Entropy Exchange in a Mixture of Ultracold Atoms

Abstract

We investigate experimentally the entropy transfer between two distinguishable atomic quantum gases at ultralow temperatures. Exploiting a species-selective trapping potential, we are able to control the entropy of one target gas in presence of a second auxiliary gas. With this method, we drive the target gas into the degenerate regime in conditions of controlled temperature by transferring entropy to the auxiliary gas. We envision that our method could be useful both to achieve the low entropies required to realize new quantum phases and to measure the temperature of atoms in deep optical lattices. We verified the thermalization of the two species in a 1D lattice.PACS numbers: 03.75. Hh, 67.85.Pq, 05.30.Jp In recent years an intense research of quantum phases common to condensed matter systems and atomic quantum gases has made remarkable progresses [1]. Some of these phases can only be reached provided that the temperature is suitably low. However, in strongly correlated quantum systems, even the temperature measurement can be a challenging task. If so, to ascertain whether a given quantum phase is accessible, it is convenient to focus on the critical value of entropy, rather than temperature. The advantage is especially clear when the strongly correlated regime is reached by sufficiently slow, entropy-preserving, transformations of the trapping potential, as it is often the case for atoms in deep optical lattices [2]. For these reasons, it is important to determine and grasp control of the entropy of degenerate quantum gases [3,4,5]. In this work, we demonstrate the reversible and controlled transfer of entropy between the two ultracold, harmonically trapped Bose gases, which is based on the use of a species-selective dipole potential (SSDP), i.e., an optical potential experienced exclusively by one species (Fig. 1) [6,7]. In particular, we drive the target gas across the threshold for Bose-Einstein condensation, by a reversible transfer of entropy to the auxiliary gas.The main idea can be understood from textbook thermodynamics. Let us consider two distinguishable gases filling an isolated box, exchanging neither particles nor energy with the outside, and imagine that only one gas (target) is compressed, e.g. through a piston permeable to the second gas (auxiliary). The temperature will increase and, in thermal equilibrium, heat, hence entropy, will transfer from the target to the auxiliary uncompressed gas. In the limit of the auxiliary gas containing a large number of particles, it stands as a thermal bath. In a more formal way, for an ideal gas of N particles, the entropy S is proportional to N log(Σ/N ), where the number of accessible single-particle states Σ increases with the energy density of states and with the average energy, i.e., the temperature. In an adiabatic compression of one single gas, the reduction of the energy density of states is compensated by a temperature raising such that Σ, hence S, remains constant. If we add the uncompressed auxiliary gas in thermal contact, the temperature rai...

Interspecies thermalization in an ultracold mixture of Cs and Yb in an optical trap

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Hartree-Fock Approach for Rotating Bose-Einstein Condensation in One-dimensional Deep Optical Lattice

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