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Ultracold Molecules: Production Techniques and Scientific Applications

David P. DeMille

Published Jan 1, 2011• 1 citation
Publisher

Abstract

Cooling and trapping of diatomic molecules is now possible. Their vibrational and rotational degrees of freedom provide new tools to address problems in fields ranging from quantum computing and simulation, to particle physics.
DOI
10.1364/qels.2011.qwb1

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Laser cooling of atoms began more than 25 years ago and has led to spectacular advances in areas such as atomic clocks, Bose-Einstein condensation, degenerate Fermi gases, and ultracold collisions. Molecules, due to their complexity relative to atoms (1), have been slower to jump on the ultracold bandwagon but are now firmly on board. Three recent papers (2-4) describe techniques for efficiently and substantially reducing the vibrational motion of molecular ensembles, thus approaching the goal of large numbers of molecules that are cold in all their degrees of freedom, both external and internal.
We propose a new method for making ultracold molecules below 300 μK by cooling via the dipole force of an intense infrared (IR) standing wave created in a high finesse cavity. Deceleration can be achieved by periodically switching an IR standing wave slightly red-detuned from the resonance frequency of a molecular vibration–rotation transition. The potential depth becomes as high as 50 mK with an incident laser power of 3 W. By properly switching the potential, one can remove the translational energy of molecules from a couple of hundred mK to 300 μK or below. Since all molecules except homonuclear diatomic molecules have one or more optical transitions in the IR range that can be used for a source of the dipole force, the present method is applicable to almost all molecules of chemical interest. Moreover, by choosing an appropriate rotation–vibration transition, one can decelerate and trap molecules in the completely ground state in any degrees of freedom. Numerical simulations showed sufficiently wide phase-space areas of decelerated molecules through use of this technique for practical applications. The effect of various parameters on the efficiency of the cavity decelerator has been examined by numerical simulations.
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