Roman V. Krems

Cold and ultracold molecules: science, technology and applications

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.

Cold controlled chemistry.

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.

Editorial Quo vadis, cold molecules?

We give a snapshot of the rapidly developing field of ultracold polar molecules abd walk the reader through the papers appearing in this topical issue.

Molecules near absolute zero and external field control of atomic and molecular dynamics

This article reviews the current state of the art in the field of cold and ultracold molecules and demonstrates that chemical reactions, inelastic collisions and dissociation of molecules at subkelvin temperatures can be manipulated with external electric or magnetic fields. The creation of ultracold molecules may allow for spectroscopy measurements with extremely high precision and tests of fundamental symmetries of nature, quantum computation with molecules as qubits, and controlled chemistry. The probability of chemical reactions and collisional energy transfer can be very large at temperatures near zero kelvin. The collision energy of ultracold atoms and molecules is much smaller than perturbations due to interactions with external electric or magnetic fields available in the laboratory. External fields may therefore be used to induce dissociation of weakly bound molecules, stimulate forbidden electronic transitions, suppress the effect of centrifugal barriers in outgoing reaction channels or tune Feshbach resonances that enhance chemical reactivity.

Cold molecules: theory, experiment, applications

COLD COLLISONS Theory of Cold Atomic and Molecular Collisions, J.M. Hutson Electric Dipoles at Ultralow Temperatures, J.L. Bohn Inelastic Collisions and Chemical Reactions of Molecules at Ultracold Temperatures, G. Quemener, N. Balakrishnan and A. Dalgarno Effects of External Electromagnetic Fields on Collisions of Molecules at Low Temperatures, T.V. Tscherbul and R.V. Krems PHOTOASSOCIATION Ultracold Molecule Formation by Photoassociation, W.C. Stwalley, P.L. Gould, and E.E. Eyler Molecular States Near a Collision Threshold, P.S. Julienne Prospects for Control of Ultracold Molecule Formation via Photoassociation with Chirped Laser Pulses, E. Luc-Koenig and F. Masnou-Seeuws Adiabatic Raman Photoassociation with Shaped Laser Pulses, E.A. Shapiro and M. Shapiro FEW- AND MANY-BODY PHYSICS Ultracold Feshbach Molecules, F. Ferlaino, S. Knoop, and R. Grimm Molecular Regimes in Ultracold Fermi Gases, D.S. Petrov, C. Salomon, and G.V. Shlyapnikov Theory of Ultracold Feshbach Molecules, T.M. Hanna, H. Martay and T. Kohler Condensed Matter Physics with Cold Polar Molecules, G. Pupillo, A. Micheli, H.P. Buchler, and P. Zoller COOLING AND TRAPPING Cooling, Trap Loading, and Beam Production Using a Cryogenic Helium Buffer Gas, W.C. Campbell and J.M. Doyle Slowing, Trapping, and Storing of Polar Molecules by Means of Electric Fields, S.Y.T. van de Meerakker, H.L. Bethlem, and G. Meijer TESTS OF FUNDAMENTAL LAWS Preparation and Manipulation of Molecules for Fundamental Physics Tests, M.R. Tarbutt, J.J. Hudson, B.E. Sauer, and E.A. Hinds Variation of the Fundamental Constants as Revealed by Molecules: Astrophysical Observations and Laboratory Experiments, V.V. Flambaum and M.G. Kozlov QUANTUM COMPUTING Quantum Information Processing with Ultracold Polar Molecules, S.F. Yelin, D. DeMille, and R. Cote COLD MOLECULAR IONS Sympathetically Cooled Molecular Ions: From Principles to First Applications, B. Roth and S. Schiller Index

Manipulation of molecules with electromagnetic fields

The goal of the present article is to review the major developments that have led to the current understanding of molecule–field interactions and experimental methods for manipulating molecules with electromagnetic fields. Molecule–field interactions are at the core of several, seemingly distinct areas of molecular physics. This is reflected in the organisation of this article, which includes sections on field control of molecular beams, external field traps for cold molecules, control of molecular orientation and molecular alignment, manipulation of molecules by non-conservative forces, ultracold molecules and ultracold chemistry, controlled many-body phenomena, entanglement of molecules and dipole arrays, and stability of molecular systems in high-frequency super-intense laser fields. The article contains 852 references.

Quantum-mechanical theory of atom-molecule and molecular collisions in a magnetic field: Spin depolarization

A theory for quantum-mechanical calculations of cross sections for atom-molecule and molecular collisions in a magnetic field is presented. The formalism is based on the representation of the wave function as an expansion in a fully uncoupled space-fixed basis. The systems considered include 1S-atom-2Sigma-molecule, 1S-atom-3Sigma-molecule, 2Sigma-molecule-2Sigma-molecule, and 3Sigma-molecule-3Sigma-molecule. The theory is used to elucidate the mechanisms for collisionally induced spin depolarization.

Interaction of NH(XΣ−3) with He: Potential energy surface, bound states, and collisional Zeeman relaxation

A detailed analysis of the He-NH((3)Sigma(-)) van der Waals complex is presented. We discuss ab initio calculations of the potential energy surface and fitting procedures with relevance to cold collisions, and we present accurate calculations of bound energy levels of the triatomic complex as well as collisional properties of NH molecules in a buffer gas of (3)He. The influence of the external magnetic field used to trap the NH molecules and the effect of the atom-molecule interaction potential on the collisionally induced Zeeman relaxation are explored. It is shown that minute variations of the interaction potential due to different fitting procedures may alter the Zeeman relaxation rate at ultralow temperatures by as much as 50%.

Fine-Structure Excitation of O I and C I by Impact with Atomic Hydrogen

Using accurate interaction potentials, we perform refined calculations of rate coefficients for the fine-structure excitations in collisions of O(3P) and C(3P) with atomic hydrogen. The results are presented in the form of analytical functions approximating the rate coefficients over a wide range of temperatures. We examine the sensitivity of the collision dynamics to variations of the interaction potentials and the couplings to electronically excited states.

The He–CaH(2Σ+) interaction. II. Collisions at cold and ultracold temperatures

We present cross sections for rotational, vibrational, and fine-structure transitions in He–CaH(2Σ) collisions at cold and ultracold temperatures calculated using the ab initio potential energy surface reported in the preceding paper. Rotational quenching is fast, vibrational quenching is slow. The spin-rotational interaction, although small and having no influence at temperatures above 10 K, changes significantly the rate coefficients for rotational quenching at lower temperatures. The theoretical rotational, vibrational, and elastic cross sections are compared with the results of a buffer gas cooling experiment carried out at a temperature of about 0.4 K. The theoretical predictions for the vibrational and elastic cross sections are larger than the measured values. The sensitivity to the potential energy surface is explored. A modified surface diminishes but does not remove the differences between theory and experiment.