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Physical Chemistry Chemical Physics

A dipolar gas of ultracold molecules

Kang-Kuen Ni, Silke Ospelkaus, David J. Nesbitt, J. Ye, D. S. Jin

Published Jan 1, 2009• 59 citations
Publisher

Abstract

Ultracold polar molecular gases promise new directions and exciting applications in collisions and chemical reactions at ultralow energies, precision measurements, novel quantum phase transitions, and quantum information science. Here we briefly discuss key experimental requirements for observing strong dipole-dipole interactions in an ultracold dipolar gas of molecules. We then survey current experimental progress in the field with a focus on our recent work creating a near quantum degenerate gas of KRb polar molecules [Ni et al., Science, 2008, 322, 231].

DOI
10.1039/b911779b

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Ultracold polar molecular gases promise new directions and exciting appli-cations in precision measurements, ultracold chemistry, electric-field controlled collisions, dipolar quantum gases, and quantum information sciences. This the-sis presents experimental realization of a near quantum degenerate gas of polar molecules, where the phase-space density of the gas achieved is more than 10 or-ders of magnitude higher than previous results. The near quantum degenerate gas of polar molecules is created using two coherent steps. First, atoms in an ul-tracold gas mixture are converted into extremely weakly bound molecules near a Fano-Feshbach resonance. Second, the weakly bound molecules are transferred to the ro-vibronic ground state using a coherent two-photon Raman technique. The fact that these ground-state molecules are polar is confirmed with a spectroscopic measurement of the permanent electric dipole moment. Finally, manipulation of the molecular hyperfine state is demonstrated; this allows molecules to be pop-ulated in a single quantum state, in particular, the lowest energy state. With an ultracold gas of molecules, full control of molecular internal state, and electric field as a new handle, ultracold molecular collisions, including ultracold chemical reactions and dipolar collisions, are studied.

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Ultracold polar molecular quantum gases promise to open new research directions ranging from the study of ultra-cold chemistry, precision measurements to novel quantum phase transitions. Based on the preparation of high-phase space density gases of polar KRb molecules [1–3], I will discuss the control of dipolar collisions and chemical reactions of polar molecules in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play a dominant role [4]. In particular, I will point out the crucial role of electric dipole-dipole interactions [5] and external confinement [6] in determining the chemical reaction rate. Finally, I will discuss prospects of reaching quantum degeneracy in bialkali samples of polar molecules and prospects for these systems as novel dipolar quantum many-body systems.

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Cold and ultracold molecules are the next wave of ultracold physics, giving rise to an exciting array of scientific opportunities, including many body physics for novel quantum phase transitions, new states of matter, and quantum information processing. Precision tests of fundamental physical laws benefit from the existence of molecular internal structure with exquisite control. The study of novel collision and reaction dynamics will open a new chapter of quantum chemistry. Cold molecules bring together researchers from a variety of fields, including atomic, molecular, and optical physics, chemistry and chemical physics, quantum information science and quantum simulations, condensed matter physics, nuclear physics, and astrophysics, a truly remarkable synergy of scientific explorations. For the past decade there have been steady advances in direct cooling techniques, from buffer-gas cooling to cold molecular beams to electro- and magneto-molecular decelerators. 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Ultracold polar molecular gases promise new directions and exciting applications in collisions and chemical reactions at ultralow energies, precision measurements, novel quantum phase transitions, and quantum information science. Here we briefly discuss key experimental requirements for observing strong dipole-dipole interactions in an ultracold dipolar gas of molecules. We then survey current experimental progress in the field with a focus on our recent work creating a near quantum degenerate gas of KRb polar molecules [Ni et al., Science, 2008, 322, 231].
Ultracold polar molecular gases promise new directions and exciting appli-cations in precision measurements, ultracold chemistry, electric-field controlled collisions, dipolar quantum gases, and quantum information sciences. This the-sis presents experimental realization of a near quantum degenerate gas of polar molecules, where the phase-space density of the gas achieved is more than 10 or-ders of magnitude higher than previous results. The near quantum degenerate gas of polar molecules is created using two coherent steps. First, atoms in an ul-tracold gas mixture are converted into extremely weakly bound molecules near a Fano-Feshbach resonance. Second, the weakly bound molecules are transferred to the ro-vibronic ground state using a coherent two-photon Raman technique. The fact that these ground-state molecules are polar is confirmed with a spectroscopic measurement of the permanent electric dipole moment. Finally, manipulation of the molecular hyperfine state is demonstrated; this allows molecules to be pop-ulated in a single quantum state, in particular, the lowest energy state. With an ultracold gas of molecules, full control of molecular internal state, and electric field as a new handle, ultracold molecular collisions, including ultracold chemical reactions and dipolar collisions, are studied.
Ultracold polar molecular quantum gases promise to open new research directions ranging from the study of ultra-cold chemistry, precision measurements to novel quantum phase transitions. Based on the preparation of high-phase space density gases of polar KRb molecules [1–3], I will discuss the control of dipolar collisions and chemical reactions of polar molecules in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play a dominant role [4]. In particular, I will point out the crucial role of electric dipole-dipole interactions [5] and external confinement [6] in determining the chemical reaction rate. Finally, I will discuss prospects of reaching quantum degeneracy in bialkali samples of polar molecules and prospects for these systems as novel dipolar quantum many-body systems.
Cold and ultracold molecules are the next wave of ultracold physics, giving rise to an exciting array of scientific opportunities, including many body physics for novel quantum phase transitions, new states of matter, and quantum information processing. Precision tests of fundamental physical laws benefit from the existence of molecular internal structure with exquisite control. The study of novel collision and reaction dynamics will open a new chapter of quantum chemistry. Cold molecules bring together researchers from a variety of fields, including atomic, molecular, and optical physics, chemistry and chemical physics, quantum information science and quantum simulations, condensed matter physics, nuclear physics, and astrophysics, a truly remarkable synergy of scientific explorations. For the past decade there have been steady advances in direct cooling techniques, from buffer-gas cooling to cold molecular beams to electro- and magneto-molecular decelerators. These techniques have allowed a large variety of molecules to be cooled for pioneering studies. Recent amazing advances in experimental techniques combining the ultracold and the ultraprecise have furthermore brought molecules to the point of quantum degeneracy. These latter indirect cooling techniques magnetically associate atoms from a Bose–Einstein condensate and/or a quantum degenerate Fermi gas, transferring at 90% efficiency highly excited Fano–Feshbach molecules, which are on the order of 10 000 Bohr radii in size, to absolute ground state molecules just a few Bohr across. It was this latter advance, together with significant breakthroughs in internal state manipulations, which inspired us to coordinate this focus issue now, and is the reason why we say the next wave of ultracold physics has now arrived. Whether directly or indirectly cooled, heteronuclear polar molecules offer distinct new features in comparison to cold atoms, while sharing all of their advantages (purity, high coherence, controllability, tunable interactions, no disorder, etc). First, they are more easily manipulated because of the strong response of their electric dipole moment to external electric fields, DC or AC. The electric dipole moment also creates the new aspect of long range interactions. Second, they have a rich internal structure, with vibrational and rotational states, fine or hyperfine structure, and Ω- or Λ-doublets. This internal structure allows for wonderful new possibilities in areas such as precision measurement and exquisite control of system dynamics. Therefore, although this focus issue contains a few articles on homonuclear molecules, more complex molecules such as benzene, and even a contribution on atomic chromium, which has a significant magnetic dipole moment, our main focus is on the heteronuclear polar case. This focus issue explores both direct and indirect cooling of mainly polar molecules, and the theory to support and inspire these advances. Thirty-eight research groups have contributed original work, and there are two review articles to complement these advances: the first covers cold and ultracold molecules broadly from few body to many body physics, including foundational theory, the technology to make them, and their scientific applications. The second is on the search for time variation of fundamental constants. The former review, which is comprehensive in nature, concludes with a list of open questions. This sets the tone for the focus issue, namely, openness, innovation, and possibility, an emphasis for which New Journal of Physics , an open-access journal of the highest quality, is especially fitted. Focus on Cold and Ultracold Molecules Contents Cold and ultracold molecules: science, technology and applications Lincoln D Carr, David DeMille, Roman V Krems and Jun Ye Ultracold molecules: new probes on the variation of fundamental constants Cheng Chin, V V Flambaum and M G Kozlov Probing the unitarity limit at low laser intensities Philippe Pellegrini and Robin Côté Single-photon molecular cooling Edvardas Narevicius, S Travis Bannerman and Mark G Raizen Quantum simulations of extended Hubbard models with dipolar crystals M Ortner, A Micheli, G Pupillo and P Zoller Collisional and molecular spectroscopy in an ultracold Bose–Bose mixture G Thalhammer, G Barontini, J Catani, F Rabatti, C Weber, A Simoni, F Minardi and M Inguscio Multi-channel modelling of the formation of vibrationally cold polar KRb molecules Svetlana Kotochigova, Eite Tiesinga and Paul S Julienne Formation of ultracold, highly polar X 1 Σ + NaCs molecules C Haimberger, J Kleinert, P Zabawa, A Wakim and N P Bigelow Quantum polarization spectroscopy of correlations in attractive fermionic gases T Roscilde, M Rodríguez, K Eckert, O Romero-Isart, M Lewenstein, E Polzik and A Sanpera Inelastic semiclassical collisions in cold dipolar gases Michael Cavagnero and Catherine Newell Quasi-universal dipolar scattering in cold and ultracold gases J L Bohn, M Cavagnero and C Ticknor Stark deceleration of lithium hydride molecules S K Tokunaga, J M Dyne, E A Hinds and M R Tarbutt Molecular vibrational cooling by optical pumping with shaped femtosecond pulses D Sofikitis, S Weber, A Fioretti, R Horchani, M Allegrini, B Chatel, D Comparat and P Pillet Deeply bound ultracold molecules in an optical lattice Johann G Danzl, Manfred J Mark, Elmar Haller, Mattias Gustavsson, Russell Hart, Andreas Liem, Holger Zellmer and Hanns-Christoph Nägerl Toward the production of quantum degenerate bosonic polar molecules, 41 K 87 Rb K Aikawa, D Akamatsu, J Kobayashi, M Ueda, T Kishimoto and S Inouye Influence of a Feshbach resonance on the photoassociation of LiCs J Deiglmayr, P Pellegrini, A Grochola, M Repp, R Côté, O Dulieu, R Wester and M Weidemüller The kinematic cooling of molecules with laser-cooled atoms Ken Takase, Larry A Rahn, David W Chandler and Kevin E Strecker Coherent collapses of dipolar Bose–Einstein condensates for different trap geometries J Metz, T Lahaye, B Fröhlich, A Griesmaier, T Pfau, H Saito, Y Kawaguchi and M Ueda High-energy-resolution molecular beams for cold collision studies L P Parazzoli, N Fitch, D S Lobser and H J Lewandowski Collisional effects in the formation of cold guided beams of polar molecules M Motsch, C Sommer, M Zeppenfeld, L D van Buuren, P W H Pinkse and G Rempe Towards sympathetic cooling of large molecules: cold collisions between benzene and rare gas atoms P Barletta, J Tennyson and P F Barker Efficient formation of ground-state ultracold molecules via STIRAP from the continuum at a Feshbach resonance Elena Kuznetsova, Marko Gacesa, Philippe Pellegrini, Susanne F Yelin and Robin Côté Emergent timescales in entangled quantum dynamics of ultracold molecules in optical lattices M L Wall and L D Carr Rotational state resolved photodissociation spectroscopy of translationally and vibrationally cold MgH + ions: toward rotational cooling of molecular ions K Højbjerre, A K Hansen, P S Skyt, P F Staanum and M Drewsen Collective transverse cavity cooling of a dense molecular beam Thomas Salzburger and Helmut Ritsch A Stark decelerator on a chip Samuel A Meek, Horst Conrad and Gerard Meijer Deceleration of molecules by dipole force potential: a numerical simulation Susumu Kuma and Takamasa Momose Ultracold molecules: vehicles to scalable quantum information processing Kathy-Anne Brickman Soderberg, Nathan Gemelke and Cheng Chin Magnetic field modification of ultracold molecule–molecule collisions T V Tscherbul, Yu V Suleimanov, V Aquilanti and R V Krems Spectroscopy of 39 K 85 Rb triplet excited states using ultracold a 3 Σ + state molecules formed by photoassociation J T Kim, D Wang, E E Eyler, P L Gould and W C Stwalley Pumping vortex into a Bose–Einstein condensate of heteronuclear molecules Z F Xu, R Q Wang and L You Intense atomic and molecular beams via neon buffer-gas cooling David Patterson, Julia Rasmussen and John M Doyle Dynamical properties of dipolar Fermi gases T Sogo, L He, T Miyakawa, S Yi, H Lu and H Pu Collisions of bosonic ultracold polar molecules in microwave traps Alexander V Avdeenkov Cold TiO(X 3 Δ)–He collisions Mei-Ju Lu and Jonathan D Weinstein Investigation of dephasing rates in an interacting Rydberg gas U Raitzsch, R Heidemann, H Weimer, B Butscher, P Kollmann, R Löw, H P Büchler and T Pfau Impact of electric fields on highly excited rovibrational states of polar dimers Rosario González-Férez and Peter Schmelcher Phase transition from straight into twisted vortex lines in dipolar Bose–Einstein condensates M Klawunn and L Santos Stimulating the production of deeply bound RbCs molecules with laser pulses: the role of spin–orbit coupling in forming ultracold molecules Subhas Ghosal, Richard J Doyle, Christiane P Koch and Jeremy M Hutson Sensitive measurement of m p / m e variance using vibrational transition frequencies of cold molecules Masatoshi Kajita
The research field of ultracold atoms has expanded from atomic and molecular physics to a variety of fields. Ultracold polar molecules have long range and anisotropic dipole-dipole interactions, and similar to atoms, can also be conveniently manipulated by laser and other electromagnetic fields. Thus, ultracold molecules offer promising applications such as ultracold chemistry, quantum simulation, and quantum information. However, due to the difficulty in creating ultracold ground state molecules, expanding the horizon of ultracold physics from atoms to molecules is still under development. In the past decade, many research groups have successfully created bi-alkali rovibrational ground state polar molecules using magneto association and stimulated Raman adiabatic passage (STIRAP). This paper presents a review of the recent progress including creating and manipulating ultracold molecules with this method, and the collision property of molecules at ultracold temperature.
Ultracold polar molecules have emerged as an exciting platform for experiments in quantum science, with many proposed applications seeking to leverage the existence of long-range dipolar interactions and the rich internal structure of long-lived states. However, the complexity of molecules also presents many experimental challenges. In this chapter, we review the experimental advances over the past two decades that have brought ultracold molecules under full quantum control. We focus on three key topics: control of ultracold molecular collisions, engineering dipolar interactions and nascent quantum simulations in optical lattices.
The realization of rovibrationally stable dense samples of ultracold diatomic molecules remains one of the main stepping stones to achieve the next slate of major goals in the field of atomic and molecular physics. Though obtaining diatomic alkali molecules was seen as a logical next step following the optical cooling of atoms, many of the possible applications currently under investigation extend beyond atomic and molecular physics. For example, spectroscopy of ultracold molecules can help in testing extensions of the Standard Model via the search for a permanent electric dipole moment of the electron (1; 2), or the energy difference between enantiomers of chiral molecules (3). Various molecular transitions can be utilized to track the time dependence of fundamental constants, including the fine structure constant and the proton to electron mass ratio (4). They also open the way for cold and ultracold chemistry, where the interacting species and products are in a coherent quantum superposition state (5) and reactions can happen via quantum tunneling. Dipolar ultracold quantum gases promise to show a plethora of new phenomena due to anisotropic long-range dipole-dipole interactions (6). Dipolar molecules in optical lattices can be employed as a quantum simulator of condensed matter systems, and they are predicted to demonstrate new quantum phases such as a dipolar crystal, supersolid, checkerboard and collapse phases (7; 8). Ultracold polar molecules also represent an attractive platform for quantum computation (9). They offer a variety of long-lived states for qubit encoding, including rotational, spin and hyperfine (if electronic and nuclear spins are non-zero), Λ and Ω-doublet states (10) and scalability to a large number of qubits. Polar molecules can be easily controlled by DC electric and magnetic fields, as well as bymicrowave and optical fields, allowing the design of various traps (11; 12). The main appeal of polar molecules for quantum information processing, however, comes from their permanent electric dipole moment, permitting them to interact via a long-range dipole-dipole interaction. The dipole-dipole interaction offers a tool to construct two-qubit gates, required for universal quantum computation (9; 13). Ultracold molecules in their ground vibrational state v = 0 (and even in specific rotational, hyperfine or Zeeman states) are required formany of these applications since they have a large permanent electric dipole moment and are stable with respect to collisions and spontaneous emission. Currently translationally ultracold (100 nK 1 mK) molecules are produced by magneto(14) and photo-association (15) techniques. In a typical photoassociation scheme, 3