  We provide a theoretical treatment of the quantum backaction of Larmor
frequency measurements on a spinor Bose-Einstein condensate by an off-resonant
light field. Two main results are presented; the first is a "quantum jump"
operator description that reflects the abrupt change in the spin state of the
atoms when a single photon is counted at a photodiode. The second is the
derivation of a conditional stochastic master equation relating the evolution
of the condensate density matrix to the measurement record. We comment on
applications of this formalism to metrology and many-body studies.
