Shun-Jin Wang

Equation of motion approach to the solution of the Anderson model

Based on an equation of motion approach the single-impurity Anderson model is reexamined. Using the cluster expansions the equations of motion of Green functions are transformed into the corresponding equations of motion of connected Green functions, which provides a natural and uniform truncation scheme. A factor of 2 missing in the Lacroixs approximation for the Kondo temperature is gained in the next higher-order truncation beyond Lacroixs. A quantitative improvement in the density of states at the Fermi level is also obtained.

Control of one- and two-photon absorption in a four-level atomic system by changing the amplitude and phase of a driving microwave field

We consider the one- and two-photon absorption spectra of a four-level Y-type atom with the two highest lying levels driven by a microwave field. We found that in the one-photon absorption case, the microwave field can lead to the probe gain, and the absorption and gain spectral structures depend strongly on the microwave field amplitude. For the two-photon absorption case, the strong microwave field can enhance the absorption. When the microwave field amplitude is reduced to a certain value, the single absorption peak in the two-photon spectrum changes into a structure of two-peak structure with different magnitudes. Moreover, the one- and two-photon absorption spectra can be modulated by the phase of the microwave field which produces a closed-loop configuration. Finally, we use the analytic solutions in terms of dressed-state basis to explain the results from our numerical calculation.

Entanglement production and decoherence-free subspace of two single-mode cavities embedded in a common environment

A system consisting of two identical single-mode cavities coupled to a common environment is investigated within the framework of algebraic dynamics. Based on the left and right representations of the Heisenberg-Weyl algebra, the algebraic structure of the master equation is explored and exact analytical solutions of this system are obtained. It is shown that for such a system, the environment can produce entanglement in contrast to its commonly believed role of destroying entanglement. In addition, the collective zeromode eigensolutions of the system are found to be free of decoherence against the dissipation of the environment. These decoherence-free states may be useful in quantum information and quantum computation.

Scalable quantum computation in decoherence-free subspaces with trapped ions

We propose a decoherence-free subspace (DFS) scheme to realize scalable quantum computation with trapped ions. The spin-dependent laser-ion coupling is exploited in the presence of Coulomb interactions. A universal set of unconventional geometric quantum gates is achieved in encoded subspaces that are immune from decoherence by collective dephasing. The scalability of the scheme for an ion-array system is demonstrated, either by adiabatically switching on and off the interactions, or by a fast gate scheme with DFS encoding and noise-decoupling techniques.

Entanglement dynamics of qubits in a common environment

We use the quantum jump approach to study the entanglement dynamics of a quantum register, which is composed of two or three dipole–dipole coupled two-level atoms, interacting with a common environment. Our investigation of entanglement dynamics reflects that the environment has dual actions on the entanglement of the qubits in the model. While the environment destroys the entanglement induced by the coherent dipole–dipole interactions, it can produce stable entanglement between the qubits prepared initially in a separable state. The analysis shows that it is the entangled decoherence-free states contained as components in the initial state that contribute to the stable entanglement. Our study indicates how the environmental noise produces the entanglement and exposes the interplay of environmental noise and coherent interactions of qubits on the entanglement.

Mean-field approximation to the master equation for sympathetic cooling of trapped bosons

We use the mean-field approximation to simplify the master equation for sympathetic cooling of bosons. For the mean single-particle occupation numbers, this approach yields the same equations as the factorization assumption introduced in an early paper. The stationary or equilibrium solution of the resulting master equation for the one-body density matrix shows that the mean-field approximation breaks down whenever the fraction of condensate bosons exceeds 10% or so of the total. Using group-theoretical methods, we also solve the time-dependent master equation for the one-body density matrix. Given the time dependence of the mean single-particle occupation numbers, this solution is obtained by quadratures. It tends asymptotically towards the equilibrium solution.

Inverse q-boson operators and their relation to photon-added and photon-depleted states

The generalized inverse of q-boson operators are introduced via their action on the q-number states. The q-analogy of photon-added and photon-depleted coherent states are constructed via the generalized inverse of q-boson operator actions on the q-coherent states. Their mathematical and quantum statistical properties are discussed in detail analytically and numerically.

Electron spin transport through an Aharonov-Bohm ring - a spin switch

Electron spin transport through an Aharonov-Bohm ring driven by time-dependent inhomogeneous magnetic fields is treated. The system possesses an su(2)(l) x su(2)(s) dynamical symmetry in both orbital angular momentum space and spin space, and is thus proved to be integrable according to algebraic dynamics. Based on the analytical solutions, the relevant physical quantities such as electric current, spin current, magnetization and conductance are calculated. It is found that for a magnetic field with pi/2 twist angle, the direction of spin-polarization will be reversed at zero magnetic flux. In the resonant rotating magnetic field, the spin transmission is oscillating with time t, and can reach unity, so that a complete spin flip can also be induced. The results obtained may be of practical significance for the design of nano-electromagnetic spin devices, such as a spin switch, in a controllable way.

Dynamical symmetry and analytical solutions of the non-autonomous quantum master equation of the dissipative two-level system: decoherence of the quantum register

Based on the non-autonomous quantum master equation, we investigate the dissipative and decoherence properties of the two-level atom system interacting with the environment of thermal quantum radiation fields. For this system, by a novel algebraic dynamic method, the dynamical symmetry of the system is found, the quantum master equation is converted into a Schrodinger-like equation and the non-Hermitian rate (quantum Liouville) operator of the master equation is expressed as a linear function of the dynamical u(2) generators. Furthermore, the integrability of the non-autonomous master equation has been proved for the first time. Based on the time-dependent analytical solutions, the asymptotic behaviour of the solution has been examined and the approach to the equilibrium state has been proved. Finally, we have studied the decoherence property of the multiple two-level atom system coupled to the thermal radiation fields, which are related to the quantum register.

Cross correlations and shot noise in a Y-shaped quantum dot

Motivated by recent experimental realization of a Y-shaped artificial Kondo impurity, we investigated the current correlations in a three-terminal Kondo dot modelled by the Anderson Hamiltonian. Using the Keldysh nonequilibrium Greens function technique, the multiterminal noise power spectrum at zero frequency is expressed in terms of the retarded (advanced) Greens function in the dot, which is valid for small voltages and low temperatures. The retarded (advanced) Greens function is determined under the truncation beyond the Lacroix approximation by the equation of motion approach and thus describes well the nonequilibrium Kondo physics. Our numerical results, along with analytical ones in some limit cases, can be tested by present technology.