Inspired by the electromagnetic duality, we propose an approach to realize the fractional angular momentum by using a cold atom which possesses a permanent magnetic dipole moment. This atom interacts with two electric fields and is trapped by a harmonic potential which enable the motion of the atom to be planar and rotationally symmetric. We show that eigenvalues of the canonical angular momentum of the atom can take fractional values when the atom is cooled down to its lowest kinetic energy level. The fractional part of canonical angular momentum is dual to that of the fractional angular momenta realized by using a charged particle. Another approach of getting the fractional angular momentum is also presented. The differences between these two approaches are investigated.In 1984, Aharonov and Casher predicted that there would exist a topology phase when a neutral particle possessing a non-vanishing magnetic dipole moment moved around a uniformly charged infinitely long filament with its direction paralleling to the filament [1]. It is named Aharonov-Casher (AC) effect.In three-dimensional space, the Hamiltonian which governs the dynamics of a neutral particle possessing a permanent magnetic dipole moment in the background of an electric field is given bywhere m is the mass of the neutral particle, p = −i ∇ is the canonical momentum, µ is the magnitude of the magnetic dipole moment, c is the speed of light in vacuum, n is the unit vector along the magnetic dipole moment and E is the electric field. In AC effect setting, the electric field is produced by a uniformly charged infinitely long filament [1]. The explicit form of the electric field in AC effect isin which λ is charges per unit length on the long filament, ǫ 0 is the permittivity of vacuum, r is the distance between the atom and the long filament and e r is the unit vector along the radial direction on the plane where the atom moves. For AC setting, the last term in Hamiltonian (1) disappears since ∇ · E AC = 0 for r = 0. Hamiltonian (1) is the non-relativistic limit of a relativistic spin-half particle which possesses a permanent magnetic dipole moment in the background of an * Electronic address: jingjian@mail. buct. edu. cn † Electronic address: dongsh2@yahoo. com electromagnetic field. When the neutral atom moves around the uniformly charged infinitely long filament, it will receive a topology phase. The acquired topology phase is given bywhich has been observed in the experiment [2]. Possible classical explanations about AC effect have been presented [3,4]. Inspired by the work of Aharonov and Casher, refs.[5-11] studied topological phases neutral particles would receive in various backgrounds. AC effect is dual to the Aharonov-Bohm (AB) effect. Over half century ago, Aharonov and Bohm predicted that a charged particle would generate a topology phase when it circled around a long-thin flux-carried solenoid. It is known as AB effect [12].The Hamiltonian which describes a charged particle in the background of magnetic potentials in three-dimensional s...