Magnetic field-induced spectroscopy of forbidden optical transitions   with application to lattice-based optical atomic clocks
  We develop a method of spectroscopy that uses a weak static magnetic field to enable direct optical excitation of forbidden electric-dipole transitions that are otherwise prohibitively weak. The power of this scheme is demonstrated using the important application of optical atomic clocks based on neutral atoms confined to an optical lattice. The simple experimental implementation of this method -- a single clock laser combined with a DC magnetic field-- relaxes stringent requirements in current lattice-based clocks (e.g., magnetic field shielding and light polarization), and could therefore expedite the realization of the extraordinary performance level predicted for these clocks. We estimate that a clock using alkaline earth-like atoms such as Yb could achieve a fractional frequency uncertainty of well below 10^-17 for the metrologically preferred even isotopes. 
A. V. Taichenachev, V. I. Yudin, C. W. Oates, C. W. Hoyt, Z. W.   Barber, and L. Hollberg