639 research outputs found
Dynamical control of qubit coherence: Random versus deterministic schemes
We revisit the problem of switching off unwanted phase evolution and
decoherence in a single two-state quantum system in the light of recent results
on random dynamical decoupling methods [L. Viola and E. Knill, Phys. Rev. Lett.
{\bf 94}, 060502 (2005)]. A systematic comparison with standard cyclic
decoupling is effected for a variety of dynamical regimes, including the case
of both semiclassical and fully quantum decoherence models. In particular,
exact analytical expressions are derived for randomized control of decoherence
from a bosonic environment. We investigate quantitatively control protocols
based on purely deterministic, purely random, as well as hybrid design, and
identify their relative merits and weaknesses at improving system performance.
We find that for time-independent systems, hybrid protocols tend to perform
better than pure random and may improve over standard asymmetric schemes,
whereas random protocols can be considerably more stable against fluctuations
in the system parameters. Beside shedding light on the physical requirements
underlying randomized control, our analysis further demonstrates the potential
for explicit control settings where the latter may significantly improve over
conventional schemes.Comment: 21 pages, 15 figures, to appear in Physical Review A, 72 (2005
Enhanced Convergence and Robust Performance of Randomized Dynamical Decoupling
We demonstrate the advantages of randomization in coherent quantum dynamical
control. For systems which are either time-varying or require decoupling cycles
involving a large number of operations, we find that simple randomized
protocols offer superior convergence and stability as compared to deterministic
counterparts. In addition, we show how randomization always allows to
outperform purely deterministic schemes at long times, including combinatorial
and concatenated methods. General criteria for optimally interpolating between
deterministic and stochastic design are proposed and illustrated in explicit
decoupling scenarios relevant to quantum information storage.Comment: 4 pages, 3 figures, replaced with final versio
Quantum Chaos, Delocalization, and Entanglement in Disordered Heisenberg Models
We investigate disordered one- and two-dimensional Heisenberg spin lattices
across a transition from integrability to quantum chaos from both a statistical
many-body and a quantum-information perspective. Special emphasis is devoted to
quantitatively exploring the interplay between eigenvector statistics,
delocalization, and entanglement in the presence of nontrivial symmetries. The
implications of basis dependence of state delocalization indicators (such as
the number of principal components) is addressed, and a measure of {\em
relative delocalization} is proposed in order to robustly characterize the
onset of chaos in the presence of disorder. Both standard multipartite and {\em
generalized entanglement} are investigated in a wide parameter regime by using
a family of spin- and fermion- purity measures, their dependence on
delocalization and on energy spectrum statistics being examined. A distinctive
{\em correlation between entanglement, delocalization, and integrability} is
uncovered, which may be generic to systems described by the two-body random
ensemble and may point to a new diagnostic tool for quantum chaos. Analytical
estimates for typical entanglement of random pure states restricted to a proper
subspace of the full Hilbert space are also established and compared with
random matrix theory predictions.Comment: 17 pages, 10 figures, revised versio
MARKOV DIFFUSIONS IN COMOVING COORDINATES AND STOCHASTIC QUANTIZATION OF THE FREE RELATIVISTIC SPINLESS PARTICLE
We revisit the classical approach of comoving coordinates in relativistic
hydrodynamics and we give a constructive proof for their global existence under
suitable conditions which is proper for stochastic quantization. We show that
it is possible to assign stochastic kinematics for the free relativistic
spinless particle as a Markov diffusion globally defined on . Then
introducing dynamics by means of a stochastic variational principle with
Einstein's action, we are lead to positive-energy solutions of Klein-Gordon
equation. The procedure exhibits relativistic covariance properties.Comment: 31 pages + 1 figure available upon request; Plain REVTe
Design for Robustness: Bio-Inspired Perspectives in Structural Engineering
Bio-inspired solutions are widely adopted in different engineering disciplines. However, in structural engineering, these solutions are mainly limited to bio-inspired forms, shapes, and materials. Nature is almost completely neglected as a source of structural design philosophy. This study lists and discusses several bio-inspired solutions classified into two main classes, i.e., compartmentalization and complexity, for structural robustness design. Different examples are provided and mechanisms are categorized and discussed in detail. Some provided ideas are already used in the current structural engineering research and practice, usually without focus on their bio-analogy. These solutions are revisited and scrutinized from a bio-inspired point of view, and new aspects and possible improvements are suggested. Moreover, novel bio-inspired concepts including delayed compartmentalization, active compartmentalization, compartmentalization in intact parts, and structural complexity are also propounded for structural design under extreme loading conditions
Dynamical control of electron spin coherence in a quantum dot
We investigate the performance of dynamical decoupling methods at suppressing
electron spin decoherence from a low-temperature nuclear spin reservoir in a
quantum dot. The controlled dynamics is studied through exact numerical
simulation, with emphasis on realistic pulse delays and long-time limit. Our
results show that optimal performance for this system is attained by a periodic
protocol exploiting concatenated design, with control rates substantially
slower than expected from the upper spectral cutoff of the bath. For a known
initial electron spin state, coherence can saturate at long times, signaling
the creation of a stable ``spin-locked'' decoherence-free subspace. Analytical
insight on saturation is obtained for a simple echo protocol, in good agreement
with numerical results.Comment: 4 pages, 4 figures with 3 of them in colo
Suppression of electron spin decoherence in a quantum dot
The dominant source of decoherence for an electron spin in a quantum dot is
the hyperfine interaction with the surrounding bath of nuclear spins. The
decoherence process may be slowed down by subjecting the electron spin to
suitable sequences of external control pulses. We investigate the performance
of a variety of dynamical decoupling protocols using exact numerical
simulation. Emphasis is given to realistic pulse delays and the long-time
limit, beyond the domain where available analytical approaches are guaranteed
to work. Our results show that both deterministic and randomized protocols are
capable to significantly prolong the electron coherence time, even when using
control pulse separations substantially larger than what expected from the {\em
upper cutoff} frequency of the coupling spectrum between the electron and the
nuclear spins. In a realistic parameter range, the {\em total width} of such a
coupling spectrum appears to be the physically relevant frequency scale
affecting the overall quality of the decoupling.Comment: 8 pages, 3 figures. Invited talk at the XXXVII Winter Colloquium on
the Physics of Quantum Electronics, Snowbird, Jan 2007. Submitted to J. Mod.
Op
Dynamical Control of Electron Spin Coherence In a Quantum Dot: A Theoretical Study
We investigate the performance of dynamical decoupling methods at suppressing electron spin decoherence from a low-temperature nuclear spin reservoir in a quantum dot. The controlled dynamics is studied through exact numerical simulation, with emphasis on realistic pulse delays and the long-time limit. Our results show that optimal performance for this system is attained by a periodic protocol exploiting concatenated design, with control rates substantially slower than expected from the upper spectral cutoff of the bath. For a known initial electron spin state, coherence can saturate at long times, signaling the creation of a stable “spin-locked” decoherence-free subspace. Analytical insight into saturation is obtained for a simple echo protocol, in good agreement with numerical results
Quantum pseudo-randomness from cluster-state quantum computation
We show how to efficiently generate pseudo-random states suitable for quantum
information processing via cluster-state quantum computation. By reformulating
pseudo-random algorithms in the cluster-state picture, we identify a strategy
for optimizing pseudo-random circuits by properly choosing single-qubit
rotations. A Markov chain analysis provides the tool for analyzing convergence
rates to the Haar measure and finding the optimal single-qubit gate
distribution. Our results may be viewed as an alternative construction of
approximate unitary 2-designs.Comment: 4 pages, 4 figures, version appearing in Phys. Rev.
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