3. An atomic or molecular beam resonator comprising source means for producing said beam, first and second state selector means for deflecting said beam, separating means for providing an interaction zone separating said first and second selector means, means for generating an oscillating electric field in said interaction zone, and a detector, said second state selector comprising dipolar assembly means having an electric field, and having three electrodes, one of said electrodes being positive and another negative with respect to said third electrode, said third electrode being disposed between said positive an negative electrodes.

4. An atomic beam resonator having a principal axis and comprising a source for producing said beam, first state selector means for producing a multipole magnetic field for focusing the atoms of one state of energy toward said axis, means providing an interaction zone, means for generating an oscillating magnetic field within said zone for producing transitions to another energy state having a magnetic dipole moment opposite to that of said one state of energy, second state selector means having a dipole magnet assembly comprising two concave shaped pole pieces and a convex shaped central pole piece placed in between said concave pieces for concentrating the magnetic lines of force of said assembly so that the atoms in said one energy state are deflected towards said axis in order to be detected by said detector.