390 research outputs found
Optimal pulse spacing for dynamical decoupling in the presence of a purely-dephasing spin-bath
Maintaining quantum coherence is a crucial requirement for quantum
computation; hence protecting quantum systems against their irreversible
corruption due to environmental noise is an important open problem. Dynamical
decoupling (DD) is an effective method for reducing decoherence with a low
control overhead. It also plays an important role in quantum metrology, where
for instance it is employed in multiparameter estimation. While a sequence of
equidistant control pulses (CPMG) has been ubiquitously used for decoupling,
Uhrig recently proposed that a non-equidistant pulse sequence (UDD) may enhance
DD performance, especially for systems where the spectral density of the
environment has a sharp frequency cutoff. On the other hand, equidistant
sequences outperform UDD for soft cutoffs. The relative advantage provided by
UDD for intermediate regimes is not clear. In this paper, we analyze the
relative DD performance in this regime experimentally, using solid-state
nuclear magnetic resonance. Our system-qubits are 13C nuclear spins and the
environment consists of a 1H nuclear spin-bath whose spectral density is close
to a normal (Gaussian) distribution. We find that in the presence of such a
bath, the CPMG sequence outperforms the UDD sequence. An analogy between
dynamical decoupling and interference effects in optics provides an intuitive
explanation as to why the CPMG sequence performs superior to any
non-equidistant DD sequence in the presence of this kind of environmental
noise.Comment: To be published in Phys. Rev. A. 15 pages, 16 figures. Presentation
of the work was improved. One Figure and some Refs. were adde
Svetlichny's inequality and genuine tripartite nonlocality in three-qubit pure states
The violation of the Svetlichny's inequality (SI) [Phys. Rev. D, 35, 3066
(1987)] is sufficient but not necessary for genuine tripartite nonlocal
correlations. Here we quantify the relationship between tripartite entanglement
and the maximum expectation value of the Svetlichny operator (which is bounded
from above by the inequality) for the two inequivalent subclasses of pure
three-qubit states: the GHZ-class and the W-class. We show that the maximum for
the GHZ-class states reduces to Mermin's inequality [Phys. Rev. Lett. 65, 1838
(1990)] modulo a constant factor, and although it is a function of the three
tangle and the residual concurrence, large number of states don't violate the
inequality. We further show that by design SI is more suitable as a measure of
genuine tripartite nonlocality between the three qubits in the the W-class
states, and the maximum is a certain function of the bipartite entanglement
(the concurrence) of the three reduced states, and only when their certain sum
attains a certain threshold value, they violate the inequality.Comment: Modified version, 5 pages, 2 figures, REVTeX
Perfect quantum transport in arbitrary spin networks
Spin chains have been proposed as wires to transport information between
distributed registers in a quantum information processor. Unfortunately, the
challenges in manufacturing linear chains with engineered couplings has
hindered experimental implementations. Here we present strategies to achieve
perfect quantum information transport in arbitrary spin networks. Our proposal
is based on the weak coupling limit for pure state transport, where information
is transferred between two end-spins that are only weakly coupled to the rest
of the network. This regime allows disregarding the complex, internal dynamics
of the bulk network and relying on virtual transitions or on the coupling to a
single bulk eigenmode. We further introduce control methods capable of tuning
the transport process and achieve perfect fidelity with limited resources,
involving only manipulation of the end-qubits. These strategies could be thus
applied not only to engineered systems with relaxed fabrication precision, but
also to naturally occurring networks; specifically, we discuss the practical
implementation of quantum state transfer between two separated nitrogen vacancy
(NV) centers through a network of nitrogen substitutional impurities.Comment: 5+7 page
Interacting with the enemy: indirect effects of personality on conspecific aggression in crickets
In animal contests, individuals respond plastically to the phenotypes of the opponents that they confront. These ‘opponent’ – or ‘indirect’ – effects are often repeatable, e.g., certain opponents consistently elicit more or less aggressiveness in others. ‘Personality’ (repeatable among-individual variance in behavior) has been proposed as an important source of indirect effects. Here, we repeatedly assayed aggressiveness of wild-caught adult male field crickets Gryllus campestris in staged dyadic fights, measuring aggressiveness of both contestants. Measurements of their personality in non-social contexts (activity and exploration behavior) enabled us to ask whether personality caused indirect effects on aggressiveness. Activity, exploration, and aggressiveness were positively associated into a behavioral syndrome eliciting aggressiveness in conspecifics, providing direct evidence for the role of personality in causing indirect effects. Our findings imply that a multivariate view of phenotypes that includes indirect effects greatly improves our ability to understand the ecology and evolution of behavior
Enhanced dynamic nuclear polarization via swept microwave frequency combs
Dynamic Nuclear Polarization (DNP) has enabled enormous gains in magnetic
resonance signals and led to vastly accelerated NMR/MRI imaging and
spectroscopy. Unlike conventional cw-techniques, DNP methods that exploit the
full electron spectrum are appealing since they allow direct participation of
all electrons in the hyperpolarization process. Such methods typically entail
sweeps of microwave radiation over the broad electron linewidth to excite DNP,
but are often inefficient because the sweeps, constrained by adiabaticity
requirements, are slow. In this paper we develop a technique to overcome the
DNP bottlenecks set by the slow sweeps, employing a swept microwave frequency
comb that increases the effective number of polarization transfer events while
respecting adiabaticity constraints. This allows a multiplicative gain in DNP
enhancement, scaling with the number of comb frequencies and limited only by
the hyperfine-mediated electron linewidth. We demonstrate the technique for the
optical hyperpolarization of 13C nuclei in powdered microdiamonds at low
fields, increasing the DNP enhancement from 30 to 100 measured with respect to
the thermal signal at 7T. For low concentrations of broad linewidth electron
radicals, e.g. TEMPO, these multiplicative gains could exceed an order of
magnitude.Comment: Contains supplementary inf
Aminoethyl benzenesulfonyl fluoride and its hexapeptide (Ac-VFRSLK) conjugate are both in vitro inhibitors of subtilisin kexin isozyme-1
AbstractUsing a number of intramolecularly quenched fluorogenic (IQF) substrates encompassing the subtilisin kexin isozyme-1 (SKI-1)-mediated cleavage sites of various viral glycoproteins, it is revealed that 4-[2-Aminoethyl Benzene] SulfonylFluoride (AEBSF) can inhibit the proteolytic activity of SKI-1 mostly in a competitive manner. The measured IC50 values range from 200 to 800 nM depending on the nature of the substrate used. This is the first in vitro demonstration of a non-peptide inhibitor of SKI-1. In an effort to enhance the selectivity and potency of SKI-1 inhibition, a hexapeptidyl derivative containing SKI-1 consensus sequence, Ac-Val-Phe-Arg-Ser-Leu-Lys-AEBSF, was prepared. The peptide sequence was derived from the primary auto-activation site of prodomain of SKI-1 itself terminating at Leu-Lys138 and contains the crucial P4-basic and P2 alkyl side chain containing hydrophobic amino acids. Like AEBSF, the hexapeptidyl-AEBSF analog blocked SKI-1 cleavages of all IQF-substrates tested but with enhanced efficiency
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