7,286 research outputs found
Exchange-Only Dynamical Decoupling in the 3-Qubit Decoherence Free Subsystem
The Uhrig dynamical decoupling sequence achieves high-order decoupling of a
single system qubit from its dephasing bath through the use of bang-bang Pauli
pulses at appropriately timed intervals. High-order decoupling of single and
multiple qubit systems from baths causing both dephasing and relaxation can
also be achieved through the nested application of Uhrig sequences, again using
single-qubit Pauli pulses. For the 3-qubit decoherence free subsystem (DFS) and
related subsystem encodings, Pauli pulses are not naturally available
operations; instead, exchange interactions provide all required encoded
operations. Here we demonstrate that exchange interactions alone can achieve
high-order decoupling against general noise in the 3-qubit DFS. We present
decoupling sequences for a 3-qubit DFS coupled to classical and quantum baths
and evaluate the performance of the sequences through numerical simulations
Quantum Impurities and the Neutron Resonance Peak in : Ni versus Zn
The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin
dynamics of an optimally doped high temperature superconductor is compared in
two samples with almost identical superconducting transition temperatures:
YBa(CuNi)O (T=80 K) and
YBa(CuZn)O (T=78 K). In the Ni-substituted
system, the magnetic resonance peak (which is observed at E40 meV in
the pure system) shifts to lower energy with a preserved E/T ratio
while the shift is much smaller upon Zn substitution. By contrast Zn, but not
Ni, restores significant spin fluctuations around 40 meV in the normal state.
These observations are discussed in the light of models proposed for the
magnetic resonance peak.Comment: 3 figures, submitted to PR
Low-energy renormalization of the electron dispersion of high-T superconductors
High-resolution ARPES studies in cuprates have detected low-energy changes in
the dispersion and absorption of quasi-particles at low temperatures, in
particular, in the superconducting state. Based on a new 1/N expansion of the
t-J-Holstein model, which includes collective antiferromagnetic fluctuations
already in leading order, we argue that the observed low-energy structures are
mainly caused by phonons and not by spin fluctuations, at least, in the optimal
and overdoped regime.Comment: 6 pages, 3 figure
A Loosely-Coupled Collaborative Integrated Environmental Modelling Framework
Integration of environmental models requires full support of the modelling community. When a large number of models are integrated, it requires consistency within scale, datasets, and model to model interactions to minimize the uncertainty among the models. The integrated environmental modelling (IEM) framework is a necessary approach to integrate multiple environmental models for a particular study. When modellers cannot afford considerable amount of time to get involved with full and tightly-integrated IEM or an IEM has very short time frame to complete, then a loosely-coupled collaborative IEM environment can provide the benefits of the integrated approach while minimizing the effort of each individual modeller. However, such a framework will require setting rules that all participants must adhere to. These rules address the issues of model inputs and model to model interaction. The framework should also provide value-added functionality to make the IEM framework more transparent and applicable
Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak in YBa2Cu3O7
The magnetic excitation spectrum of a YBa_2 Cu_3 O_7 crystal containing 0.5%
of nonmagnetic (Zn) impurities has been determined by inelastic neutron
scattering. Whereas in the pure system a sharp resonance peak at E ~ 40 meV is
observed exclusively below the superconducting transition temperature T_c, the
magnetic response in the Zn-substituted system is broadened significantly and
vanishes at a temperature much higher than T_c. The energy-integrated spectral
weight observed near q = (pi,pi) increases with Zn substitution, and only about
half of the spectral weight is removed at T_c
Efficient Coherent Control by Optimized Sequences of Pulses of Finite Duration
Reliable long-time storage of arbitrary quantum states is a key element for
quantum information processing. In order to dynamically decouple a spin or
quantum bit from a dephasing environment, we introduce an optimized sequence of
control pulses of finite durations \tau\pp and finite amplitudes. The
properties of this sequence of length stem from a mathematically rigorous
derivation. Corrections occur only in order and \tau\pp^3 without
mixed terms such as T^N\tau\pp or T^N\tau\pp^2. Based on existing
experiments, a concrete setup for the verification of the properties of the
advocated realistic sequence is proposed.Comment: 8 pages, 1 figur
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