STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.     

Ground state description of bound polarons in parabolic quantum wires

The Feynman-Haken variational path integral theory is, for the first time, generalized to calculate the ground-state energy of an electron coupled simultaneously to a Coulomb potential and to a longitudinal-optical (LO) phonon field in parabolic quantum wires. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron-phonon coupling constant than the Coulomb binding parameter and monotonically stronger as the effective wire radius decreases. We apply our calculations to several semiconductor quantum wires and find that the polaronic correction can be considerably large. Received 16 November 1998     

Electron self-trapping at quantum and classical critical points

Using Feynman path integral technique estimations of the ground state energy have been found for a conduction electron interacting with order parameter fluctuations near quantum critical points. In some cases only singular perturbation theory in the coupling constant emerges for the electron ground state energy. It is shown that an autolocalized state (quantum fluctuon) can be formed and its characteristics have been calculated depending on critical exponents for both weak and strong coupling regimes. The concept of fluctuon is considered also for the classical critical point (at finite temperatures) and the difference between quantum and classical cases has been investigated. It is shown that, whereas the quantum fluctuon energy is connected with a true boundary of the energy spectrum, for classical fluctuon it is just a saddle-point solution for the chemical potential in the exponential density of states fluctuation tail.     

Quantum computation based on magic-angle-spinning solid state nuclear magnetic resonance spectroscopy

Magic-angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy is shown to be a promising technique for implementing quantum computing. The theory underlying the principles of quantum computing with nuclear spin systems undergoing MAS is formulated in the framework of formalized quantum Floquet theory. The procedures for realizing state labeling, state transformation and coherence selection in Floquet space are given. It suggests that by this method, the largest number of qubits can easily surpass that achievable with other techniques. Unlike other modalities proposed for quantum computing, this method enables one to adjust the dimension of the working state space, meaning the number of qubits can be readily varied. The universality of quantum computing in Floquet space with solid state NMR is discussed and a demonstrative experimental implementation of Grover's search is given. Received 19 April 2001     

Hartree-Fock approximation of bipolaron state in quantum dots and wires

The bipolaronic ground state of two electrons in a spherical quantum dot or a quantum wire with parabolic boundaries is studied in the strong electron-phonon coupling regime. We introduce a variational wave function that can conveniently conform to represent alternative ground state configurations of the two electrons, namely, the bipolaronic bound state, the state of two individual polarons, and two nearby interacting polarons confined by the external potential. In the bipolaron state the electrons are found to be separated by a finite distance about a polaron size. We present the formation and stability criteria of bipolaronic phase in confined media. It is shown that the quantum dot confinement extends the domain of stability of the bipolaronic bound state of two electrons as compared to the bulk geometry, whereas the quantum wire geometry aggravates the formation of stable bipolarons.     

Effect of confined LO-phonons interactions on polarons in quantum well wires

Summary  The energy of an electron interacting with the confined longitudinaloptical (LO) phonons in a rectangular quantum well wire is calculated as a function of the length of one size of the wires by a modified Lee-Low-Pines variational method. It is shown that in a very small wire the electron self-energy has a peak. But for larger wires, the self-energies increase monotonically. It is also found that the energies of the quasi-one-dimensional polaron are related to the sizes of the crosssection rather than to the cross-sectional area of the wire. The authors of this paper have agreed to not receive the proofs for correction.     

Insufficiency of the quantum state for deducing observational probabilities

It is usually assumed that the quantum state is sufficient for deducing all probabilities for a system. This may be true when there is a single observer, but it is not true in a universe large enough that there are many copies of an observer. Then the probability of an observation cannot be deduced simply from the quantum state (say as the expectation value of the projection operator for the observation, as in traditional quantum theory). One needs additional rules to get the probabilities. What these rules are is not logically deducible from the quantum state, so the quantum state itself is insufficient for deducing observational probabilities. This is the measure problem of cosmology.     

Rayleigh-Schrödinger perturbation theory for electron-phonon interaction in two dimensional quantum dots with asymmetric parabolic potential

Within the framework of the second-order Rayleigh-Schr?dinger perturbation theory, we investigate the effects of the interaction of the electron and longitudinal-optical phonons in two-dimensional semiconductive quantum dots with respect to a general potential. We propose a simple expression for the ground state energy, and compare it with those obtained by Landau-Pekar strong coupling theory. It is shown both analytically and numerically that the results obtained from the second-order Rayleigh-Schr?dinger perturbation theory could be better than those from Landau-Pekar strong coupling theory when the coupling constant is sufficiently small. Moreover, some interesting problems, such as polarons in quasi-one-dimensional quantum wires, and quasi-zero-dimensional asymmetric or symmetric quantum dots can be easily discussed only by taking different limits. After the numerical calculations, we find that there exists a simple dimensional scaling and symmetry relation for the ground state polaron energy. Furthermore, we apply our results to some weak-coupling polar semiconductors such as GaAs, CdS. It is shown that the polaronic effects are found to be quiet appreciable if the confinement lengths and smaller than a few nanometers. Received: 3 December 1997 / Revised: 6 July 1998 / Accepted: 17 September 1998     

The effect of a strong magnetic field on the binding energy and the photoionization process in quantum-well wires

Within the effective-mass approximation, we have investigated the influence of a strong magnetic field on the ground state binding energy and the photon energy dependence of the photoionization cross-section of a shallow donor impurity in a quasi-one-dimensional rectangular quantum wire with infinite and finite potential barriers, using a variational approach. It is found that the binding energy and the photoionization cross-section as a function of photon energy were drastically dependent on the sizes of the wire, the potential well heights and the applied magnetic field.     

Effect of Hydrostatic Pressure on Interband Transitions in Coupled Quantum Wires

We calculate the exciton binding energy and interband optical absorption in a rectangular coupled quantum wire under the hydrostatic pressure in the effective-mass approximation, using the variational approach. It is found that the interband optical absorption strongly depend on the hydrostatic pressure and the coupling parameter, and that the magnitude of the absorption coefficient for the HH1-E1 transition in the coupled quantum wire is larger than that of the single quantum wire.     

抛物量子点中弱耦合磁极化子的性质

应用线性组合算符和幺正变换方法研究了抛物量子点中磁极化子的基态性质。得出基态能和基态束缚能随有效束缚强度增大而减小,随回旋频率增大而增大。当有效柬缚强度给定,基态能量随电子-体纵光学声子耦合强度增加而减小。当有效束缚强度l0>0.3时,电子-体纵光学声子耦合强度的变化对量子点中弱耦合磁极化子的基态能量的影响变得显著。当有效束缚强度l0<0.3时,电子-体纵光学声子耦合强度的变化对基态能量影响很小。由于有效束缚强度与量子点受限强度的平方根成反比,所以量子点受限越强,基态能量、基态束缚能越大,电子一体纵光学声子耦合强度和磁场的变化对量子点的影响相对越小;当量子点受限变弱时,电子-声子耦合强度变化对量子点的影响变大,磁场对量子点的影响也变大,所以在量子点中,极化子对量子点的影响不容忽略。     

Antenna model for wire lasers

An antenna model is proposed for long (L > A) lasers with subwavelength cross sections (wire lasers). It is shown that the far-field pattern of the wire lasers is determined by the ratio of the wavelength to the length. The radiation of the wire laser is predicted to be concentrated in a narrow beam theta approximately radical(2lambda/L) for laser modes where the longitudinal phase velocity is in synchronism with the velocity of light in air. Experimental results obtained using a terahertz quantum cascade wire laser are in agreement with the model.     

抛物形量子点中弱耦合极化子的性质

采用线性组合算符和幺正变换方法研究了抛物形量子点中弱耦合极化子的基态能量和束缚能。计算结果表明,基态能量和束缚能随有效束缚强度增加而减小。随着有效束缚强度逐渐加大,最后逐渐趋于体结构极化子的基态能量。当有效束缚强度给定,基态能随电子-体纵光学声子耦合强度增加而减小。由于有效束缚强度与量子点受限强度平方根成反比,所以量子点受限越强,基态能和束缚能越大,电子-体纵光学声子耦合强度的变化对量子点的影响越小。当量子点受限变弱时,电子-体纵光学声子耦合强度变化对量子点的影响变大。所以在量子点弱受限区域,极化子对量子点的影响不容忽略。     

非对称量子点中极化子的性质

采用线性组合算符和幺正变换方法研究了非对称量子点中强、弱耦合极化子的性质,导出了非对称量子点中强、弱耦合极化子的基态能量和基态结合能随量子点的横向、纵向有效受限长度,电子-声子耦合强度的变化关系.通过数值计算,结果表明,量子点中强、弱耦合极化子的基态能量和基态结合能随量子点的横向、纵向有效受限长度的减小而迅速增大.     

Statistical-Mechanical Entropy of a Black Hole with a Global Monopole to All Orders in Planck Length

Considering corrections to all orders in the Planck length on the quantum state density from the generalized uncertainty principle, we calculate the statistical entropy of the scalar field in the global monopole black hole spacetime without any artificial cutoff. It is shown that the entropy is proportional to the horizon area.     

Fresnel-Transform's Quantum Correspondence and Quantum Optical ABCD Law

Corresponding to the Fresnel transform there exists a unitary operator in quantum optics theory, which could be known the Fresnel operator (FO). We show that the multiplication rule of the FO naturally leads to the quantum optical ABCD law. The canonical operator methods as mapping of ray-transferABCD matrix is explicitly shown by the normally ordered expansion of the FO through the coherent state representation and the technique of integration within an ordered product of operators. We show that time evolution of the damping oscillator embodies the quantum optical ABCD law.     

Controlled Teleportation of Multi-Qudit Quantum Information

We present a controlled teleportation scheme for teleporting an arbitrary superposition state of an M-qudit quantum system. The scheme employs only one entangled state as quantum channel, which consists of the qudits from Alice, Bob and every agent. The quantum operations used in the teleportation process are a series of qudit Bell measurements, single-qudit projective measurements, qudit H-gates, qudit-Pauli gates and qudit phase gates. It is shown that the original state can be restored by the receiver only on the condition that all the agents collaborate. If any agent does not cooperate, the original state can not be fully recovered.     

Quantum light memory using quantum dot spins in a microdisk cavity via Raman process

A solid state quantum circuit where an ensemble of self-assembled quantum dots in a microdisk cavity served as long-lived quantum light memory, is investigated. It is shown that via laser coupling Raman process, the coherent transfer between the light field (qubits) and the ensemble spin states of the quantum dots can be efficient and fast. The coherence properties of the system are analyzed, which enables us to obtain a long coherence time.     

Efficient Construction of High-Dimensional Cluster State

We present a scheme for efficiently constructing high-dimensional cluster state using probabilistic entangling quantum gates. It is shown that the required computational overhead scales efficiently both with lip and n even if all the entangling quantum gates only succeed with an arbitrary small probability, where p is the success probability of the entangling quantum gate and n is the number of qubits in the computation.     

Quantum Fisher Information of Superpositions with 3-Qubit Ghz-Class State and W State

We investigate the quantum Fisher information in a general superposition of a GHZ-class state and a W state. It is shown that the mean spin direction is determined by the four coefficients of superposition and the relative phase between the GHZ-class state and the W state and it is independent of the relative phase between the GHZ-class state; while the length of mean spin is only determined by the four superposition coefficients. It is also shown that the quantum Fisher information of the superposition state is determined by the four superposition coefficients and the two relative phases.