This paper presents a review of the current state of the art in the research field of cold and ultracold molecules. It serves as an introduction to the focus issue of New Journal of Physics on Cold and Ultracold Molecules and describes new prospects for fundamental research and technological development. Cold and ultracold molecules may revolutionize physical chemistry and few-body physics, provide techniques for probing new states of quantum matter, allow for precision measurements of both fundamental and applied interest, and enable quantum simulations of condensed-matter phenomena. Ultracold molecules offer promising applications such as new platforms for quantum computing, precise control of molecular dynamics, nanolithography and Bose-enhanced chemistry. The discussion is based on recent experimental and theoretical work and concludes with a summary of anticipated future directions and open questions in this rapidly expanding research field.

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Cold and ultracold molecules: science, technology and applications

Collisions of molecules in a thermal gas are difficult to control. Thermal motion randomizes molecular encounters and diminishes the effects of external radiation or static electromagnetic fields on intermolecular interactions. The effects of the thermal motion can be reduced by cooling molecular gases to low temperatures. At temperatures near or below 1 K, the collision energy of molecules becomes less significant than perturbations due to external fields. At the same time, inelastic scattering and chemical reactions may be very efficient in low-temperature molecular gases. The purpose of this article is to demonstrate that collisions of molecules at temperatures below 1 K can be manipulated by external electromagnetic fields and to discuss possible applications of cold controlled chemistry. The discussion focuses on molecular interactions at cold (0.001-2 K) and ultracold (<0.001 K) temperatures and is based on both recent theoretical and experimental work. The article concludes with a summary of current challenges for theory and experiment in the research of cold molecules and cold chemistry.

Cold controlled chemistry.

Theoretical progress in many-body physics with ultracold dipolar gases

M. A. Baranov,†,‡,§ M. Dalmonte,†,⊥ G. Pupillo,†,‡,∇ and P. Zoller*,†,‡ †Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck, Austria ‡Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria RRC “Kurchatov Institute”, Kurchatov Square 1, 123182, Moscow, Russia Dipartimento di Fisica dell’Universita di Bologna, via Irnerio 46, 40126 Bologna, Italy ISIS (UMR 7006) and IPCMS (UMR 7504), Universite de Strasbourg and CNRS, Strasbourg, France

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Condensed matter theory of dipolar quantum gases.

The ability to cool and slow atoms with light for subsequent trapping allows investigations of the properties and interactions of the trapped atoms in unprecedented detail. By contrast, the complex structure of molecules prohibits this type of manipulation, but magnetic trapping of calcium hydride molecules thermalized in ultra-cold buffer gas and optical trapping of caesium dimers generated from ultra-cold caesium atoms have been reported. However, these methods depend on the target molecules being paramagnetic or able to form through the association of atoms amenable to laser cooling, respectively, thus restricting the range of species that can be studied. Here we describe the slowing of an adiabatically cooled beam of deuterated ammonia molecules by time-varying inhomogeneous electric fields and subsequent loading into an electrostatic trap. We are able to trap state-selected ammonia molecules with a density of 10(6) cm(-3) in a volume of 0.25 cm3 at temperatures below 0.35 K. We observe pronounced density oscillations caused by the rapid switching of the electric fields during loading of the trap. Our findings illustrate that polar molecules can be efficiently cooled and trapped, thus providing an opportunity to study collisions and collective quantum effects in a wide range of ultra-cold molecular systems.

Electrostatic trapping of ammonia molecules

Ultimately, the resolution of any spectroscopic experiment is limited by the interaction time between the particles that are to be examined and the measuring device. The obtainable spectroscopic resolution in a molecular beam experiment can be considerably improved using samples of slow molecules, as produced, for example, in a Stark-decelerator. This is demonstrated here by measuring the inversion tunneling spectrum of ${}^{15}{\rm ND}_3$
 using a pulsed molecular beam that has been decelerated to about 52 m/s. Hyperfine resolved inversion transitions in ${}^{15}{\rm ND}_3$
 in the $\left\vert J,K\right > = \left\vert 1,1\right > $
 state (around 1.43 GHz) are induced in a microwave region, and ${}^{15}{\rm ND}_3$
 molecules that have undergone the transition are subsequently detected using a UV-laser based ionization detection scheme. To increase the signal intensity, the decelerated molecular beam is both transversally and longitudinally focused into the laser detection region. The observed spectral width of individual hyperfine transitions in the fully resolved spectrum is about 1 kHz, and the standard deviation of the best fit is 62 Hz.

Decelerated molecular beams for high-resolution spectroscopy

A series of pulsed electric fields can be arranged such that it creates a traveling potential well in which neutral dipolar molecules can be confined. This provides a method to transport, to decelerate, and to cool a sample of neutral molecules while maintaining the initial phase-space density. This method is described using the concept of phase stability. The oscillating motion of molecules in the traveling potential well, reaching a minimum velocity spread corresponding to a translational temperature of 4 mK, is experimentally observed.

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Trapping neutral molecules in a traveling potential well.

We report the direct imaging of electrically active Si dopants near the GaAs(110) surface with a scanning tunneling microscope at a temperature of 4.2 K. In the filled state images, we observe patterns of rings which are centered around the individual doping atoms. We believe these ring patterns are induced by the individual impurities, which, due to their charge, disturb the local potential and cause oscillations in the charge density, also called Friedel oscillations. In the empty state images no Friedel oscillations can be observed.

Direct observation of Friedel oscillations around incorporated SiGa dopants in GaAs by low-temperature scanning tunneling microscopy.

Zn- and Cd-induced features at the GaAs(110) and InP(110) surfaces studied by low-temperature scanning tunneling microscopy

Tunable pulsed infrared (IR) radiation from a free electron laser is used to selectively excite fullerenes via their vibrational modes to very high internal energies. After absorbing several hundred IR photons, the molecules can autoionize. Ion creation is unexpectedly slow, occurring at times beyond 0.1 ms after laser excitation, but is found to be significantly faster for electronically preexcited molecules. It is concluded that reaching the first electronically excited state is a high hurdle on the way from hot molecules to ions.

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A.J.A. van Roij

Papers

Decelerated molecular beams for high-resolution spectroscopy

Trapping neutral molecules in a traveling potential well.

Direct observation of Friedel oscillations around incorporated SiGa dopants in GaAs by low-temperature scanning tunneling microscopy.

Zn- and Cd-induced features at the GaAs(110) and InP(110) surfaces studied by low-temperature scanning tunneling microscopy

Shedding new light on thermionic electron emission of fullerenes