Dept. of Physics & Astronomy
Physics S-114
Stony Brook University
Stony Brook, NY 11794-3800
Tel. 631 632-4497 and -8043
Fax 631 632-8176
People

 

Research

Introduction
 

 

In our laboratory we create, control, and manipulate quantum-degenerate atomic gases. The broad goal of our research is to produce "designer materials" - a billion times less dense and cooler than conventional materials - with precisely controllable quantum properties that will allow for fundamental studies at the boundary between atomic physics, quantum optics, solid-state physics, mathematical physics, and quantum information science.

Our main research focus currently is on multicomponent Bose-Einstein condensates in periodic optical potentials (optical lattices) and on their manybody quantum behavior both in the regimes of weak and strong quantum correlations.

 

 
Research highlights


Nonadiabatic diffraction of matter waves. We identified a novel regime for the diffraction of atoms from a standing wave of lightIn our experiment, we studied the dynamics of a condensate resonantly driven to the orbitals of a deep, state-selective optical lattice. Weak driving produces Rabi oscillations between lattice and condensate. However, for stronger driving these oscillations become less pronounced and are accompanied by a dynamical "digging" of holes into the condensate wavefunction, as evidenced by diffractive transfer of atoms to higher momenta. A theoretical analysis of the coupled dynamics reveals coherent, nonadiabatic transitions between optical potentials that are driven by zero-point motion and vanish only for extremely strong driving (in the latter case, the population/diffractive dynamics decouple, and conventional, i.e. adiabatic, Kapitza-Dirac diffraction is recovered). Besides its relevance for interferometry applications, our work sheds light on the notion of dressed potentials in ultracold atomic physics. [article] [poster]




Abating the effects of disorder with weak interactions. In studies of the dynamics of a Bose-Einstein cpicondensate in a tilted incommensurate optical lattice, we demonstrated a nontrivial interplay between interactions and disorder. When a weak constant force is applied to atoms moving in a lattice potential, they undergo periodic motion known as Bloch oscillations (rather than accelerating as free particles). It is well established that for condensates these oscillations damp out due to either the presence of disorder or collisional interactions. However, we observed that when interactions and disorder are present simultaneously, they can compete with each other such that more interactions actually mean less, rather than more, damping. This screening effect vanishes once the interactions become stronger than the disorder (strong interactions cooperate with disorder, cf. eg. our earlier work). Our results are of relevance for the description of systems ranging from superfluid helium in porous materials to superconductivity in granular or disordered materials.[article] [poster]


   

Crossing the quantum-to-classical boundary with a coupled-rotor system. By exposing a Bose-Einstein condensate to a periodically-flashed, two-color standing wave of light, we have for the first time realized a system of two coupled, kicked quantum rotors, and have discovered a quantum-to-classical transition. The simple quantum kicked rotor, which one obtains when using a single-color standing wave only, is a well-studied paradigm system for quantum chaos. Here, unless very specific conditions are met, the atomic velocities remain frozen in time due to quantum interference, despite continuous periodic kicking. However, already when just two such off-resonantly kicked rotors are coupled, new physics can appear. In our work, we found that the ability of each rotor to freeze the atoms' velocities can be destroyed, and that the coupling between the two rotors instead produces classical momentum diffusion. Our work shows that classical behavior can indeed be an innate property of simple quantum systems, and thus sheds light on the fundamental question how the quantum and classical worlds are connected. [article] [poster] [PHYS.ORG]

 

 
   

Probing an ultracold-atom crystal using atomic matter waves. We explored the scattering of atomic matter waves from ultracold atoms confined in an optical lattice. By “shining” a one-dimensional Bose gas (probe) onto a Mott insulator (target), we observed Bragg diffraction peaks that reveal the spatial ordering and localization of atoms on individual lattice sites. Our in-situ interrogation technique is non-destructive, and it precludes limitations on spatial resolution since the de Boglie wavelength of the incident atoms can be tuned freely. For weak confinement, we observed inelastic excitations of atoms in the target, which connect to a quantum "Newton's cradle" in the free-atom limit. Furthermore, we used atomic de Broglie waves to detect forced antiferromagnetic ordering in an atomic spin mixture, demonstrating the suitability of our method for the non-destructive detection of spin-ordered phases in strongly correlated atomic gases. Our work extends matter-wave diffraction, a technique that has already proven useful in various scientific disciplines, to the realm of ultracold quantum matter. Our method is similar to the diffraction of neutrons for the characterization of solid-state systems, but at energies that are a billion times lower. It also provides a nice example of wave-particle duality, where ultracold atoms serve both as localized particles and as coherent waves diffracting from them. [article] [poster] [PHYS.ORG]

 

 
   

Impurities make an ultracold-atom crystal behave like an amorphous glass.  We studied one-dimensional gases of bosonic atoms subject to localized atomic impurities and observed behavior consistent with an exotic quantum phase called a Bose glass. Two-species mixtures of quantum gases can form so-called "quantum emulsions" (see next paragraph). When one of the two species is pinned in place, its frozen emulsion pattern can effectively act as a static disorder potential for the more mobile species. It is predicted that such atomic impurities can cause a disorder-driven quantum phase transition from a perfectly-ordered Mott insulator crystal to an amorphous Bose glass. In our experiment, we created "frozen" atomic impurities by loading the atoms of one species onto the sites of a deep, species-specific optical lattice. We found that the other, more mobile species is driven to a localized, insulating state with a flat excitation profile, consistent with the expectation for a Bose glass. One key motivation for the use of atomic impurity disorder, as opposed to the standard optical disorder potentials, is the difference in spatial correlation properties. Indeed, we found that using uncorrelated atomic disorder much more easily drives the system into the amorphous phase. [article] [poster]

 

 
   

Dressings, dips, and emulsions in ultracold mixtures. We reported novel many-body effects for a two-component mixtuimgPRL105re of ultracold bosonic atoms in an optical lattice. In our setup, one component feels a much deeper lattice than the other, which confines these atoms to the lattice sites, while the other component forms a frictionless superfluid background. An atom on a given site locally displaces the superfluid around it, thereby dressing itself with a density dip. Such a dressed atom ("polaron") is predicted to be less mobile than a bare atom, since it has to carry its dip along as it hops from site to site. Also, when two or more dressed atoms get close, the dips may cause mutual attraction and clustering, similar to what would happen to bowling balls placed on a mattress. On the other hand, the density of the superfluid surrounding the atoms can be made to dip so strongly that the superfluid effectively breaks up into many tiny droplets, leading to a "quantum emulsion" akin to a shaken vinaigrette. Consistent with these two scenarios, we observed that the apparent superfluid coherence of a given component near the superfluid-to-Mott transition is reduced by a second component, most significantly when the latter either experiences a strongly localizing lattice potential or none at all. [article] [poster]

 

 
 

 

Adding a spin to nonlinear atom optics. We have observed collinear four-wave mixing of two-component imgPRL104matter-waves. Even though the first atomic four-wave mixing (FWM) experiment was performed a decade ago, no experiment to date has exploited the possibilities arising from the internal degrees of freedom. We report the first experimental demonstration of atomic FWM with two internal states and distinct, macroscopically occupied momentum modes. Our system is promising for the creation and observation of counter-propagating atomic wave packets with macroscopic spin entanglement and/or spin squeezing. The collinearity should be especially useful for possible applications in sub-shot-noise atom interferometry. Moreover, we find that FWM can also occur after the release of a homo-nuclear bosonic mixture trapped in a state-dependent optical lattice. We show that FWM can mimic in-situ interaction effects, and thus needs to be taken into account in such studies. [article] [poster]

 

 
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2012/07/30