541 research outputs found
Dynamical fidelity of a solid-state quantum computation
In this paper we analyze the dynamics in a spin-model of quantum computer.
Main attention is paid to the dynamical fidelity (associated with dynamical
errors) of an algorithm that allows to create an entangled state for remote
qubits. We show that in the regime of selective resonant excitations of qubits
there is no any danger of quantum chaos. Moreover, in this regime a modified
perturbation theory gives an adequate description of the dynamics of the
system. Our approach allows to explicitly describe all peculiarities of the
evolution of the system under time-dependent pulses corresponding to a quantum
protocol. Specifically, we analyze, both analytically and numerically, how the
fidelity decreases in dependence on the model parameters.Comment: 9 pages, 6 figures, submitted to PR
Breakdown of Universality in Quantum Chaotic Transport: the Two-Phase Dynamical Fluid Model
We investigate the transport properties of open quantum chaotic systems in
the semiclassical limit. We show how the transmission spectrum, the conductance
fluctuations, and their correlations are influenced by the underlying chaotic
classical dynamics, and result from the separation of the quantum phase space
into a stochastic and a deterministic phase. Consequently, sample-to-sample
conductance fluctuations lose their universality, while the persistence of a
finite stochastic phase protects the universality of conductance fluctuations
under variation of a quantum parameter.Comment: 4 pages, 3 figures in .eps format; final version to appear in
Physical Review Letter
Decay of the classical Loschmidt echo in integrable systems
We study both analytically and numerically the decay of fidelity of classical
motion for integrable systems. We find that the decay can exhibit two
qualitatively different behaviors, namely an algebraic decay, that is due to
the perturbation of the shape of the tori, or a ballistic decay, that is
associated with perturbing the frequencies of the tori. The type of decay
depends on initial conditions and on the shape of the perturbation but, for
small enough perturbations, not on its size. We demonstrate numerically this
general behavior for the cases of the twist map, the rectangular billiard, and
the kicked rotor in the almost integrable regime.Comment: 8 pages, 3 figures, revte
Ehrenfest times for classically chaotic systems
We describe the quantum mechanical spreading of a Gaussian wave packet by
means of the semiclassical WKB approximation of Berry and Balazs. We find that
the time scale on which this approximation breaks down in a chaotic
system is larger than the Ehrenfest times considered previously. In one
dimension \tau=\fr{7}{6}\lambda^{-1}\ln(A/\hbar), with the Lyapunov
exponent and a typical classical action.Comment: 4 page
Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2
We have performed low-temperature specific heat and thermal conductivity
measurements on the Ni-pnictide superconductors BaNiAs
(=0.7 K and SrNiP (=1.4 K). The temperature
dependences and of the two compounds are similar to the
results of a number of s-wave superconductors. Furthermore, the concave field
responses of the residual for BaNiAs rules out the presence of
nodes on the Fermi surfaces. We postulate that fully gapped superconductivity
could be universal for Ni-pnictide superconductors. Specific heat data on
BaLaNiAs shows a mild suppression of and
relative to BaNiAs.Comment: 5 pages, 3 figures, to be published in J. Phys.: Conf. Se
Identification of target-specific bioisosteric fragments from ligand-protein crystallographic data
Bioisosteres are functional groups or atoms that are structurally different but that can form similar intermolecular interactions. Potential bioisosteres were identified here from analysing the X-ray crystallographic structures for sets of different ligands complexed with a fixed protein. The protein was used to align the ligands with each other, and then pairs of ligands compared to identify substructural features with high volume overlap that occurred in approximately the same region of geometric space. The resulting pairs of substructural features can suggest potential bioisosteric replacements for use in lead-optimisation studies. Experiments with 12 sets of ligand-protein complexes from the Protein Data Bank demonstrate the effectiveness of the procedure
Fractional plateaus in the Coulomb blockade of coupled quantum dots
Ground-state properties of a double-large-dot sample connected to a reservoir
via a single-mode point contact are investigated. When the interdot
transmission is perfect and the dots controlled by the same dimensionless gate
voltage, we find that for any finite backscattering from the barrier between
the lead and the left dot, the average dot charge exhibits a Coulomb-staircase
behavior with steps of size e/2 and the capacitance peak period is halved. The
interdot electrostatic coupling here is weak. For strong tunneling between the
left dot and the lead, we report a conspicuous intermediate phase in which the
fractional plateaus get substantially altered by an increasing slope.Comment: 6 pages, 4 figures, final versio
Zeeman smearing of the Coulomb blockade
Charge fluctuations of a large quantum dot coupled to a two-dimensional lead
via a single-mode good Quantum Point Contact (QPC) and capacitively coupled to
a back-gate, are investigated in the presence of a parallel magnetic field. The
Zeeman term induces an asymmetry between transmission probabilities for the
spin-up and spin-down channels at the QPC, producing noticeable effects on the
quantization of the grain charge already at low magnetic fields. Performing a
quantitative analysis, I show that the capacitance between the gate and the
lead exhibits - instead of a logarithmic singularity - a reduced peak as a
function of gate voltage. Experimental applicability is discussed.Comment: 5 pages, 3 figures (Final version
Capacitance of a quantum dot from the channel-anisotropic two-channel Kondo model
We investigate the charge fluctuations of a large quantum dot coupled to a
two-dimensional electron gas via a quantum point contact following the work of
Matveev. We limit our discussion to the case where exactly two channels enter
the dot and we discuss the role of an anisotropy between the transmission
coefficients (for these two channels) at the constriction. Experimentally, a
channel-anisotropy can be introduced applying a relatively weak in-plane
magnetic field to the system when only one ``orbital'' channel is open. The
magnetic field leads to different transmission amplitudes for spin-up and
spin-down electrons.
In a strong magnetic field the anisotropic two-channel limit corresponds to
two (spin-polarized) orbital channels entering the dot.
The physics of the charge fluctuations can be captured using a mapping on the
channel-anisotropic two-channel Kondo model. For the case of weak reflection at
the point contact this has already briefly been stressed by one of us in PRB
{\bf 64}, 161302R (2001). This mapping is also appropriate to discuss the
conductance behavior of a two-contact set-up in strong magnetic field.
Here, we elaborate on this approach and also discuss an alternative solution
using a mapping on a channel-isotropic Kondo model. In addition we consider the
limit of weak transmission.
We show that the Coulomb-staircase behavior of the charge in the dot as a
function of the gate voltage, is already smeared out by a small
channel-anisotropy both in the weak- and strong transmission limits.Comment: 17 pages, 4 figures, 1 Table; Expands cond-mat/0101126; Sec. VI on
2-contact setup added (Final version for PRB
First-principles study of illite-smectite and implications for clay mineral systems
Illite-smectite interstratified clay minerals are ubiquitous in sedimentary basins and they have been linked to the maturation, migration and trapping of hydrocarbons(1), rock cementation(2), evolution of porewater chemistry during diagenesis(3) and the development of pore pressure(4). But, despite the importance of these clays, their structures are controversial. Two competing models exist, each with profoundly different consequences for the understanding of diagenetic processes: model A views such interstratified clays as a stacking of layers identical to endmember illite and smectite layers, implying discrete and independently formed units (fundamental particles)(5), whereas model B views the clays as composed of crystallites with a unique structure that maintains coherency over much greater distances, in line with local charge balance about interlayers(6). Here we use first-principles density-functional theory to explore the energetics and structures of these two models for an illite-smectite interstratified clay mineral with a ratio of 1:1 and a Reichweite parameter of 1. We find that the total energy of model B is 2.3 kJ atom(-1) mol(-1) lower than that of model A, and that this energy difference can be traced to structural distortions in model A due to local charge imbalance. The greater stability of model B requires re-evaluation of the evolution of the smectite-to-illite sequence of clay minerals, including the nature of coexisting species, stability relations, growth mechanisms and the model of fundamental particles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62760/1/nature01155.pd
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