3 research outputs found
Universal pulse sequence to minimize spin dephasing in the central spin decoherence problem
We present a remarkable finding that a recently discovered [G. S. Uhrig,
Phys. Rev. Lett. 98, 100504 (2007)] series of pulse sequences, designed to
optimally restore coherence to a qubit in the spin-boson model of decoherence,
is in fact completely model-independent and generically valid for arbitrary
dephasing Hamiltonians given sufficiently short delay times between pulses. The
series maximizes qubit fidelity versus number of applied pulses for
sufficiently short delay times because the series, with each additional pulse,
cancels successive orders of a time expansion for the fidelity decay. The
"magical" universality of this property, which was not appreciated earlier,
requires that a linearly growing set of "unknowns" (the delay times) must
simultaneously satisfy an exponentially growing set of nonlinear equations that
involve arbitrary dephasing Hamiltonian operators.Comment: Published in PRL, revise
Advantages of Randomization in Coherent Quantum Dynamical Control
Control scenarios have been identified where the use of randomized design may
substantially improve the performance of dynamical decoupling methods [L. F.
Santos and L. Viola, Phys. Rev. Lett. {\bf 97}, 150501 (2006)]. Here, by
focusing on the suppression of internal unwanted interactions in closed quantum
systems, we review and further elaborate on the advantages of randomization at
long evolution times. By way of illustration, special emphasis is devoted to
isolated Heisenberg-coupled chains of spin-1/2 particles. In particular, for
nearest-neighbor interactions, two types of decoupling cycles are contrasted:
inefficient averaging, whereby the number of control actions increases
exponentially with the system size, and efficient averaging associated to a
fixed-size control group. The latter allows for analytical and numerical
studies of efficient decoupling schemes created by exploiting and merging
together randomization and deterministic strategies, such as symmetrization,
concatenation, and cyclic permutations. Notably, sequences capable to remove
interactions up to third order are explicitly constructed. The consequences of
faulty controls are also analyzed.Comment: 27 pages, 7 figure
Semiconductor Spintronics
Spintronics refers commonly to phenomena in which the spin of electrons in a
solid state environment plays the determining role. In a more narrow sense
spintronics is an emerging research field of electronics: spintronics devices
are based on a spin control of electronics, or on an electrical and optical
control of spin or magnetism. This review presents selected themes of
semiconductor spintronics, introducing important concepts in spin transport,
spin injection, Silsbee-Johnson spin-charge coupling, and spindependent
tunneling, as well as spin relaxation and spin dynamics. The most fundamental
spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling.
Depending on the crystal symmetries of the material, as well as on the
structural properties of semiconductor based heterostructures, the spin-orbit
coupling takes on different functional forms, giving a nice playground of
effective spin-orbit Hamiltonians. The effective Hamiltonians for the most
relevant classes of materials and heterostructures are derived here from
realistic electronic band structure descriptions. Most semiconductor device
systems are still theoretical concepts, waiting for experimental
demonstrations. A review of selected proposed, and a few demonstrated devices
is presented, with detailed description of two important classes: magnetic
resonant tunnel structures and bipolar magnetic diodes and transistors. In most
cases the presentation is of tutorial style, introducing the essential
theoretical formalism at an accessible level, with case-study-like
illustrations of actual experimental results, as well as with brief reviews of
relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure