282 research outputs found
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
Dynamics of entanglement in a one-dimensional Ising chain
The evolution of entanglement in a one-dimensional Ising chain is numerically
studied under various initial conditions. We analyze two problems concerning
the dynamics of the entanglement: (i) generation of the entanglement from the
pseudopure separable state and (ii) transportation of the entanglement from one
end of the chain to the other. The investigated model is a one-dimensional
Ising spin-1/2 chain with nearest-neighbor interactions placed in an external
magnetic field and irradiated by a weak resonant transverse field. The
possibility of selective initialization of partially entangled states is
considered. It was shown that, in spite of the use of a model with the direct
interactions between the nearest neighbors, the entanglement between remote
spins is generated.Comment: 19 pages, 7 figure
Tribenzoatobismuth(III): a new Âpolymorph
A new polymorph (ÎČ) was obtained for an active pharmaceutical ingredient, bisÂmuth tribenzoate, [Bi(C6H5CO2)3]. The new ÎČ-polymorph is 1.05 times denser than the previously known polymorph [Rae et al. (1998 â¶). Acta Cryst. B54, 438â442]. In the ÎČ-polymorph, the Bi atom is linked with three benzoate anions, each of them acting as a bidentate ligand, and these assemblies with C
3 point symmetry can be considered as âmolÂeculesâ. The structure of the ÎČ-polymorph has no polymeric chains, in contrast to the previously known polymorph. The âmolÂeculesâ in the ÎČ-polymorph are stacked along [001], so that the phenyl rings of the neighbouring molÂecules are parallel to each other. Based on the pronounced difference in the crystal structures, one can suppose that two polymorphs should differ in the dissolution kinetics and bioavailability
Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60
We present an NMR study of Na2C60 and K4C60, two compounds that are related
by electron-hole symmetry in the C60 triply degenerate conduction band. In both
systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements
detect a gap in the electronic structure, most likely related to
singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or
C60^{4-}. However, the extended temperature range of the measurements presented
here (10 K to 700 K) allows to reveal deviations with respect to this general
trend, both at high and low temperatures. Above room temperature, 1/T1 deviates
from the activated law that one would expect from the presence of the gap and
saturates. In the same temperature range, a lowering of symmetry is detected in
Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60,
modifications of the 13C spectra lineshapes also indicate a structural
modification. We discuss this high temperature deviation in terms of a coupling
between JTD and local symmetry. At low temperatures, 1/TT tends to a
constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual
metallic character, which emphasizes the proximity of metallic and insulting
behaviors in alkali fullerides.Comment: 12 pages, 13 figure
Entanglement in a Solid State Spin Ensemble
Entanglement is the quintessential quantum phenomenon and a necessary
ingredient in most emerging quantum technologies, including quantum repeaters,
quantum information processing (QIP) and the strongest forms of quantum
cryptography. Spin ensembles, such as those in liquid state nuclear magnetic
resonance, have been powerful in the development of quantum control methods,
however, these demonstrations contained no entanglement and ultimately
constitute classical simulations of quantum algorithms. Here we report the
on-demand generation of entanglement between an ensemble of electron and
nuclear spins in isotopically engineered phosphorus-doped silicon. We combined
high field/low temperature electron spin resonance (3.4 T, 2.9 K) with
hyperpolarisation of the 31P nuclear spin to obtain an initial state of
sufficient purity to create a non-classical, inseparable state. The state was
verified using density matrix tomography based on geometric phase gates, and
had a fidelity of 98% compared with the ideal state at this field and
temperature. The entanglement operation was performed simultaneously, with high
fidelity, to 10^10 spin pairs, and represents an essential requirement of a
silicon-based quantum information processor.Comment: 4 pages, 3 figures plus supporting information of 4 pages, 1 figure
v2: Updated reference
Maternal immunization against Group B streptococcus: World Health Organization research and development technological roadmap and preferred product characteristics.
Group B streptococcus, found in the vagina or lower gastrointestinal tract of about 10-40% of women of reproductive age, is a leading cause of early life invasive bacterial disease, potentially amenable to prevention through maternal immunization during pregnancy. Following a consultation process with global stakeholders, the World Health Organization is herein proposing priority research and development pathways and preferred product characteristics for GBS vaccines, with the aim to facilitate and accelerate vaccine licensure, policy recommendation for wide scale use and implementation
Reply to "Localized Behavior near the Zn Impurity in YBa2Cu4O8 as Measured By Nuclear Quadrupole Resonance"
Julien et al. have commented on two of our publications claiming that we have
made erroneous interpretations of the nuclear magnetic resonance (NMR) and
nuclear quadrupole resonance (NQR) data. Specifically, they believe that their
model of an extended staggered moment about a Zn impurity is the only
interpretation of the data [Julien et al., Phys. Rev Lett. 84, 3422 (2000)].
Not only does their claim ignore models presented by other authors, we show
that the model of Julien et al. [Phys. Rev Lett. 84, 3422 (2000)] does not
consistently reproduce all of the NMR data.Comment: 16 pages in PDF forma
An all silicon quantum computer
A solid-state implementation of a quantum computer composed entirely of
silicon is proposed. Qubits are Si-29 nuclear spins arranged as chains in a
Si-28 (spin-0) matrix with Larmor frequencies separated by a large magnetic
field gradient. No impurity dopants or electrical contacts are needed.
Initialization is accomplished by optical pumping, algorithmic cooling, and
pseudo-pure state techniques. Magnetic resonance force microscopy is used for
readout. This proposal takes advantage of many of the successful aspects of
solution NMR quantum computation, including ensemble measurement, RF control,
and long decoherence times, but it allows for more qubits and improved
initialization.Comment: ReVTeX 4, 5 pages, 2 figure
Spin-Gap Proximity Effect Mechanism of High Temperature Superconductivity
When holes are doped into an antiferromagnetic insulator they form a slowly
fluctuating array of ``topological defects'' (metallic stripes) in which the
motion of the holes exhibits a self-organized quasi one-dimensional electronic
character. The accompanying lateral confinement of the intervening
Mott-insulating regions induces a spin gap or pseudogap in the environment of
the stripes. We present a theory of underdoped high temperature superconductors
and show that there is a {\it local} separation of spin and charge, and that
the mobile holes on an individual stripe acquire a spin gap via pair hopping
between the stripe and its environment; i.e. via a magnetic analog of the usual
superconducting proximity effect. In this way a high pairing scale without a
large mass renormalization is established despite the strong Coulomb repulsion
between the holes. Thus the {\it mechanism} of pairing is the generation of a
spin gap in spatially-confined {\it Mott-insulating} regions of the material in
the proximity of the metallic stripes. At non-vanishing stripe densities,
Josephson coupling between stripes produces a dimensional crossover to a state
with long-range superconducting phase coherence. This picture is established by
obtaining exact and well-controlled approximate solutions of a model of a
one-dimensional electron gas in an active environment. An extended discussion
of the experimental evidence supporting the relevance of these results to the
cuprate superconductors is given.Comment: 30 pages, 2 figure
The physics of dipolar bosonic quantum gases
This article reviews the recent theoretical and experimental advances in the
study of ultracold gases made of bosonic particles interacting via the
long-range, anisotropic dipole-dipole interaction, in addition to the
short-range and isotropic contact interaction usually at work in ultracold
gases. The specific properties emerging from the dipolar interaction are
emphasized, from the mean-field regime valid for dilute Bose-Einstein
condensates, to the strongly correlated regimes reached for dipolar bosons in
optical lattices.Comment: Review article, 71 pages, 35 figures, 350 references. Submitted to
Reports on Progress in Physic
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