20,000 research outputs found
Combined quantum state preparation and laser cooling of a continuous beam of cold atoms
We use two-laser optical pumping on a continuous atomic fountain in order to
prepare cold cesium atoms in the same quantum ground state. A first laser
excites the F=4 ground state to pump the atoms toward F=3 while a second
pi-polarized laser excites the F=3 -> F'=3 transition of the D2 line to produce
Zeeman pumping toward m=0. To avoid trap states, we implement the first laser
in a 2D optical lattice geometry, thereby creating polarization gradients. This
configuration has the advantage of simultaneously producing Sisyphus cooling
when the optical lattice laser is tuned between the F=4 -> F'=4 and F=4 -> F'=5
transitions of the D2 line, which is important to remove the heat produced by
optical pumping. Detuning the frequency of the second pi-polarized laser
reveals the action of a new mechanism improving both laser cooling and state
preparation efficiency. A physical interpretation of this mechanism is
discussed.Comment: Minor changes according to the recommendations of the referee: -
Corrected Fig.1. - Split the graph of Fig.6 for clarity. - Added one
reference. - Added two remarks in the conclusion. - Results unchange
Quantum Computation and Simulation using Fermion-Pair Registers
We propose and analyze an approach to realize quantum computation and
simulation using fermionic particles under quantum gas microscopes. Our work is
inspired by a recent experimental demonstration of large-scale quantum
registers, where tightly localized fermion pairs are used to encode qubits
exhibiting long coherence time and robustness against laser intensity noise. We
describe how to engineer the SWAP gate and high-fidelity controlled-phase gates
by adjusting the fermion hopping as well as Feshbach interaction strengths.
Combined with previously demonstrated single-qubit rotations, these gates
establish the computational universality of the system. Furthermore, we show
that 2D quantum Ising Hamiltonians with tunable transverse and longitudinal
fields can be efficient simulated by modulating Feshbach interaction strengths.
We present a sample-efficient protocol to characterize engineered gates and
Hamiltonian dynamics based on an improved classical shadow process tomography
that requires minimal experimental controls. Our work opens up new
opportunities to harness existing ultracold quantum gases for quantum
information sciences
The organic chemistry in the innermost, infalling envelope of the Class 0 protostar L483
Context: The protostellar envelopes, outflow and large-scale chemistry of
Class~0 and Class~I objects have been well-studied, but while previous works
have hinted at or found a few Keplerian disks at the Class~0 stage, it remains
to be seen if their presence in this early stage is the norm. Likewise, while
complex organics have been detected toward some Class~0 objects, their
distribution is unknown as they could reside in the hottest parts of the
envelope, in the emerging disk itself or in other components of the
protostellar system, such as shocked regions related to outflows.
Aims: In this work, we aim to address two related issues regarding
protostars: when rotationally supported disks form around deeply embedded
protostars and where complex organic molecules reside in such objects.
Methods: We observed the deeply embedded protostar, L483, using Atacama Large
Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a
high angular resolution down to ~0.1 (20~au) scales.
Results: We find that the kinematics of CS~-- and
HCN~-- are best fitted by the velocity profile from infall under
conservation of angular momentum and not by a Keplerian profile. The spatial
extents of the observed complex organics are consistent with an estimated ice
sublimation radius of the envelope at ~50~au, suggesting that the complex
organics exist in the hot corino of L483.
Conclusions: We find that L483 does not harbor a Keplerian disk down to at
least ~au in radius. Instead, the innermost regions of L483 are undergoing
a rotating collapse. This result highlights that some Class~0 objects contain
only very small disks, or none at all, with the complex organic chemistry
taking place on scales inside the hot corino of the envelope, in a region
larger than the emerging disk.Comment: 19 pages, 11 figure
Magnetic properties of two-phase superconductors
We have recently proposed a theoretical model for superconductors endowed
with two distinct superconducting phases, described by two scalar order
parameters which condensate at different critical temperatures. On analyzing
the magnetic behavior of such systems, we have found some observable
differences with respect to the case of ordinary Ginzburg-Landau
superconductors. In particular, at low temperature the London penetration
length is strongly reduced and the Ginzburg-Landau parameter k becomes a
function of temperature. By contrast, in the temperature region between the two
phase transitions k is constant and the system is a type-I or a type-II
superconductor depending on the ratio between the critical temperatures.Comment: revtex, 5 pages, 1 eps figur
Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A
Magnetic fields are believed to play a crucial role in the process of star
formation. We compare high-angular resolution observations of the submillimeter
polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a
low-mass protostar, with models of the collapse of magnetized molecular cloud
cores. Assuming a uniform dust alignment efficiency, we computed the Stokes
parameters and synthetic polarization maps from the model density and magnetic
field distribution by integrations along the line-of-sight and convolution with
the interferometric response. The synthetic maps are in good agreement with the
data. The best-fitting models were obtained for a protostellar mass of 0.8
solar masses, of age 9e4 yr, formed in a cloud with an initial mass-to-flux
ratio ~2 times the critical value. The magnetic field morphology in NGC 1333
IRAS 4A is consistent with the standard theoretical scenario for the formation
of solar-type stars, where well-ordered, large-scale, rather than turbulent,
magnetic fields control the evolution and collapse of the molecular cloud cores
from which stars form.Comment: 4 pages, 5 figures. Accepted by Astronomy and Astrophysic
Local probing of the field emission stability of vertically aligned multiwalled carbon nanotubes
Metallic cantilever in high vacuum atomic force microscope has been used as
anode for field emission experiments from densely packed vertically aligned
multi-walled carbon nanotubes. The high spatial resolution provided by the
scanning probe technique allowed precise setting of the tip-sample distance in
the submicron region. The dimension of the probe (curvature radius below 50nm)
allowed to measure current contribution from sample areas smaller than 1um^2.
The study of long-term stability evidenced that on these small areas the field
emission current remains stable (within 10% fluctuations) several hours (at
least up to 72 hours) at current intensities between 10-5A and 10-8A.
Improvement of the current stability has been observed after performing
long-time Joule heating conditioning to completely remove possible adsorbates
on the nanotubes.Comment: 15 pages, 7 figure
Second discontinuity in the specific heat of two-phase superconductors
The recently proposed theoretical model of superconductors endowed with two
distinct superconducting phases and critical temperatures is further analyzed
by introducing two distinct order parameters, described by two scalar fields
which condensate at different temperatures. We find some deviations in basic
thermodynamical quantities with respect to the Ginzburg-Landau one-phase
superconductors. In particular, by contrast to the usual case where only one
jump in specific heat takes place at the normal-superconductor transition
temperature, we actually predict an additional discontinuity for C_V when
passing from a superconducting phase to the other one.Comment: revtex, 4 pages, 1 figur
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