1,184 research outputs found
Simple groups separated by finiteness properties
We show that for every positive integer there exists a simple group that
is of type but not of type . For
these groups are the first known examples of this kind. They also provide
infinitely many quasi-isometry classes of finitely presented simple groups. The
only previously known infinite family of such classes, due to Caprace--R\'emy,
consists of non-affine Kac--Moody groups over finite fields. Our examples arise
from R\"over--Nekrashevych groups, and contain free abelian groups of infinite
rank.Comment: 25 pages. v2: incorporated comments v3: final version, to appear,
Invent. Mat
Concatenated dynamical decoupling in a solid-state spin bath
Concatenated dynamical decoupling (CDD) pulse sequences hold much promise as
a strategy to mitigate decoherence in quantum information processing. It is
important to investigate the actual performance of these dynamical decoupling
strategies in real systems that are promising qubit candidates. In this Rapid
Communication, we compute the echo decay of concatenations of the Hahn echo
sequence for a solid-state electronic spin qubit in a nuclear spin bath using a
cluster expansion technique. We find that each level of concatenation reverses
the effect of successive levels of intrabath fluctuations. On the one hand,
this advances CDD as a versatile and realistic decoupling strategy. On the
other hand, this invalidates, as overly optimistic, results of the simple pair
approximation used previously to study restoration, through CDD, of coherence
lost to a mesoscopic spin bath
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