17,337 research outputs found
Randomized Dynamical Decoupling Techniques for Coherent Quantum Control
The need for strategies able to accurately manipulate quantum dynamics is
ubiquitous in quantum control and quantum information processing. We
investigate two scenarios where randomized dynamical decoupling techniques
become more advantageous with respect to standard deterministic methods in
switching off unwanted dynamical evolution in a closed quantum system: when
dealing with decoupling cycles which involve a large number of control actions
and/or when seeking long-time quantum information storage. Highly effective
hybrid decoupling schemes, which combine deterministic and stochastic features
are discussed, as well as the benefits of sequentially implementing a
concatenated method, applied at short times, followed by a hybrid protocol,
employed at longer times. A quantum register consisting of a chain of spin-1/2
particles interacting via the Heisenberg interaction is used as a model for the
analysis throughout.Comment: 7 pages, 2 figures. Replaced with final version. Invited talk
delivered at the XXXVI Winter Colloquium on the Physics of Quantum
Electronics, Snowbird, Jan 2006. To be published in J. Mod. Optic
What to Fix? Distinguishing between design and non-design rules in automated tools
Technical debt---design shortcuts taken to optimize for delivery speed---is a
critical part of long-term software costs. Consequently, automatically
detecting technical debt is a high priority for software practitioners.
Software quality tool vendors have responded to this need by positioning their
tools to detect and manage technical debt. While these tools bundle a number of
rules, it is hard for users to understand which rules identify design issues,
as opposed to syntactic quality. This is important, since previous studies have
revealed the most significant technical debt is related to design issues. Other
research has focused on comparing these tools on open source projects, but
these comparisons have not looked at whether the rules were relevant to design.
We conducted an empirical study using a structured categorization approach, and
manually classify 466 software quality rules from three industry tools---CAST,
SonarQube, and NDepend. We found that most of these rules were easily labeled
as either not design (55%) or design (19%). The remainder (26%) resulted in
disagreements among the labelers. Our results are a first step in formalizing a
definition of a design rule, in order to support automatic detection.Comment: Long version of accepted short paper at International Conference on
Software Architecture 2017 (Gothenburg, SE
Velocity Tails for Inelastic Maxwell Models
We study the velocity distribution function for inelastic Maxwell models,
characterized by a Boltzmann equation with constant collision rate, independent
of the energy of the colliding particles. By means of a nonlinear analysis of
the Boltzmann equation, we find that the velocity distribution function decays
algebraically for large velocities, with exponents that are analytically
calculated.Comment: 4 pages, 2 figure
Efficient decoupling schemes with bounded controls based on Eulerian orthogonal arrays
The task of decoupling, i.e., removing unwanted interactions in a system
Hamiltonian and/or couplings with an environment (decoherence), plays an
important role in controlling quantum systems. There are many efficient
decoupling schemes based on combinatorial concepts like orthogonal arrays,
difference schemes and Hadamard matrices. So far these (combinatorial)
decoupling schemes have relied on the ability to effect sequences of
instantaneous, arbitrarily strong control Hamiltonians (bang-bang controls). To
overcome the shortcomings of bang-bang control Viola and Knill proposed a
method called Eulerian decoupling that allows the use of bounded-strength
controls for decoupling. However, their method was not directly designed to
take advantage of the composite structure of multipartite quantum systems. In
this paper we define a combinatorial structure called an Eulerian orthogonal
array. It merges the desirable properties of orthogonal arrays and Eulerian
cycles in Cayley graphs (that are the basis of Eulerian decoupling). We show
that this structure gives rise to decoupling schemes with bounded-strength
control Hamiltonians that can be applied to composite quantum systems with few
body Hamiltonians and special couplings with the environment. Furthermore, we
show how to construct Eulerian orthogonal arrays having good parameters in
order to obtain efficient decoupling schemes.Comment: 8 pages, revte
Shock waves in superconducting cosmic strings: growth of current
Intrinsic equations of motion of superconducting cosmic string may admit
solutions in the shock-wave form that implies discontinuity of the current term
\chi. The hypersurface of discontinuity propagates at finite velocity
determined by finite increment \Delta \chi =\chi_+ -\chi_-. The current
increases \chi_+>\chi_- in stable shocks but transition between spacelike (\chi
>0) and timelike (\chi<0) currents is impossible.Comment: 13 pages, 3 figure
Correlated errors can lead to better performance of quantum codes
A formulation for evaluating the performance of quantum error correcting
codes for a general error model is presented. In this formulation, the
correlation between errors is quantified by a Hamiltonian description of the
noise process. We classify correlated errors using the system-bath interaction:
local versus nonlocal and two-body versus many-body interactions. In
particular, we consider Calderbank-Shor-Steane codes and observe a better
performance in the presence of correlated errors depending on the timing of the
error recovery. We also find this timing to be an important factor in the
design of a coding system for achieving higher fidelities.Comment: 5 pages, 3 figures. Replaced by the published version. Title change
Decoherence under many-body system-environment interactions: a stroboscopic representation based on a fictitiously homogenized interaction rate
An environment interacting with portions of a system leads to
multiexponential interaction rates. Within the Keldysh formalism, we
fictitiously homogenize the system-environment interaction yielding a uniform
decay rate facilitating the evaluation of the propagators. Through an injection
procedure we neutralize the fictitious interactions. This technique justifies a
stroboscopic representation of the system-environment interaction which is
useful for numerical implementation and converges to the natural continuous
process. We apply this procedure to a fermionic two-level system and use the
Jordan-Wigner transformation to solve a two-spin swapping gate in the presence
of a spin environment.Comment: 11 pages, 3 figures, title changed, some typos change
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