108 research outputs found
Superfluid phase transition and strong-coupling effects in an ultracold Fermi gas with mass imbalance
We investigate the superfluid phase transition and effects of mass imbalance
in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation)
crossover regime of an cold Fermi gas. We point out that the Gaussian
fluctuation theory developed by Nozi\`eres and Schmitt-Rink and the -matrix
theory, that are now widely used to study strong-coupling physics of cold Fermi
gases, give unphysical results in the presence of mass imbalance. To overcome
this problem, we extend the -matrix theory to include higher-order pairing
fluctuations. Using this, we examine how the mass imbalance affects the
superfluid phase transition. Since the mass imbalance is an important key in
various Fermi superfluids, such as K-Li Fermi gas mixture, exciton
condensate, and color superconductivity in a dense quark matter, our results
would be useful for the study of these recently developing superfluid systems.Comment: 7 pages, 4 figures, Proceedings of QFS-201
Tests with a Carlina-type diluted telescope; Primary coherencing
Studies are under way to propose a new generation of post-VLTI
interferometers. The Carlina concept studied at the Haute- Provence Observatory
is one of the proposed solutions. It consists in an optical interferometer
configured like a diluted version of the Arecibo radio telescope: above the
diluted primary mirror made of fixed cospherical segments, a helium balloon (or
cables suspended between two mountains), carries a gondola containing the focal
optics. Since 2003, we have been building a technical demonstrator of this
diluted telescope. First fringes were obtained in May 2004 with two
closely-spaced primary segments and a CCD on the focal gondola. We have been
testing the whole optical train with three primary mirrors. The main aim of
this article is to describe the metrology that we have conceived, and tested
under the helium balloon to align the primary mirrors separate by 5-10 m on the
ground with an accuracy of a few microns. The servo loop stabilizes the mirror
of metrology under the helium balloon with an accuracy better than 5 mm while
it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have
obtained the white fringes of metrology; i.e., the three mirrors are aligned
(cospherized) with an accuracy of {\approx} 1 micron. We show data proving the
stability of fringes over 15 minutes, therefore providing evidence that the
mechanical parts are stabilized within a few microns. This is an important step
that demonstrates the feasibility of building a diluted telescope using cables
strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could
be one of the first members of a new class of telescopes named diluted
telescopes.Comment: 18 pages, 17 figures, A&A, accepte
The Carlina-type diluted telescope: Stellar fringes on Deneb
Context. The performance of interferometers has largely been increased over
the last ten years. But the number of observable objects is still limited due
to the low sensitivity and imaging capability of the current facilities.
Studies have been done to propose a new generation of interferometers. Aims.
The Carlina concept studied at the Haute-Provence Observatory consists in an
optical interferometer configured as a diluted version of the Arecibo radio
telescope: above the diluted primary mirror made of fixed co-spherical
segments, a helium balloon or cables suspended between two mountains and/or
pylons, carries a gondola containing the focal optics. This concept does not
require delay lines. Methods. Since 2003, we have been building a technical
demonstrator of this diluted telescope. The main goals of this project were to
find the opto-mechanical solutions to stabilize the optics attached under
cables at several tens of meters above the ground, and to characterize this
diluted telescope under real conditions. In 2012, we have obtained metrology
fringes, and co-spherized the primary mirrors within one micron accuracy. In
2013, we have tested the whole optical train: servo loop, metrology, and the
focal gondola. Results. We obtained stellar fringes on Deneb in September 2013.
In this paper, we present the characteristics of these observations: quality of
the guiding, S /N reached, and possible improvements for a future system.
Conclusions. It is an important step that demonstrates the feasibility of
building a diluted telescope using cables strained between cliffs or pylons.
Carlina, like the MMT or LBT, could be one of the first members of a new class
of telescopes named Large Diluted Telescopes. Its optical architecture has many
advantages for future projects: Planet Formation Imager, Post-ELTs,
Interferometer in space.Comment: 8 pages, 7 figures, Astronomy & Astrophysic
Ultrasensitive force and displacement detection using trapped ions
The ability to detect extremely small forces is vital for a variety of
disciplines including precision spin-resonance imaging, microscopy, and tests
of fundamental physical phenomena. Current force-detection sensitivity limits
have surpassed 1 (atto ) through coupling of micro or
nanofabricated mechanical resonators to a variety of physical systems including
single-electron transistors, superconducting microwave cavities, and individual
spins. These experiments have allowed for probing studies of a variety of
phenomena, but sensitivity requirements are ever-increasing as new regimes of
physical interactions are considered. Here we show that trapped atomic ions are
exquisitely sensitive force detectors, with a measured sensitivity more than
three orders of magnitude better than existing reports. We demonstrate
detection of forces as small as 174 (yocto ), with a
sensitivity 390 using crystals of Be
ions in a Penning trap. Our technique is based on the excitation of normal
motional modes in an ion trap by externally applied electric fields, detection
via and phase-coherent Doppler velocimetry, which allows for the discrimination
of ion motion with amplitudes on the scale of nanometers. These experimental
results and extracted force-detection sensitivities in the single-ion limit
validate proposals suggesting that trapped atomic ions are capable of detecting
of forces with sensitivity approaching 1 . We anticipate that
this demonstration will be strongly motivational for the development of a new
class of deployable trapped-ion-based sensors, and will permit scientists to
access new regimes in materials science.Comment: Expanded introduction and analysis. Methods section added. Subject to
press embarg
Stamp transferred suspended graphene mechanical resonators for radio-frequency electrical readout
We present a simple micromanipulation technique to transfer suspended
graphene flakes onto any substrate and to assemble them with small localized
gates into mechanical resonators. The mechanical motion of the graphene is
detected using an electrical, radio-frequency (RF) reflection readout scheme
where the time-varying graphene capacitor reflects a RF carrier at f=5-6 GHz
producing modulation sidebands at f +/- fm. A mechanical resonance frequency up
to fm=178 MHz is demonstrated. We find both hardening/softening Duffing effects
on different samples, and obtain a critical amplitude of ~40 pm for the onset
of nonlinearity in graphene mechanical resonators. Measurements of the quality
factor of the mechanical resonance as a function of DC bias voltage Vdc
indicate that dissipation due to motion-induced displacement currents in
graphene electrode is important at high frequencies and large Vdc
Control of microwave signals using circuit nano-electromechanics
Waveguide resonators are crucial elements in sensitive astrophysical
detectors [1] and circuit quantum electrodynamics (cQED) [2]. Coupled to
artificial atoms in the form of superconducting qubits [3, 4], they now provide
a technologically promising and scalable platform for quantum information
processing tasks [2, 5-8]. Coupling these circuits, in situ, to other quantum
systems, such as molecules [9, 10], spin ensembles [11, 12], quantum dots [13]
or mechanical oscillators [14, 15] has been explored to realize hybrid systems
with extended functionality. Here, we couple a superconducting coplanar
waveguide resonator to a nano-coshmechanical oscillator, and demonstrate
all-microwave field controlled slowing, advancing and switching of microwave
signals. This is enabled by utilizing electromechanically induced transparency
[16-18], an effect analogous to electromagnetically induced transparency (EIT)
in atomic physics [19]. The exquisite temporal control gained over this
phenomenon provides a route towards realizing advanced protocols for storage of
both classical and quantum microwave signals [20-22], extending the toolbox of
control techniques of the microwave field.Comment: 9 figure
Finite temperature phase diagram of a polarised Fermi condensate
The two-component Fermi gas is the simplest fermion system displaying
superfluidity, and as such finds applications ranging from the theory of
superconductivity to QCD. Ultracold atomic gases provide an exceptionally clean
realization of this system, where the interatomic interaction and the atom
species population are both independent, tuneable parameters. This allows one
to investigate the Fermi gas with imbalanced spin populations, which had
previously been experimentally elusive, and this prospect has stimulated much
theoretical activity. Here we show that the finite temperature phase diagram
contains a region of phase separation between the superfluid and normal states
that touches the boundary of second-order superfluid transitions at a
tricritical point, reminiscent of the phase diagram of He-He mixtures.
A variation of interaction strength then results in a line of tricritical
points that terminates at zero temperature on the molecular Bose-Einstein
condensate (BEC) side. On this basis, we argue that tricritical points will
play an important role in the recent experiments on polarised atomic Fermi
gases.Comment: 6 pages, 4 figures. Manuscript extended and figures modified. For
final version, see Nature Physic
Imbalanced Superfluid Phase of a Trapped Fermi Gas in the BCS-BEC Crossover Regime
We theoretically investigate the ground state of trapped neutral fermions
with population imbalance in the BCS-BEC crossover regime. On the basis of the
single-channel Hamiltonian, we perform full numerical calculations of the
Bogoliubov-de Gennes equation coupled with the regularized gap and number
equations. The zero-temperature phase diagram in the crossover regime is
presented, where the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state
governs the weak-coupling BCS region of a resonance. It is found that the FFLO
oscillation vanishes in the BEC side, in which the system under population
imbalance turns into a phase separation (PS) between locally binding superfluid
and fully polarized spin domains. We also demonstrate numerical calculations
with a large particle number O(10^5), comparable to that observed in recent
experiments. The resulting density profile on a resonance yields the PS, which
is in good agreement with the recent experiments, while the FFLO modulation
exists in the pairing field. It is also proposed that the most favorable
location for the detection of the FFLO oscillation is in the vicinity of the
critical population imbalance in the weak coupling BCS regime, where the
oscillation periodicity becomes much larger than the interparticle spacing.
Finally, we analyze the radio-frequency (RF) spectroscopy in the imbalanced
system. The clear difference in the RF spectroscopy between BCS and BEC sides
reveals the structure of the pairing field and local ``magnetization''.Comment: 16 pages, 13 figures, replaced by the version to appear in J. Phys.
Soc. Jp
Relativistic quantum effects of Dirac particles simulated by ultracold atoms
Quantum simulation is a powerful tool to study a variety of problems in
physics, ranging from high-energy physics to condensed-matter physics. In this
article, we review the recent theoretical and experimental progress in quantum
simulation of Dirac equation with tunable parameters by using ultracold neutral
atoms trapped in optical lattices or subject to light-induced synthetic gauge
fields. The effective theories for the quasiparticles become relativistic under
certain conditions in these systems, making them ideal platforms for studying
the exotic relativistic effects. We focus on the realization of one, two, and
three dimensional Dirac equations as well as the detection of some relativistic
effects, including particularly the well-known Zitterbewegung effect and Klein
tunneling. The realization of quantum anomalous Hall effects is also briefly
discussed.Comment: 22 pages, review article in Frontiers of Physics: Proceedings on
Quantum Dynamics of Ultracold Atom
Many-Body Physics with Ultracold Gases
This article reviews recent experimental and theoretical progress on
many-body phenomena in dilute, ultracold gases. Its focus are effects beyond
standard weak-coupling descriptions, like the Mott-Hubbard-transition in
optical lattices, strongly interacting gases in one and two dimensions or
lowest Landau level physics in quasi two-dimensional gases in fast rotation.
Strong correlations in fermionic gases are discussed in optical lattices or
near Feshbach resonances in the BCS-BEC crossover.Comment: revised version, accepted for publication in Rev. Mod. Phy
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