Manipulating ultracold polar molecules with microwave radiation: The influence of hyperfine structure

We calculate the microwave spectra of ultracold (KRb)-K-40-Rb-87 alkali-metal dimers, including hyperfine interactions and in the presence of electric and magnetic fields. We show that microwave transitions may be used to transfer molecules between different hyperfine states, but only because of the presence of nuclear quadrupole interactions. Hyperfine splittings may also complicate the use of ultracold molecules for quantum computing. The spectrum of molecules oriented in electric fields may be simplified dramatically by applying a simultaneous magnetic field.