Giovanni Barontini

Double Species Bose-Einstein Condensate with Tunable Interspecies Interactions

We produce Bose-Einstein condensates of two different species, 87Rb and 41K, in an optical dipole trap in proximity of interspecies Feshbach resonances. We discover and characterize two Feshbach resonances, located around 35 and 79 G, by observing the three-body losses and the elastic cross section. The narrower resonance is exploited to create a double species condensate with tunable interactions. Our system opens the way to the exploration of double species Mott insulators and, more in general, of the quantum phase diagram of the two-species Bose-Hubbard model.

Observation of Heteronuclear Atomic Efimov Resonances

Building on the recent experimental observation with ultracold atoms, we report the first experimental evidence of Efimov physics in a heteronuclear system. A mixture of ;{41}K and ;{87}Rb atoms was cooled to few hundred nanokelvins and stored in an optical dipole trap. Exploiting a broad interspecies Feshbach resonance, the losses due to three-body collisions were studied as a function of the interspecies scattering length. We observe an enhancement of the three-body collisions for three distinct values of the interspecies scattering lengths, both positive and negative, where no Feshbach resonances are expected. We attribute the two features at negative scattering length to the existence of two kinds of Efimov trimers, KKRb and KRbRb.

Controlling the dynamics of an open many-body quantum system with localized dissipation.

We experimentally investigate the action of a localized dissipative potential on a macroscopic matter wave, which we implement by shining an electron beam on an atomic Bose-Einstein condensate (BEC). We measure the losses induced by the dissipative potential as a function of the dissipation strength observing a paradoxical behavior when the strength of the dissipation exceeds a critical limit: for an increase of the dissipation rate the number of atoms lost from the BEC becomes lower. We repeat the experiment for different parameters of the electron beam and we compare our results with a simple theoretical model, finding excellent agreement. By monitoring the dynamics induced by the dissipative defect we identify the mechanisms which are responsible for the observed paradoxical behavior. We finally demonstrate the link between our dissipative dynamics and the measurement of the density distribution of the BEC allowing for a generalized definition of the Zeno effect. Because of the high degree of control on every parameter, our system is a promising candidate for the engineering of fully governable open quantum systems.

Degenerate Bose-Bose mixture in a three-dimensional optical lattice

Quantum degenerate gases are formidable systems to shine light on fundamental quantum phenomena occurring at extremely low temperatures, such as superconductivity and superfluidity. In combination with optical lattices and scattering resonances, degenerate gases give rise to strongly correlated systems, enriching even further the breadth of phenomena that can be directly probed. Indeed, the pioneering experiment on the superfluid to Mott-insulator transition [1] has shown how physical models long studied in the field of condensed matter can be realized almost ideally. With two different atomic species, the wealth of quantum phases grows to a daunting complexity [2], only marginally explored by experiments. Actually, experiments with heteronuclear mixtures in three-dimensional (3D) optical lattice have been performed very recently only for Fermi-Bose systems [3, 4], while Fermi-Fermi and Bose-Bose mixtures are yet uncharted territory. The importance of mixtures in optical lattices is hardly overstated: association of dipolar molecules [5], mapping of spin arrays [6], schemes for quantum calculation [7], and implementation of disorder [8] represent only a few major research lines that potentially will greatly benefit from such systems. In particular, Bose-Bose mixtures seem well suited for all these purposes, provided that collisional losses are adequately suppressed. This work reports the first realization of a degenerate Bose-Bose heteronuclear mixture in a 3D optical lattice. Exploiting the large mass difference, we investigate the regime where one species lies well in the superfluid domain, while the other exhibits the disappearence of the interference pattern usually associated with the transition from a superfluid to a Mott insulator. We focus only

Entropy Exchange in a Mixture of Ultracold Atoms

We investigate experimentally the entropy transfer between two distinguishable atomic quantum gases at ultralow temperatures. Exploiting a species-selective trapping potential, we are able to control the entropy of one target gas in presence of a second auxiliary gas. With this method, we drive the target gas into the degenerate regime in conditions of controlled temperature by transferring entropy to the auxiliary gas. We envision that our method could be useful both to achieve the low entropies required to realize new quantum phases and to measure the temperature of atoms in deep optical lattices. We verified the thermalization of the two species in a 1D lattice.

Scattering in mixed dimensions with ultracold gases.

We experimentally investigate the mix-dimensional scattering occurring when the collisional partners live in different dimensions. We employ a binary mixture of ultracold atoms and exploit a species-selective 1D optical lattice to confine only one atomic species in 2D. By applying an external magnetic field in proximity of a Feshbach resonance, we adjust the free-space scattering length to observe a series of resonances in mixed dimensions. By monitoring 3-body inelastic losses, we measure the magnetic field values corresponding to the mix-dimensional scattering resonances and find a good agreement with the theoretical predictions based on simple energy considerations.

Observation of local temporal correlations in trapped quantum gases.

We measure the temporal pair correlation function g(2)(τ) of a trapped gas of bosons above and below the critical temperature for Bose-Einstein condensation. The measurement is performed in situ by using a local, time-resolved single-atom sensitive probing technique. Third- and fourth-order correlation functions are also extracted. We develop a theoretical model and compare it with our experimental data, finding good quantitative agreement. We discuss, finally, the role of interactions. Our results promote temporal correlations as new observables to study the dynamical evolution of ultracold quantum gases.

Deterministic generation of multiparticle entanglement by quantum Zeno dynamics.

Entangling atoms by persistent poking In quantum mechanics, repeated measurements targeting a particular unoccupied state of the system can keep that state from being occupied. Barontini et al. used this so-called quantum Zeno effect to restrict the dynamics of an ensemble of 36 87Rb atoms acting as qubits and residing in an optical cavity. The measurement of the cavity transmission blocked off the collective state in which all qubits were in their ground state. The ensuing dynamics resulted in the entanglement of the atoms, creating a potential resource for quantum information processing. Science, this issue p. 1317 The dynamics of an ensemble of 36 atoms of rubidium-87 in a cavity is restricted by measuring the cavity’s transmission. Multiparticle entangled quantum states, a key resource in quantum-enhanced metrology and computing, are usually generated by coherent operations exclusively. However, unusual forms of quantum dynamics can be obtained when environment coupling is used as part of the state generation. In this work, we used quantum Zeno dynamics (QZD), based on nondestructive measurement with an optical microcavity, to deterministically generate different multiparticle entangled states in an ensemble of 36 qubit atoms in less than 5 microseconds. We characterized the resulting states by performing quantum tomography, yielding a time-resolved account of the entanglement generation. In addition, we studied the dependence of quantum states on measurement strength and quantified the depth of entanglement. Our results show that QZD is a versatile tool for fast and deterministic entanglement generation in quantum engineering applications.

Thermodynamics of strongly correlated one-dimensional Bose gases

We investigate the thermodynamics of one-dimensional Bose gases in the strongly correlated regime. To this end, we prepare ensembles of independent 1D Bose gases in a two-dimensional optical lattice and perform high-resolution in situ imaging of the column-integrated density distribution. Using an inverse Abel transformation we derive effective one-dimensional line-density profiles and compare them to exact theoretical models. The high resolution allows for a direct thermometry of the trapped ensembles. The knowledge about the temperature enables us to extract thermodynamic equations of state such as the phase-space density, the entropy per particle and the local pair correlation function.

Macroscopic Zeno effect and stationary flows in nonlinear waveguides with localized dissipation.

We theoretically demonstrate the possibility of observing the macroscopic Zeno effect for nonlinear waveguides with localized dissipation. We show the existence of stable stationary flows, which are balanced by losses in the dissipative domain. The macroscopic Zeno effect manifests itself in the nonmonotonic dependence of the stationary flow on the strength of the dissipation. In particular, we highlight the importance of the dissipation parameters in observing the phenomenon. Our results are applicable to a large variety of systems, including the condensates of atoms or quasiparticles and optical waveguides.