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
 

 

Macroscopic objects tend to behave classically, but their behavior near zero temperature can be fundamentally different. When a trapped atomic gas is cooled down and the atoms' thermal velocities become small, the uncertainty principle dictates that their position becomes undefined -- the atoms smear out over the entire gas volume and, if bosonic, occupy a single quantum state. The resulting dilute-gas Bose-Einstein condensate, or BEC, forms a giant coherent matter wave with superfluid properties.

In our laboratory, we load ultracold atoms from BECs into optical lattices, i.e. potentials realized by standing waves of laser light, in order to create and explore engineered quantum systems. With the help of such optical lattices, it thus becomes possible to address cutting-edge topics ranging from condensed matter physics to quantum information science, through direct quantum simulation. For example, the behavior of atoms in an optical lattice closely mimics that of electrons in a solid, but at a length scale that is three orders of magnitude larger and with exquisite control over all relevant parameters in a naturally defect-free system. Moreover, atoms in optical lattices can act as localized qubits (spins) with controllable interactions, making lattice-based atomic quantum systems a versatile platform for studying the fundamental science driving the development of modern quantum technologies.


 
Research highlights



Over the past decade, we have pioneered the manipulation of
atomic mixtures in state-dependent optical lattices to create quantum systems that combine particle-like/strongly-correlated and wave-like/superfluid behavior, and in which the internal and external degrees of freedom are dynamically coupled. Our research centers on topics in atom optics, solid-state physics and the physics of dissipative quantum systems.

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Spontaneous emission of matter waves. We demonstrated the spontaneous emission of matter .waves from a tunable open quantum system. The quantitative understanding of spontaneous emission harks back to the early days of quantum electrodynamivcs (QED), when in 1930 Weisskopf and Wigner, using Dirac's radiation theory, calculated the transition rate of an excited atom undergoing radiative decay. Their model, which describes the emission of a photon through coherent coupling of the atom’s dipole moment to the continuum of vacuum modes, reflects the view that spontaneous emission into free space, driven by vacuum fluctuations, is inherently irreversible. We realized the Weisskopf-Wigner model in a novel context that allowed us to go beyond the model's usual assumptions. For this purpose, we created an array of microscopic atom traps in an optical lattice that, driven by tunable microwave radiation, individually emit single atoms, rather than single photons, into the surrounding vacuum. Our ultracold system, which provides a matter-wave analog of photon emission in a photonic-bandgap material, revealed behavior beyond standard exponential decay with its associated Lamb shift. It includes partial backflow of radiation into the emitter, and the formation of a long-predicted, exotic bound state in which the emitted particle hovers around the emitter in an evanscent wave.
Our system provides a flexible experimental platform for simulating open-system quantum electrodynamics and for studying dissipative many-body physics with ultracold atoms.
[article] [SharedIt] [Nature News & Views] [pro-physik.de] [SBU News][nanowerk][opli][STRN] [Gizmodo][G.|UK][G.|AU]


In preceding theoretical work, we used the Weisskopf-Wigner formalism to revisit a proposal by the group of J.I.Cirac at MPQ and determine the details of the (non-)Markovian decay and emission dynamics considering an isolated matter-wave emitter in our waveguide geometry. In our work, we also calculated the spatial structure of the bound state for negative excitation energy, which is in excellent agreement with our measurements.
[article] [poster]




Diffracting matter waves without a potential. We explored a novel regime of diffraction arising in the dynamical response of an atomic Bose-Einstein condensate coupled to a optical standing wave. Throughout the long history of diffraction, it has generally been taken for granted that the diffracting object can be described through a (position-dependent) potential with absorptive or dispersive character. This includes diffraction from material structures and gratings, ponderomotive potentials, as well as Kapitza-Dirac diffraction of atoms from a standing wave of light.  The latter can be described via an optical potential, and is one of the standard tools in atom optics and metrology. We demonstrated that Kapitza-Dirac diffraction (i.e. the diffraction of matter from light) can in fact occur without a potential being present. In our experiments, we studied the dynamics of a condensate after switching on a resonant driving to the orbitals of a deep, state-selective optical lattice.  We observed diffractive dynamics that remains coupled to coherent oscillations of the internal degree of freedom, violating the usual adiabatic (Born-Oppenheimer) approximation needed for the introduction of an optical potential.  This represents a hitherto unexplored regime in which the dressed states undergoing diffraction are strongly mixed by zero-point motion.
Our study sheds new light on the role of potentials in diffraction, and on the nature of dressed states arising in Rabi-type dynamics. We explored its utility for atom interferometry and future realizationa of dissipative quantum systems. [article] [poster]




Abating the effects of disorder with weak interactions. In studies of the dynamics of a Bose-Einstein copicndensate 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. The understanding of how classical dynamics can emerge in closed quantum systems is a problem of fundamental importance. Remarkably, while classical behavior usually arises from coupling to thermal fluctuations or random spectral noise, it may also be an innate property of certain isolated, periodically driven quantum systems. 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 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-imgPRL105component mixture 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-componenimgPRL104t matter-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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