Superfluid Fraction in an Interacting Spatially Modulated Bose-Einstein Condensate.

At zero temperature, a Galilean-invariant Bose fluid is expected to be fully superfluid. Here we investigate theoretically and experimentally the quenching of the superfluid density of a dilute Bose-Einstein condensate due to the breaking of translational (and thus Galilean) invariance by an external 1D periodic potential. Both Leggett's bound fixed by the knowledge of the total density and the anisotropy of the sound velocity provide a consistent determination of the superfluid fraction. The use of a large-period lattice emphasizes the important role of two-body interactions on superfluidity.

Realization of a Townes Soliton in a Two-Component Planar Bose Gas.

Most experimental observations of solitons are limited to one-dimensional (1D) situations, where they are naturally stable. For instance, in 1D cold Bose gases, they exist for any attractive interaction strength g and particle number N. By contrast, in two dimensions, solitons appear only for discrete values of gN, the so-called Townes soliton being the most celebrated example. Here, we use a two-component Bose gas to prepare deterministically such a soliton: Starting from a uniform bath of atoms in a given internal state, we imprint the soliton wave function using an optical transfer to another state. We explore various interaction strengths, atom numbers, and sizes and confirm the existence of a solitonic behavior for a specific value of gN and arbitrary sizes, a hallmark of scale invariance.

Magnetic Dipolar Interaction between Hyperfine Clock States in a Planar Alkali Bose Gas.

In atomic systems, clock states feature a zero projection of the total angular momentum and thus a low sensitivity to magnetic fields. This makes them widely used for metrological applications like atomic fountains or gravimeters. Here, we show that a mixture of two such nonmagnetic states still displays magnetic dipole-dipole interactions comparable to the one expected for the other Zeeman states of the same atomic species. Using high-resolution spectroscopy of a planar gas of ^{87}Rb atoms with a controlled in plane shape, we explore the effective isotropic and extensive character of these interactions and demonstrate their tunability. Our measurements set strong constraints on the relative values of the s-wave scattering lengths a_{ij} involving the two clock states.

Sound Propagation in a Uniform Superfluid Two-Dimensional Bose Gas.

In superfluid systems several sound modes can be excited, such as, for example, first and second sound in liquid helium. Here, we excite running and standing waves in a uniform two-dimensional Bose gas and we characterize the propagation of sound in both the superfluid and normal regimes. In the superfluid phase, the measured speed of sound is in good agreement with the prediction of a two-fluid hydrodynamic model, and the weak damping is well explained by the scattering with thermal excitations. In the normal phase we observe a stronger damping, which we attribute to a departure from hydrodynamic behavior.

Relaxation Dynamics in the Merging of N Independent Condensates.

Controlled quantum systems such as ultracold atoms can provide powerful platforms to study nonequilibrium dynamics of closed many-body quantum systems, especially since a complete theoretical description is generally challenging. In this Letter, we present a detailed study of the rich out-of-equilibrium dynamics of an adjustable number N of uncorrelated condensates after connecting them in a ring-shaped optical trap. We observe the formation of long-lived supercurrents and confirm the scaling of their winding number with N in agreement with the geodesic rule. Moreover, we provide insight into the microscopic mechanism that underlies the smoothening of the phase profile.

Exploring the thermodynamics of a universal Fermi gas.

One of the greatest challenges in modern physics is to understand the behaviour of an ensemble of strongly interacting particles. A class of quantum many-body systems (such as neutron star matter and cold Fermi gases) share the same universal thermodynamic properties when interactions reach the maximum effective value allowed by quantum mechanics, the so-called unitary limit. This makes it possible in principle to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory. Previous work on the thermodynamics of a two-component Fermi gas led to thermodynamic quantities averaged over the trap, making comparisons with many-body theories developed for uniform gases difficult. Here we develop a general experimental method that yields the equation of state of a uniform gas, as well as enabling a detailed comparison with existing theories. The precision of our equation of state leads to new physical insights into the unitary gas. For the unpolarized gas, we show that the low-temperature thermodynamics of the strongly interacting normal phase is well described by Fermi liquid theory, and we localize the superfluid transition. For a spin-polarized system, our equation of state at zero temperature has a 2 per cent accuracy and extends work on the phase diagram to a new regime of precision. We show in particular that, despite strong interactions, the normal phase behaves as a mixture of two ideal gases: a Fermi gas of bare majority atoms and a non-interacting gas of dressed quasi-particles, the fermionic polarons.

Quench-induced supercurrents in an annular Bose gas.

We create supercurrents in annular two-dimensional Bose gases through a temperature quench of the normal-to-superfluid phase transition. We detect the magnitude and the direction of these supercurrents by measuring spiral patterns resulting from the interference of the cloud with a central reference disk. These measurements demonstrate the stochastic nature of the supercurrents. We further measure their distribution for different quench times and compare it with predictions based on the Kibble-Zurek mechanism.

Experimental realization of plaquette resonating valence-bond states with ultracold atoms in optical superlattices.

The concept of valence-bond resonance plays a fundamental role in the theory of the chemical bond and is believed to lie at the heart of many-body quantum physical phenomena. Here we show direct experimental evidence of a time-resolved valence-bond quantum resonance with ultracold bosonic atoms in an optical lattice. By means of a superlattice structure we create a three-dimensional array of independent four-site plaquettes, which we can fully control and manipulate in parallel. Moreover, we show how small-scale plaquette resonating valence-bond (RVB) states with s- and d-wave symmetry can be created and characterized. We anticipate our findings to open the path towards the creation and analysis of many-body RVB states in ultracold atomic gases.

Experimental realization of strong effective magnetic fields in an optical lattice.

We use Raman-assisted tunneling in an optical superlattice to generate large tunable effective magnetic fields for ultracold atoms. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of 1 flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for noninteracting particles. We provide a measurement of the local phase acquired from Raman-induced tunneling, demonstrating time-reversal symmetry breaking of the underlying Hamiltonian. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the magnetic field is directly revealed.

Fermi-liquid behavior of the normal phase of a strongly interacting gas of cold atoms.

We measure the magnetic susceptibility of a Fermi gas with tunable interactions in the low-temperature limit and compare it to quantum Monte Carlo calculations. Experiment and theory are in excellent agreement and fully compatible with the Landau theory of Fermi liquids. We show that these measurements shed new light on the nature of the excitations of the normal phase of a strongly interacting Fermi gas.

Tan's two-body contact across the superfluid transition of a planar Bose gas.

Tan's contact is a quantity that unifies many different properties of a low-temperature gas with short-range interactions, from its momentum distribution to its spatial two-body correlation function. Here, we use a Ramsey interferometric method to realize experimentally the thermodynamic definition of the two-body contact, i.e., the change of the internal energy in a small modification of the scattering length. Our measurements are performed on a uniform two-dimensional Bose gas of 87Rb atoms across the Berezinskii-Kosterlitz-Thouless superfluid transition. They connect well to the theoretical predictions in the limiting cases of a strongly degenerate fluid and of a normal gas. They also provide the variation of this key quantity in the critical region, where further theoretical efforts are needed to account for our findings.

Collective oscillations of an imbalanced Fermi gas: axial compression modes and polaron effective mass.

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.

Six-and-half years' experience with the St. Jude BioImplant porcine prosthesis.

BACKGROUND AND AIMS OF THE STUDY: In this study, we reviewed our experience in heart valve replacement with the St. Jude BioImplant heart valve, which is a low-profile, low-pressure glutaraldehyde-fixed porcine prosthesis mounted on a flexible Delrin stent. METHODS: During the period May 1989-January 1996, 117 patients were implanted with 132 BioImplant prostheses; three patients were lost to follow up and excluded from the series. Mean age was 67.5 +/- 9.8 years (range: 19 to 82 years); myocardial revascularization was performed in 22 (19.3%) patients. In-hospital mortality rate was 6% (7/117 patients). By January 1996, 114 patients (53 males, 61 females), in whom 59 aortic, 35 mitral, 15 mitro-aortic and five tricuspid prostheses had been implanted, were eligible for the analysis. Mean follow up was 40.4 +/- 21.7 months (range: 1 to 76 months). RESULTS: The survival probability of survivors was 72.1 +/- 6.5 at 77 months. Seventeen patients died during follow-up. The mean NYHA class improved from 3.1 +/- 0.6 preoperatively to 1.4 +/- 0.6 postoperatively. The freedom probabilities were respectively 89.5 +/- 5.3% from thromboembolism, 93.2 +/- 3.7% from infective endocarditis, 84.5 +/- 10.3% from structural dysfunction, 99.1 +/- 0.9% from non-structural dysfunction, and 80.1 +/- 10.2% from reoperation. The freedom probability for valve-related events was respectively 75.3 +/- 12.3%, 98.0 +/- 1.9% and 67.2 +/- 17.2% for patients who underwent mitral, aortic and mitro-aortic heart valve replacement (p = 0.05 comparing only patients who underwent mitral or aortic replacement); moreover the freedom probability from valve-related events was 71.6 +/- 11.2% in patients aged < or = 65 years and 90.2 +/- 6.6% in patients aged > 65 years (p = 0.006). CONCLUSIONS: The BioImplant heart valve, in our experience, seems to be a valuable device which shows a mid-term performance similar to that of other porcine prostheses.

A simple way to treat chronic atrial fibrillation during mitral valve surgery: the epicardial radiofrequency approach.

OBJECTIVE: We describe an original radiofrequency ablation technique to treat chronic atrial fibrillation in patients undergoing mitral valve surgery. Most of the procedure is carried out epicardially, in order to avoid an undue increase of surgical time and trauma. METHODS: The ablations are performed using a temperature-controlled multipolar radiofrequency catheter. Two encircling lesions around the ostia of the right and of the left pulmonary veins are carried out epicardially, usually before cardiopulmonary bypass. Through a conventional left atriotomy the ablation procedure is completed with two endocardial lesions connecting the two encirclings between them and to the mitral valve annulus. After the mitral valve procedure is performed, the left appendage is sutured. RESULTS: From February 1998 to May 1999, 40 patients with chronic atrial fibrillation (43. 1+/-51.9 months) underwent combined radiofrequency ablation and mitral valve surgery. Mean left atrial diameter was 56.8+/-10.7 mm. Mean cardiopulmonary bypass and aortic cross-clamp time were, respectively, 119.1+/-26.3 and 76.7+/-21.0 min. Mean postoperative blood loss was 287.2+/-186.6 ml. No reexploration for bleeding occurred. One patient died of pneumonia 12 days after operation. No patient needed permanent pacemaker implantation. Mean postoperative hospital stay was 7.3+/-5.6 days. At follow-up (mean 11.6+/-4.7 months), 30/39 (76.9%) of the patients were in stable sinus rhythm. All patients in sinus rhythm 3 months after operation recovered both left and right atrial contractility at echocardiographic control (mean 7.3+/-3.4 months). The left atrial diameter decreased significantly in patients recovering sinus rhythm. CONCLUSIONS: Epicardial radiofrequency ablation is a safe means to achieve surgical ablation of atrial fibrillation with a high success rate. The simplicity of the technique and the low procedure-related risk should dictate combined treatment virtually in all patients with atrial fibrillation undergoing open heart operations.

Timing for surgical treatment in native infective endocarditis. A seven-year experience.

BACKGROUND: Early surgical intervention in infective endocarditis is performed only when there is persistence of sepsis, hemodynamic instability or when arterial embolism has occurred, otherwise a 4-week antibiotic therapy before surgery is considered necessary. Our 7-year experience in the surgical treatment of native endocarditis in 28 patients, is here revised focusing on the timing of surgery. METHODS: Patients were retrospectively divided into group A (n. 16) with blood cultures that became negative before surgery and group B (n. 11) with blood cultures positive at the time of urgent surgery. One patient with constantly negative blood cultures was not assigned to any group. In group A antibiotic therapy was administered until 3 consecutive blood cultures became negative and the patients were then operated on the basis of echocardiographic findings after a mean duration of antibiotic therapy of 17.4 +/- 6.3 days. RESULTS: Twenty-six patients out of 28 underwent replacement of the infected valve (mechanical bileaflets in 16 patients, porcine stented in 7 and porcine stentless in 3). Valve repair was performed in 2 patients. Overall operative mortality was) 7.1% (2/28); death occurred in 2 patients of group B, operated on for cardiogenic shock. Two/26 patients died (1 acute renal failure and 1 stroke) at a mean follow-up of 32.5 +/- 24.8 (range 3-95) months. CONCLUSIONS: Patients who underwent surgery for infective endocarditis after blood culture negativization showed no mortality and no recurrence of disease even if a 4 weeks antibiotic course was not completed. This experience suggests that earlier operations can be performed safely, lowering the incidence of hemodynamic impairment and arterial embolism.

[Epicardial ablation of atrial fibrillation during heart valve surgery]. / Ablazione epicardica della fibrillazione atriale durante chirurgia valvolare.

BACKGROUND: We report the results of an intraoperative ablation procedure for combined treatment of atrial fibrillation (AF) in patients affected by heart valve disease. METHODS: From February 1998 to June 2000, 80 patients scheduled for heart valve operations underwent combined surgical treatment of AF. Seventy-eight patients had mitral valve disease and 2 had aortic regurgitation; 74 patients were affected by chronic AF (mean 50 +/- 74 months, range 6-480 months) and 6 had paroxysmal AF. A left atrial set of radiofrequency ablations (mainly epicardial) was performed in all patients. RESULTS: Thirty-five patients underwent conservative mitral valve surgery, 43 had mitral valve replacement and 2 had aortic valve replacement. The combination of the ablation procedure did not lead to a substantial prolongation of cardiopulmonary and aortic cross clamp time and did not increase perioperative morbidity. No procedure-related complications were recorded. Operative mortality was favorably comparable with that of valvular surgery alone (2.5%). Mean hospital stay was 6.8 +/- 4.4 days. At follow-up (16.2 +/- 9.2 months, range 3-28 months), 61 patients (78.2%) were in stable sinus rhythm; all of them recovered left and right atrial contractility as assessed by Doppler echocardiography. CONCLUSIONS: The combined treatment of AF with a radiofrequency ablation surgical technique is effective in restoring stable sinus rhythm and atrial contractility. The procedure is low risk thereby allowing a prompt clinical recovery after operation. It should therefore be considered in all patients with AF undergoing open-heart surgery.

The equation of state of a low-temperature Fermi gas with tunable interactions.

Interacting fermions are ubiquitous in nature, and understanding their thermodynamics is an important problem. We measured the equation of state of a two-component ultracold Fermi gas for a wide range of interaction strengths at low temperature. A detailed comparison with theories including Monte-Carlo calculations and the Lee-Huang-Yang corrections for low-density bosonic and fermionic superfluids is presented. The low-temperature phase diagram of the spin-imbalanced gas reveals Fermi liquid behavior of the partially polarized normal phase for all but the weakest interactions. Our results provide a benchmark for many-body theories and are relevant to other fermionic systems such as the crust of neutron stars.