813 research outputs found
Electromechanical Quantum Simulators
Digital quantum simulators are among the most appealing applications of a
quantum computer. Here we propose a universal, scalable, and integrated quantum
computing platform based on tunable nonlinear electromechanical
nano-oscillators. It is shown that very high operational fidelities for single
and two qubits gates can be achieved in a minimal architecture, where qubits
are encoded in the anharmonic vibrational modes of mechanical nanoresonators,
whose effective coupling is mediated by virtual fluctuations of an intermediate
superconducting artificial atom. An effective scheme to induce large
single-phonon nonlinearities in nano-electromechanical devices is explicitly
discussed, thus opening the route to experimental investigation in this
direction. Finally, we explicitly show the very high fidelities that can be
reached for the digital quantum simulation of model Hamiltonians, by using
realistic experimental parameters in state-of-the art devices, and considering
the transverse field Ising model as a paradigmatic example.Comment: 14 pages, 8 figure
Spectroscopic analysis of the two subgiant branches of the globular cluster NGC1851
NGC1851 possibly shows a spread in [Fe/H], but the relation between this
spread and the division in the SGB is unknown. We obtained blue (3950-4600 A)
intermediate resolution (R~8,000) spectra for 47 stars on the bright and 30 on
the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The determination of
the atmospheric parameters to extremely high internal accuracy leads to small
errors when comparing different stars in the cluster. We found that the b-SGB
is slightly more metal-poor than the f-SGB, with [Fe/H]=-1.227+/-0.009 and
[Fe/H]=-1.162+/- 0.012, respectively. This implies that the f-SGB is only
slightly older by ~0.6 Gyr than the b-SGB if the total CNO abundance is
constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is
consistent with what is found for HB stars, if b-SGB are the progenitors of red
HB stars, and f-SGB those of blue HB ones. The abundances of the n-capture
elements Sr and Ba have a bimodal distribution, reflecting the separation
between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both
groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting
that there is a real spread in the abundances of n-capture elements. There is
some correlation between C and Ba abundances, while the same correlation for Sr
is much more dubious. We identified six C-rich stars, which have a moderate
overabundance of Sr and Ba and rather low N abundances. This group of stars
might be the progenitors of these on the anomalous RGB in the (v, v-y) diagram.
These results are discussed within different scenarios for the formation of
NGC1851. It is possible that the two populations originated in different
regions of an inhomogeneous parent object. However, the striking similarity
with M22 calls for a similar evolution for these two clusters. Deriving
reliable CNO abundances for the two sequences would be crucial.Comment: Accepted by Astronomy and Astrophysics; 16 pages, 20 figure
The helium content of globular clusters: NGC6121 (M4)
He has been proposed as a key element to interpret the observed multiple MS,
SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars
belonging to the bluer part of the HB, are thought to be more He rich (\Delta
Y=0.03 or more) and more Na-rich/O-poor than those located in the redder part.
This hypothesis was only partially confirmed in NGC 6752, where stars of the
redder zero-age HB showed a He content of Y=0.25+-0.01, fully compatible with
the primordial He content of the Universe, and were all Na-poor/O-rich. Here we
study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus
O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic
facility. In addition to He, O, Na, and Fe abundances were estimated. Stars
turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He
content with a mean value of Y=0.29+-0.01(random)+-0.01(systematic). The high
He content of blue HB stars in M4 is also confirmed by the fact that they are
brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are
He-enhanced by \Delta Y=0.02-0.03 with respect to the RHB stars. The whole
sample of stars has a metallicity of [Fe/H]=-1.06+-0.02 (internal error). This
is a rare direct measurement of the (primordial) He abundance for stars
belonging to the Na-rich/O-poor population of GC stars in a temperature regime
where the He content is not altered by sedimentation or extreme mixing as
suggested for the hottest, late helium flash HB stars. Our results support
theoretical predictions that the Na-rich/O-poor population is also more He-rich
than the Na-poor/O-rich generation and that a leading contender for the 2^{nd}
parameter is the He abundance.Comment: 18 pages, 8 figures, accepted for publication on Ap
Low-energy excitations in the S=(1/2) molecular nanomagnet K6[V<sub>15</sub><sup>IV</sup>As6O42(H2O)]·8H2O from proton NMR and µSR
Zero- and longitudinal-field muon-spin-rotation (µSR) and 1H NMR measurements on the S=(1/2) molecular nanomagnet K6[V15IVAs6O42(H2O)]·8H2O are presented. In LF experiments, the muon asymmetry P(t) was fitted by the sum of three different exponential components with fixed weights. The different muon relaxation rates lambdai (i=1,2,3) and the low-field H=0.23 T 1H NMR spin-lattice relaxation rate 1/T1 show a similar behavior for T>50 K: starting from room temperature they increase as the temperature is decreased. The increase of lambdai and 1/T1 can be attributed to the "condensation" of the system toward the lowest-lying energy levels. The gap Delta~550 K between the first and second S=(3/2) excited states was determined experimentally. For T<2 K, the muon relaxation rates lambdai stay constant, independently of the field value H<=0.15 T. The behavior for T<2 K strongly suggests that, at low T, the spin fluctuations are not thermally driven but rather originate from quasielastic intramolecular or intermolecular magnetic interactions. We suggest that the very-low-temperature relaxation rates could be driven by energy exchanges between two almost degenerate S=(1/2) ground states and/or by quantum effects
S-mixing and quantum tunneling of the magnetization in molecular nanomagnets
The role of -mixing in the quantum tunneling of the magnetization in
nanomagnets has been investigated. We show that the effect on the tunneling
frequency is huge and that the discrepancy (more than 3 orders of magnitude in
the tunneling frequency) between spectroscopic and relaxation measurements in
Fe can be resolved if -mixing is taken into account.Comment: REVTEX, 10 pages, 3 jpg figures, to appear in PR
Spin dynamics in rare earth single molecule magnets from muSR and NMR in [TbPc] and [DyPc]
The spin dynamics in [TbPc] and [DyPc] single
molecule magnets have been investigated by means of muon and nuclear
spin-lattice relaxation rate measurements. The correlation time for the spin
fluctuations was found to be close to 0.1 ms already at 50 K, about two orders
of magnitude larger than the one previously found in other lanthanide based
single molecule magnets. In [TbPc] two different regimes for the
spin fluctuations have been evidenced: a high temperature activated one
involving spin fluctuations across a barrier separating
the ground and first excited states and a low temperature regime involving
quantum fluctuations within the twofold degenerate ground-state. In
[DyPc] a high temperature activated spin dynamics is also evidenced
which, however, cannot be explained in terms of a single spin-phonon coupling
constant.Comment: 4 pages, 4 figure
Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for
The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the
paramagnetic phase of La_{2-x}Sr_xCuO_4 (for ) due to the
injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation
time T_1 measurements. The results are discussed in the framework of the
connection between T_1 and the in-plane magnetic correlation length
. It is found that at high temperatures the system remains in
the renormalized classical regime, with a spin stiffness constant
reduced by small doping to an extent larger than the one due to Zn doping. For
the effect of doping on appears to level off. The
values for derived from T_1 for K are much larger
than the ones estimated from the temperature behavior of sublattice
magnetization in the ordered phase (). It is argued that these
features are consistent with the hypothesis of formation of stripes of
microsegregated holes.Comment: 10 pages, 3 figure
Evidence for impurity-induced frustration in La2CuO4
Zero-field muon spin rotation and magnetization measurements were performed
in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by
M=Mg2+ spinless impurity. It is shown that while the doping dependence of the
sublattice magnetization (M(x)) is nearly the same for both compounds, the
N\'eel temperature (T_N(x)) decreases unambiguously more rapidly in the
Zn-doped compound. This difference, not taken into account within a simple
dilution model, is associated with the frustration induced by the Zn2+ impurity
onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin
stiffness is derived and found to be reduced by Zn doping more significantly
than expected within a dilution model. The effect of the structural
modifications induced by doping on the exchange coupling is also discussed.Comment: 4 pages, 4 figure
Strong electronic correlations in LiZnPc organic metals
Nuclear magnetic resonance, electron paramagnetic resonance and magnetization
measurements show that bulk LiZnPc are strongly correlated one-dimensional
metals. The temperature dependence of the nuclear spin-lattice relaxation rate
and of the static uniform susceptibility on approaching room
temperature are characteristic of a Fermi liquid. Moreover, while for the electrons are delocalized down to low temperature, for a
tendency towards localization is noticed upon cooling, yielding an increase
both in and . The -dependence of the effective density of
states at the Fermi level displays a sharp enhancement for , at the half filling of the ZnPc lowest unoccupied molecular orbitals. This
suggests that LiZnPc is on the edge of a metal-insulator transition where
enhanced superconducting fluctuations could develop.Comment: 5 pages, 4 figure
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