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arXiv:0902.2558 · Science

Probing Interactions Between Ultracold Fermions

Gretchen K. Campbell, Martin M. Boyd, Jan W. Thomsen, Michael J. Martin, Sebastian Blatt, Matthew D. Swallows, Travis Nicholson, Tara M. Fortier, Chris W. Oates, Scott A. Diddams, N. Lemke, Pascal Naidon, Paul S. Julienne, Jun Ye, Andrew D. Ludlow

Published Apr 16, 2009• 121 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1126/science.1169724

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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.
Optical lattice clocks with extremely stable frequency are possible when many atoms are interrogated simultaneously, but this precision may come at the cost of systematic inaccuracy resulting from atomic interactions. Density-dependent frequency shifts can occur even in a clock that uses fermionic atoms if they are subject to inhomogeneous optical excitation. However, sufficiently strong interactions can suppress collisional shifts in lattice sites containing more than one atom. We demonstrated the effectiveness of this approach with a strontium lattice clock by reducing both the collisional frequency shift and its uncertainty to the level of 10(-17). This result eliminates the compromise between precision and accuracy in a many-particle system; both will continue to improve as the number of particles increases.
We investigate the influence of atomic motion on precision Rabi spectroscopy of ultracold fermionic atoms confined in a deep one-dimensional optical lattice. We analyze the spectral components of longitudinal sideband spectra and present a model to extract information about the transverse motion and sample temperature from their structure. Rabi spectroscopy of the clock transition itself is also influenced by atomic motion in the weakly confined transverse directions of the optical lattice. By deriving Rabi flopping and Rabi line shapes of the carrier transition, we obtain a model to quantify trap-state-dependent excitation inhomogeneities. The inhomogeneously excited ultracold fermions become distinguishable, which allows sss-wave collisions. We derive a detailed model of this process and explain observed density shift data in terms of a dynamic mean-field shift of the clock transition.
We study ultracold collisions in fermionic ytterbium by precisely measuring the energy shifts they impart on the atoms' internal clock states. Exploiting Fermi statistics, we uncover p-wave collisions, in both weakly and strongly interacting regimes. With the higher density afforded by two-dimensional lattice confinement, we demonstrate that strong interactions can lead to a novel suppression of this collision shift. In addition to reducing the systematic errors of lattice clocks, this work has application to quantum information and quantum simulation with alkaline-earth atoms.
We discuss two possible configurations for optical lattice clocks; a one-dimensional (1D) lattice loaded with spinpolarized fermions, and a three-dimensional (3D) lattice loaded with bosons. In the former scheme, collisional frequency shifts are suppressed by the quantum statistical property of identical fermions. This Pauli blocking of collisions is critically dependent on the degree of spin polarization of the fermionic atoms, which we carefully investigated in the Rabi excitation process of the clock transition. In the latter scheme, a single occupancy lattice suppresses bunching of bosons and collisional frequency shifts. We demonstrate a frequency comparison of these two optical lattice clocks based on fermionic 87Sr and bosonic 88Sr. Operating these clocks parallely has yielded a stability approaching 10−17. Such measurements are an important step to ascertain the lattice clocks' uncertainty at the 10−17 level and beyond.
Recent experiments have measured collisional frequency shifts in polarized fermionic alkaline-earth atoms using 1S0-3P0 Rabi spectroscopy. Here, we provide a first-principles nonequilibrium theoretical description of the interaction frequency shifts starting from the microscopic many-body Hamiltonian. Our formalism describes the dependence of the frequency shift on excitation inhomogeneity, interactions, temperature, and many-body dynamics, provides a fundamental understanding of the effects of the measurement process, and explains the observed density shift data.