Feshbach resonances are the essential tool to control the interaction between atoms in ultracold quantum gases. They have found numerous experimental applications, opening up the way to important breakthroughs. This review broadly covers the phenomenon of Feshbach resonances in ultracold gases and their main applications. This includes the theoretical background and models for the description of Feshbach resonances, the experimental methods to find and characterize the resonances, a discussion of the main properties of resonances in various atomic species and mixed atomic species systems, and an overview of key experiments with atomic Bose-Einstein condensates, degenerate Fermi gases, and ultracold molecules.

Feshbach resonances in ultracold gases

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

Advances in atomic physics, such as cooling and trapping of atoms and molecules and developments in frequency metrology, have added orders of magnitude to the precision of atom-based clocks and sensors. Applications extend beyond atomic physics and this article reviews using these new techniques to address important challenges in physics and to look for variations in the fundamental constants, search for interactions beyond the standard model of particle physics, and test the principles of general relativity.

Search for New Physics with Atoms and Molecules

Spinor Bose–Einstein condensates

The technique of stimulated Raman adiabatic passage (STIRAP), which allows efficient and selective population transfer between quantum states without suffering loss due to spontaneous emission, was introduced in 1990 (Gaubatz \emph{et al.}, J. Chem. Phys. \textbf{92}, 5363, 1990). Since then STIRAP has emerged as an enabling methodology with widespread successful applications in many fields of physics, chemistry and beyond. This article reviews the many applications of STIRAP emphasizing the developments since 2000, the time when the last major review on the topic was written (Vitanov \emph{et al.}, Adv. At. Mol. Opt. Phys. \textbf{46}, 55, 2001). A brief introduction into the theory of STIRAP and the early applications for population transfer within three-level systems is followed by the discussion of several extensions to multi-level systems, including multistate chains and tripod systems. The main emphasis is on the wide range of applications in atomic and molecular physics (including atom optics, cavity quantum electrodynamics, formation of ultracold molecules, precision experiments, etc.), quantum information (including single- and two-qubit gates, entangled-state preparation, etc.), solid-state physics (including processes in doped crystals, nitrogen-vacancy centers, superconducting circuits, etc.), and even some applications in classical physics (including waveguide optics, frequency conversion, polarization optics, etc.). Promising new prospects for STIRAP are also presented (including processes in optomechanics, detection of parity violation in molecules, spectroscopy of core-nonpenetrating Rydberg states, and population transfer with X-ray pulses).

Stimulated Raman adiabatic passage in physics, chemistry, and beyond

We report on the direct conversion of laser-cooled $^{41}\mathrm{K}$ and $^{87}\mathrm{Rb}$ atoms into ultracold $^{41}\mathrm{K}^{87}\mathrm{Rb}$ molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.

Coherent transfer of photoassociated molecules into the rovibrational ground state

We review our experimental progress toward achieving the production of quantum degenerate bosonic polar molecules, 41K87Rb. Our primary goal is to transfer quantum degenerate Feshbach molecules into the absolute ground state via the stimulated Raman adiabatic passage (STIRAP). We have achieved a dual-species Bose–Einstein condensate of 41K and 87Rb in a magnetic trap, which consists of NK=1.4×104 potassium atoms and NRb=3.5×104 rubidium atoms. An optimum optical transition for STIRAP is being identified by a separate experiment based on photoassociation in a dual-species magneto-optical trap of 41K and 87Rb.

https://iopscience.iop.org/article/10.1088/1367-2630/11/5/055035/pdf

Toward the production of quantum degenerate bosonic polar molecules, 41K87Rb

We propose and experimentally investigate a scheme for narrow-line cooling of KRb molecules in the rovibrational ground state. We show that the spin-forbidden $\mathrm{X^1\Sigma^+} \rightarrow \mathrm{b^3\Pi_{0^+}}$ transition of KRb is ideal for realizing narrow-line laser cooling of molecules because it has highly diagonal Franck-Condon factors and narrow linewidth. In order to confirm the prediction, we performed the optical and microwave spectroscopy of ultracold $^{41}$K$^{87}$Rb molecules, and determined the linewidth ($2\pi\times$ 4.9(4) kHz) and Franck-Condon factors for the $\mathrm{X^1\Sigma^+} (v''=0) \rightarrow \mathrm{b^3\Pi_{0^+}} (v'=0)$ transition (0.9474(1)). This result opens the door towards all-optical production of polar molecules at sub-microkelvin temperatures.

Prospects for narrow-line cooling of KRb molecules in the rovibrational ground state

Experimental techniques to manipulate cold molecules have seen great development in recent years. The precision measurements of cold molecules are expected to give insights into fundamental physics. Here we use a rovibrationally pure sample of ultracold KRb molecules to improve the measurement on the stability of electron-to-proton mass ratio $$\left( {\mu = \frac{{m_{\mathrm{e}}}}{{M_{\mathrm{p}}}}} \right)$$
 . The measurement is based upon a large sensitivity coefficient of the molecular spectroscopy, which utilizes a transition between a nearly degenerate pair of vibrational levels each associated with a different electronic potential. Observed limit on temporal variation of μ is $$\frac{1}{\mu }\frac{{d\mu }}{{dt}} = (0.30 \pm 1.0) \times 10^{ - 14} \, {\mathrm{year}}^{ - 1}$$
 , which is better by a factor of five compared with the most stringent laboratory molecular limits to date. Further improvements should be straightforward, because our measurement was only limited by statistical errors. Ultracold molecules are suitable platforms for precision measurements due to their internal degrees of freedom. Here the authors derive a limit on the variation of the electron-to-proton mass ratio by using the spectroscopy of ultracold KRb molecules.

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Measurement of the variation of electron-to-proton mass ratio using ultracold molecules produced from laser-cooled atoms

We have investigated the collisional properties of $^{41}\text{K}$ atoms at ultracold temperatures. To demonstrate the possibility of using $^{41}\text{K}$ as a coolant, a Bose-Einstein condensate of $^{41}\text{K}$ atoms in the stretched state $(F=2,{m}_{F}=2)$ was created by direct evaporative cooling in a magnetic trap. An upper bound of the three-body loss coefficient for atoms in the condensate was determined to be $4(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}29}\text{ }{\text{cm}}^{\ensuremath{-}6}\text{ }{\text{s}}^{\ensuremath{-}1}$. A Feshbach resonance in the $F=1,\text{ }{m}_{F}=\ensuremath{-}1$, state was observed at 51.35(10) G, which is in good agreement with theoretical prediction.

Jun Kobayashi

Papers

Coherent transfer of photoassociated molecules into the rovibrational ground state

Toward the production of quantum degenerate bosonic polar molecules, 41K87Rb

Prospects for narrow-line cooling of KRb molecules in the rovibrational ground state

Measurement of the variation of electron-to-proton mass ratio using ultracold molecules produced from laser-cooled atoms

Direct evaporative cooling of K 41 into a Bose-Einstein condensate