Introduction to Ultracold Molecules: New Frontiers in Quantum andChemical Physics

Molecules cooled to ultralow temperatures providefundamental new insights to molecular interaction and

reaction dynamics in the quantum regime. In recent years,researchers from various scientific disciplines such as atomic,optical, and condensed matter physics, physical chemistry, andquantum science have joined force to explore many emergentand exciting research topics that are enabled by cold molecules,including cold chemistry, strongly correlated quantum systems,novel quantum phases, and precision measurement. Thisthematic issue provides a set of comprehensive reviews onthis increasingly active and important research field, coveringrecent developments such as experimental techniques forpreparing cold molecular gases, implementation of control forboth internal and external molecular degrees of freedom, theoryframeworks to describe molecular interactions and manipu-lation, and future outlook for many-body quantum physics andchemistry.Building on the strong tradition of molecular beams in both

physics and chemistry communities, researchers working oncold molecules have made steady advances in lowering thetemperature of the beam and manipulating the beam for novelapplications. In the paper “The buffer gas beam: An intense,cold, and slow source for atoms and molecules” by Hutzler, Lu,and Doyle, the authors provide a comprehensive review of thestate-of-the-art in buffer gas cooling, which is fundamentallydifferent from supersonic expansion-based cooling and canbring the temperature of a molecular sample to a few Kelvins inthe lab rest frame. Another extremely powerful approach takesadvantage of the capability in controlling the motion of neutralmolecules via inhomogeneous electric or magnetic fields. In thereview “Manipulation and control of molecular beams” by vande Meerakker, Bethlem, Vanhaecke, and Meijer, an author-itative description is provided of the molecular Starkdecelerator and its application to a variety of pioneeringexperiments. The underlying design principles for thedecelerator and a careful treatment of the interaction ofmolecules with electric fields is given, leading to elucidatingdiscussions on the control of molecular packets in both positionand velocity spaces, as well as the trapping of molecules in thelaboratory frame. Similarly, based on the interaction ofmolecules with inhomogeneous magnetic fields, Nareviciusand Raizen describe how molecular motions can bemanipulated with magnetic fields in their review “Towardscold chemistry with magnetically decelerated supersonicbeams”. The unique capability of magnetic deceleration forboth atoms and molecules greatly facilitates investigations ofimportant atom−molecule collisions and reactions at lowenergies.These powerful techniques have allowed a large variety of

molecules to be brought to a few tens of milliKelvin or even afew milliKelvin and have enabled applications ranging fromnovel molecular scattering studies at low collision energies tohigh-resolution spectroscopy. The samples of cold moleculesthat are produced via these molecular beam technologies also

provide excellent starting conditions for further cooling ofmolecules, with current experimental efforts exploringpossibilities such as laser cooling and direct molecularevaporation.In a completely different approach, atoms are cooled to

ultralow temperatures and then brought together to form coldmolecules via magneto-association or photoassociation or acombined process. In their review “Ultracold molecules formedby photoassociation: Heteronuclear dimers, inelastic collisions,and interactions with ultrashort laser pulses”, Ulmanis,Deiglmayr, Repp, Wester, and Weidemuller discuss themethodology, benefits, and challenges in photoassociation ofmolecules out of ultracold atomic gases. To form a moleculefrom a pair of free atoms requires detailed theoreticalknowledge of the interaction properties of the colliding atomicpair and the energy level structure of the target molecule andprecise experimental control of optical and magnetic fields. Thisapproach has produced some of the coldest samples ofmolecules in their ground state. Koch and Shapiro in theirreview of “Coherent control of ultracold photoassociation”provide an excellent discussion of fundamental concepts incoherent control in the context of producing cold molecules viaphotoassociation using adiabatic passage and spectrally shapedultrashort laser pulses. These theoretical insights play animportant role in improving the molecule production efficiencyin experiment. By combining magnetoassociation and stimu-lated Raman adiabatic passage, the field has finally witnessedthe production of ultracold molecules that are at the verge ofquantum degeneracy. The precise control of both the internaland external degrees of freedom in a molecule has enabled thefirst experiments of ultracold chemistry where all degrees offreedom of the colliding species are controlled and quantized,including vibration, rotation, electron and nuclear spin,orientation, and translation. Quemener and Julienne title theirreview “Ultracold molecules under control!” to emphasize thisnew scientific realm. Their paper provides an overview oftheoretical tools that are being developed to treat theinteractions, collisions, and reactions of ultracold molecules inthe quantum regime, where the de Broglie wavelength of thecolliding molecules is much larger than the typical range ofchemical interactions. The intrinsic quantum dynamics ofcollisions thus becomes highly sensitive to weak long-rangeinteractions that can be brought under experimental control.An important scientific direction with ultracold molecules is

the study of strongly correlated many-body quantum systems.Baranov, Dalmonte, Pupillo, and Zoller provide a review of thistopic in “Condensed matter theory of dipolar quantum matter”.The connections and differences between quantum gases ofdipolar molecules and traditional condensed-matter systems areexplained, highlighting the inherent interdisciplinary nature of

Special Issue: 2012 Ultracold Molecules

Published: September 12, 2012

Editorial

pubs.acs.org/CR

This article not subject to U.S. Copyright.Published 2012 by the American ChemicalSociety

4801 dx.doi.org/10.1021/cr300342x | Chem. Rev. 2012, 112, 4801−4802

these studies. Covering many recent theoretical developmentson the study of dipolar many-body quantum systems, theauthors discuss both the mean-field treatment for dipolarBose−Einstein condensates and dipolar paired-fermion super-fluids and the many-body analysis of strongly correlated dipolargases in optical lattices and low-dimensional geometries.Finally, study of novel collision and reaction dynamics in theultralow energy range is now opening a new chapter forquantum chemistry. Nesbitt provides some unique physicalchemistry perspectives for the field of ultracold molecules in hisreview of “Toward state-to-state dynamics in ultracoldcollisions: Lessons from high-resolution spectroscopy of weaklybound molecular complexes”. One of the most important goalsin ultracold molecules research is to enable investigation of themost fundamental dynamics in the breaking and making ofchemical bonds, under full control of both the internal andexternal quantum states of the molecules. Nesbitt provides anelegant connection between high-resolution molecular spec-troscopy and ultracold collisional dynamics, where the wealthof high-resolution spectral signatures in weakly boundcomplexes permits access to collision and chemically reactivedynamics arising from pure quantum states of the complex,thus offering important insight and intuition into the dynamicsof ultracold collisional processes.

Deborah S. JinJun YeJILA, National Institute of Standards and Technology andUniversity of ColoradoBoulder, Boulder, Colorado80309-0440, United States

AUTHOR INFORMATION

Biography

Deborah Jin (left) is an experimental physicist who studies ultracoldquantum gases at JILA in Boulder, Colorado. She is a NIST Fellow, aJILA Fellow, and an Adjoint Professor of Physics at the University ofColorado. Dr. Jin is a member of the National Academy of Sciencesand has received a MacArthur fellowship, the Benjamin FranklinMedal in physics, and Sigma Xi’s William Proctor Prize for ScientificAchievement. Her research interests include experimental investigationof ultracold Fermi gases, the creation and exploration of an ultracoldgas of polar molecules, and studies of strongly interacting Bose−Einstein condensates.

Jun Ye (right) is a Fellow of JILA, the National Institute of Standardsand Technology and the University of Colorado. He is also a Fellow ofNIST, a Fellow of the American Physical Society, a Fellow of theOptical Society of America, and a member of the National Academy ofSciences. His research focuses on the frontier of light-matterinteractions and includes precision measurement, quantum physicsand cold matters, optical frequency metrology, and ultrafast scienceand quantum control. Awards and honors include Frew Fellowshipfrom the Australian Academy of Science, I. I. Rabi Prize from theAmerican Physical Society, European Frequency and Time ForumAward, Carl Zeiss Research Award, William F. Meggers Award andAdolph Lomb Medal from the Optical Society of America, Arthur S.Flemming Award, Presidential Early Career Award for Scientists andEngineers, U.S. Commerce Department Gold Medals, FriedrichWilhem Bessel Award from Alexander von Humboldt Foundation,and Samuel Wesley Stratton Award from NIST. His research group'sweb page is http://jila.colorado.edu/YeLabs/.

Chemical Reviews Editorial

dx.doi.org/10.1021/cr300342x | Chem. Rev. 2012, 112, 4801−48024802