We show that atomic motion in a hollow cylindrical waveguide of circular cr
oss section is subject to a quantised torque arising from the orbital angul
ar momentum effects associated with any excited waveguide mode of order l >
0. In addition to its axial motion due to light pressure, an appropriately
detuned atom immersed in such a mode may be trapped radially in vibrationa
l states and be made to exhibit novel rotational effects associated with th
e light torque, including the recently predicted rotational frequency shift
. (C) 1999 Elsevier Science B.V. All rights reserved.