Marco Fattori

Anderson localization of a non-interacting Bose-Einstein condensate.

Anderson localization of waves in disordered media was originally predicted fifty years ago, in the context of transport of electrons in crystals. The phenomenon is much more general and has been observed in a variety of systems, including light waves. However, Anderson localization has not been observed directly for matter waves. Owing to the high degree of control over most of the system parameters (in particular the interaction strength), ultracold atoms offer opportunities for the study of disorder-induced localization. Here we use a non-interacting Bose–Einstein condensate to study Anderson localization. The experiment is performed with a one-dimensional quasi-periodic lattice—a system that features a crossover between extended and exponentially localized states, as in the case of purely random disorder in higher dimensions. Localization is clearly demonstrated through investigations of the transport properties and spatial and momentum distributions. We characterize the crossover, finding that the critical disorder strength scales with the tunnelling energy of the atoms in the lattice. This controllable system may be used to investigate the interplay of disorder and interaction (ref. 7 and references therein), and to explore exotic quantum phases.

Observation of an Efimov spectrum in an atomic|[nbsp]|system

In 1970, Vitaly Efimov predicted that three interacting particles can form an infinite series of bound trimer states, even when none of the two-particle subsystems is stable. Experimental evidence for such an exotic state was obtained in 2006, but now an Efimov spectrum, containing two such states with the predicted scaling between them, has been observed.

Comparing Contact and Dipolar Interactions in a Bose-Einstein Condensate

We have measured the relative strength epsilon dd of the magnetic dipole-dipole interaction compared with the contact interaction in a dipolar chromium Bose-Einstein condensate. We analyze the asymptotic velocities of expansion of the condensate with different orientations of the atomic magnetic moments. By comparing the experimental results with numerical solutions of the hydrodynamic equations for dipolar condensates, we obtain epsilon dd = 0.159+/-0.034. We use this result to determine the s-wave scattering length a = (5.08+/-1.06 x 10(-9)) m = (96+/-20) a0 of 52Cr. This is fully consistent with our previous measurements on the basis of Feshbach resonances and therefore confirms the validity of the theoretical approach used to describe the dipolar Bose-Einstein condensate.

Atom Interferometry with a Weakly Interacting Bose-Einstein Condensate

We demonstrate the operation of an atom interferometer based on a weakly interacting Bose-Einstein condensate. We strongly reduce the interaction induced decoherence that usually limits interferometers based on trapped condensates by tuning the s-wave scattering length almost to zero via a magnetic Feshbach resonance. We employ a 39K condensate trapped in an optical lattice, where Bloch oscillations are forced by gravity. The fine-tuning of the scattering length down to 0.1 a_(0) and the micrometric sizes of the atomic sample make our system a very promising candidate for measuring forces with high spatial resolution. Our technique can be in principle extended to other measurement schemes opening new possibilities in the field of trapped atom interferometry.

Atom interferometry gravity-gradiometer for the determination of the Newtonian gravitational constant G

Abstract.We developed a gravity-gradiometer based on atom interferometry for the determination of the Newtonian gravitational constant G. The apparatus, combining a Rb fountain, Raman interferometry and a juggling scheme for fast launch of two atomic clouds, was specifically designed to reduce possible systematic effects. We present instrument performances and preliminary results for the measurement of G with a relative uncertainty of 1%. A discussion of projected accuracy for G measurement using this new scheme shows that the results of the experiment will be significant to discriminate between previous inconsistent values.

Feshbach resonances in ultracold 39K

We discover several magnetic Feshbach resonances in collisions of ultracold 39K atoms, by studying atom losses and molecule formation. Accurate determination of the magnetic-field resonance locations allows us to optimize a quantum collision model for potassium isotopes. We employ the model to predict the magnetic-field dependence of scattering lengths and of near-threshold molecular levels. Our findings will be useful to plan future experiments on ultracold 39K atoms and molecules.

Test of the Universality of the Three-Body Efimov Parameter at Narrow Feshbach Resonances

We measure the critical scattering length for the appearance of the first three-body bound state, or Efimov three-body parameter, at seven different Feshbach resonances in ultracold ^{39}K atoms. We study both intermediate and narrow resonances, where the three-body spectrum is expected to be determined by the nonuniversal coupling of two scattering channels. Instead, our observed ratio of the three-body parameter with the van der Waals radius is approximately the same universal ratio as for broader resonances. This unexpected observation suggests the presence of a new regime for three-body scattering at narrow resonances.

Quantum phase transitions with parity-symmetry breaking and hysteresis

Symmetry-breaking quantum phase transitions play a key role in several condensed matter, cosmology and nuclear physics theoretical models1-3. Its observation in real systems is often hampered by finite temperatures and limited control of the system parameters. In this work we report for the first time the experimental observation of the full quantum phase diagram across a transition where the spatial parity symmetry is broken. Our system is made of an ultra-cold gas with tunable attractive interactions trapped in a spatially symmetric double-well potential. At a critical value of the interaction strength, we observe a continuous quantum phase transition where the gas spontaneously localizes in one well or the other, thus breaking the underlying symmetry of the system. Furthermore, we show the robustness of the asymmetric state against controlled energy mismatch between the two wells. This is the result of hysteresis associated with an additional discontinuous quantum phase transition that we fully characterize. Our results pave the way to the study of quantum critical phenomena at finite temperature4, the investigation of macroscopic quantum tunneling of the order parameter in the hysteretic regime and the production of strongly quantum entangled states at critical points5.

MAGIA?using atom interferometry to determine the Newtonian gravitational constant

We describe our experiment MAGIA (misura accurata di G mediante interferometria atomica), in which we will use atom interferometry to perform a high precision measurement of the Newtonian gravitational constant G. Free-falling laser-cooled atoms in a vertical atomic fountain will be accelerated due to the gravitational potential of nearby source masses (SMs). Detecting this acceleration with techniques of Raman atom interferometry will enable us to assign a value to G. To suppress systematic effects we will implement a double-differential measurement. This includes launching two atom clouds in a gradiometer configuration and moving the SMs to different vertical positions. We briefly summarize the general idea of the MAGIA experiment and put it in the context of other high precision G-measurements. We present the current status of the experiment and report on analyses of the expected measurement accuracy.

Measurement of the mobility edge for 3D Anderson localization

The mobility edge characterizes the transition from localization to diffusion. This key parameter in Anderson localization was measured for a system of ultracold atoms in a tunable disordered potential created by laser speckles.