Evidence for cooling in an optical lattice by amplitude modulation

Abstract

We report on a generic cooling technique for atoms trapped in optical lattices. It consists in modulating the lattice depth with a proper frequency sweeping. This filtering technique removes the most energetic atoms, and provides with the onset of thermalization a cooling mechanism reminiscent of evaporative cooling. However, the selection is here performed in quasi-momentum space rather than in position space. Interband selection rules are used to protect the population with a zero quasi-momentum, namely the Bose Einstein condensate. Direct condensation of thermal atoms in an optical lattice is also achieved with this technique. It offers an interesting complementary cooling mechanism for quantum simulations performed with quantum gases trapped in optical lattices.Atomic and molecular physics have been strongly impacted by cooling techniques [1,2]. Laser cooling has triggered a boost of research activities to reach very low temperatures [3][4][5] and therefore to improve the control on the external degrees of freedom of atoms, with many applications in metrology [6,7]. Laser cooling down to degeneracy has been demonstrated only recently [8,9]. Those cooling techniques are tailor-made for a given species. Lasers shall indeed address specific atomic lines. Furthermore, the temperature achieved with laser cooling is strongly dependent on the width of the excited state of the considered cycling transition [10]. In contrast, evaporative cooling is a much more generic technique. The filtering technique of the most energetic atoms can be easily transposed from one species to another, and the cooling occurs through the re-thermalization of the atomic cloud. Only the cooling rate depends on the species. Evaporative cooling has originally been envisioned for hydrogen atoms for which laser cooling is not expected to be efficient [11], and successfully implemented on lasercooled alkali atoms to reach quantum degeneracy in nondissipative traps [12][13][14].In this article, we report on a generic cooling method inspired by evaporative cooling but adapted to atoms trapped in an optical lattice. It is indeed of upmost importance to find methods to decrease the temperature of atoms trapped in optical lattices since such systems are currently used to perform quantum simulations, and temperature may constitute an obstacle for some challenging experiments [19,20]. The advantage of optical lattices lies in their tunability: the geometry of the lattices can be easily modified, they can be made spin dependent and can be readily modulated in phase and amplitude. This latter possibility is at the origin of the so-called Floquet engineering which opens up interesting perspectives for generating effective hamiltonians and engineered artificial gauge fields [21,22].The method put forward in this article exploits interband transitions excited by amplitude modulation for non-zero quasi-momentum [23]. The modulation frequency is scanned through all the values of the considered interband transition. The chosen excited band obeys a selection ru...

Parametric Excitation of Ultracold Sodium Atoms in an Optical Dipole Trap

“…The parametric heating of bosons has been realized in a periodically-driven 2D lattice, in which exponential decay rates are observed [9]. Direct condensation of thermal atoms in an optical lattice is also achieved using this technique [10]. Furthermore, the creation of bosonic fractional quantum Hall states is expected in periodically-driven optical lattices [11].…”

Suppressing dissipation in a Floquet-Hubbard system

“…A natural extension of this scheme employs additional control frequencies to cancel further resonances, in the spirit of pulse-shaping techniques in laser-driven molecular dynamics [34]. Moreover, tuneable couplings to higher bands can be an asset, rather than a nuisance, for instance, in designing novel cooling schemes, not only for optical lattices [17,35] but also in condensed matter [36]. More generally, this kind of precisely engineered dissipation is a valuable resource for engineering specific quantum states, particularly in the many-body context [37].…”

Controlling vortical motion of particles in two-dimensional driven superlattices

“…Our setup is hence a driven superlattice formed by the superposition of different sublattices, each driven with a distinct driving phase φ mn . Possible experimental realizations of such a 2D potential include holographic optical lattices [21,[40][41][42][43] or optical superlattices [44] with the lattice depth modulated via standard amplitude modulation techniques [45,46]. The rotational dynamics of particles in such a setup could be observed with colloidal particles or with cold atoms in the classically describable regime of microkelvin temperatures [12,21].…”