Phonon-assisted absorption of hypersound in a rectangular quantum wire

The mechanisms of the Landau-Rumer process and of the process of two-phonon decay of microwave phonons are considered for a rectangular quantum wire. For GaAs wires with a free surface and various transverse dimensions, the coefficients of absorption of confined microwave phonons of the lowest hybrid width mode are calculated. The phononic and electronic mechanisms of hypersound absorption in rectangular wires and unbounded solids are compared. At low temperatures, the inclusion of cubic anharmonicity in a wire leads to an exponential temperature dependence of the hypersound absorption for the Landau-Rumer process and to the dominant absorption due to decay processes; in the latter case, the exponent of the frequency dependence decreases. At sufficiently high temperatures, the phonon-assisted absorption dominates over the electron-assisted absorption in a quantum wire of a nondegenerate material.     

Quantenfeldtheorie wechselwirkender Vielteilchensysteme zur semiklassischen Beschreibung von kollektiver Bewegung mit großen Amplituden

By using path integral methods a collective quantum field theory of interacting many-body systems is developed, the classical limit of which is given by the time-dependent mean-field approximation. In this way the mean-field approximation is embedded into the full quantum mechanics and the quantum corrections to the “classical” mean-field approximation can be systematically evaluated. By including the dominant quantum corrections to the mean-field approximation a semiclassical theory of large amplitude collective motions in many-body-systems, which show a highly nonlinear dynamic and are not accessible to perturbation theoretical methods, is derived. The semiclassical theory is developed explicitly for bound states and decay processes like nuclear fission. In the case of bound states this leads to the quantization of the time-dependent Hartree-Fock-Theory, which is demonstrated for a uniform nuclear rotation.     

On transformations of physical systems

A universal, unified theory of transformations of physical systems based on the propositions of probabilistic physics is developed. This is applied to the treatment of decay processes and intramolecular rearrangements. Some general features of decay processes are elucidated. A critical analysis of the conventional quantum theories of decay and of Slater's quantum theory of intramolecular rearrangements is given. It is explained why, despite the incorrectness of the decay theories in principle, they can give correct estimations of decay rate constants. The reasons for the validity of the Arrhenius formula for the temperature dependence of an intramolecular rearrangement rate constant are discussed. A criterion for the possibility of a proper intramolecular rearrangement is given. The issue of causality in quantum physics is settled.     

Size-dependence of the anharmonicities in the vibrational potential of colloidal CdSe nanocrystals

We investigated multi-phonon processes in two types of CdSe nanocrystals, spherical quantum dots and nanorods by temperature-dependent Raman spectroscopy. We find the anharmonic constants relating to the two- and three-phonon decay channels to systematically change with changing nanocrystal size. The contributions from two-phonon processes decrease with decreasing diameter, while the contributions from higher-order processes increase. The anharmonicities in the vibrational potential increase due to the phonon confinement, and we did not find a shape dependence.     

微腔中单量子点的受激辐射行为

研究了微腔中单模光场与一个三能级量子点相互作用系统.利用系统主方程作数值模拟计算微腔中单量子点的净受激辐射率随量子点和腔模耦合强度的变化,同时研究激光能级间衰减率γ23对净受激辐射率的影响.发现净受激辐射率随泵浦强度增大而呈现较快趋于饱和的曲线,这些饱和曲线随着耦合强度的增加又较快地趋于一个极值.而激光能级间衰减率γ23的增大使该激光的净受激辐射率的曲线组表现出相反的规律.     

强弱耦合板-腔系统的声辐射模态计算与分析EI北大核心CSCD

针对板-腔耦合系统的声辐射模态(ARM)计算问题,提出了一种基于能量原理的声辐射模态计算方法,该方法从能量原理的动力学方程构建起声压模态幅值和结构模态幅值的关系,通过将声势能表示为结构模态幅值向量的二次型形式,得到板-腔耦合系统的声辐射模态,弥补了前人理论在解决声腔为阻抗壁面和结构-声为强耦合条件时的不足。通过数值算例验证了本文计算方法的正确性和有效性,在此基础上分析了壁面和结构-声耦合条件变化对声辐射模态特性的影响。结果表明:声辐射模态辐射效率曲线会在声腔模态频率处产生峰值,阻抗壁面的引入会降低声辐射模态辐射效率在峰值处的幅值,并且阻抗值越小,幅值衰减效应越明显,具体表现为声势能曲线在辐射效率峰值频率处幅值会下降;强耦合条件下低频段声势能响应主要由弹性板结构模态激发,响应峰值密度更高,幅值更低。低频同频宽的声辐射模态辐射效率峰值数更少,峰值频率更高。     

Nonequilibrium Relaxation of Bose-Einstein Condensates: Real-Time Equations of Motion and Ward Identities

We present a field-theoretical method to obtain consistently the equations of motion for small amplitude condensate perturbations in a homogeneous Bose-condensed gas directly in real time. It is based on a linear response and combines the Schwinger-Keldysh formulation of nonequilibrium quantum field theory with the Nambu-Gor'kov formalism of quasiparticle excitations in the condensed phase and the tadpole method in quantum field theory. This method leads to causal equations of motion that allow us to study the nonequilibrium evolution as an initial value problem. It also allows us to extract directly the Ward identities, which are a consequence of the underlying gauge symmetry and which in equilibrium lead to the Hugenholtz-Pines theorem. An explicit one-loop calculation of the equations of motion beyond the Hartree-Fock-Bogoliubov approximation reveals that the nonlocal, absorptive contributions to the self-energies corresponding to the Beliaev and Landau damping processes are necessary to fulfill the Ward identities in or out of equilibrium. It is argued that a consistent implementation at low temperatures must be based on the loop expansion, which is shown to fulfill the Ward identities order by order in perturbation theory.     

Ward identities and chiral anomalies in stochastic quantization

We derive the Ward identities (WI) for vector and axial currents in stochastic quantization at any given fictitious time t. This is achieved through a functional integral representation of the fermionic Langevin equations. The currents for this effective field theory differ in general from the naive ones; if stochastic regularization is used they are both conserved. We establish the connection between those WI and the field theory ones. The physical source of chiral anomalies is identified: these result from the quantum fluctuations in the fictitious time evolution of the system. In this context, both a traditional regularization method (Pauli-Villars) and stochastic regularization are considered.     

Influence of thermal photons on the fundamental model of quantum optics with a coherent pump

A quantum system consisting of a two-level atom interacting with a single field mode of a high-Qcavity under influence of a coherent pump is considered. The analytical solutions for the P and Q distribution functions are obtained in the limit of large Rabi frequencies. In the presence of thermal photons, the P distribution function loses its property of restriction by the range on the complex plane and becomes an analytical function. When the ratio of the atomic decay rate to the cavity mode damping rate is smaller than 4, the effect of phase bistability appears. Absorptive optical bistability is absent in this case. On the basis of the system of Fokker-Planck equations for the quasi-probabilities corresponding to the atom being on the upper and lower atomic levels, computer simulation of the stochastic trajectory of motion for the system is presented.     

Finite temperature theory of metastable anharmonic potentials

The decay rate for a particle in a metastable cubic potential is investigated in the quantum regime by the Euclidean path integral method in semiclassical approximation. The imaginary time formalism allows one to monitor the system as a function of temperature. The family of classical paths, saddle points for the action, is derived in terms of Jacobian elliptic functions whose periodicity sets the energy-temperature correspondence. The period of the classical oscillations varies monotonically with the energy up to the sphaleron, pointing to a smooth crossover from the quantum to the activated regime. The softening of the quantum fluctuation spectrum is evaluated analytically by the theory of the functional determinants and computed at low T up to the crossover. In particular, the negative eigenvalue, causing an imaginary contribution to the partition function, is studied in detail by solving the Lamè equation which governs the fluctuation spectrum. For a heavvy particle mass, the decay rate shows a remarkable temperature dependence mainly ascribable to a low lying soft mode and, approaching the crossover, it increases by a factor five over the predictions of the zero temperature theory. Just beyond the peak value, the classical Arrhenius behavior takes over. A similar trend is found studying the quartic metastable potential but the lifetime of the latter is longer by a factor ten than in a cubic potential with same parameters. Some formal analogies with noise-induced transitions in classically activated metastable systems are discussed.     

A scheme of quantum repeaters with single atom and cavity-QED

To implement long-distance quantum communication, quantum repeaters have been proposed. The distribution and storage of quantum entanglement are essential to implement quantum repeaters. Here, we propose a new quantum repeaters protocol which is based on single atom-cavity QED. We use simple long-life two-level atoms to store quantum entanglement unlike three-level atoms which are commonly used in other quantum repeaters proposals. The property of long life-time (T₁) and transverse decay time (T₂) between excited level and ground level, such as rare-earth atoms, may store quantum entanglement as long as possible. Modulations of cavity mode and rate of coupling between cavity mode and output mode are also key steps to our scheme. And the efficiency of our protocol is analyzed by quantum trajectory theory.     

Chiral invariant Gross-Neveu model: Classical versus quantum

We present a unification of different and independently investigated aspects of the chiral invariant Gross-Neveu model. Special emphasis is placed on the relevance of classical (c-number, non Grassmann) spinor solutions of the G-N field equations for the construction, and thus understanding of the respective quantized Fermi model. To get an insight into the “quantum meaning of classical field theory” if specialized to the G-N case, we perform the path integral quantization procedure which first leads to the Fermi oscillator problem, and then, after appropriate generalizations, to the quantum Fermi G-N model. Path integrals are carried out with respect to c-number spinor paths only, and in fact no reference is necessary to the Grassmann algebra methods, which are conventionally used to integrate out fermions.     

Quantum Dynamics in the Fermi–Pasta–Ulam Problem

We study a quantum chain of oscillators with nonlinear quartic interactions, under the “narrow packet” approximation. We analyse the dynamics of quantum corrections and the conditions at which the quantum solution for average complex amplitude converges to the corresponding classical unstable solution which describes the four-wave decay processes of phonons. We develop an asymptotic theory by using a small quasiclassical parameter, and determine the characteristic time scale for which the evolution of decay processes is essentially specified by quantum effects. AMS Subject classification: Primary: 35F10 Secondary: 35Q40, 35B40     

Macroscopic description of time evolution of quantum systems

The evolution of a quantum system in which a specified set of variables is subjected to measurement over the time interval [t_o, t] is considered. It is shown that if the specified set of variables is macroscopically complete (i.e., it permits continuous measurement), then the expected values of these variables satisfy a closed system of integral equations. The quantum state of the system is then described by a Gibbs density matrix.     

Difference equations in spin chains with a boundary

Correlation functions and form factors in vertex models or spin chains are known to satisfy certain difference equations called the quantum Knizhnik-Zamolodchikov equations. We find similar difference equations for the case of semi-infinite spin chain systems with integrable boundary conditions. We derive these equations using the properties of the vertex operators and the boundary vacuum state, or alternatively through corner transfer matrix arguments for the eight-vertex model with a boundary. The spontaneous boundary magnetization is found by solving such difference equations. The boundary S-matrix is also proposed and compared, in the sine-Gordon limit, with Ghoshal-Zamolodchikov's result. The axioms satisfied by the form factors in the boundary theory are formulated.