Quantum entanglement in an oscillating macroscopic mirror

In this paper, we revisit the problem of quantum entanglement in an oscillating macroscopic mirror previously studied by Marshall et al. consisting of a modified Michelson interferometer where one of the mirrors is free to oscillate about its center of mass. A photon incident upon the oscillating mirror becomes entangled with the mirror, driving the mirror into a superposition of quantum states. Once the photon and mirror decouple, the mirror returns to its initial state. The purpose of our investigations was to optimize the parameter regime, taking into consideration the current state of technology and the demands imposed by the need to maintain a stable environment in the presence of thermal noise. Optimization should not demand ultra-low temperatures and this is reflected in our results. Our results also show that if the separation between states is maintained at 10^-14 m, the mirror size is reduced, making it easier to induce superposition in the mirror. The critical nature of mirror reflectivity and its connection to cavity decay rate was also revealed by our investigations. The results obtained through our investigations could be useful in quantum error correction, where decoherence negatively affects the results of computations performed by quantum computers. Finally, we note that we are only concerned with an isolated system, where no losses to the external environment occur and any decoherence that occurs within the system remains internal to the system; that is, any mention of decoherence refers specifically to recoverable decoherence.     

Stimulated emission of electromagnetic waves by Josephson vortices

We propose a new type of microwave generator in which the Josephson junction is used as an active medium. We find the modulation instability in a soliton chain moving in the Josephson junction coupled with a retarding system. The development of this instability leads to wave amplification in the transmission line. This effect is similar to that of grouping in BWT and TWT and results in the effective transformation of the soliton energy into the microwave radiation energy.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 3–4, pp. 287–291, March–April, 1995.     

High-field Josephson effect caused by the inside-grain vortices

We present a theoretical prediction of the new effects for critical currents across single grain boundaries in polycrystalline superconductors being under an applied magnetic field. It was shown that Josephson oscillation should be observed not only in the case of the increasing of the magnetic flux through the junction but also when vortex penetrates inside grain. In the intergrain Josephson junction the critical transport current is strongly dependent on the normalized grain size ratio, grain anisotropy ratio and grain coupling strength. These results imply that achieving a high degree of texture along these parameters is important for the obtaining of very high critical currents in pure polycrystalline samples. Also, it is shown that in contrast to the well-known Fraunhofer dependence, the period of oscillations corresponds to adding flux quantums per two grains.     

Line vortices in a three-dimensional ordered Josephson medium

Two equilibrium configurations of a line vortex in a three-dimensional ordered Josephson medium are considered: (i) the vortex core is at the center of a cell and (ii) the vortex core is on a contact. Infinite systems of equations describing these configurations are derived. In going to a finite system, the currents far away from the center are neglected. A new technique for solving the finite system of equations is suggested. It does not require smallness of phase discontinuities at all vortex cells and, therefore, can be applied for any values of pinning parameter I down to zero. The structures and energies of both equilibrium states for isolated line vortices are calculated for any I from the range considered. For I >0.3, a vortex can be thought of as fitting a square of 5×5 cells. For lower I, the vortex energy can be expressed as a sum of the energies of the small discrete core and the quasi-continuous outside. The core energy is comparable to the energy of the outside and is a major contributor to the vortex energy when I is not too small. For any I, the energy of the vortex centered on the contact is higher than the energy of the configuration centered at the center of the cell.     

The Josephson junction as a macroscopic radiating atom

We investigate the electrodynamic properties of a Josephson junction emitting coherent radiation and establish that it can be regarded as a two-level atom within the framework of the microscopic theory of superconductivity.     

Generation of vortices by a streamwise oscillating cylinder

The wake of a streamwise oscillating cylinder is presently investigated. The Reynolds number investigated is 300, based on the cylinder diameterd. The cylinder oscillates at an amplitude of 0.5d and a frequencyf _e/f_s=1.8, wheref _e is the cylinder oscillating frequency andf _s is the natural vortex shedding frequency of a stationary cylinder. Under these conditions the flow is essentially two dimensional. A two-dimensional direct numerical simulation (DNS) scheme has been developed to calculate the flow. The DNS results display a street of binary vortices, each containing two counter-rotating vortical structures, symmetrical about the centerline, which is in excellent agreement with measurements. The drag and lift on the cylinder have been examined. The time averaged drag and lift are 1.4 and 0, respectively, which are the same as those on a stationary cylinder at the sameRe. However, the fluctuating drag was high, about 2.68. It has been found that, being symmetrically formed about the centerline, the binary vortices induce an essentially zero fluctuating lift, which may have a profound implication in flow control and engineering.     

Quantum electrodynamic fluctuations of the macroscopic Josephson phase

We study the equilibrium dynamics of the relative phase in a superconducting Josephson link taking into account the quantum fluctuations of the electromagnetic vacuum. The photons act as a superohmic heat bath on the relative Cooper pair number and thus, indirectly, on the macroscopic phase difference φ. This leads to an enhancement of the mean square 〈φ²〉 that adds to the spread due to the Coulomb interaction carried by the longitudinal electromagnetic field. We also include the interaction with the electronic degrees of freedom due to quasiparticle tunneling, which couple to the phase and only indirectly to the particle number. The simultaneous inclusion of both the radiation field fluctuations and quasiparticle tunneling leads to a novel type of particle-bath Hamiltonian in which the quantum particle couples through its position and momentum to two independent bosonic heat baths. We study the interplay between the two mechanisms in the present context and find interference contributions to the quantum fluctuations of the phase. We explore the observability of the QED effects discussed here.     

Aspects of macroscopic quantum tunnelling in small Josephson junctions

Summary We study the effect of temperature on the lifetime of the resonant macroscopic quantum tunnelling voltage state. In view of experiments we consider a merit factor in order to balance between measuring extremely small signals of longer duration or larger signals of shorter duration.     

组合Bose场的分数自旋和分数统计性

对组合Bose子场，采用FS (Faddeev-Senjanovic) 路径积分量子化方法进行量子化.从量子Noether定理出发，给出量子分数自旋和分数统计性质. 关键词： 路径积分量子化 分数自旋 分数统计     

Directed motion of vortices in a current-driven Josephson junction network

Dynamics of directed motion of vortices in a Josephson junction network (JJN) with a ladder structure is studied using a numerical simulation. By applying spatial and temporal modulation of external bias currents, directed motion of vortices occurs in the absence of a ratchet-type asymmetric potential. In the present system, the asymmetry of the directed motion emerges as a dynamical effect due to the modulated bias current. Some dynamical effects such as mode-locking and vortex–antivortex excitation are relevant to the directed dynamics. We clarify the details of the directed motion of vortices in the JJN.     

Interaction of Josephson and Abrikosov vortices in a Y-Ba-Cu-O superconductor

The penetration of weak magnetic fields (0-n × 10⁻² T) into a Y-Ba-Cu-O ceramic supercon-ductor is studied. The nucleation fields for Abrikosov and Josephson vortices (fluxons) and the boundaries of the range in which the amplitude of the high-temperature superconductor response depends on the superposition of the dc field and weak variable fields caused by the fluxon mobility are experimentally determined.     

Interaction between optical phonons and Josephson vortices in layered superconductors

The effect of optical phonons on the stability of the in-phase motion regime of Josephson vortices in a layered superconductor has been studied by using an expression for the dielectric constant of a layered high-temperature superconductor. The dispersion characteristics of linear waves have been analyzed. It has been shown that the in-phase motion regime of the Josephson vortices can be spontaneously established in the layered superconductor.     

Electrically tunable macroscopic quantum tunneling in a graphene-based Josephson junction

Stochastic switching-current distribution in a graphene-based Josephson junction exhibits a crossover from the classical to quantum regime, revealing the macroscopic quantum tunneling of a Josephson phase particle at low temperatures. Microwave spectroscopy measurements indicate a multiphoton absorption process occurring via discrete energy levels in washboard potential well. The crossover temperature for macroscopic quantum tunneling and the quantized level spacing are controlled with the gate voltage, implying its potential application to gate-tunable superconducting quantum bits.     

Generation of macroscopic entangled coherent states with large Josephson junctions

We propose a simple and experimental architecture to generate macroscopic entanglement in a solid system which consists of two large Josephson junctions and a flux qubit. Through quantum measuring of flux qubit, entangled coherent states of two large Josephson junctions are obtained. The concurrence of entangled coherent states can be accommodated by adjusted systematic parameters. We also give a brief discussion on the experimental feasibility of this proposal.     

Fractional vortices and composite domain walls in flat nanomagnets

We provide a simple explanation of complex magnetic patterns observed in ferromagnetic nanostructures. To this end we identify elementary topological defects in the field of magnetization: ordinary vortices in the bulk and vortices with half-integer winding numbers confined to the edge. Domain walls found in experiments and numerical simulations in strips and rings are composite objects containing two or more of the elementary defects.