We report quantum degeneracy in a gas of ultra-cold fermionic 87 Sr atoms. By evaporatively cooling a mixture of spin states in an optical dipole trap for 10.5 s, we obtain samples well into the degenerate regime with T /TF = 0.26−.06 . The main signature of degeneracy is a change in the momentum distribution as measured by time-of-flight imaging, and we also observe a decrease in evaporation efficiency below T /TF ∼ 0.5. 87 Sr has a very large nuclear spin, I=9/2, which may allow studies of novel magnetic phenomena due to enlarged SU(N) symmetry of the interaction Hamiltonian for N = 2I + 1 [17][18][19]. High resolution spectroscopy technologies are the most advanced in strontium because of the use of narrow intercombination transitions in 87 Sr for optical frequency standards [20], and these tools have motivated proposals for applications in quantum information [21][22][23] and quantum simulation of many-body phenomena [17,24]. Strontium also has a number of stable bosons which have recently been brought into the quantum degenerate regime [25][26][27], which makes Fermi-Bose mixtures with relatively small mass differences available [28]. There is also the potential for manipulating interactions on small spatial and temporal scales with low-loss optical Feshbach resonances [29,30]. Details about our apparatus can be found in [26,31,32]. Atoms are trapped from a Zeeman slowed beam in a magneto-optical trap (MOT) operating on the (5s 2 ) 1 S 0 − (5s5p) 1 P 1 transition at 461 nm. Since this transition is not closed, approximately 1 in 10 5 excitations results in an atom decaying through the (5s5d) 1 D 2 state to the (5s5p) 3 P 2 state, which has an 9 minute lifetime [33] and can be trapped in the quadrupole mag- FIG. 1. (color online) Partial level diagram for88 Sr (--) and 87 Sr (-) including hyperfine structure and isotope shifts. For 87 Sr, total quantum number F is indicated.netic field of the MOT [34][35][36][37]. The magnetic trap has a lifetime of about 25 s, which is limited by background pressure and blackbody radiation [36]. This allows us to accumulate atoms over a period of 30 s and trap a significant number in spite of the low natural abundance of 87 Sr (7%).After accumulation, atoms in the 3 P 2 state are returned to the ground state via excitation to the (5s4d) 3 D 2 state [32]. This repumping is achieved by applying 3 W/cm 2 of 3.012 µm light for 60 ms. The transition has hyperfine structure due to the nuclear spin of I = 9/2, which spreads the transition over ∼3 GHz. Individual hyperfine transitions are not resolved in the repumping efficiency curve because of the high intensity of the repump laser and length of time over which the repumping laser is applied [38]. The 3 µm laser is tuned 1 GHz blue of the 87 Sr centroid, which effectively repumps transitions from the F = 11/2 and F = 13/2 3 P 2 levels. During the repumping stage, the 461 nm MOT is left on so that atoms returned to the ground state are recaptured and cooled. We typically recapture 3 × 10 7 atoms at temperatures of a few milliKelvi...

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We demonstrate control of the collapse and expansion of an 88 Sr Bose-Einstein condensate using an optical Feshbach resonance (OFR) near the 1 S0-3 P1 intercombination transition at 689 nm. Significant changes in dynamics are caused by modifications of scattering length by up to ±10 a bg , where the background scattering length of 88 Sr is a bg = −2 a0 (1 a0 = 0.053 nm). Changes in scattering length are monitored through changes in the size of the condensate after a time-of-flight measurement. Because the background scattering length is close to zero, blue detuning of the OFR laser with respect to a photoassociative resonance leads to increased interaction energy and a faster condensate expansion, while red detuning triggers a collapse of the condensate. The results are modeled with the time-dependent non-linear Gross-Pitaevskii equation.The ability to tune interactions in ultracold atomic gases makes these systems ideal for exploring many-body physics [1] and has enabled some of the most important recent advances in atomic physics, such as investigation of the Bose-Einstein condensate (BEC)-Bardeen-CooperSchrieffer crossover regime [1] and creation of quantum degenerate molecules [2,3]. Magnetic Feshbach resonances [4], which are the standard tool for changing atomic interactions, have proven incredibly powerful, but they are also limited because the methods for creating magnetic fields preclude high-frequency spatial and temporal modulation. Also, in atoms with non-degenerate ground states, such as alkaline-earth-metal atoms, magnetic Feshbach resonances do not exist.These limitations can be overcome by using an optical Feshbach resonance (OFR), which tunes interatomic interactions by coupling a colliding atom pair to a bound molecular level of an excited state potential with a laser tuned near a photoassociative resonance [5]. Optical Feshbach resonances may open new avenues of research in nonlinear matter waves [6][7][8] and quantum fluids [9][10][11], and could be very valuable for experiments with fermionic alkaline-earth atoms [12,13] Early experiments on OFRs [22-24] used strong dipoleallowed transitions in alkali-metal atoms to alter atomic collision properties, but substantial change in the atomatom scattering length was accompanied by rapid atom losses. Tuning of interactions in alkali-metal atoms, but with smaller atom loss, was recently obtained with a magnetic Feshbach resonance using an AC Stark shift of the closed channel to modify the position of the resonance [25,26]. Recently, a multiple-laser optical method was proposed for wider modulation of the interaction strength near a magnetic Feshbach resonance [27]. Unfortunately, none of these hybrid variations are feasible for atoms lacking magnetic Feshbach resonances.Ciurylo et al. [28,29] predicted that an OFR induced by a laser tuned near a weakly allowed transition should tune the scattering length with significantly less induced losses. This can be done with divalent atoms, such as strontium and ytterbium, by exciting near an intercombinatio...

In high energy and condensed matter physics, particle exchange plays an essential role in the understanding of long-range interactions. For example, the exchange of massive bosons leads to the Yukawa potential [1,2]. Phonon exchange between electrons gives rise to Cooper pairing in superconductors [3]. When a Bose-Einstein condensate (BEC) of Cs is embedded in a degenerate Fermi gas of Li, we show that interspecies interactions can give rise to an effective trapping potential, damping, and attractive boson-boson interactions mediated by fermions. The latter, related to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism [4], results from a coherent three-body scattering process. Such mediated interactions are expected to form novel magnetic phases [5] and supersolids [6]. We show that for suitable conditions, the mediated interactions can convert a stable BEC into a train of "Bose-Fermi solitons" [7,8].