1,964 research outputs found
Compressive and tensile axial strain reduced critical currents in Bi-2212 conductors
Mono and multifilamentary wires of BSCCO-2212 in Ag matrix are investigated in an axial strain experiment. The superconducting samples are soldered to a substrate that is bend in order to achieve a compressive or tensile axial strain. The I/sub c/-strain dependence is measured in magnetic fields up to 16 T at 4.2 K and the strain is varied from -2% to +1.2%. In these Bi-2122 samples any strain-induced I/sub c/ reduction is irreversible. Moreover a significant rise in I/sub c/ was never observed after changing the strain. Special attention is paid to the tensile axial strain regime (0 to 0.4%). A small but significant reduction in I/sub c/ is found in this case. The strain behaviour of these wires indicates that the I/sub c/ reduction is due to fractures in the superconducting filament
Euler characteristic and quadrilaterals of normal surfaces
Let be a compact 3-manifold with a triangulation . We give an
inequality relating the Euler characteristic of a surface normally embedded
in with the number of normal quadrilaterals in . This gives a relation
between a topological invariant of the surface and a quantity derived from its
combinatorial description. Secondly, we obtain an inequality relating the
number of normal triangles and normal quadrilaterals of , that depends on
the maximum number of tetrahedrons that share a vertex in .Comment: 7 pages, 1 figur
Field dependence of the critical current and its relation to the anisotropy of BSCCO conductors and coils
The design of HTS magnets is often based on the properties of a number of short samples that are presumed to be representative of the conductor to be used. Variability in conductor properties and inhomogeneity in the magnetic field distribution within the magnets, coupled with conductor anisotropy, provide a significant challenge to accurately predict the field dependence of the magnet critical current. This work is based on measured superconducting properties of Bi-2212 and Bi-2223 conductors at 4.2 K in parallel and perpendicular magnetic fields up to 33 T. Properties of double pancake units and stacks, from the same or similar conductor batches, are presented, based on measurements at self-field and in applied co-axial background magnetic fields up to 19 T. Modeling of this data is based on short sample properties in perpendicular field; the average grain misalignment is used as the parameter to quantify the anisotropy. Correlations and discrepancies between the measured data and models based on short sample data are discussed for Bi-2212 and Bi-2223 conductors
Quantum shape effects on Zeeman splittings in semiconductor nanostructures
We develop a general method to calculate Zeeman splittings of electrons and
holes in semiconductor nanostructures within the tight-binding framework. The
calculation is carried out in the electron-hole picture and is extensible to
the excitonic calculation by including the electron-hole Coulomb interaction.
The method is suitable for the investigation of quantum shape effects and the
anisotropy of the g-factors. Numerical results for CdSe and CdTe nanostructures
are presented
Prospects for a mHz-linewidth laser
We propose a new light source based on having alkaline-earth atoms in an
optical lattice collectively emit photons on an ultra-narrow clock transition
into the mode of a high Q-resonator. The resultant optical radiation has an
extremely narrow linewidth in the mHz range, even smaller than that of the
clock transition itself due to collective effects. A power level of order
is possible, sufficient for phase-locking a slave optical local
oscillator. Realizing this light source has the potential to improve the
stability of the best clocks by two orders of magnitude.Comment: minor revisions + shortening; factor 2 algebra mistake correcte
Limits to the critical current in Bi2Sr2Ca2Cu3Ox tape conductors: The parallel path model
An extensive overview of a model that describes current flow and dissipation in high-quality Bi2Sr2Ca2Cu3Ox superconducting tapes is provided. The parallel path model is based on a superconducting current running in two distinct parallel paths. One of the current paths is formed by grains that are connected at angles below 4°. Dissipation in this strongly linked backbone occurs within the grains and is well described by classical flux-creep theory. The other current path, the weakly linked network, is formed by superconducting grains that are connected at intermediate angles (4°â8°) where dissipation occurs at the grain boundaries. However, grain boundary dissipation in this weakly linked current path does not occur through Josephson weak links, but just as in the strongly linked backbone, is well described by classical flux creep. The results of several experiments on Bi2Sr2Ca2Cu3Ox tapes and single-grained powders that strongly support the parallel path model are presented. The critical current density of Bi2Sr2Ca2Cu3Ox tapes can be scaled as a function of magnetic field angle over the temperature range from 15 K to 77 K. Expressions based on classical flux creep are introduced to describe the dependence of the critical current density of Bi2Sr2Ca2Cu3Ox tapes on the magnetic field and temperature
Non-equilibrium quantum condensation in an incoherently pumped dissipative system
We study spontaneous quantum coherence in an out of equilibrium system,
coupled to multiple baths describing pumping and decay. For a range of
parameters describing coupling to, and occupation of the baths, a stable
steady-state condensed solution exists. The presence of pumping and decay
significantly modifies the spectra of phase fluctuations, leading to
correlation functions that differ both from an isolated condensate and from a
laser.Comment: 5 pages, 2 eps figure
Steady state entanglement in open and noisy quantum systems at high temperature
We show that quantum mechanical entanglement can prevail even in noisy open
quantum systems at high temperature and far from thermodynamical equilibrium,
despite the deteriorating effect of decoherence. The system consists of a
number N of interacting quantum particles, and it can interact and exchange
particles with some environment. The effect of decoherence is counteracted by a
simple mechanism, where system particles are randomly reset to some standard
initial state, e.g. by replacing them with particles from the environment. We
present a master equation that describes this process, which we can solve
analytically for small N. If we vary the interaction strength and the reset
against decoherence rate, we find a threshold below which the equilibrium state
is classically correlated, and above which there is a parameter region with
genuine entanglement.Comment: 5 pages, 3 figure
A Stochastic Liouville Equation Approach for the Effect of Noise in Quantum Computations
We propose a model based on a generalized effective Hamiltonian for studying
the effect of noise in quantum computations. The system-environment
interactions are taken into account by including stochastic fluctuating terms
in the system Hamiltonian. Treating these fluctuations as Gaussian Markov
processes with zero mean and delta function correlation times, we derive an
exact equation of motion describing the dissipative dynamics for a system of n
qubits. We then apply this model to study the effect of noise on the quantum
teleportation and a generic quantum controlled-NOT (CNOT) gate. For the quantum
CNOT gate, we study the effect of noise on a set of one- and two-qubit quantum
gates, and show that the results can be assembled together to investigate the
quality of a quantum CNOT gate operation. We compute the averaged gate fidelity
and gate purity for the quantum CNOT gate, and investigate phase, bit-flip, and
flip-flop errors during the CNOT gate operation. The effects of direct
inter-qubit coupling and fluctuations on the control fields are also studied.
We discuss the limitations and possible extensions of this model. In sum, we
demonstrate a simple model that enables us to investigate the effect of noise
in arbitrary quantum circuits under realistic device conditions.Comment: 36 pages, 6 figures; to be submitted to Phys. Rev.
A Study on the Noise Threshold of Fault-tolerant Quantum Error Correction
Quantum circuits implementing fault-tolerant quantum error correction (QEC)
for the three qubit bit-flip code and five-qubit code are studied. To describe
the effect of noise, we apply a model based on a generalized effective
Hamiltonian where the system-environment interactions are taken into account by
including stochastic fluctuating terms in the system Hamiltonian. This noise
model enables us to investigate the effect of noise in quantum circuits under
realistic device conditions and avoid strong assumptions such as maximal
parallelism and weak storage errors. Noise thresholds of the QEC codes are
calculated. In addition, the effects of imprecision in projective measurements,
collective bath, fault-tolerant repetition protocols, and level of parallelism
in circuit constructions on the threshold values are also studied with emphasis
on determining the optimal design for the fault-tolerant QEC circuit. These
results provide insights into the fault-tolerant QEC process as well as useful
information for designing the optimal fault-tolerant QEC circuit for particular
physical implementation of quantum computer.Comment: 9 pages, 9 figures; to be submitted to Phys. Rev.
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