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

It has long been predicted that the scattering of ultracold atoms can be altered significantly through a so-called ‘Feshbach resonance’. Two such resonances have now been observed in optically trapped Bose–Einstein condensates of sodium atoms by varying an external magnetic field. They gave rise to enhanced inelastic processes and a dispersive variation of the scattering length by a factor of over ten. These resonances open new possibilities for the study and manipulation of Bose–Einstein condensates.

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Observation of Feshbach resonances in a Bose–Einstein condensate

Feshbach Resonances in Ultracold Gases

Energy levels of light nuclei A=8,9,10

The method of complex coordinate rotation and its applications to atomic collision processes

New experimental results on muon-catalyzed d-t fusion are reported. Unexpected target-density effects have been discovered both in dt\ensuremath{\mu} molecular formation in t\ensuremath{\mu}+${\mathrm{D}}_{2}$ collisions and in the effective sticking probability. The dt\ensuremath{\mu} formation rate is significantly enhanced at high density, presumably because of a strong three-body contribution. The origin of the observed reduction of the sticking probability at high density remains unclear. Both effects increase the number of fusions per muon that can be achieved; 150\ifmmode\pm\else\textpm\fi{}4(stat.)\ifmmode\pm\else\textpm\fi{}20(syst.) fusions per muon have been observed.

Observation of unexpected density effects in muon-catalyzed d-t fusion.

Measurements of the absolute neutron yield and the time dependence of the appearance of neutrons resulting from muon-catalyzed fusion have been carried out in high-density deuterium-tritium mixtures. The temperature dependence of the resonant dt..mu..-molecular formation process has been determined in the range 100 to 540 K. Mesomolecular formation is found to be resonant for DT as well as D/sub 2/ target molecules. The sticking probability and other fundamental parameters have been measured for the first time.

Experimental investigation of muon-catalyzed d-t fusion

We have used a colliding-beam method to find two resonances in the photodetachment of electrons from H/sup -/. A nitrogen laser beam is directed at variable angle across the 800-meV H/sup -/ beam at the Clinton P. Anderson Meson Physics Facility (LAMPF), resulting in a center-of-mass photon beam wavelength which is continuously tunable from the visible to the vacuum ultraviolet. Our preliminary measurements of the two resonances observed near 11 eV agree well with theoretical predictions within our experimental resolution of 10 meV. (AIP)

https://link.aps.org/pdf/10.1103/PhysRevLett.39.1236

Observation of Resonances near 11 eV in the Photodetachment Cross Section of the H − Ion

Photoejection of both electrons from the H/sup -/ ion by a single incident photon has been observed with use of a crossed relativistic beam technique. The dipole selection rules for a single-photon pick out the /sup 1/P/sup 0/ final state. The relative cross section was measured over a 0.30 eV range of photon energies near threshold in steps of 0.007 eV with a resolution of 0.007 eV full width at half maximum. The energy dependence of the cross section in this region was consistent with a Wannier threshold law.

Observation of two-electron photoionization of the H/sup -/ ion near threshold

Relativistic kinematics with an 800-MeV H/sup -/ beam at the Clinton P. Anderson Meson Physics Facility (LAMPF) at Los Alamos allows the observation, with very little background, of resonant structures in the photodetachment spectrum of H/sup -/ in intense electric fields. We report studies of the /sup 1/P resonances near n = 2 in H/sup -/ in electric fields as high as 1.3 MV/cm. The splitting of the /sup 1/P Feshbach resonance can be explained by Stark mixing with a nearly degenerate /sup 1/S resonance. For field values above those required to quench the Feshbach resonances a /sup 1/D state is observed, apparently induced by Stark mixing with the /sup 1/P shape resonance. The systematics of the behavior of these resonances are compared with the results of a simple perturbation model.

P. A. M. Gram

Papers

Observation of unexpected density effects in muon-catalyzed d-t fusion.

Experimental investigation of muon-catalyzed d-t fusion

Observation of Resonances near 11 eV in the Photodetachment Cross Section of the H − Ion

Observation of two-electron photoionization of the H/sup -/ ion near threshold

Effects of strong electric fields on resonant structures in H - photodetachment