Atomic parity violation in 0-to-0 two-photon transitions
  We present a method for measuring atomic parity violation in the absence of static external electric and magnetic fields. Such measurements can be achieved by observing the interference of parity conserving and parity violating two-photon transition amplitudes between energy eigenstates of zero electronic angular momentum. General expressions for induced two-photon transition amplitudes are derived. The signal-to-noise ratio of a two-photon scheme using the 6s^2 1S0 to 6s6p 3P0 transition in ytterbium is estimated. 
D. R. Dounas-Frazer, K. Tsigutkin, D. English, and D. Budker
physics.atom-ph