116,925 research outputs found
Large angle reorientation manoeuvre of spacecraft using robust backstepping control
The nonlinear control design problem for large angle reorientation manoeuvre of
spacecraft has a proper structure for the direct application of backstepping design as
its dynamics and kinematics are naturally in a cascade form. In this paper, the
robustness of the backstepping control against the uncertainties in the moment of
inertia matrix is investigated and a sufficient condition for the robust stability is
derived. Numerical simulations show the validity of the condition
Reversible and irreversible dynamics of a qubit interacting with a small environment
We analyze the dynamics of a system qubit interacting by means a sequence of
pairwise collisions with an environment consisting of just two qubits. We show
that the density operator of the qubits approaches a common time averaged
equilibrium state, characterized by large fluctuations, only for a random
sequence of collisions. For a regular sequence of collisions the qubitstates of
the system and of the reservoir undergo instantaneous periodic oscillations and
do not relax to a common state. Furthermore we show that pure bipartite
entanglement is developed only when at least two qubits are initially in the
same purestate while otherwise also genuine multipartite entanglement builds
up.Comment: 5 pages, 4 figure
Quantum network architecture of tight-binding models with substitution sequences
We study a two-spin quantum Turing architecture, in which discrete local
rotations \alpha_m of the Turing head spin alternate with quantum controlled
NOT-operations. Substitution sequences are known to underlie aperiodic
structures. We show that parameter inputs \alpha_m described by such sequences
can lead here to a quantum dynamics, intermediate between the regular and the
chaotic variant. Exponential parameter sensitivity characterizing chaotic
quantum Turing machines turns out to be an adequate criterion for induced
quantum chaos in a quantum network.Comment: Accepted for publication in J. mod. Optics [Proc. Workshop
"Entanglement and Decoherence", Gargnano (Italy), Sept 1999], 3 figure
Electron spin relaxation in carbon nanotubes
The long standing problem of inexplicably short spin relaxation in carbon
nanotubes (CNTs) is examined. The curvature-mediated spin-orbital interaction
is shown to induce fluctuating electron spin precession causing efficient
relaxation in a manner analogous to the Dyakonov-Perel mechanism. Our
calculation estimates longitudinal (spin-flip) and transversal (decoherence)
relaxation times as short as 150 ps and 110 ps at room temperature,
respectively, along with a pronounced anisotropic dependence. Interference of
electrons originating from different valleys can lead to even faster dephasing.
The results can help clarify the measured data, resolving discrepancies in the
literature.Comment: 9 pages, 3 figure
Waiting time dynamics of priority-queue networks
We study the dynamics of priority-queue networks, generalizations of the
binary interacting priority queue model introduced by Oliveira and Vazquez
[Physica A {\bf 388}, 187 (2009)]. We found that the original AND-type protocol
for interacting tasks is not scalable for the queue networks with loops because
the dynamics becomes frozen due to the priority conflicts. We then consider a
scalable interaction protocol, an OR-type one, and examine the effects of the
network topology and the number of queues on the waiting time distributions of
the priority-queue networks, finding that they exhibit power-law tails in all
cases considered, yet with model-dependent power-law exponents. We also show
that the synchronicity in task executions, giving rise to priority conflicts in
the priority-queue networks, is a relevant factor in the queue dynamics that
can change the power-law exponent of the waiting time distribution.Comment: 5 pages, 3 figures, minor changes, final published versio
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