Bose-Einstein Condensation of Alkaline Earth Atoms:Ca40

Abstract

We have achieved Bose-Einstein condensation of 40 Ca, the first for an alkaline earth element. The influence of elastic and inelastic collisions associated with the large ground-state s-wave scattering length of 40 Ca was measured. From these findings, an optimized loading and cooling scheme was developed that allowed us to condense about 2·10 4 atoms after laser cooling in a two-stage magnetooptical trap and subsequent forced evaporation in a crossed dipole trap within less than 3 s. The condensation of an alkaline earth element opens novel opportunities for precision measurements on the narrow intercombination lines as well as investigations of molecular states at the 1 S-3 P asymptotes.PACS numbers: 03.75.Hh, 67.85.HjThe first Bose-Einstein condensate (BEC) of a dilute quantum gas in 1995 has opened completely new avenues in physics. In subsequent years this quantum degenerate state could be reached with different species (for a recent list of references see, e. g., [1]). By far most research has been performed on alkali atomic and molecular BECs apart from hydrogen, metastable helium, chromium, and ytterbium. So far, no member of the alkaline earth elements could be brought to quantum degeneracy despite considerable effort [2,3]. The alkaline earth elements have unique properties, e. g., their narrow intercombination transitions or their ground state without a magnetic moment. Due to the non-degenerate ground state in 40 Ca and in the other alkaline earth elements, the associated simpler molecule structure allows for more accurate investigations of collisions [4,5]. Moreover, the vanishing magnetic moment in the ground and excited state will allow for novel applications in atom interferometry not hampered by phase shifts due to magnetic fields. Calcium has a particularly large ground state scattering length that not only made it interesting but also difficult to realize a BEC.Calcium (like the other alkaline earth elements) shares these properties with ytterbium which has a similar electronic structure [6] but has a five hundred-fold larger line width of the 1 S 0 -3 P 1 intercombination transition. This 370 Hz line width at 657 nm made calcium for some time the optical frequency standard with the lowest uncertainty in the visible [7]. Both technologies, optical frequency metrology and the generation of a BEC, can now be combined for novel applications.In this letter we report on the preparation of a 40 Ca BEC. Starting point for our experiment is a magnetooptical trap (MOT) on the 1 S 0 -1 P 1 transition in the singlet system. The vanishing ground-state magnetic moment prevents sub-Doppler cooling of 40 Ca in a MOT. To cool the atoms further we use a second MOT stage on the narrow intercombination line 1 S 0 -3 P 1 allowing for temperatures in the MOT as low as 15 µK. For efficient cooling we increase the scattering rate of this transition by quenching the upper state [8]. During both MOT stages an optical dipole trap is overlapped with the MOT. In order not to interfere with the narrow line cooling we choo...

Ultracold Fermi gases with emergent SU(N) symmetry

“…In 2009 at PTB (Germany) a BEC of 40 Ca was reported [68]. This was achieved in spite of the inelastic collisions associated with its large ground state s-wave scattering length (18nm < a s < 37nm) [58], by using a large volume optical trap for initial cooling.…”

Bose-Einstein Condensation ofSr84

“…Each beam has a waist of approximately 100 µm in the trapping region. This is slightly different than other realizations of quantum degeneracy in two-electron atoms, in which at least one beam had a waist that was significantly smaller [16,23,24,26,27].…”

“…Here we report the observation of Bose-Einstein condensation of 84 Sr. To date, quantum degeneracy with alkaline earth metal atoms and similar elements has been reported in a collection of ytterbium isotopes [16,23,24,25], and very recently in 40 Ca [26] and 84 Sr [27], demonstrating increasing interest in these systems. Starting with a natural-abundance strontium source, our success in forming a relatively large Bose-Einstein condensate of 84 Sr, which has a natural abundance of 0.6%, demonstrates the power of laser cooling in strontium using the (5s 2 ) 1 S 0 -(5s5p) 3 P 1 narrow intercombination line at 689 nm [28].…”

Quantum treatment of two-stage sub-Doppler laser cooling of magnesium atoms

“…Besides, subrecoil temperatures can be obtained by using evaporative cooling technique, getting Bose-Einstein condensation [35][36][37]. Unfortunately, researchers have not been able to reach the same great success with Mg atoms.…”