Atomic absorption of twisted light

In undergraduate quantum mechanics courses, it’s a common exercise to derive the likelihood that an electron in an atom will absorb a photon. Conservation of angular momentum leads to so-called selection rules that dictate the possible changes in the electron’s quantum state. The calculations usually assume that the photon comes from a uniform, plane-wave light beam and thus carries only spin angular momentum. But light can also carry orbital angular momentum (see the article by Miles Padgett, Johannes Courtial, and Les Allen, Physics Today, May 2004, page 35 ). Such twisted light has a helical wavefront that spirals along the beam direction, and the selection rules for the absorption of twisted light are much more complicated. In an international collaboration, an experimental group at Mainz University led by Ferdinand Schmidt-Kaler has now used a single trapped calcium ion to experimentally measure how the photon-absorption rate depends on the ion’s position within the beam and on the beam’s orbital angular momentum and polarization. The results agree within 3% with the theoretical predictions derived for various kinds of twisted beams by team member Andrei Afanasev and his fellow theorists at the George Washington University and the College of William and Mary. (The figure illustrates the predicted contours for a linearly polarized Bessel–Gauss beam.) Better knowledge of matter’s interaction with twisted photon states could lead to new methods for beam polarimetry and characterization and for quantum information processing. (A. Afanasev et al., New J. Phys, in press .)