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Science

Coaxing quantumness in a molecular gas

Jelena Stajic

Published Feb 21, 2019• 0 citations
Publisher

Abstract

Cold Molecules A dilute atomic gas cooled down to very cold temperatures can enter the so-called quantum degenerate regime, where quantum properties of the gas come to the fore. This regime has been achieved for both bosonic and fermionic atoms, but molecules, with their many internal states, present a special challenge. De Marco et al. cooled a bulk gas of fermionic potassium-rubidium molecules to quantum degeneracy (see the Perspective by Zelevinsky). The authors first cooled atomic potassium and rubidium gases separately, then bound them together into potassium-rubidium molecules, and finally brought the molecules down to their ground state. The density profile of the molecular gas revealed the system's quantum nature, which in turn kept the gas stable by suppressing chemical reactions. Science , this issue p. [853][1]; see also p. [820][2] [1]: /lookup/doi/10.1126/science.aau7230 [2]: /lookup/doi/10.1126/science.aav9149

DOI
10.1126/science.363.6429.831-n

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Cold Molecules
A dilute atomic gas cooled down to very cold temperatures can enter the so-called quantum degenerate regime, where quantum properties of the gas come to the fore. This regime has been achieved for both bosonic and fermionic atoms, but molecules, with their many internal states, present a special challenge. De Marco et al. cooled a bulk gas of fermionic potassium-rubidium molecules to quantum degeneracy (see the Perspective by Zelevinsky). The authors first cooled atomic potassium and rubidium gases separately, then bound them together into potassium-rubidium molecules, and finally brought the molecules down to their ground state. The density profile of the molecular gas revealed the system's quantum nature, which in turn kept the gas stable by suppressing chemical reactions.
Science , this issue p. 853; see also p. 820
Atomic Physics When atomic gases, such as those of alkali elements, are cooled to very low temperatures, they can reach a state of quantum degeneracy, where their quantum nature comes to the fore. In this process, the very last step is evaporative cooling, in which the hottest atoms are coaxed into leaving the gas. Hu et al. devised a protocol that evades the evaporative cooling step, is faster, and suffers less atom loss. The method rests on iteratively manipulating the laser beams of an optical lattice in which a gas of 87Rb atoms is held so that the gas becomes progressively denser. The method should be widely applicable to other atomic species. Science , this issue p. [1078][1] [1]: /lookup/doi/10.1126/science.aan5614
Experimental realization of a quantum degenerate gas of molecules would provide access to a wide range of phenomena in molecular and quantum sciences. However, the very complexity that makes ultracold molecules so enticing has made reaching degeneracy an outstanding experimental challenge over the past decade. We now report the production of a degenerate Fermi gas of ultracold polar molecules of potassium-rubidium. Through coherent adiabatic association in a deeply degenerate mixture of a rubidium Bose-Einstein condensate and a potassium Fermi gas, we produce molecules at temperatures below 0.3 times the Fermi temperature. We explore the properties of this reactive gas and demonstrate how degeneracy suppresses chemical reactions, making a long-lived degenerate gas of polar molecules a reality.
Packing rubidium into quantum degeneracy When atomic gases, such as those of alkali elements, are cooled to very low temperatures, they can reach a state of quantum degeneracy, where their quantum nature comes to the fore. In this process, the very last step is evaporative cooling, in which the hottest atoms are coaxed into leaving the gas. Hu et al. devised a protocol that evades the evaporative cooling step, is faster, and suffers less atom loss. The method rests on iteratively manipulating the laser beams of an optical lattice in which a gas of 87 Rb atoms is held so that the gas becomes progressively denser. The method should be widely applicable to other atomic species. Science , this issue p. 1078
Quantum Gases Cold atomic gases are extremely flexible systems; the ability to tune interactions between fermionic atoms can, for example, cause the gas to undergo a crossover from weakly interacting fermions to weakly interacting bosons via a strongly interacting unitary regime. Murthy et al. studied this crossover in a gas of fermions confined to two dimensions. The formation of atomic pairs occurred at much higher temperatures in the unitary regime than previously thought. Science , this issue p. [452][1] [1]: /lookup/doi/10.1126/science.aan5950
Quantum Gases Atomic gases can be cooled to a fraction of a degree above absolute zero and—if they are also dense enough—can enter a regime where their quantum properties come to the fore. In reaching this so-called quantum degenerate regime, particle loss through three-body collisions is usually a foe rather than a friend. Dogra et al. predict theoretically that a weakly interacting 3D gas of bosonic atoms can be made more degenerate by exploiting, rather than fighting, three-body particle loss. For the scheme to work, the gas should reside in a homogeneous trap, and the cooling must start from a particular subset of initial conditions, both of which should be realizable in the lab. Phys. Rev. Lett. 123 , 020405 (2019).
Abstract The control of molecules is key to the investigation of quantum phases, in which rich degrees of freedom can be used to encode information and strong interactions can be precisely tuned1. Inelastic losses in molecular collisions2–5, however, have greatly hampered the engineering of low-entropy molecular systems6. So far, the only quantum degenerate gas of molecules has been created via association of two highly degenerate atomic gases7, 8. Here we use an external electric field along with optical lattice confinement to create a two-dimensional Fermi gas of spin-polarized potassium–rubidium (KRb) polar molecules, in which elastic, tunable dipolar interactions dominate over all inelastic processes. Direct thermalization among the molecules in the trap leads to efficient dipolar evaporative cooling, yielding a rapid increase in phase-space density. At the onset of quantum degeneracy, we observe the effects of Fermi statistics on the thermodynamics of the molecular gas. These results demonstrate a general strategy for achieving quantum degeneracy in dipolar molecular gases in which strong, long-range and anisotropic dipolar interactions can drive the emergence of exotic many-body phases, such as interlayer pairing and p-wave superfluidity.