Continuous loading of a magnetic trap

Abstract

We have realized a scheme for continuous loading of a magnetic trap (MT).52 Cr atoms are continuously captured and cooled in a magneto-optical trap (MOT). Optical pumping to a metastable state decouples atoms from the cooling light. Due to their high magnetic moment (6 µB), low-field seeking metastable atoms are trapped in the magnetic quadrupole field provided by the MOT. Limited by inelastic collisions between atoms in the MOT and in the MT, we load 10 8 metastable atoms at a rate of 10 8 atoms/s below 100 µK into the MT. Optical repumping after the loading allows us to realize a MT of ground state chromium atoms. Although multiply loading of a MT has been achieved [10] experiments so far suffer from the absence of a method for efficient cw loading of atoms into a BEC. Hence to date a matter wave analogon to the continuous wave optical laser has not been realized. Alternatively, a cw atom laser based on magnetic guiding in combination with atomic collisions was suggested [11]. In addition, cw loading of low-dimensional optical traps with laser cooled atoms was proposed [12] and recently demonstrated [13].In this letter we report on the cw loading of a three dimensional conservative trap with laser cooled atoms that are decoupled from all light fields present. We show that atoms can be optically pumped within a chromium magneto-optical trap (MOT) [14,15] into metastable "dark" states and stored in a MT built up by the quadrupole magnetic field of the MOT. We present results of systematic studies on the loading process and on the lifetime of the MT. Using the cw loading mechanism and a final repumping process we obtain good starting conditions for experiments towards degenerate quantum gases with ground state chromium atoms.Our cw loading scheme consists of an atomic reservoir and a conservative trap overlapped in space and time. The reservoir is prepared by cooling atoms in a MOT on a transition |g → |e (FIG. 1). A weak decay channel |e → |d allows the transfer of reservoir atoms into an additional long lived and trapped state |d in which atoms can be accumulated. In our realization low field seeking Zeeman substates of |d are trapped in the magnetic quadrupole field of the MOT. The loading can be very efficient if |d atoms are decoupled from the MOT light and if their kinetic energy is smaller than the conservative trap depth. A large decay rate branching ratio (Γ eg /Γ ed ≫ 1) assures a steady state MOT in thermal equilibrium and is expected to greatly reduce reabsorption of transfer photons by atoms in the MT [16]. MT FIG. 1. Relevant part of the52 Cr level scheme. The MOT involves all levels and transitions, the continuous loading process of the magnetic trap (MT) relies on the Λ-system depicted in black (levels |g , |e , |d ).Chromium combines the desired Λ-like level scheme (FIG. 1, black levels and transitions) with a high magnetic moment of up to 6 µ B (µ B =Bohr's magneton). Due to its isotopic composition (3 bosons:52 Cr (84%), 50 Cr (4%), 54 Cr (2%), and one fermion: 53 Cr (10%) ) it is a promising e...

Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

“…The only other parameter which is not negligible is the inelastic two-body loss parameter, which is measured independently in our experiment (see below). The main conclusion of our analysis at high RF frequencies is that inelastic collisions with atoms from the MOT limit the total number of atoms that are accumulated in the MT, and produce a strong heating: the temperature of the atoms in the MT, which, in absence of collisions, could be as low at T MOT /3 [2], raises to about 100 µK for large RF frequencies. This drastically limits the achieved steady state phase-space density.…”

“…As in [2] and [6], chromium atoms in the 7 S 3 ground state are captured in a MOT, and depumped to dark metastable states 5 D 4,3 . We observe different regimes as a function of the RF frequency.…”

“…Spontaneous emission from the excited state of the cooling transition depumps atoms into a dark metastable state, which can be trapped, either optically, or magnetically. This provides an interesting means to continuously accumulate atoms in a magnetic trap [2], or for a continuous loading of an optical or magnetic waveguide. Cold metastable atoms are directly produced inside the waveguide or the trap, which reduces heating associated with the sequential loading from a MOT.…”

Collisional properties of trapped cold chromium atoms

“…Processes occurring in the cold regime are sensitive to the details of the interaction potentials between the colliding systems over an extended range of internuclear separations. Recent experiments with chromium [4,5,6,7] emphasize the need for theoretical studies of Cr scattering properties. The interest in cooling Cr stems from its particular properties; in its ground state 7 S 3 , it possesses a very large magnetic moment, 6 µ B (µ B : Bohr magneton), making it an ideal atom for buffer-cooling in a purely magnetic trap [7], as well as for magneto-optical trapping [4].…”

Continuous all-optical deceleration and single-photon cooling of molecular beams

“…This relationship demonstrates the connection between the deceleration force and the final effective temperature in this scheme. After their longitudinal momentum space density has been compressed, molecules can be subjected to a similar single-photon cooling step in position space through position-sensitive optical pumping into a trap [23,45,50,52]. In principle, each molecule needs to spontaneously emit only two photons during the entire process from beam to trap, demonstrating the large capacity each spontaneously emitted photon has for carrying away entropy.…”