Alkaline Earth Atoms: Strictly speaking, the alkaline earth (Group II) atoms comprise beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). However, zinc (Zn), cadmium (Cd), mercury (Hg), and ytterbium (Yb) are isoelectronic to the Group II atoms and the following discussion applies to them equally as well. Alkaline earth atoms have two valence electrons, which give rise to singlet (S=0) and triplet (S=1) spin configurations. The singlet manifold contains the absolute ground state, 1S0, from which there is a strong laser cooling transition to the 1P1 excited state. The triplet manifold contains low-lying 3S1, 3P0,1,2, and 3D1,2,3 states, which provide access to a rich array of physics not available in other laser cooled atomic systems.
One advantage of the triplet manifold is that narrow line laser cooling on the spin forbidden 1S0 → 3P1 [1] and long wavelength 3P2 → 3D3 [2] transitions have Doppler limited temperatures in the 1 – 10 μK range, compared to the typical 100 μK Doppler limited temperatures of alkali atoms.
The doubly forbidden 1S0 → 3P0 transition has a sub-Hz linewidth and is being investigated as an optical frequency standard with a potential fractional accuracy of 10-17 [3-6]. Such accurate frequency measurements are currently being used to test the time-variation of fundamental constants [7,8], and may lead to a redefinition of the second.
Quantum information processing schemes have also been proposed for alkaline earth atoms involving collisions between ground state atoms [9], collisions between ground state and metastable state atoms [10], and photoassociation transitions [11]. The qubits in these proposals are the nuclear spin states of the 1S0 electronic level, which are expected to have long coherence times (~1000 s has been demonstrated in liquid xenon experiments [12,13]).
Cold Collisions: Anisotropic atom-atom interactions lead to internal state changing collisions, while isotropic interactions do not disrupt the internal degrees-of-freedom of the constitutes of the collision.
Anisotropic Interactions: The 3P2 metastable state has an electric quadrupole moment, which gives rise to anisotropic interactions between atoms [14,15]. This anisotropy leads to inelastic, spin-changing collisions [16], but also converts spin angular momentum into translational angular momentum leading to the Einstein-de Haas effect [17,18].
Isotropic Interactions: The 1S0 ground state has a closed valance shell, which minimizes the probability of inelastic collisions with other atoms or molecules [19]. As a result, ultracold alkaline earth atoms are a good candidate for sympathetically cooling other atomic and molecular species in an analogous manner to helium buffer gas cooling in cryogenic systems [20].
Precision Measurements: Advancing the frontier of precision measurements with ultracold atoms is motivated by both practical goals and fundamental pursuits. On the practical side, great strides have been made recently in atomic clocks [3-6], magnetometers [21], gyroscopes [22], and gravimeters [22]. Similarly, fundamental pursuits in search of physics beyond the Standard Model are setting new limits on the time-variation of fundamental constants [7,8], the strength of non-Newtonian gravitational interactions [23-25], and the size of permanent electric dipole moments [26]. Ultracold Yb atoms, with a heavy mass and readily accessible electronic transitions, are uniquely suited for many of these pursuits.
Atom-Surface Interactions: We aim to extend the study of atom-surface interactions into a regime where the Newtonian gravitational force law is untested and theoretical predictions suggest it may be violated [23-25]. However, on such short length scales (< 10 μm), gravitational forces are minute compared to electromagnetic (van der Waals and Casimir) forces, which complicates their measurement. Our experiment has the capability to modify either the electromagnetic force (by changing the atomic state) or the gravitational force (by changing the mass distribution under the surface) in such a manner that the two effects can be readily distinguished. Additionally, we have the option of using (bosonic) spin-0 isotopes to minimize potential magnetic interactions, or (fermonic) spin-1/2 isotopes to study theories predicting exotic spin-mass couplings. As a result, our carefully designed spectroscopic scheme can normalize out the electromagnetic background forces leaving behind a possible gravitational signal with 1000 times more sensitivity than the current best measurements.
Electric Dipole Moments: A permanent electric dipole moment (EDM) of a fundamental particle, nucleus, atom, or molecule violates both parity (P) and time-reversal (T) symmetries. The Standard Model contains only enough CP-violation to generate tiny EDMs, however various extensions to the Standard Model [e.g. Supersymmetry (SUSY)] predict EDMs that are 10 orders of magnitude larger. The current experimental limit on the electron EDM, |de| < 1.6 x 10-27 e-cm [26], already constrains the minimal supersymmetric extension to the Standard Model (MSSM). Ongoing EDM searches strive to tighten constraints on SUSY and/or probe for physics beyond the Standard Model [27]. Yb atoms are a novel choice for this endeavor since they allow for an electron EDM search in the metastable 3P2 state and a nuclear EDM (Schiff moment) search in the 1S0 ground state.
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