Full observation of single-atom dynamics in cavity QED

We report the use of broadband heterodyne spectroscopy to perform continuous measurement of the interaction energy E_int between one atom and a high-finesse optical cavity, during individual transit events of ≈250 μs duration. We achieve a fractional sensitivity ≈4×10^-4/ to variations in E_int/? within a measurement bandwidth that covers 2.5 decades of frequency (1–300 kHz). Our basic procedure is to drop cold cesium atoms into the cavity from a magnetooptic trap while monitoring the cavity’s complex optical susceptibility with a weak probe laser. The instantaneous value of the atom–cavity interaction energy, which in turn determines the coupled system’s optical susceptibility, depends on both the atomic position and (Zeeman) internal state. Measurements over a wide range of atom–cavity detunings reveal the transition from resonant to dispersive coupling, via the transfer of atom-induced signals from the amplitude to the phase of light transmitted through the cavity. By suppressing all sources of excess technical noise, we approach a measurement regime in which the broadband photocurrent may be interpreted as a classical record of conditional quantum evolution in the sense of recently developed quantum trajectory theories. Received: 2 June 1998 / Revised version: 4 December 1998 / Published online: 31 March 1999     

Quantum Correlation in Circuit QED Under Various Dissipative Modes

Dynamical evolutions of quantum correlations in circuit quantum electrodynamics (circuit-QED) are investigated under various dissipative modes. The influences of photon number, coupling strength, detuning and relative phase angle on quantum entanglement and quantum discord are compared as well. The results show that quantum discord may be less robust to decoherence than quantum entanglement since the death and revival also appears. Under certain dissipative mode, the decoherence subspace can be formed in circuit-QED due to the cooperative action of vacuum field. Whether a decoherence subspace can be formed not only depends on the form of quantum system but also relates closely to the dissipative mode of environment. One can manipulate decoherence through manipulating the correlation between environments, but the effect depends on the choice of initial quantum states and dissipative modes. Furthermore, we find that proper relative phase of initial quantum state provides one means of suppressing decoherence.     

Lightfront DHW Functions and Strong Field QED

We investigate strong-field effects in QED using a phase space approach based on the Dirac–Heisenberg–Wiger function. We calculate the electron–positron DHW function in a null (laser) field background exactly, using lightfront field theory. The DHW function exhibits the distinct features of strong-field QED: in particular we explicitly identify the effective fermion mass for arbitrary temporal pulse profiles.     

Gauge dependence of Green’s functions in QCD and QED

When all Green’s functions are known in a given gauge we may raise the question of whether it is possible or not to derive the corresponding ones in a different gauge. The answer is negative in QCD but affirmative in QED provided that we confine ourselves to the covariant gauge characterized by a gauge parameter α. We shall discuss the physical significance of this conclusion.     

充分发挥例题作用

物理学因其自身的学科特点和规律而在培养学生的创新思维方面有着其他学科无法替代的作用.充分发挥例题的作用,是培养和发展学生创新思维和创新能力的有效途径之一.1 通过例题教学巩固深化物理知识 教师有针对性地设计例题,引导学生探     

Green Functions for the Wrong-Sign Quartic

It has been shown that the Schwinger-Dyson equations for non-Hermitian theories implicitly include the Hilbert-space metric. Approximate Green functions for such theories may thus be obtained, without having to evaluate the metric explicitly, by truncation of the equations. Such a calculation has recently been carried out for various PT-symmetric theories, in both quantum mechanics and quantum field theory, including the wrong-sign quartic oscillator. For this particular theory the metric is known in closed form, making possible an independent check of these approximate results. We do so by numerically evaluating the ground-state wave-function for the equivalent Hermitian Hamiltonian and using this wave-function, in conjunction with the metric operator, to calculate the one- and two-point Green functions. We find that the Green functions evaluated by lowest-order truncation of the Schwinger-Dyson equations are already accurate at the 6% level. This provides a strong justification for the method and a motivation for its extension to higher order and to higher dimensions, where the calculation of the metric is extremely difficult.     

Retarded Green Functions and Modified Dispersion Relations

We analyse a three-field model, which describes a relativistic two-level atom interactingwith a radiation bath. From the one-loop retarded propagators at finite temperature we extract the transition rates and the modifications of the dispersion relations. To further investigate the relationships between propagators and these physical quantities, we analyse a non-equilibrium situation in which an additional atom is present in the bath. Preliminary results indicate that transition rates can still be extracted from the (retarded) propagator. This approach couldtherefore be useful in relating high-frequency (trans-Planckian) dispersion relations to the physical processes occurring at these scales.     

Renormalons in QED

Four-dimensional Quantum Electrodynamics is studied in the limit of a large number of leptons (N→∞) up to terms of order 1/N inclusive. It is proven that closed analytic expressions can be given for the Borel transform of Green's functions. Furthermore, an appropriate renormalization scheme is introduced. In this scheme, up to first order in the 1/N expansion, all the renormalization group functions are polynomials. Since renormalon singularities are explicitly known, the Borel summability of QED is discussed.     

Generalized Green Functions and Current Correlations in the TASEP

We study correlation functions of the totally asymmetric simple exclusion process (TASEP) in discrete time with backward sequential update. We prove a determinantal formula for the generalized Green function which describes transitions between positions of particles at different individual time moments. In particular, the generalized Green function defines a probability measure at staircase lines on the space-time plane. The marginals of this measure are the TASEP correlation functions in the space-time region not covered by the standard Green function approach. As an example, we calculate the current correlation function that is the joint probability distribution of times taken by selected particles to travel given distance. An asymptotic analysis shows that current fluctuations converge to the Airy₂ process.     

Introduction of Anomalous Green Functions into Quantum Chromodynamics

The nonperturbative vacuum structure of Quantum Chromodynamic is studied with the help of a generalization of the formalism of Green functions which corresponds to the method of Gorkov and Nambu in the theory of superconductivity. Taking into account the existence of gluon condensation the selfenergy of the gluon-quasi-particles is calculated with the help of modified rules for Feynman diagrams. The resulting integral equations for the effective field parameters contain particular solutions with an energy gap in the spectrum of the quasi-particles and a phase transition at a critical momentum.     

Bremsstrahlung in self-field QED

We present a fully relativistic formulation of the theory of electron-nucleus Bremsstrahlung, within the context of self-field QED, as advanced recently by Barut and his co-workers. The Bremsstrahlung emission cross-section, reported here, is also shown to reduce to the standard correct nonrelativistic limit, in the dipole approximation.     

Exact results in QED

We consider several applications of the simplest nonlinear QED phenomena described by the light-by-light (LBL) scattering tensor. Among the relevant processes we present the splitting of high energy photon in a Coulomb field, calculate the asymptotics of differential photon photon elastic scattering. We show that LBL mechanism of the four photon mode of neutral pion decay have a dominant role compared, for instance, with the quark loop Feynman amplitude contribution. The mechanisms of creation of two and three gluon jets at colliding electro-positron beams is analyzed. We calculate also the contribution of LBL mechanism to the ortho-positronium decay width. One Of the important application is the analytic calculation of the QED contribution to the anomalous magnetic moment of the muon arising from LBL mechanism realized through electron positron loops, which is enhanced by the logarithm of the ratio of muon to electron masses. The modification of the QED kernel, which takes into account the QED polarization operator is used to extract the pure strong interaction contribution. We consider as well the problem of the Coulomb law modification. At second part of review we consider Moeller scattering process and RC to it. We show that RC are in agreement with renormalization group approach and could be taken into account in form of Drell-Yan process cross-section.     

Renormalization schemes in QED

We discuss the method of dimensional renormalization as applied to quantum electrodynamics. Then we give a general method which allows one to compare the various quantities like coupling constants and masses that appear in different renormalization schemes.     

Gell-Mann-Low function in QED

The Gell-Mann-Low function β(g) in QED (g is the fine structure constant) is reconstructed. At large g, it behaves as β_∞ g ^α with α≈1 and β_∞≈1.     

Multiparticle bound states in QED

The relation between multiparticle Schrödinger equations and the underlying field theory for weakly coupled systems is clarified. A systematic perturbation theory for the energy levels is presented the first term of which is the eigenvalue of a Schrödinger equation with relativistic kinematics.     

Polarizability sum rules in QED

The well founded total photoproduction and the, assumed substraction free, longitudinal photoproduction polarizability sum rules are checked in QED at the lowest non-trivial order. The first one is shown to hold, whereas the second one turns out to need a subtraction, which makes its usefulness for determining the electromagnetic polarizabilities of the nucleons quite doubtful.     

Dimensional regularization in massless QED

Using dimensional regularization, the cross section for scattering of massless electrons in an external potential is examined to order α³. The masses of the photon and the electron are put equal to zero from the beginning, and the continuous number of dimensions, n, serves as a regularizing parameter for all ultraviolet divergences as well as for those associated with the two kinds of massless particles. The result is a double- and single-pole structure around n = 4 for the separate terms in the cross section. These poles are found to cancel provided the hard bremsstrahlung is also taken into account.