛ$. Subsequently the other basic HQET parameters $λ_1$ and $λ_2$, and the heavy quark masses $m_b$ and $m_c$ can also be consistently determined.

hep-ph↗

arXiv1998

Nonperturbative Description of Deep Inelastic Structure Functions in Light-Front QCD

We explore the deep inelastic structure functions of hadrons nonperturbatively in an inverse power expansion of the light-front energy of the probe in the framework of light-front QCD. We arrive at the general expressions for various structure functions as the Fourier transform of matrix elements of different components of bilocal vector and axial vector currents on the light-front in a straightforward manner. The complexities of the structure functions are mainly carried by the multi-parton wave functions of the hadrons, while, the bilocal currents have a dynamically dependent yet simple structure on the light-front in this description. We also present a novel analysis of the power corrections based on light-front power counting which resolves some ambiguities of the conventional twist analysis in deep inelastic processes. Further, the factorization theorem and the scale evolution of the structure functions are presented in this formalism by using old-fashioned light-front time-ordered perturbation theory with multi-parton wave functions. Nonperturbative QCD dynamics underlying the structure functions can be explored in the same framework. Once the nonperturbative multi-parton wave functions are known from low-energy light-front QCD, a complete description of deep inelastic structure functions can be realized.

hep-ph↗

arXiv1998

Deep Inelastic Structure Functions in Light-Front QCD: Radiative Corrections

Recently, we have introduced a unified theory to deal with perturbative and non-perturbative QCD contributions to hadronic structure functions in deep inelastic scattering. This formulation is realized by combining the coordinate space approach based on light-front current algebra techniques and the momentum space approach based on Fock space expansion methods in the Hamiltonian formalism of light-front field theory. In this work we show how a perturbative analysis in the light-front Hamiltonian formalism leads to the factorization scheme we have proposed recently. The analysis also shows that the scaling violations due to perturbative QCD corrections can be rather easily addressed in this framework by simply replacing the hadron target by dressed parton target and then carrying out a systematic expansion in the coupling constant $α_s$ based on the perturbative QCD expansion of the dressed parton target. The tools employed for this calculation are those available from light-front old-fashioned perturbation theory. We present the complete set of calculations of unpolarized and polarized deep inelastic structure functions to order $α_s$. We extract the relevant splitting functions in all the cases. We explicitly verify all the sum rules to order $α_s$. We demonstrate the validity of approximations made in the derivation of the new factorization scheme. This is achieved with the help of detailed calculations of the evolution of structure function of a composite system carried out using multi-parton wavefunctions.

hep-ph↗

arXiv1998

Effective Field Theory of Heavy Mesons

In this paper we present a detailed formulation for a recently proposed effective field theory to describe the nonperturbative QCD dynamics of heavy mesons. This effective theory incorporates with heavy quark symmetry (HQS) and the heavy quark effective theory (HQET). Heavy mesons in this theory are constructed as composite particles of a heavy quark bounded with the light degrees of freedom. The heavy meson properties in the heavy quark limit and the $1/m_Q$ corrections can then be explicitly evaluated from this effective theory. All the basic parameters of the HQET, namely, the heavy quark mass $m_Q$, the heavy meson residual mass $\overlineΛ$, and the HQS breaking mass parameters $λ_1$ and $λ_2$, are consistently determined. $λ_1$ is found to be small due to a large cancellation between the heavy quark kinetic energy and the chromo-electric interaction between the heavy quark and light degrees of freedom. We also evaluate the Isgur-Wise function, the decay constant, and the axial-vector coupling constant of heavy mesons.

hep-ph↗

arXiv1998

The quantum Frenkel-Kontorova model: a squeezed state approach

The squeezed state is used to study the one-dimensional quantum mechanical Frenkel Kontorova model. A set of coupled equations for the particle's expectation value and the fluctuations for the ground state are derived. It is shown that quantum fluctuations renormalize the standard map to an effective sawtooth map. The mechanism underling provides an alternative and simple explanation of dynamical localization in quantum chaos.

chao-dyn↗