646 research outputs found
Fine tuning of phase qubit parameters for optimization of fast single-pulse readout
We analyze a two-level quantum system, describing the phase qubit, during a
single-pulse readout process by a numerical solution of the time-dependent
Schroedinger equation. It has been demonstrated that the readout error has a
minimum for certain values of the system`s basic parameters. In particular, the
optimization of the qubit capacitance and the readout pulse shape leads to
significant reduction of the readout error. It is shown that in an ideal case
the fidelity can be increased to almost 97% for 2 ns pulse duration and to 96%
for 1 ns pulse duration.Comment: 4 pages, 5 figure
An ab initio theory of double odd-even mass differences in nuclei
Two aspects of the problem of evaluating double odd-even mass differences D_2
in semi-magic nuclei are studied related to existence of two components with
different properties, a superfluid nuclear subsystem and a non-superfluid one.
For the superfluid subsystem, the difference D_2 is approximately equal to
2\Delta, the gap \Delta being the solution of the gap equation. For the
non-superfluid subsystem, D_2 is found by solving the equation for two-particle
Green function for normal systems. Both equations under consideration contain
the same effective pairing interaction. For the latter, the semi-microscopic
model is used in which the main term calculated from the first principles is
supplemented with a small phenomenological addendum containing one
phenomenological parameter supposed to be universal for all medium and heavy
atomic nuclei.Comment: 7 pages, 10 figures, Report at Nuclear Structure and Related Topics,
Dubna, Russia, July 2 - July 7, 201
Electronic structure of turbostratic graphene
We explore the rotational degree of freedom between graphene layers via the
simple prototype of the graphene twist bilayer, i.e., two layers rotated by
some angle . It is shown that, due to the weak interaction between
graphene layers, many features of this system can be understood by interference
conditions between the quantum states of the two layers, mathematically
expressed as Diophantine problems. Based on this general analysis we
demonstrate that while the Dirac cones from each layer are always effectively
degenerate, the Fermi velocity of the Dirac cones decreases as ; the form we derive for agrees with that found via a
continuum approximation in Phys. Rev. Lett., 99:256802, 2007. From tight
binding calculations for structures with we
find agreement with this formula for . In contrast, for
this formula breaks down and the Dirac bands become
strongly warped as the limit is approached. For an ideal system
of twisted layers the limit as is singular as for the Dirac point is fourfold degenerate, while at one has the
twofold degeneracy of the stacked bilayer. Interestingly, in this limit
the electronic properties are in an essential way determined \emph{globally},
in contrast to the 'nearsightedness' [W. Kohn. Phys. Rev. Lett., 76:3168,
1996.] of electronic structure generally found in condensed matter.Comment: Article as to be published in Phys. Rev B. Main changes: K-point
mapping tables fixed, several changes to presentation
Optimal fast single pulse readout of qubits
The computer simulations of the process of single pulse readout from the
flux-biased phase qubit is performed in the frame of one-dimensional
Schroedinger equation. It has been demonstrated that the readout error can be
minimized by choosing the optimal pulse duration and the depth of a potential
well, leading to the fidelity of 0.94 for 2ns and 0.965 for 12ns sinusoidal
pulses.Comment: 4 pages, 6 figure
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