10 research outputs found
Coherent dynamics in long fluxonium qubits
We analyze the coherent dynamics of a fluxonium device (Manucharyan et al
2009 Science 326 113) formed by a superconducting ring of Josephson junctions
in which strong quantum phase fluctuations are localized exclusively on a
single weak element. In such a system, quantum phase tunnelling by
occurring at the weak element couples the states of the ring with supercurrents
circulating in opposite directions, while the rest of the ring provides an
intrinsic electromagnetic environment of the qubit. Taking into account the
capacitive coupling between nearest neighbors and the capacitance to the
ground, we show that the homogeneous part of the ring can sustain
electrodynamic modes which couple to the two levels of the flux qubit. In
particular, when the number of Josephson junctions is increased, several
low-energy modes can have frequencies lower than the qubit frequency. This
gives rise to a quasiperiodic dynamics, which manifests itself as a decay of
oscillations between the two counterpropagating current states at short times,
followed by oscillation-like revivals at later times. We analyze how the system
approaches such a dynamics as the ring's length is increased and discuss
possible experimental implications of this non-adiabatic regime.Comment: 20 pages, 8 figures (new, substantially revised version
Full counting statistics of a chaotic cavity with asymmetric leads
We study the statistics of charge transport in a chaotic cavity attached to
external reservoirs by two openings of different size which transmit non-equal
number of quantum channels. An exact formula for the cumulant generating
function has been derived by means of the Keldysh-Green function technique
within the circuit theory of mesoscopic transport. The derived formula
determines the full counting statistics of charge transport, i.e., the
probability distribution and all-order cumulants of current noise. It is found
that, for asymmetric cavities, in contrast to other mesoscopic systems, the
third-order cumulant changes the sign at high biases. This effect is attributed
to the skewness of the distribution of transmission eigenvalues with respect to
forward/backward scattering. For a symmetric cavity we find that the third
cumulant approaches a voltage-independent constant proportional to the
temperature and the number of quantum channels in the leads.Comment: new section on probability distribution and new references adde
Clar Sextet Analysis of Triangular, Rectangular and Honeycomb Graphene Antidot Lattices
Pristine graphene is a semimetal and thus does not have a band gap. By making
a nanometer scale periodic array of holes in the graphene sheet a band gap may
form; the size of the gap is controllable by adjusting the parameters of the
lattice. The hole diameter, hole geometry, lattice geometry and the separation
of the holes are parameters that all play an important role in determining the
size of the band gap, which, for technological applications, should be at least
of the order of tenths of an eV. We investigate four different hole
configurations: the rectangular, the triangular, the rotated triangular and the
honeycomb lattice. It is found that the lattice geometry plays a crucial role
for size of the band gap: the triangular arrangement displays always a sizable
gap, while for the other types only particular hole separations lead to a large
gap. This observation is explained using Clar sextet theory, and we find that a
sufficient condition for a large gap is that the number of sextets exceeds one
third of the total number of hexagons in the unit cell. Furthermore, we
investigate non-isosceles triangular structures to probe the sensitivity of the
gap in triangular lattices to small changes in geometry
Voltage and temperature dependence of current noise in double-barrier normal-superconducting structures
We study theoretically the current-noise energy (voltage bias and temperature) dependence for a N-N'-S structure, where N and S stand for bulk normal metal and superconductor, respectively, and N' for a short diffusive normal metal. Using quasiclassical theory of current fluctuations we determine the noise for arbitrary distributions of channel transparencies on both junctions. The differential Fano factor turns out to depend on both junction transparencies and the ratio of the two conductances. We discuss analytically the coherent and incoherent regimes and the case when one of the two conductances dominates the other one. Measurement of differential conductance and noise can be used to probe the channel distribution of the interfaces. We discuss recent experiments in the light of our results. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200673.23.-b Electronic transport in mesoscopic systems, 72.70.+m Noise processes and phenomena, 74.45.+c Proximity effects; Andreev effect; SN and SNS junctions, 74.40.+k Fluctuations (noise, chaos, nonequilibrium sc, localization, etc.) ,