Thursday, April 30, 2009

Probing Interactions Between Ultracold Fermions

By G. K. Campbell, ... , Jun Ye, A. D. Ludlow

At ultracold temperatures, the Pauli exclusion principle suppresses collisions between identical fermions. This has motivated the development of atomic clocks with fermionic isotopes. However,by probing an optical clock transition with thousands of lattice-confined, ultracold fermionic strontium atoms, we observed density-dependent collisional frequency shifts. These collision effects were measured systematically and are supported by a theoretical description attributing them to inhomogeneities in the probe excitation process that render the atoms distinguishable. This work also yields insights for zeroing the clock density shift.

Thursday, April 23, 2009

Repulsive Fermi Gas in a harmonic trap: Ferromagnetism and spin textures
by L. LeBlanc, ...., A. Paramekanti

We study ferromagnetism in a repulsively interacting two-component Fermi gas in a harmonic trap at zero net magnetization. Within a local density approximation, the two components phase-separate beyond a critical interText Coloraction strength, with one species having a higher density at the trap center. The mean field release energy depends on the interaction strength and contains a sharp signature of this transition. To go beyond the local density approximation, we derive an energy functional which includes a term that depends on the local magnetization gradient and acts as a `surface tension'. Using this energy functional, we numerically study the energetics of some candidate spin textures which may be stabilized in a harmonic trapping potential. We find that a hedgehog state has a lower energy than an `in-out' domain wall state in an isotropic trap. Upon inclusion of trap anisotropy we find that the hedgehog magnetization profile gets distorted due to the surface tension term, this distortion being more apparent for small atom numbers. We estimate that the magnetic dipole interaction does not play a significant role in this system. We consider implications for experiments on trapped two-component Fermi systems such as Li-6 and K-40 gases.

**Groupmeeting by Lindsay LeBlanc**

Tuesday, April 21, 2009

Observation of Rydberg blockade between two atoms
by E. Urban, ...., T. Walker and M. Saffman

Blockade interactions whereby a single particle prevents the flow or excitation of other particles provide a mechanism for control of quantum states, including entanglement of two or more particles. Blockade has been observed for electrons1, 2, 3, photons4 and cold atoms5. Furthermore, dipolar interactions between highly excited atoms have been proposed as a mechanism for 'Rydberg blockade'6, 7, which might provide a novel approach to a number of quantum protocols8, 9, 10, 11. Dipolar interactions between Rydberg atoms were observed several decades ago1213, 14, 15, 16, 17, 18. However, to harness Rydberg blockade for controlled quantum dynamics, it is necessary to achieve strong interactions between single pairs of atoms. Here, we demonstrate that a single Rydberg-excited rubidium atom blocks excitation of a second atom located more than 10 mum away. The observed probability of double excitation is less than 20%, consistent with a theoretical model of the Rydberg interaction augmented by Monte Carlo simulations that account for experimental imperfections. and have been studied recently in a many-body regime using cold atoms

**Groupmeeting by Xingxing Xing**

Wednesday, April 8, 2009

Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms
by Andre Schirotzek, ...., and Martin Zwierlein

We have observed Fermi polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid coexisting with a Fermi sea.

**Groupmeeting by Dylan Jervis**

Wednesday, April 1, 2009

Violation of the principles of Complementarity, and its implications
by Shahriar Afshar

Bohr's principle of complementarity predicts that in a welcher weg ("which-way") experiment, obtaining fully visible interference pattern should lead to the destruction of the path knowledge. Here I report a failure for this prediction in an optical interferometry experiment. Coherent laser light is passed through a dual pinhole and allowed to go through a converging lens, which forms well-resolved images of the respective pinholes, providing complete path knowledge. A series of thin wires are then placed at previously measured positions corresponding to the dark fringes of the interference pattern upstream of the lens. No reduction in the resolution and total radiant flux of either image is found in direct disagreement with the predictions of the principle of complementarity. In this paper, a critique of the current measurement theory is offered, and a novel nonperturbative technique for ensemble properties is introduced. Also, another version of this experiment without an imaging lens is suggested, and some of the implications of the violation of complementarity for another suggested experiment to investigate the nature of the photon and its "empty wave" is briefly discussed.

**Groupmeeting by Zach Medendorp**