Wang Shun-Jin

Coherent Coupling of Double Quantum Dots Embedded in a Mesoscopic Ring

We theoretically study the properties of the ground state of a series-coupled double quantum dot embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that two dots can be coupled coherently, which is reflected in the appearance of parity effects and the complex current-phase relation in this system. This system might be a possible candidate for future device applications.

Fano Resonance and Persistent Currents in a Mesoscopic Open Ring with a Flux Loop in Side-Branch

Persistent circulating currents in a mesoscopic open ring with side-branch structures are derived by transfer matrix method in the framework of quantum waveguide theory on networks. The behavior of transmission probability may show the Fano resonance in the presence of geometric scattering of side-branch structures. It is shown that persistent circulating currents in the main loop with equal length of both arms in the absence of magnetic flux occur near the Fermi wave vectors where the Fano resonance appears. The numerical results show that a larger persistent current is associated with a stronger Fano resonance. The persistent circulating currents can be controlled by the flux of side-branch structure.

An Exact Solution to the Master Equation for the Dissipative Harmonic Oscillator Driven by an External Field

By using the algebraic structure in the operator dual space in the master equation for the driven dissipative harmonic oscillator, we have rewritten the master equation as a Schrodinger-like equation. Then we have used three gauge transformations and obtained an exact solution to the master equation in the particle number representation.

An Exact Solution and the Pancharatnam Phase for the Generalized Two-Mode Optical System

Using the algebraic dynamical method, we obtain the exact solution for the generalized two-mode optical system. From the solution, the Pancharatnam phase and the mean values of the number operators of the system are calculated. It is emphasized that the system can be used as a quantum memory.

Finite-size effect and Kondo screening effect in an A-B ring with a quantum dot

The properties of the ground state of a closed dot–ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that at T=0, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature. The physical quantities depend sensitively on both the parity of the system and the size of the ring; the rich physical behaviour can be attributed to the coexistence of both the finite-size effect and the Kondo screening effect. It is also possible to detect the Kondo screening cloud by measuring the persistent current or the zero field impurity susceptibility χimp directly in future experiments.

Reversible Decoherence of a Mesoscopic Superposition Driven by a Time-Dependent External Field

By using the normal-ordering technique in the coherent state representation, we discuss the dynamical evolution of the coherence of a Schrodinger cat state in a high Q cavity coupled to another resonator in the presence of external fields. During the progression of the coherence, we pay particular attention to the effect of the external fields, as well as to the initial phase of the Schrodinger cat state.

Eigenstates of q-Deformed Two-Photon Annihilation Operators and Their Nonclassical Properties

The generalized inverses of q-boson operators denoted by are introduced via their acting on the q-number states. The even and odd number eigenstates of two photon operators are constructed. It is shown that these states can show quantum-squeezing. However, their even number states always show quantum-antibunching, while their odd number states always show quantum-bunching. This result is different from that in recent paper (C.L. Mehta, A.K. Roy and G.M. Saxena, Phys. Rev. A46 (1992) 1565) where the odd number eigenstates of operators with q ≡ 1 show bunching for some parameter values and antibunching for other parameter values.

An Approach to the Study of Quantum Phase Problem Based on Action-Angle Wigner Distribution

Quantum phase distribution is expressed in terms of action-angle Wigner distribution function. It turns out to coincide with the limit case of Pegg–Barnett theory. This discrete phase space approach, in which some concepts such as quantum phase operator are not needed, can express phase-related quantities in a unified way. The expectation values and variances of and are the same as those of Susskind–Glogower theory. The phase-(particle) number uncertainty has a simple form in this formalism.

Exact Solutions of Non-Autonomous Quantum Systems with Quadratic Hamiltonian*

Based on algebraic dynamics, we present an algorithm to obtain exact solutions of the Schrodinger equation of non-autonomous quantum systems with Hamiltonian expressed in quadratic function of creation and annihilation operators of bosons. The Hamiltonian is treated as a linear function of generators of a symplectic group. Similar to the canonical transformation of classical dynamics, we employ a set of gauge transformations to gradually transform the Hamiltonian to a linear function of Cartan operators. The exact solutions are obtained by inverse gauge transformations. When the system is autonomous, this algorithm can obtain the normal mode of the Hamiltonian, as well as the eigenstates and eigenvalues.

A Unified Algebraic Dynamical Solution to General Two-Photon Algebra Systems

We use squeezing and displacement operators and apply algebraic dynamics to develop a unified solution of general time-dependent two-photon algebra systems. A set of orthogonal-and-normalized solutions are derived with the ground state being the conventional squeezed state. Landau system is given as an example.